The ancient Celts thrived in Europe, earning a reputation as fierce warriors. They sacked Rome around 390 B.C. and, decades later, were received by Alexander the Great while he was conquering his empire.

It's no surprise, then, that archaeologists find many weapons in Celtic graves. However, over the decades, some of those weapons have been found in burials of women, hinting that they may have used these weapons. What's more, recent genetic studies suggest that elite Celts inherited power through maternal lines.

The Celts lived throughout Europe, from what is now Spain to Turkey. They shared a similar language and culture, and clues of female power have been found throughout their old lands and hinted at in ancient texts. This raises an interesting question: Did the Celts have women who fought in battle?

Opinions vary, but some experts Live Science interviewed thought it was possible that Celtic women could have fought in battle, although perhaps not regularly. One problem is that there is an ongoing debate among scholars as to whether the inhabitants of the British Isles around 2,000 years ago were, in fact, Celtic.

Genetic hints

Although it's not directly related to answering the question about Celtic women warriors, a few recent genetic studies have found that elite Celtic women held powerful roles. A 2024 study in the journal Nature Human Behaviour that looked at ancient DNA in several Celtic burials from Germany found a matrilineal inheritance of power, and a 2025 study of burials from Iron Age England revealed that men married into local, powerful female lineages.

This supports textual evidence that Celtic women had more power than many of their female counterparts across Europe. Women "of the Celts did generally enjoy more freedom and opportunity than their counterparts in the Greek and Roman world," Philip Freeman, a humanities professor at Pepperdine University, wrote in his book "War, Women and Druids: Eyewitness Reports and Early Accounts of the Celts" (University of Texas Press, 2002). "It is hard to imagine a Greek wife joining her husband on the battlefield or even leading an army, as did the British queen Boudicca."

"Flashing-eyed" women

Another piece of possible evidence is a particularly colorful ancient quote about the Gauls, a Celtic group that lived in what is now France.

The Roman soldier and historian Ammianus Marcellinus (who lived circa A.D. 330 to 395) wrote in his book "Res Gestae" ("Achievements") that "a whole group of foreigners cannot stand against one of them in a quarrel if he involves his wife, stronger than him by far and flashing-eyed — least of all when she swells her neck and gnashes her teeth and, swinging her vast, white arms, proceeds to rain down punches mixed with kicks, like shots discharged from the twisted cords of a catapult" (translation by Gavin Kelly).

In other words, don't mess with a Gallic wife.

But we have to be careful with the Ammianus quote as it refers to brawling rather than warfare, said Gavin Kelly, a professor of Latin literature and Roman history at the University of Edinburgh. We shouldn't take the quote to mean that Celtic women fought in battle, Kelly said. He doesn't know if Celtic women fought in battle but, if they did, Kelly suspects it was rare.

Weapons in burials

Rachel Pope, an archaeology professor at the University of Liverpool who has studied the Celts extensively, told Live Science in an email that "we have good evidence now across Europe that a small number of women were buried with weapons." In a 2021 Journal of Archaeological Research study, she mentioned cases in which women were buried with daggers or spears, including a 2,500-year-old cemetery in eastern France where some women were buried with daggers.

Some of the best evidence is from Britain, where "we have osteological evidence from three hillforts now that [some] women took part in combat," Pope said, noting that there "is evidence too for women fighters in northern Britain, where women are buried with chariots."

Simon Mays, a senior human skeletal biologist at Historic England who has studied a woman buried with weapons on the Isles of Scilly, off the southwest coast of Britain, said women in the region were probably involved in battle. There "is little doubt that violence was a routine part of Iron Age life, and [thus] female involvement is likely," Mays told Live Science in an email.

However, just because someone fought in battle doesn't make them a "warrior," Emmet Taylor, a lecturer at the School of Irish Learning at University College Cork, told Live Science in an email. Taylor noted that it "is important for us to think about martial identities complexly," and in ancient and early medieval Ireland, there is little evidence that women were allowed to join social groups that could be considered warriors.

Can ancient Britons be considered Celts?

Most famously, there is Boudica (sometimes spelled Boudicca), a queen of the Iceni (a people in England) who led a revolt against the Roman Empire around A.D. 60. She led her forces to several victories against the Romans before being defeated at the Battle of Watling Street in A.D. 61, after which she died — either by illness or by taking poison to avoid being captured by the Romans.

However, whether the Iceni or other ancient groups in Britain were Celtic is a matter of debate among scholars. The ancient geographer Strabo (who lived circa 64 B.C. to A.D. 24) actually "distinguished Britons from Celts," John Collis, a professor emeritus of archaeology at the University of Sheffield, wrote in his book "The Celts: Origins, Myths and Inventions" (Tempus, 2003).

Terms such as "Celt" and "Gaul" were "never used for the inhabitants of the British Isles except in the most general way for all the inhabitants of western Europe including non Indo-European speakers such as Basques," Collis wrote.

However, although ancient writers didn't refer to people in Britain as Celts, they did note their cultural and linguistic similarities, Freeman wrote. Following this line of thought, it's not too far-fetched to call Boudica a Celt.

Miranda Aldhouse-Green, a professor emeritus of archaeology at Cardiff University in the U.K., told Live Science in an email that she prefers "to refer to Boudica as British rather than Celtic" but noted that Boudica's rebellion is part of substantial evidence for warrior women in Britain.

"There is archaeological evidence for war-women in Britain in the form of Iron Age chariot [burials]," she said. For example, she noted the story of Cartimandua of the Brigantes in Yorkshire, who allied herself with Rome and fought against anti-Roman groups in the region.

"So yes, women did fight particularly in Britain," Aldhouse-Green said. But there is "less evidence" on the European continent for women fighting in battle, she noted.

In Britain the evidence for women in battle is very strong. "In the texts, we have Boudica, raising an army and razing three cities, and [the ancient writer] Tacitus tells us women commanding combat was not unusual," Pope said.

See how much you know about the celts with our Celtic quiz!

Cats are known for the whiskers on their faces. However, if you look closely, you can see that they also have whiskers on their legs. So why do cats have leg whiskers?

To answer why they're useful, it's important to understand what whiskers are. Technically known as vibrissae, whiskers are specialized hairs that are stiffer and several times thicker than other hairs. Although whiskers themselves do not contain nerves, their bases are rooted in specialized receptacles loaded with nerve endings, making whiskers very sensitive.

"Facial whiskers are as sensitive as human fingertips," Robyn Grant, a sensory biologist at Manchester Metropolitan University in the U.K., told Live Science.

A cat's whiskers around the nose, which are also known as the mystacial vibrissae, are the longest whiskers they have, according to the veterinary reference book "The Feline Patient" (Wiley, 2025). Although there is variation among cats, on average there are 12 mystacial whiskers set in four rows on each side of the muzzle.

Facial whiskers help cats hunt when light is limited or when their vision is poor. Although cats are good at seeing distant objects, "cats can't really focus on anything less than 10 inches (25 centimeters) from their eyes, so they use their whiskers to figure out where things are," Jonathan Losos, an evolutionary biologist at Washington University in St. Louis and author of "The Cat's Meow: How Cats Evolved from the Savanna to Your Sofa" (Viking, 2023), told Live Science.

In addition, cats have a tuft of whiskers known as superciliary vibrissae above each eye. These whiskers help protect the cat's vision by triggering a blink when stimulated by anything that might fly into their eye, according to The Feline Patient.

Cats also have chin whiskers (mandibular vibrissae) and whiskers on each side of the head, between the ear and chin (genal vibrissae). These also help cats stay aware of the space around them.

Leg whiskers

Cats also possess a tuft of four to six whiskers on the back of each front leg, each about 0.8 to 1.2 inches (2 to 3 centimeters) long. These are known as carpal vibrissae.

"Carpal refers to the bones that, in humans, are in our wrists," Losos said. "Carpal whiskers are attached to cat forelegs near the carpal bones." However, in cats, the carpal bones are not located in the same place as the human wrist. Rather, they "are further up the leg, because cats essentially walk on their toes and not their palms," he explained.

Cats are not the only animals with carpal whiskers. "Most carnivores have them, and many rodents do, as do lemurs and armadillos," Losos said. "Dogs don't have them, but other canine species do. Rabbits don't have them either, nor do ungulates — that is, hooved animals, such as horses and cattle."

The species that have carpal whiskers "tend to be the ones that either grasp things with their forelimbs or climb up trees," Losos said. "The idea is that if it's hard for the animals to see what they're grasping, carpal whiskers can help supplement their vision. Perhaps cats have them and dogs don't because cats can move their forearms laterally from side to side and put their palms together to grab things, whereas dogs can't."

It remains uncertain what role carpal whiskers play in cats, but they likely "guide locomotion — that is, safe forepaw placement — especially during complex locomotion, such as climbing," Grant said. "They might guide prey handling too."

How much of a cat fan are you? Find out by taking our cat quiz!

Redheads are a rare breed, with people who sport ginger locks making up less than 2% of the global population. With such low numbers, would it be possible for red hair to go "extinct," leaving people with natural hair hues ranging from black to blond, with nary a hint of red?

In a word, no. Experts told Live Science about how the genetics of red hair means the trait is sticking around.

"There is no evidence that redheads are going extinct," said Katerina Zorina-Lichtenwalter, a research associate at the University of Colorado Boulder's Institute for Behavioral Genetics who has studied red hair inheritance. She told Live Science that the genes responsible for red hair "are harmless, they do not interfere with reproduction and they are not linked to a disease that would prevent people from reaching reproductive age."

Nevertheless, the myth of an impending red hair extinction isn't a new one.

"The 'Is red hair becoming extinct?' question has been around for decades," Ian Jackson, a professor emeritus at the University of Edinburgh's Institute of Genetics and Cancer, told Live Science. "There have also been extinction stories about blond hair that go back to the mid-19th century."

The logic behind the supposed redhead extinction usually goes something like this: Red hair is rare; in the world's redhead hotspots in the United Kingdom and Ireland, only about 10% of the population is categorized as redheads. And red hair is a recessive trait, meaning two redhead carriers have to have get together to have redheaded children.

There's even the question of whether potentially deadly conditions, such as melanoma, might wipe out redheads, who are especially susceptible to sunburn. While there are people of color with red hair (Malcolm X famously had reddish hair), red hair is often linked with pale skin and, therefore, elevated skin-cancer risk. Redheads' higher melanoma risk isn't only because they burn more easily. It's also due to harmful chemistry from pheomelanin, the pigment that helps create the reddish hair hue, and reduced DNA‑repair protection, which is linked to variants of the gene that causes red hair.

Keeping the flame alive

But these issues won't doom redheads to extinction, experts said.

First, let's look at genetics. A person needs to inherit the recessive version of MC1R — a gene involved in pigment production — from both parents. Someone with only one copy may not have red hair at all, but they can still pass the variant to their children. So, the role of carrying red hair genes through the generations is shared by people of all hair colors, not just redheads.

This hidden-carrier effect is why a recessive trait can remain in a population even when it is uncommon to see. Jackson gave a simple example: If a variant is carried by half the population, only about a quarter of couples would both carry it. Among those couples, roughly one-quarter of children would inherit two copies.

For red hair to truly disappear, the relevant variants would have to be steadily removed from the gene pool. In evolutionary terms, that would usually mean people carrying these variants — or people with red hair — were having fewer children than everyone else.

"The only reason things go extinct in large populations is because you don't produce as many kids as the others, to put it in blunt terms, and I don't know that there's any evidence for that," Jonathan Rees, professor emeritus of dermatology at the University of Edinburgh, told Live Science.

Second, let's look at skin cancer risk. Rees said people with MC1R variants have a greater skin-cancer risk "somewhere in the region of two to four fold."

But skin cancer risk does not automatically equal evolutionary decline. Many skin cancers occur after people have already had children. Mortality rates for melanoma in the U.K. are the highest in people aged 90 and older, according to Cancer Research UK, after they've had more than enough time to pass on their genes.

Theoretically, before the invention of sunscreen and modern medicine, it may have been more likely for redheads to have died of skin cancer before they had children. Given that the trait has persisted, though, are there any evolutionary benefits to having red hair?

Red hair may carry surprising health advantages. For instance, recent research suggested the red‑hair pigment pheomelanin can help regulate excess cellular cysteine, which can be harmful at high concentrations.

"The vitamin D story is the one that people most often peddle," Rees said. The idea being that lighter pigmentation of the skin (and red hair being linked to lighter skin) increases the amount of vitamin D that people can make during the gloomy winters. But he added that the evidence there is lacking, especially because it can be hard to define who, exactly, is a redhead. It's a spectrum: where does red hair end and strawberry blond hair begin? That makes it hard to study without sequencing the genomes of large swathes of the population.

However, the frequency of red hair could still fluctuate. Jackson said the frequency may "go down somewhat as people become more mobile" and have children with partners from regions where MC1R variants are less common. But that would be a change in frequency, not extinction.

As long as MC1R variants persist silently in carriers — and as long as redheads are not having significantly fewer children than everyone else — the world's redheads are not about to disappear.

Beyond familiar particles such as electrons and protons, scientists have discovered a zoo of "quasiparticles" with exotic names, such as magnons, angulons, dropletons and polaritons. But what are quasiparticles, exactly? And given the "quasi" in their name, are they considered real particles?

One way to understand quasiparticles is to explore what a particle is. The standard mental picture of a particle "is of a discrete object, like a ball," Douglas Natelson, a condensed matter physicist at Rice University in Houston, told Live Science. However, this classic image of particles changed with the advent of quantum physics, which revealed that the universe becomes fuzzy at its smallest levels. For example, in 1924, French physicist Louis de Broglie showed that particles such as electrons could also behave as waves ‪—‬ a discovery that earned him a Nobel Prize.

Modern physics suggests that particles are excitations in fields that permeate the entire universe, a bit like a ripple in a pond, Ross McKenzie, a theoretical condensed matter physicist and professor emeritus at the University of Queensland in Australia, told Live Science. Each type of particle has its own corresponding field. For instance, photons of light are ripples of the electromagnetic field, McKenzie noted.

The quantum nature of particles means they can not only fly through an empty vacuum but also ripple through matter. For instance, photons can zip through transparent and translucent materials, and electrons can flow inside wires.

If you can picture a standard particle, such as an electron, as a ripple traveling within a material, you can also envision other kinds of excitations within matter. These other types of ripples are quasiparticles.

For example, think of "the wave" done at a sporting event. "You can watch it propagate around a football stadium; it has a location and a speed," Natelson said. "It only exists within the stadium, though, and it's made up of the collective response of all the interacting fans."

In much the same way, "quasiparticles can only exist within some medium or material, because they are built up from the response of that material's constituents or building blocks," Natelson said; in contrast, particles such as electrons and protons "can exist in free space."

In other words, quasiparticles can't exist in a vacuum and they can't exist on their own. They're reliant on particles acting together and being in a material where they can emerge, just as a wave carried out in a stadium can only exist when there is a group of people there who can perform it.

A philosophical question concerning quasiparticles is whether they are real, McKenzie noted. After all, "quasi" in Latin means "almost."

"Fundamental particles can exist in isolation in a vacuum, and quasiparticles need many interacting particles to exist," McKenzie said. "Still, I would say quasiparticles are, for all intents and purposes, just as real as particles, in how you can detect them and manipulate them."

A zoo of quasiparticles

The concept of quasiparticles originated with theoretical physicist Lev Landau in the 1950s, McKenzie said, and the idea ultimately helped Landau win the Nobel Prize in physics in 1962. Scientists have now proposed the existence of dozens of quasiparticle types, including the following:

• The phonon, which is a quasiparticle of sound — it is the smallest packet of vibrational energy that makes up sound in matter, McKenzie said.
• The electron hole, often simply called a hole, is the positively charged vacancy left behind once an electron has left its original place, McKenzie noted.
• The electron quasiparticle, which essentially consists of an electron and its interactions with its surrounding environment. This combined package means the electron quasiparticle requires more force to move, so it effectively has more mass than a regular electron, McKenzie explained.
• The exciton, a quasiparticle made of an electron and a hole orbiting each other.
• The anyon, a kind of quasiparticle so far only seen in two-dimensional systems, which may carry only a fraction of an electric charge.

"How many kinds of quasiparticles are there?" McKenzie said. "Just like there are an infinite number of possible states of matter, I would say that, in principle, there are an infinite number of types of quasiparticles."

Scientists describe activity in material as quasiparticles "because it simplifies everything dramatically," McKenzie said. In the same vein, Natelson said that "very often the math behind physical phenomena in solids is described well by quasiparticles."

Quasiparticles are not simply useful for bookkeeping. For instance, researchers typically describe the behavior of electricity in electronic devices in forms of quasiparticles such as holes and excitons, Natelson said.

All in all, the answer to the question of whether quasiparticles are real is a matter of interpretation. If the question is whether or not a quasiparticle is an elementary particle such as an electron, the answer is no — they cannot exist on their own, and they depend on the materials in which they manifest.

However, if the question is whether or not quasiparticles are real phenomena, the answer is yes — they are things that scientists can measure that can behave like particles and effectively have many of the same properties that other particles do.

Nowadays, the word "weapon" may evoke modern-day technologies such as firearms, blades and missiles. Yet archaeological evidence shows that weapons have existed for hundreds of thousands of years. Of course, early humans wielded weapons that looked quite different from those used today.

So what is the oldest weapon?

Generally, weapons are defined as objects used to impart violence on other living beings, said Ben Fitzhugh, a professor of anthropology at the University of Washington in Seattle. That includes objects used in interpersonal conflict among hominins (a group that includes modern Homo sapiens and our relatives that evolved following the split from chimps and bonobos) and for hunting, Fitzhugh told Live Science.

But archaeologists don't automatically consider just any object that could cause harm to be a weapon; context and intent matter, Rolf Warming, founding director of the Society for Combat Archaeology and a doctoral candidate in archaeology at Stockholm University, told Live Science in an email. For instance, experts excavating a site with potential weapons might look for evidence of hunting, such as butchered animal bones; or artifacts like pointed bone tools that can be used to craft weapons. They may also consider what objects were made out of and residue or wear marks on the weapons themselves, which can help determine how they were used.

According to Warming, archaeologists generally consider three finds as contenders for the oldest evidence of weaponry in the archaeological record: the Clacton spear, the Schöningen spears and throwing sticks, and the Kathu Pan 1 spear points.

Clacton spear

Scientists have estimated that the Clacton spear is over 400,000 years old. Experts think the uncovered 15-inch-long (37 centimeters) spear point was broken off from a longer shaft, perhaps by accident while the user was hunting, or deliberately as a ritual. It's made from yew wood and was worked with a tool to a sharp point on one end. The broken end was never found.

Excavated in 1911 from Clacton-on-Sea in Essex, England, the Clacton spear is now on display in London's Natural History Museum, where it is labeled as the oldest preserved wooden spear fragment in the world.

Archaeologists believe it was fashioned by either Homo heidelbergensis or Neanderthals — two hominins that predate H. sapiens. In fact, the spear appears to be older than our species, as the oldest known H. sapiens fossils date to around 300,000 years ago. The Clacton spear's shape, size and wood suggest that it was a thrusting weapon and that its makers engaged in cooperative hunting strategies.

Schöningen spears and throwing sticks

Since 1992, archaeologists have excavated between 20 and 25 wooden weapons at a site in Schöningen, Germany, including complete and fragmented spears, and throwing sticks made predominantly from spruce and pine wood.

They're considered the oldest completely preserved wooden weapons in the world, according to the Schöningen Research Museum; today, archaeologists' varying estimates place the weapons' age between 200,000 and 300,000 years old. Their creation has historically been attributed to H. heidelbergensis, an early human species that lived about 700,000 to 200,000 years ago throughout Europe, parts of Africa, and possibly in what is now China. More recent dating suggests the spears could potentially be the work of Neanderthals, according to a 2025 study.

Scientists think these spears, which average just over 6.6 feet (2 meters) long, were used as thrusting weapons, throwing weapons or some combination of both to hunt larger game mammals, like horses, according to a 2024 study. The shorter throwing sticks are believed to be projectile weapons and average around 26 inches (65 centimeters) long.

"In fact, the [Schöningen weapons] were found in association with butchered horses," Fitzhugh noted. The spears were sharpened, hardened and blackened using fire. "These things were shaped in a dynamically sophisticated way," he said.

According to Warming, the Schöningen weapons suggest hominins had clear material knowledge and used different hunting tactics depending how far they were from their target.

Kathu Pan 1 spear points

Hafted spear tips — stone points attached to some form of handle or shaft — became more common in the archaeological record between 200,000 and 300,000 years ago across Africa and Eurasia, and were previously thought to be created by Neanderthals or H. sapiens.

However, 500,000-year-old stone points excavated from the Kathu Pan archaeological site in South Africa have signs of wear on the edges, and the shape of their base is consistent with hafted spears, suggesting they were used as weapons. Their age predates both Neanderthals and H. sapiens, and the location they were found in suggests they could have been created by H. heidelbergensis, who lived in regions of southern Africa at that time. Archeologists think they could have been used as spear ambush weapons.

But some researchers have contested this evidence, arguing that it's "indirect" or even controversial, as no wooden shafts were uncovered with the spear points to solidify them as weapons as opposed to stone tools.

Something older?

These are some of the earliest examples of weapons in the archaeological record, but there may be older weapons that have not survived throughout time.

While hominins likely used organic materials like wood, sinew, fiber and adhesives to create objects and weapons, these materials break down or are destroyed over time, Warming noted.

Studying these ancient weapons gives us perspective about the present. After all, weapons are more than tools of harm: Warming said they are material clues that give us glimpses into how our early relatives combined materials, used their imaginations, shared knowledge and cooperated in groups.

Yet among these three important finds, Warming said that when inquiring about the oldest known weapon in the world, that "Clacton is the safest answer."

Can you identify these historical objects of war? Test your smarts with our weapons of the world quiz!

Animals come in a wide range of shapes and sizes, from spindly stick insects to blubbery whales. Round animals are harder to come by, but they're more common in the ocean than on land. So what is the roundest animal, and why are round animals uncommon?

At a glance, round animals might not seem that out of place on land. After all, small mammals like rabbits and pikas often look a bit spherical when their fur is puffed out and they're sitting. But Chris Law, an evolutionary biologist at the University of Washington in Seattle, said this shape is mostly an illusion.

"They're physically not as round as they appear to be," he told Live Science.

A sphere has the lowest surface-area-to-volume ratio of any shape, which is ideal for heat conservation. Law said the ability to curl or scrunch into a sphere can be essential to the survival of small animals, which lose heat more quickly than larger animals do.

Rolling into a circle can also offer protection. To safeguard their vital organs, animals like armadillos and hedgehogs roll into balls when threatened, leaving only their armor or spikes exposed to predators. But when you look at the skeletons of these supposedly round critters and their shape when they're not scrunched up, they aren't round at all, Law said.

In fact, there are several reasons why round creatures are rarely found on land. Being perfectly round all the time would make it difficult to fit into small spaces, which is a key survival technique for avoiding predators, he added, and with the effects of gravity, having a lot of unsupported round mass puts strain on an animal's body. Overly round mammals, like obese cats and dogs, often suffer joint issues and other health problems.

"As a round animal, you're probably going to lose mobility, and you're going to be picked off by predators pretty easily," Law told Live Science.

There are exceptions, however. For example, beetles are small enough not to be affected as much by gravity and can fit into nooks and crannies without much wiggling. Their round shape coupled with their hard carapace makes them more difficult for predators to eat. And rain frogs puff up into a round shape to keep predators from removing them from their hiding spots.

If we're counting animals that are only round sometimes, then pill bugs (also known as rolly pollies) might take first place for the roundest land animal. But if not, rain frogs, and beetles like ladybugs, are the top contenders for the roundest land animal. But even these animals aren't perfectly round; ladybugs have flat bottoms to make it easier to move around, and rain frogs are less spherical when not puffed up.

Round sea animals

Being round at sea is a little more feasible than it is on land. In fact, sometimes, it's critical to survival.

Karly E. Cohen, a biomechanist at Friday Harbor Labs in Washington, studies lumpsuckers, a type of fish she describes as "quite round." These bulbous fish have suction cups made of enamel on their stomachs, which they use to latch on to surfaces at the bottom of the ocean. Lumpsuckers have armor, also made of enamel, covering their round shape. According to Cohen, lumpsuckers' roundness plays a huge part in keeping them attached to the seafloor or other surface. The curve of their bodies, coupled with the tooth armor, modifies the drag around the animal, creating a force that pushes them down rather than shooting them into the water column.

Although gravity still works underwater, buoyancy helps push objects up, meaning the effects of gravity do not feel as strong at sea as they do on land. According to Cohen, lumpsuckers can get around just fine, if a bit more slowly than their more streamlined neighbors. And like pufferfish and porcupine fish, lumpsuckers' round shape and uninviting exterior make them tough to swallow ‪—‬ literally.

"There's no real good way to eat an apple whole; you have to take a bite," Cohen told Live Science. "And then this thing is armored, so it's very hard to take a bite."

All about symmetry

Despite their spherical shape, lumpsuckers don't quite win the award for the roundest animal. Lumpsuckers, like most animals, have bilateral symmetry, which means they are divided into two symmetrical sides across one plane of symmetry. But there are animals that have radial symmetry, which means they are symmetrically arranged in a circle around a central point with multiple planes of symmetry.

Echinoderms ‪—‬ which include sea stars, sea urchins and sand dollars ‪—‬ have pentaradial symmetry, which means they have five planes of symmetry arranged around a central point. According to Laurent Formery, a developmental biologist at the Oceanological Observatory of Banyuls-sur-Mer in France who studies echinoderms, scientists aren't entirely sure of the benefits of pentaradial symmetry or why echinoderms are the only modern animals that have it.

However, Formery thinks being divided into five sections instead of two may give echinoderms better sensory perception than bilaterally symmetrical animals. Echinoderms have decentralized nervous systems, meaning that instead of a brain, they have a series of neurons distributed evenly across their body, and photoreceptors all over their skin.

"They are kind of like a big crawling eye and brain, so they are receiving information from everywhere," he told Live Science. "They have a completely different way, that is difficult for us to imagine, of interacting with their environment."

Because of this unique organization, radially symmetrical animals can sense predators from any direction. They can also easily switch which segment of their body is in the lead, making it easier for them to change directions to capture prey.

Discounting their spikes, certain species of sea urchin are almost perfectly spherical. Like lumpsuckers, sea urchins' spherical shape, coupled with their spines, makes them difficult meals for predators, Formery said. Sponges lack any symmetry, so they can develop all sorts of body shapes. One species, called the death ball sponge, looks like a Sputnik lamp, with many hook-covered spheres attached to its spindly "arms."'

Under the sea, urchins in the genus Histocidaris, like Histocidaris purpurata and Histocidaris formosa, are the closest to perfect spheres, making them the top candidate for roundest animal.

Although roundness might be uncommon in the animal kingdom, Cohen said it's fascinating to study these animals and the reasons for their adaptations.

"As somebody that's interested in the tools and ways organisms work in their environments," she said, "they're a real treasure trove of evolution."

What do you know about the animal kingdom? Test your knowledge with our animal quiz!

Albert Einstein gave us the theory of relativity, explained the photoelectric effect and predicted the existence of gravitational waves. He revolutionized our understanding of space, time and gravity.

In fact, he is so famous for his brilliance that "Einstein" is synonymous with "genius." But being a genius doesn't mean you're infallible. So was Einstein wrong about anything?

"For sure, he was wrong about tons of things," Nicolás Yunes, a theoretical physicist at the University of Illinois Urbana-Champaign, told Live Science. "We remember him for the things he was right about, for the most part, because the things he was right about shocked the scientific world and eventually those ripples affected everyone on Earth."

For example, one of his most famous predictions was marred by a mathematical mistake. In 1916, Einstein correctly realized that the acceleration of matter could produce ripples in space-time that we now call gravitational waves. But when he and the physicist Nathan Rosen described these waves using mathematics, they ran into a problem: Any solution to the equations of general relativity that would allow for gravitational waves "blew up," Yunes said. "They had these singularities … these divergences. And solutions that diverge and explode like that, they can't be physical representations of reality."

As a result, Einstein changed his mind and decided that gravitational waves couldn't exist. He wrote up his results and submitted them to the journal Physical Review, which had recently begun sending papers to outside experts for peer review. An anonymous reviewer caught an error in Einstein's math, and when Einstein learned about it, he was so furious that he withdrew the paper and submitted it to a different journal.

But the reviewer had spotted a real error: Einstein's mathematical infinities were a coordinate artifact. Much like Earth's lines of longitude seem to converge to a "singularity" at the North Pole even though nothing unusual is happening there on the ground, Einstein's math could have been fixed by using a different set of coordinates.

Without Einstein's knowledge, the reviewer befriended and demonstrated the error to Einstein's assistant, who explained it to Einstein. Einstein corrected the error and republished the paper with the opposite conclusion, showing that gravitational waves do, in fact, exist.

Calculating "impossible infinities"

Remarkably, Einstein stumbled on the same error twice — at least, it seemed that way. When examining the mathematics around black holes, he once again calculated impossible infinities ‪—‬ this time, at the edge of a black hole. And once again, he concluded that this must mean the object couldn't exist.

"Einstein remained skeptical of the existence of black holes," John D. Norton, a professor in the Department of History and Philosophy of Science at the University of Pittsburgh, told Live Science in an email. "He was resolute in his judgment that there would be a singularity in space-time already at the event horizon of a black hole, which is now merely regarded as marking the point of no return for those falling into a black hole."

But this time, Einstein was not convinced that he made a mistake. Norton argues this wasn't just stubbornness; it reflected a philosophical belief in how Einstein thought mathematics and physics should relate to each other.

"He was unmoved by alternative analyses that found his mathematical infinities to be merely artifacts of the particular mathematical methods he preferred," Norton said.

Einstein and quantum mechanics

Einstein's resistance to quantum mechanics is probably his most famous wrong turn. His central objection was to a phenomenon called quantum entanglement: the idea that two particles can be linked in such a way that measuring one instantly affects the other, no matter the distance between them. In a 1947 letter to his colleague Max Born, he wrote, "I cannot seriously believe in it because the theory cannot be reconciled with the idea that physics should represent a reality in time and space, free from spooky actions at a distance."

Einstein thought this instantaneous phenomenon seemed to violate special relativity, which says that nothing can travel faster than light. As a result, he believed quantum mechanics must be incomplete and that there was some deeper, unknown description of reality that would restore order.

"He died not liking quantum mechanics," Yunes said. It wasn't until 1964, almost a decade after Einstein's death, that John Bell proved entanglement was real.

"Today, a lot of the technology that we have relies on quantum mechanics, and so we know it's correct," Yunes said. "But it's still incompatible with general relativity, with [Einstein's] classical theory."

"It's possible that general relativity is wrong," Yunes said. "It's also possible that quantum mechanics is not the right description when you're talking about systems that are very strongly gravitating and you're looking at Planck-scale-type dynamics," which are incredibly small scales at which quantum effects dominate. For instance, the center of a black hole, which is both compressed to the quantum scale but also sits inside the most intense gravitational field in the universe, is a place where general relativity predicts a singularity that quantum mechanics has no way to describe.

But many of Einstein's mistakes still moved science forward. "General relativity is a good example," Norton said. "Einstein based it on the idea that he was generalizing the principle of relativity to acceleration and on what he soon called Mach's principle. Neither proved compatible with his final general theory of relativity."

Einstein's mistakes might come as a surprise to others, but they didn't to him. Once, when writing a book with his collaborator Leopold Infeld, Infeld told him he was taking special care with it because Einstein's name would appear on it. Einstein laughed and said, "There are incorrect papers under my name, too."

See how much you know about Albert Einstein with our Einstein quiz!

Fast charging seems to be almost everywhere. Many smartphones can go from nearly empty to more than 50% charged in about half an hour, while some electric vehicles can add hundreds of miles of range during a quick charging stop.

But batteries aren't perfect; their capacity degrades over time. Given that fast charging delivers more power in a shorter amount of time, does fast charging damage batteries?

Scientists say the answer is yes, but it's more complicated than you might think. Fast charging can accelerate some types of battery degradation, but modern batteries are designed with safeguards to help limit the damage.

How does fast charging work?

Rechargeable lithium-ion batteries — the most common battery type in the world — work by moving lithium ions between two electrodes called a cathode and an anode. During charging, lithium ions travel through the battery and are stored in the anode until the battery is used again.

The main difference between fast charging and regular charging is how quickly those ions move. Compared with regular charging, which can take hours, fast charging can refill a battery in an hour or less.

"Regular charging applies a lower current, allowing lithium ions to intercalate [move into microscopic holes] into the anode gradually, which generates little heat and causes minimal mechanical stress," Zhiyuan Jiang, an associate professor in the Department of Chemical Engineering and Technology at Xi'an Jiaotong University in China, told Live Science via email. "Fast charging increases the current [and] power significantly to shorten charging time."

Not all batteries are designed for fast charging. A battery's ability to handle high charging speeds depends on its materials, internal structure and battery management system, Jiang explained. Fast-charging batteries often use specialized electrode materials or thinner electrodes and electrolytes that allow the lithium ions to move more easily. Manufacturers may also redesign the battery's internal architecture to reduce resistance and heat buildup.

Stanislaw Zankowski, a battery researcher at the University of Oxford, compared the process to traffic moving through a city.

"You could think about charging a battery as transporting people through roads, intersections and buildings," Zankowski told Live Science. "Fast charging is really a question of how efficiently you can move all that traffic without creating bottlenecks."

What type of damage could fast charging cause?

All lithium-ion batteries lose capacity over time, even when they are treated carefully. But fast charging can speed up some of the chemical processes responsible for that aging.

One of the biggest concerns is a process called lithium plating. During rapid charging, lithium ions may not have enough time to settle properly inside the anode. Instead, some lithium can accumulate as metallic deposits on the electrode's surface. These deposits can reduce the amount of lithium available to store energy, thereby lowering the battery's capacity. In extreme cases, the lithium can form needle-like structures called dendrites that puncture internal battery components and create safety hazards.

Fast charging can also generate more heat. Heat is a natural byproduct of electrical resistance in the battery. The faster a battery charges, the more heat it produces.

"For charging a small battery with a small current, that amount of heat will be also relatively small," Zankowski said. "So, it's not really a safety problem, but as we increase the size of the battery, the amount of current that we'll be pushing during charging and the amount of heat will also increase quite a lot. And as a result, we can't really charge larger batteries as quickly, mostly because of the safety margin."

Higher temperatures can accelerate chemical reactions that gradually degrade battery materials. In some extreme cases, overheating can cause batteries to swell, catch fire or even explode ‪—‬ a process known as thermal runaway.

Fortunately, most modern smartphones, laptops and electric vehicles contain sophisticated battery management systems that monitor voltage, current and temperature during charging. That's why you may get a heat warning from your smartphone if you leave it in the sun. If temperatures climb too high, these systems automatically slow charging to protect the battery.

Best tips for battery life

So what's the best way to protect a battery while charging it quickly?

Both Zankowski and Jiang emphasized that temperature is key. It's best to avoid charging devices in hot environments, such as inside a parked car or in direct sunlight. Extremely cold temperatures can also be harmful because they make it harder for the lithium ions to move through the battery.

"Ideally, the temperature range should be around 20 to 25 degrees Celsius [68 to 77 degrees Fahrenheit] for charging," Zankowski said. "So, just like a comfortable temperature for a human being, right?"

Experts also recommend that you avoid keeping devices like laptops constantly plugged in, as this can degrade battery performance. Jiang suggested implementing the "shallow charge, shallow discharge" habit.

"Keep your battery between 20% and 80% for daily use," he said. "It is not necessary to charge to 100% every time."

Can you match these ancient devices to their pictures? Find out with our computing quiz!

With violent conflicts happening across the world right now, war feels like a permanent condition. But have humans ever lived without war?

The answer depends on how you define "war." If war means a fight between two governments, then yes, there have been peaceful periods "because for nearly 99% of human history, there were no governments," said Ian Morris, a historian at Stanford University and author of "War! What Is It Good For? Conflict and the Progress of Civilization from Primates to Robots" (Farrar, Straus and Giroux, 2014).

Violence, on the other hand, has always been with us. If the question becomes "Have people ever lived without violence?" then "the answer is pretty clearly no — people have always fought and killed each other," Morris told Live Science in an email.

The theory of a peaceful prehistory

According to a review of studies published in 2022, war was rare or nonexistent in early human history when people lived as nomadic hunter gatherers. Peter Stearns, a professor emeritus of history at George Mason University and author of "Peace in World History" (Routledge, 2014), agrees with this idea. There was "little or no war in hunting and gathering [cultures] before the rise of agriculture," he told Live Science in an email.

This idea is based on archaeological records. Researchers examining ancient human bones from around the world have looked for skeletal evidence of war injuries, for instance of multiple individuals with unhealed wounds from stabbing, slicing and blunt force trauma who were buried in mass graves. However, they found little evidence of war injuries before 8000 B.C. After that, when humans began transitioning from nomadic life to permanent settlements, such injuries appeared widely.

This doesn't mean that lethal violence was absent in prehistoric societies. Researchers in Kenya found 27 skeletons dating to 10,000 years ago at the archaeological site of Nataruk; these skeletons show signs of violent death, which some anthropologists interpret as evidence of intergroup violence among early hunter-gatherers. And at Jebel Sahaba, a prehistoric cemetery in Sudan, archaeologists unearthed 13,000-year-old remains bearing signs of intergroup attacks.

But these cases of interpersonal violence don't count as war, because of how researchers usually define it. "Researchers who focus on war as a specific category of violence usually define it by saying it has to be violence organized by a government or else collective violence that kills more than a certain number of people," Morris said. "Prehistoric societies rarely had formal governments and hunter-gatherer bands rarely had more than a few dozen members, so if you define war as a conflict run by a government or one that kills >100 people, then by definition there can't have been wars in [early] prehistory."

David Christian, a historian and professor emeritus in the Department of History and Archaeology at Macquarie University in Australia, echoed this idea. "For much of human history communities were so small that it is not clear if we can equate violence with war," he told Live Science in an email. "I guess we can say that humans have always been capable of violence and as communities got larger that violence began to take on forms that we might want to describe as 'war.'"

Peace between rival powers

Once large kingdoms and empires appeared, war became very common. A doctoral thesis written by Jared Morgan McKinney, now an assistant professor of international security studies at Air War College in Alabama, focused on periods of peace between major powers. He concluded that war was basically the norm throughout history and that famous "peaceful" eras, like the Pax Romana (Roman Peace), usually just meant one powerful group had beaten everyone else into submission.

But there were exceptions. "Wars are expensive and risky," said Peter Frankopan, a professor of global history at the University of Oxford. "So in many periods in history, stability and peace have been achieved by rivals, adversaries and neighbors being able to match each other's capabilities."

McKinney's thesis identified several periods when rival powers managed to avoid war.

From around 1400 to 1200 B.C., the big powers of the ancient Middle East — mainly Egypt and the Hittite Empire, an ancient civilization in modern-day Turkey — went through two unusually long stretches without major wars. Peace was possible because the "Great Kings" recognized one another as equals and settled their territorial and political differences through formal treaties rather than war.

For most of their history, Rome and Persia were at war. Then, from roughly A.D. 387 to around 501, the two superpowers mostly stopped fighting. Historians call this the "Long Fifth Century" and have put forward many explanations for this period of peace, including that both sides faced serious outside threats that made fighting less affordable and that the two sides developed a language of "brotherhood" that acknowledged each other as equals rather than enemies, McKinney wrote in his thesis.

Between roughly A.D. 1000 and 1200, the rich Song dynasty in China secured peace with its militant northern neighbors ‪—‬ the Liao and the Jin‪ —‬ by paying them regularly to keep the peace. Although these monetary gifts (or bribes) looked like weakness, the payments were tiny compared with what China was earning through trade, according to McKinney.

Another example, according to Morris, is the "long peace" between China, Korea and Japan between about 1600 and 1850. While European states spent those centuries competing for power and territory through war, the East Asian states lived in relative peace. "In Europe, we've tended to be a lot more aggressive and competitive — which gives the impression that war is a 'natural state' of being," Frankopan said.

A notable period of relative peace in North America was the Long Peace among the Iroquois nations, McKinney told Live Science in an email. For roughly three centuries, from about 1450 to 1777, five (and later six) Native American nations who had previously engaged in violent and costly conflict forged a peaceful relationship known as the "Haudenosaunee Confederacy" or "League of Five Nations."

In South America, McKinney pointed to the South American Long Peace. This refers to the absence of major interstate wars between sovereign nations in South America ever since 1935.

"War is 'normal' in history," McKinney said. But, as these examples show, "patterns have exceptions."

Can you identify these historical objects of war? Test your smarts with our weapons of the world quiz.

When you click to enter a website or try to log in or fill out a form, you may be asked to identify motorcycles from a grid of grainy images, decipher a string of convoluted characters, or click a box that states "I am not a robot."

These tests are called CAPTCHAs, which stands for "Completely Automated Public Turing test to tell Computers and Humans Apart." As their name suggests, they are meant to help a website distinguish if an action is coming from a human or a bot, since the aforementioned tasks are theoretically easy for a human and difficult for automated software to perform. This, in turn, blocks bots from spamming comments, downloading files, taking over accounts, or executing any other action on a website.

But as computer models increasingly gain the ability to solve CAPTCHAs, thanks to advancing artificial intelligence (AI) — and with puzzles getting weirder and more difficult for humans to complete — does this mean CAPTCHAs are still useful?

CAPTCHAs were introduced in the late 1990s to address "a very simple, but very difficult problem," Andreas Plesner, a computer scientist at ETH Zurich, told Live Science. "If I don't interact with a person physically … is it a computer? Or is it human?" Some of the first CAPTCHAs, still common on websites today, were composed of distorted text, since text-reading software at that time had trouble interpreting warped words.

But over time, text-reading software improved and new types of CAPTCHAs were developed. For instance, reCAPTCHA, one of the most popular CAPTCHA services, has an image-based test that asks users to identify objects such as traffic lights, motorcycles or bicycles from a grid of Google Street View photos. This was developed after Google acquired the service in 2009.

"The bet was that recognizing objects in messy, real-world photos was still a uniquely human skill," Ng Chong, chief of information technology and director of United Nations University's Campus Computing Centre in Tokyo, told Live Science in an email.

As time went on, CAPTCHA design continued to advance. In 2014, Google came out with reCAPTCHA v2, which analyzed computer mouse behavior by asking people to click a checkbox to test if a user was human. If the behavior is deemed suspicious, determined by factors like how a user interacts with the site beforehand or the timing of their click, the street-image grid pops up as an additional puzzle.

However, more recently, technology has improved to a point where image recognition is no longer a human-specific skill. As early as 2016, researchers found that low-cost deep learning technologies could solve reCAPTCHAv2 around 70% of the time. By 2024, Plesner and his colleagues developed an AI model that could solve the puzzles correctly 100% of the time. Earlier in 2026, Chong noted that he built a tool that could mimic human-like browsing behavior and sometimes bypass reCAPTCHA v2 without triggering the image grid at all. When the grid was triggered, the tool used AI to solve it within a few tries.

"When both the challenge and the behavioral layer are defeated by commodity tools running on a single laptop, the fundamental premise of CAPTCHA, that there are tasks humans can do but machines can't, stops holding," Chong wrote.

Looking ahead

So does that mean CAPTCHAs are completely obsolete? Not quite. Although the model Plesner and his colleagues developed breezed past reCAPTCHAv2, "there were a lot of the safety measures that were not tied to being able to solve it, but more tied to how you solve it," he said. For example, while conducting their research, Plesner noted that his team used a virtual private network (VPN) that changed IP addresses for each test, since a single IP address with a high volume of solved CAPTCHAs faced tasks with increasing difficulty, or got blocked entirely.

Modern CAPTCHAs focus on these background clues and tactics, rather than the puzzle itself. This includes Google's reCAPTCHA v3, Friendly CAPTCHA, hCAPTCHA and Cloudflare's Turnstile, among others, which run without sending a puzzle at all. They instead look at whether the action is coming from a real attested device (rather than from automated code), whether an IP address has had a high volume of automated requests in the past, how a user navigates a webpage, what the user's cookie history is, and a slew of other factors to determine possible malicious intent.

As the tug-of-war continues, CAPTCHA puzzles are still widespread. After all, they’ve been the status quo for decades, are easy to set up and are relatively cost-effective, Chong said. But these tasks have some other drawbacks. Although bots can increasingly solve the puzzles with ease, CAPTCHAs can be a headache to get through for humans and can be seen as discriminatory against those with disabilities, notably visual disabilities, as a researcher noted in a 2022 conference paper.

The crescendoing complexity of CAPTCHA puzzles has even been the subject of parody, with developer Neal Agarwal creating a free satirical game called "I'm Not a Robot." Users must solve a series of increasingly convoluted verification checks — scoring a point for each stage they pass, which eventually transcend into the absurd.

So, as machines get smarter, the answer may not be to find more difficult puzzles. "If a CAPTCHA can only be solved by someone with a Ph.D. in mathematics, then it's not very useful," Plesner said. "The internet needs to be used by everyone."

Can you match these ancient devices to their pictures? Find out with our computing quiz!

If you push two metal plates together on Earth, nothing happens. But if you take those same plates into the vacuum of space, they can fuse into a single piece of metal.

This phenomenon, called cold welding, has been a known hazard for spacecraft engineers for a long time. So what's actually happening at the atomic level, and why does space make it so much easier?

The answer comes down to a lack of oxygen in space, experts told Live Science.

Metals are made of lattices — structures where atoms are bonded to one another. But atoms near the surface of a metal aren't bonded to anything on the outward-facing side. If given the chance, they'll "reach out" and share electrons with the surface of another piece of metal.

But on Earth, nearly every metal surface is coated in an oxide layer. The layer is just a few atoms thick and forms when metal meets oxygen. "Once the oxide is formed, it's over," Julia Greer, a materials scientist at Caltech, told Live Science. "Then it can't cold weld anymore, because the oxygen basically passivates these bonds."

That thin oxide acts like an insulating wrapper. Without it, the free electrons at the surface of one metal piece stop recognizing which atom they belong to. "Those electrons don't know if it's in this piece or if it's in that piece, so they begin sharing the electrons, and essentially that cold welds things together," Sven Bilén, a professor of engineering design and aerospace engineering at Penn State, told Live Science.

In space, there's no oxygen to rebuild that layer once it's gone. The cold and the radiation make things worse. Bombardment from solar and ionic radiation in orbit can scour metal surfaces clean, Greer said, leaving freshly exposed atoms primed to bond. "Everything in space is conducive to cold welding," she said.

Metal surfaces are never perfectly smooth, either. At a microscopic level, they're jagged ‪—‬ more like tiny mountain ranges than flat plains, said Zachary Cordero, an aerospace engineer at MIT.

Pressing two surfaces together, especially with any sliding or vibration, can shear off the oxide layer that formed while the metal was on Earth and flatten those peaks into metal-to-metal contact. "You're breaking up the surface oxide, and you're forming metallurgical bonds," Cordero said.

Why cold welding worried early spacecraft engineers

Cold welding in space has long been a problem. "If there is cold welding, things can become stuck in place," Cordero told Live Science. "If you have a deployable structure and there's cold welding, you might freeze the mechanism, or a door might become locked, or something might become immobilized, which you don't want."

For example, say you were to add a metal screw to a metal door. After a while, you would not be able to unscrew it because it would have become part of the door.

Bilén pointed to NASA's Galileo probe, which launched in 1989: lubricant loss and launch vibrations during launch are thought to have stripped the oxide layer from parts of its furled high-gain antenna. When engineers attempted to deploy the antenna in 1991, it never fully opened.

Some metals are more troublesome than others. Gold and platinum don't form an oxide layer at all, even on Earth, which makes them notoriously prone to cold welding. "Gold definitely is a very notorious metal for cold welding," Greer said, adding that gold's softness lets it conform easily to whatever surface it touches and it bonds even more easily.

How to prevent cold welding in space

To prevent components from accidentally fusing in orbit, engineers rely on a few strategies. One is anodizing, a process that locks an artificial oxide layer onto a metal surface. Another method is to coat moving parts with dry lubricants, such as molybdenum disulfide, to physically keep surfaces from touching.

A third strategy is to pair dissimilar metals — for instance, gold next to a "body-centered" metal, like molybdenum — so their atomic structures don't mesh as neatly. "Their packing order is not quite perfectly aligned, and so there'll be a lot more energetic kind of barrier to overcome," Greer said.

Before launch, hardware also gets shaken on vibration tables and cycled through extreme hot-and-cold swings inside vacuum chambers, simulating the stresses of liftoff and orbit to catch problems on the ground.

Even with all of those precautions, cold welding can still happen. Bilén recalled bolts in his own lab's vacuum chamber fusing shut after a move across campus. They eventually had to be drilled out. "It happens even on Earth," he said.

See how much you know about human exploration into space with our spaceflight quiz!

Ancient Roman transportation engineers built a vast network of roads that stretched across Europe, North Africa and parts of the Middle East. This complex road system was a key to travel and trade in the Roman Empire, and many of the centuries-old streets had a reputation for being extremely straight, although not all of them were.

For instance, Via Appia (Appian Way), which connected Rome to the port of Brundisium in southern Italy, was more than 300 miles (500 kilometers) long, and sizable parts of it were straight. Another Roman road, Stane Street in southern England, was built to connect London to Chichester. Much of the road, which stretches roughly 57 miles (92 km), is straight. The Middle East also had straight Roman roads, including a coastal avenue from Antioch, Turkey, to what is now Gaza.

A recent mapping project was able to map about 186,400 miles (300,000 km) of roads and more are probably undiscovered. But how did the Romans avoid unnecessary twists and turns to ensure straight streets? The answer may lie in three surveying instruments the Romans used.

Surveying roads

In some instances, the Romans built on top of older roads that existed before they conquered an area. Their "road network incorporated older roads from a broad range of different societies and polities," Marion Kruse, an associate professor of classics at the University of Cincinnati, told Live Science in an email.

But when the Romans built new roads, they used a few different tools to help plan them.

"Three instruments were used consistently by Roman road builders: the dioptra, the groma, and the chorobatus [or chorobates]," Adriana Panaite, a researcher at the Vasile Pârvan Institute of Archaeology in Romania who has studied Roman roads extensively, told Live Science in an email.

While the dioptra is known from ancient texts, no example of it has ever been found in an archaeological dig, according to M.J.T. Lewis, who was a historian at the University of Hull in the U.K. In his book "Surveying Instruments of Greece and Rome" (Cambridge University Press, 2001), Lewis noted that the design of the dioptra varied considerably. The different designs tended to include a stand and a disc-shaped base with a tube-shaped sighting instrument attached. An ancient surveyor could look through the tube and see a distant object without extraneous light interfering, allowing for a better view.

The chorobatus was used to measure horizontal planes. Around 20 feet (6 meters) long, the chorobatus was a beam of wood on legs and looked like a small table, Lewis wrote. It likely had small weights hanging from it to show that the beam was level. No ancient example of the chorobatus has survived, and the exact design and way it was used are unclear. Ancient texts indicate that it acted as a builder's level that could help set up level points and determine elevations.

But the most important tool Roman surveyors used was the groma, Joseph Lewis, an archaeologist at the University of Cambridge who has conducted extensive research on Roman roads, told Live Science in an email. The "groma was the principal tool of the mensor ‪—‬ the land surveyor ‪—‬ when planning long, straight alignments," he noted. "These alignments were then often used when constructing roads across gentle terrain."

The groma, also called a gruma or croma, consisted of a vertical pole with a horizontal X-shaped cross on top and four small weights hanging down from strings at the ends of the crossbeams, Lewis said. The groma was most useful for establishing right angles.

Multiple Roman surveyors could use the weights on their poles to ensure any road they were building was going in the right direction.

"A surveyor at one end will direct other surveyors to move their poles until they are in line with one another," Lewis explained. "Once the direction had been established, Roman surveyors observed the landscape and adjusted the course of a road to accommodate/avoid obstacles such as steep sections which would have posed difficulties for wheeled vehicles, locations to ford rivers, or to connect pre-existing settlements."

However, the techniques used to build roads throughout the Roman Empire probably varied somewhat, meaning experts "should be cautious about assuming that there was a single 'Roman' technique for road building," Kruse said. He noted that the Roman Empire covered a vast area and lasted for a long period of time.

"It seems safe to assume that practices varied across time and space," Kruse explained.

Not all roads were straight

One reason for the variation in Roman roads was likely the diversity of the laborers who built them. "In all likelihood, road-building work was done by a mix of soldiers, slaves ‪—‬ especially prisoners ‪—‬ and free locals called upon to help as part of a 'corvee' obligation imposed by their local community on Rome's instructions," Richard Talbert, a professor emeritus of history at the University of North Carolina at Chapel Hill, told Live Science in an email. But paid laborers were probably used for some skilled tasks, such as building bridges, Talbert said.

While Roman roads have a reputation for being precisely laid out and straight, not all of them were.

"It is commonly mentioned that Roman roads were exceptionally straight. This is partially true: The Romans had engineers and a large labor force that in some cases managed to modify the landscape to enable straight roads," Tom Brughmans, a classical archaeologist at Aarhus University in Denmark who is part of a team that helped create an updated map of the Roman road system, told Live Science in an email.

"We believe that the Romans preferred relatively straight roads in places where there was very little friction offered by the topography," such as flatland areas, Brughmans said. But in areas with more difficult terrain, such as mountainous regions, the roads would often not run straight.

Brughmans said he expects "future research will show that, in general, [Roman roads] are less straight than modern roads, given the need for motorized vehicles to avoid sharp turns when driving at speed."

From Augustus to Nero, see how much you know about ancient Rome's famous leaders with our Roman emperor quiz!

The Amazon is the largest rainforest in the world, spanning more than 2 million square miles (5.2 million square kilometers) — an area 12 times the size of California. It influences global water cycles, stores years of global carbon emissions, supports 47 million people, and is home to the greatest concentration of biodiversity on Earth.

But the Amazon rainforest is also disappearing, with 17% of it already cut down or destroyed and largely replaced with agriculture. Other grave threats, such as oil drilling and illegal mining, continue to whittle it down. The next century may have outsize importance, as the forest could reach a "tipping point."

So what will the Amazon rainforest look like in 100 years?

The answer depends on a number of compounding threats, Bernardo Flores, a researcher with the EqualSea Lab at the University of Santiago de Compostela in Spain, told Live Science.

Encroaching farmland and organized crime are a couple of the problems chipping away at the Amazon. But those work in tandem with what he considers the three main threats: climate change, which can lead to extreme weather events, "like wetter wet seasons and drier dry seasons," deforestation and fire.

As the Amazon loses more of its forest, it triggers a feedback loop. "You have less rainfall; then you have less forest, [then] less rainfall, less forest," Flores explained. "That ultimately leads to "a global scale feedback involving the Amazon: More forest loss [leads to] more global warming. More global warming, more forest loss."

As forests get drier, it becomes easier for wildfires to burn more areas. Roads also degrade the forest, and "wherever you have roads, you have people doing illegal activities, illegal logging … then this leads to [more] forest fires," Flores said.

The "arc of deforestation" — a roughly 310,000-square-mile (800,000 square km) border along the Amazon considered the largest deforestation frontier in the world — offers a preview of what much of the Amazon could ultimately look like, according to Flores. The forests that remain there have higher tree mortality and more canopy gaps, and they are often "covered with lianas," or woody vines, that become an ecological problem, he said. Lianas compete with trees for light and nutrients in the soil, and significantly reduce not only a tree's chance of survival but also the overall diversity of trees in a forest. "When the whole forest is covered in lianas, you don't see the forest anymore," he added.

Invasive grasses introduced by cattle farmers will likely proliferate in the decades ahead, but "only a few parts" of the Amazon could become "a savanna, because a savanna is a native, biodiverse ecosystem," he said. Invasive grasses "exclude native species, reduce biodiversity" and would not allow native savanna grasses to replace the forest, Flores said. Instead, one possibility is a "degraded open-canopy ecosystem," where native, naturally fire-tolerant trees, combined with invasive grasses, vines and ferns, proliferate, Flores told Live Science.

Wildlife would quickly be affected as well. Aquatic species are especially vulnerable, Flores said. "When you start having these droughts that will simply last for one, two, three years," wetlands will dry out and become flammable, he explained. That could lead to "very quick extinctions in those areas."

The destruction of the Amazon rainforest would be disastrous for the Indigenous people living there, Christian Poirier, program director of Amazon Watch, an environmental and Indigenous rights advocacy group, told Live Science. "Imagine having your backyard bulldozed and your water source poisoned," he said. "You probably need to move from where you live, and that's exactly what's happening in the Amazon."

A devastated Amazon would also lead to "a more chaotic global climate system," Flores said. There could be less rainfall across parts of South America, and global warming will worsen. Earth could eventually reach a tipping point where ice sheets melt, ocean currents malfunction and the collapse of the Amazon accelerate warming all at once, pushing the planet to "cross the tipping point and transition to a much warmer climate," he said, leading to potentially irreversible consequences.

Unlike other major climate risks, such as the potential of the Greenland Ice Sheet melting and contributing to sea level rise, deforestation can in theory be reversed more easily by reforestation, said Arie Staal, an assistant professor of ecosystem resilience at Utrecht University in the Netherlands.

"That gives us a knob to turn that we don't have for other possible tipping points on Earth," he told Live Science. "It is clear that we really need to stop deforestation in the Amazon. And there's hope."

Editor's note: This article was updated at 6:23 p.m. EDT on June 22 to fix the conversion of roughly 310,000 square miles to 800,000 square kilometers.

Rainforest quiz: Can you sort the largest rainforests on Earth?

It's after dark, and the lights go out; a thunderstorm has knocked out your power. At first, it's pitch-black. But slowly, after a while, the light from the moon makes it possible to see your surroundings. The light didn't change; your eyes did.

But why does it take our eyes so long to fully adjust to the dark?

It has to do with the types of cells in the eye and how they evolved, experts told Live Science.

Our eyes use two kinds of cells to sense light. These cells, known as photoreceptors, are called rods and cones, based on their shapes, Alapakkam Sampath, a retinal neuroscientist at UCLA, told Live Science.

Cones are responsible for color vision. Humans possess three kinds of cone cells, which detect red, green or blue light. The colors people can see are a mix of red, green and blue, Anand Swaroop, chief of the Neurobiology Neurodegeneration & Repair Laboratory at the National Eye Institute, told Live Science.

Rods cannot discriminate colors, but they are far more sensitive to light. Each is capable of detecting a single photon, or particle of light. "Rods are what allow you to see in dim light," Swaroop said.

The extraordinary sensitivity of rods has a price, though: Once each rod detects a photon, it can take up to an hour for it to regenerate its ability to see light, Johan Pahlberg, chief of the photoreceptor physiology group at the National Eye Institute, told Live Science.

The compound that helps rods sense light is called rhodopsin, which is derived from vitamin A, Sampath said. Once each rhodopsin molecule absorbs light, it gets "bleached," meaning it can no longer detect light.

Specifically, rhodopsin is made of two compounds: opsin and retinal, Pahlberg said. When rhodopsin absorbs light, the retinal changes from bent to straight and detaches from the opsin. This retinal eventually makes its way into a part of the eye known as the retinal pigment epithelium, where it can get repaired and resume a bent shape. This retinal can then make its way back to an opsin and reattach to create a functional rhodopsin.

If all of the rods in a human eye are bleached, it may take 45 minutes to an hour for all of them to regenerate, Pahlberg said. However, some rods may regenerate within 10 to 15 minutes to grant some level of vision in dim light, he added.

Rods and cones line the retina, the back part of the eyeball. Rods outnumber cones in most mammal retinas. "In each human eye, there are roughly 6 million cones and 100 million rods," Swaroop said. Sampath noted that "since it takes a lot of time for rods to regenerate, your eyes have a huge number of them to compensate."

Cones are found mostly in the center of the retina, where the eye's lens focuses most of the light it receives, while rods dominate the rest of the retina, Sampath explained. This reflects how during the day, we rely mostly on cone cells for our vision. "Rods are mostly not functional at all during that time," he said.

The human eye does have a quicker way to adjust somewhat to the dark. It can enlarge the pupil, the dark hole at the front of the eye, to let in more light, explained Mark Fairchild, professor of color science at the Rochester Institute of Technology in New York, in an article in The Conversation. However, the vast majority of the eye's adaptation to darkness is due to its rods, and the long amount of time it takes for rods to regenerate explains why it takes so long for eyes to fully adapt to the dark.

Before artificial lighting, humans would typically not experience quick shifts from light to dark. Instead, the ability for human eyes to adapt to the dark would have relied on the setting of the sun, which might take about the same time as rod cells take to regenerate. "There was no evolutionary pressure for it to be faster," Sampath said.

Rods are the most vulnerable cells in the retina to disease and dysfunction, Pahlberg said. This is why older adults often have trouble driving at night. "My colleagues are developing a diagnostic test as a normal part of an eye exam to measure the human adaptation to the dark to see how it changes with age," Sampath noted.

If you've ever washed uncooked rice, you've seen the pale, milky-white water that rinses away. Billions of people across the world eat rice every day. So should everybody who cooks rice wash it beforehand?

This question has been the subject of a number of scientific studies, which have looked at how washing rice affects its texture, nutrition and how well it removes unwanted substances, such as dust, arsenic and microplastics.

Rice is traditionally grown in shallowly flooded fields, as it requires constant irrigation to grow. When the grains are ready, they are harvested and milled to remove the inedible hulls, yielding brown rice, which can be further milled to remove the bran layer, producing white rice.

The act of milling can damage rice kernels, leaving a layer of starch (the substance that makes up much of rice, potatoes and wheat) on the outside. When rice is rinsed, some of this starch is washed away.

In a 2017 study, scientists had speculated that washing rice might change the texture of cooked rice by rinsing off starch that would make grains stick together.

However, subsequent research found that "washing rice makes no difference to the stickiness of the cooked rice," Evangeline Mantzioris, an accredited practicing dietitian at Adelaide University in Australia, told Live Science.

Specifically, a 2019 study showed that "the stickiness of rice is not from the surface starch, amylose, but from another starch within the grain called amylopectin," Mantzioris said. "This is what leaches out during cooking and impacts the stickiness."

The 2019 study found the amount of amylopectin that leaches out of rice during cooking does not depend on whether it is washed. "It is the variety of rice that is important," Mantzioris said. In tests with 10-gram (0.4 ounces) samples of three kinds of rice that were all washed the same amount of time with increasing amounts of water and cooked for 30 minutes, the scientists found that "if you want sticky rice for your dish, you are best using glutinous rice. Medium-grain and jasmine rice are less sticky."

Traditionally, rice was washed for health and safety reasons, "to rinse off dust, insects, little stones and small parts of the husk," Mantzioris said.

However, nowadays, "rice sold in supermarkets and reputable retailers is generally produced under strict quality standards," Bo Wang, a food scientist at Adelaide University, told Live Science. "It is typically cleaned [with machines, such as sifters and blowers], dried, dehusked, milled, graded and packaged before reaching consumers. The drying step particularly reduces moisture content, helping maintain quality and limiting microbial growth during storage. As a result, rice is already a relatively safe product, and washing is generally not required to make it safe for consumption."

Still, "one additional consideration is that some rice may contain naturally occurring inorganic arsenic absorbed from soil and water," Permal Deo, a food scientist with a background in molecular biology at Adelaide University, told Live Science. "Rinsing may help remove some arsenic on the grain surface."

Rinsing rice can also rinse off microplastics, Mantzioris said. A 2021 study found that washing rice before cooking it reduced the amount of plastic contaminating it by 20 to 40%. (It remains uncertain what effect microplastics have on our health, although evidence is growing that microplastics could be harmful.)

Mantzioris noted that washing rice does reduce levels of some important nutrients that naturally dissolve in water, such as copper, iron, zinc and vanadium. However, rice at best offers only a small percentage of one's daily intake of these nutrients. Washing rice, therefore, "is unlikely to affect you nutritionally," she said.

"Long story short, for most consumers, a gentle rinse once or twice before cooking is usually sufficient," Wang said.

This article is for informational purposes only and is not meant to offer medical advice.

Do you know where pumpkins and blueberries come from? Find out with our fruits and vegetables quiz!

As water molecules move around the planet through the water cycle, they take on many forms, moving from solid to liquid to gas and back again. They can make up snowpacks melting in the spring, a river rushing to the ocean, clouds carried on sea breezes, and even pee flushed down the toilet.

But with this complex cycle of evaporation, condensation and precipitation continuing over and over, has the water coming out of your faucet been inside a dinosaur or a mammoth at some point? And does that mean all the water on Earth has been peed before?

The answer depends on how you approach the question.

For Neil Donahue, director of the Steinbrenner Institute for Environmental Education and Research and a professor of chemistry and chemical engineering at Carnegie Mellon University, the answer is likely yes, based on a back-of-the-envelope calculation. But this math is based on a few assumptions, Donahue told Live Science over email.

The first is that the average person weighs about 110 pounds (50 kilograms) and pees about 0.26 gallons (1 liter) per day, which is roughly equivalent to 2.2 pounds (1 kg). Then, Donahue made a "WILD assumption" that all animals pee roughly the same proportion — 1% of their body weight — which he said is "probably wrong" but helpful for a ballpark estimate.

Next, if the total mass of all chordates (the taxonomic group that includes mammals, birds, fish,) is approximately 2.2 billion tons of water and carbon (2 billion metric tons), this equates to around 0.02 gigatonnes — or 8,000 Olympic-sized swimming pools — of pee every day, Donahue said. For the sake of the calculation, he assumed that the weight of chordates has remained constant over time.

On the water side of the equation, the U.S. Geological Survey estimates that Earth has about 1.4 billion gigatons of water, including that in oceans, ice caps, glaciers, lakes, rivers, groundwater and vapor.

If you divide the weight of the world's water by 0.02 gigatonnes of pee each day, it would take around 70 billion days, or about 191 million years, "to pee out the whole ocean," Donahue said. Given that chordates emerged at least 500 million years ago, this is "plenty of time" for all water to become pee at one time or another, he said.

David Kreamer, a professor of hydrology at the University of Nevada, Las Vegas, agreed that when "you go back in history to the dinosaurs and things like that, that's a lot of pee." But he also said calculations that depend on assumptions and generalizations have a large margin of error.

Instead, when it comes to whether every last drop of water on Earth has been peed at one time or another, Kreamer said the answer is no.

For one, water does not move through the water cycle at a constant speed, he noted; water can be trapped as glacial ice for hundreds of thousands of years, and "there is some deep groundwater that's been underground for tens of thousands of years."

What's more, some water hasn't ever been in the water cycle, he said. This water, known as juvenile water, is trapped deep below the planet's surface and "hasn't really emerged from the depths of the Earth ever in Earth's history," Kreamer told Live Science.

Over time, juvenile water can be brought to the surface through volcanic activity in the form of steam or lava. Trapped deep in Earth's crust, "it's released when magma dissolves it, and when a volcano blows up into the atmosphere," Kreamer explained, noting that during eruptions, "there's a certain amount of water or moisture that's released" alongside ash and rock. This water would then enter the water cycle and could eventually find its way into and out of an animal.

Ultimately, at any given time, at sites of volcanic activity around the world, there are streams of new water reaching the surface for the first time. That water "hasn't been peed," Kreamer said ‪—‬ at least, not yet.

Editor's note: This article was updated at 1:41 p.m. EDT on June 15 to fix a math error: 1% of 2.2 billion tons is around 0.02 gigatonnes (8,000 Olympic-sized swimming pools) of pee, not 0.2 gigatonnes (80,000 Olympic-sized swimming pools) as was previously stated. This means it would take approximately 191 million years to "pee out the whole ocean," not 19 million as was previously stated.

As a result, researcher Neil Donahue changed his answer from "emphatically yes" to "likely yes." Despite this error, Donahue says that would still have been enough time if you go back to when chordates first emerged. Previously, he had gone back only to the end of the dinosaur age and said that mammals alone could have done it.

Caves pockmark the world, extending deep into the bowels of Earth. Many caves are record holders, including one in Kentucky that's the longest known cave system, another in Vietnam that's the largest cave, and one in Mexico that holds the title for the longest underwater cave. But caves are known for being dark abysses, so which is the deepest cave on Earth? And which strange animals live at their deepest depths?

There's no clear winner, Live Science found. Instead, there are two caves that alternate as the record holders for the deepest cave on our planet: Veryovkina and Krubera-Voronya (also spelled Voronja), limestone caves in Abkhazia, an autonomous republic in the Republic of Georgia.

Both of these caves are more than 6,560 feet (2,000 meters, or 1.2 miles) deep, and they sit on the same remote mountain range, known as the Gagra Range. Explorers and scientists frequently update each cave's measurements.

According to a list of the world's deepest caves maintained by geologist Paul Burger, a regional hydrologist in Alaska for the National Park Service, Veryovkina is currently in the lead, with a depth of 7,257 feet (2,212 m, or 1.37 miles). Krubera is close behind, at 7,215 feet (2,199 m, or 1.37 miles).

"Both caves are frequently reported with vertical errors of plus/minus tens of feet so first and second place can vary depending on how those data are interpreted," Burger, who is also the author of the book "Cave Exploring: The Definitive Guide to Caving Technique, Safety, Gear, and Trip Leadership" (Myotis Publishing, 2022), told Live Science in an email.

The two caves are located in the Arabika Massif, a glacially eroded karst in the Caucasus. Karst is a type of landscape formed by soluble rocks like marble, gypsum and, in this case, limestone from the Late Jurassic and Early Cretaceous periods, or from about 163.5 million to 100.5 million years ago. Over time, the limestone lays down in layers, and then it gets compressed by tectonic forces and turns into vertical walls of rock, Hazel Barton, a microbiologist and geologist at the University of Alabama, told Live Science.

"If you can imagine all those layers, you've got a BLT sandwich," she said. If you turn that BLT vertically, "when the water goes in, it's always going to look for the easiest way down." The water travels through cracks, eventually hitting the water table. "It's basically a massive bathtub down there," Barton said. Once the water hits the water table, it becomes groundwater and circulates in subterranean streams and rivers.

The reason this region has so many deep caves has to do with mountain building, Barton said "The limestone beds have been tilted near vertically, so the easiest route for the water is just straight down. You also have a large plateau that can collect the water, which helps push it one way or another."

Life in the caves

These caves aren't cozy. They're extremely dark, wet and cold environments. Temperatures inside caves in this region average 35 to 37 degrees Fahrenheit (2 to 3 degrees Celsius) throughout the year, said Ana Sofia Reboleira, a subterranean ecologist at the University of Lisbon in Portugal.

These conditions are harsh, and any organisms living there must adapt in order to survive. The deeper you go into caves, the more limited the nutrients become there. Animals need to slow their metabolism so they can go long stretches without eating, which allows them to live longer, Reboleira said. Their features are also adapted to life underground; most animals living at such depths lack pigment and eyes. They also have long appendages, hair and antennae to help them move in the darkness and sense air vibrations inside the cave.

One of these deep-dwelling species is a type of wingless insect known as a springtail, which Reboleira and her colleagues discovered on a 2010 expedition to Krubera at about 6,500 feet (1,980 m, or 1.2 miles) below the surface. The hexapod, named Plutomurus ortobalaganensis, feeds on fungi and decomposing organic matter. Sixteen years later, it still holds the record for the deepest known land animal.

Subterranean microbes have also evolved mechanisms to survive deep underground. At extreme depths, these microbes rely on chemolithoautotrophy, a metabolic process that enables them to gain energy by oxidizing the bedrock and turning it into food, Barton explained.

By understanding these subterranean environments, scientists can learn more about what's happening underneath our feet. "Caves are windows to the vast dimension of the underworld," Reboleira said.

Caves are ultimately part of a system that is fundamental for all ecosystems on our planet. They provide water purification by filtering water their their many layers, play an important role in nutrient cycling, and sustain the carbon cycle by storing carbon in stone and hosting organisms that turn carbon dioxide into organic matter.

"Subterranean ecosystems are absolutely vital and strategic for ensuring life on Earth," Reboleira said.

How much do you know about our blue planet? Test your terran knowledge with our Earth quiz!

Of the four fundamental forces of nature, gravity is the one we experience most directly ‪—‬ it's what keeps our feet on the ground and the sun in the sky. Yet we still can't pin down its exact strength. Since the 1980s, scientists have made more than a dozen measurements to calculate the precise value of gravity, and many of those numbers contradict one another.

So why is it so hard to figure out how strong gravity is?

One problem is that gravity is weak. Gravity feels strong because we constantly feel the pull of Earth. But the force of gravity between any two objects in everyday life — or any two objects that can fit in an experimental lab — is extraordinarily weak.

"It's weak, and you have to measure this against the background of the Earth's gravitational field," Stephan Schlamminger, a physicist at the National Institute of Standards and Technology, told Live Science. "If we measure gravity, we have to use everyday objects, because these are the only ones where we know the mass. What you have to do in the lab is basically use two very controlled masses, bring them close together, and measure the force between them."

In an April 2026 study, Schlamminger and colleagues replicated a precision experiment to determine the strength of gravity and calculated a value different from the previous result. They used 13 tons (12 metric tons) of mercury to run their experiment, but even then, "the change in the gravitational field was only a millionth of the change that we have here from local gravity," he said.

The team’s measured value was 6.67387x10^-11 m³kg^-1s^-2, which was 0.0235% lower than the previous result — a small difference in everyday terms, but significant in the field of metrology.

Christian Rothleitner, a physicist at the German National Metrology Institute, co-authored a comprehensive review of all gravity measurements with Schlamminger in 2017 but was not involved in the new study.

"This small force has to be determined to six or more decimal places," Rothleitner told Live Science in an email. "This is equivalent to trying to measure the weight of 7 human cells."

Physics, engineering and psychology

One explanation for the discrepancy in values could be that all of the measurements are so imprecise that the true value lies somewhere within them. But each experiment reports a small margin of error, and those ranges don't overlap.

Schlamminger thinks there are three possible reasons for this.

"I have it as a handy-dandy acronym: It's PEP: P stands for physics, E stands for engineering, and the second P stands for psychology," Schlamminger said. "It's sorted by excitement."

The least probable explanation, he said, is the physics one: Maybe there's some element of physics that scientists don't yet understand. Just as general relativity extended scientists' understanding of gravity, there may be another realm of physics yet to be discovered.

"I think it's a remote possibility, but we should not exclude it," Schlamminger said.

Then, there's the engineering explanation: Every experiment uses a slightly different setup, resulting in different values. Some use a torsion balance, a device that senses tiny forces by measuring the twisting of a small fiber. Others use pendulums or free-falling objects. Each approach has its own potential sources of error, and those mistakes are difficult to untangle from the gravitational signal.

"I personally do not believe that the reason lies in the physics, but in the measurement technology," Rothleitner said.

Human error is another part of the engineering explanation. "Such an experiment requires expert knowledge in many areas of physics and measurement technology," Rothleitner said. "You cannot be an expert in all those fields. This kind of measurement is on the cutting edge of measurement science."

The most likely possibility, Schlamminger said, relates to psychology.

"There is a driver for these people who measure these numbers to give really, really small uncertainties" — that is, margins of error — "because it makes them famous," Schlamminger said. "Because the pressure is there, the uncertainties may be a little bit too small, and that's why they don't agree with each other."

In the end, though, a precise measurement of gravity may not matter. We know the product of G times Earth's mass, and that's enough for practical applications like launching rockets into space. That may be all we need for now.

"The value of Newton's gravitational constant is rather of academic interest," Rothleitner said. "If it were different, nations would have spent much more effort in determining it better."

Schlamminger still finds it exciting, though. "We live in a society where we think everything is discovered," he said. "But if you look, there's still terra incognita. There are still problems, and the problems may be small, but they're still problems we can solve and contribute to and find mesmerizing and intriguing. And this is one of those problems."

Whether using family records, DNA tests or genealogy websites, many people can trace their family histories back generations. The world-record holder for the longest family tree is Chinese philosopher Confucius (551 to 479 B.C.), whose family tree extends more than 80 generations from his ancestors in the eighth century B.C. to his living descendants. That's almost 3,000 years.

But our species has been around for 300,000 years, based on scientific dates of the oldest known fossils. So how many generations do we go back as a species?

To find out, you need to know how long modern humans (Homo sapiens) have existed and the length of a generation, according to Matthew Hahn, a population geneticist at Indiana University Bloomington. The number of human generations that have lived would be equal to the time since H. sapiens emerged as a species divided by the length of a generation, also called the generation interval.

The generation interval is typically defined as the average age at which humans have children, Hahn said. It tends to be longer for men than for women because men can have children later in life, Hahn explained.

There are many estimates of our species' generation interval, each of which produces a different answer to the question of how many human generations have come before us.

For example, a 2003 study of Icelanders published in the American Journal of Human Genetics based generation interval estimates on the country's extensive records from churches and other sources. Using these records, researchers at the company deCODE Genetics created a genealogical database of the country's entire population. They found that the average generation interval in Iceland over the past 300 years was 30.3 years.

A 2005 study used data about the average age at which European women had children between 1960 and 2000 and estimated the generation intervals for men to arrive at an average generation interval of 29.1 years.

But those are estimates of the generation interval in the recent past. A study led by Hahn and published in the journal Science Advances in 2023 estimated the generation interval over the past 250,000 years. Hahn's study put together data from two others. A 2017 study of Icelanders, also led by deCODE Genetics and published in the journal Nature, found that as parents age, the blend of mutations that arise in their children changes. Using this data, Hahn and colleagues built a model of the mix of new mutations that you would expect to find in a group of people according to the generation interval at the time.

"If you know the types of mutations that individuals leave to their children according to their age, if you have a collection of mutations, you can try to estimate how old the mixture of individuals was," Hahn said.

A 2020 study in the journal PLOS Biology estimated when millions of mutations found in humans today first arose. Hahn and colleagues grouped the mutations from the 2020 paper according to when they arose and then determined the blend of new mutations that popped up during each time period. With that information, they could estimate the generation interval for each time span. While the generation interval varied over the course of an estimated 250,000 years, it was an estimated 26.9 years, on average. Using that generation interval, there would have been an estimated 11,152 generations of humans over 300,000 years, Hahn said.

Moisès Coll Macià, an evolutionary biologist and population geneticist who works as a postdoctoral researcher at the Institute of Evolutionary Biology in Barcelona, Spain, told Live Science that while a generation interval of 26.9 years is "not unimaginable," he prefers to give a range of possible generation intervals.

In Coll Macià's view, the lower bound would be the generation interval for one of our closest living relatives, chimpanzees (Pan troglodytes). Because humans and chimps share a common ancestor that lived during the Miocene epoch (23 million to 5 million years ago), you would expect that past human generations would have had a generation interval somewhere between that of contemporary humans and that of contemporary chimps, Coll Macià said. Chimpanzees have an estimated generation time of about 24.6 years, according to a 2012 paper in the journal PNAS.

As for the upper bound in modern humans, Coll Macià suggested 26 to 30 years. That's based on a 2016 PNAS study that analyzed fragments of Neanderthal DNA found in ancient and contemporary human genomes to estimate the human generation interval across the past 45,000 years.

Based on Coll Macià's upper-bound generation interval of about 30 years, there have been at least 10,000 generations of humans. Based on the lower-bound generation interval of 24.6 years, there have been at most 12,195 generations. These numbers attest that the human family tree is pretty tall, however you look at it.

See how much you know about early humans with our human evolution quiz!

Most long-distance travelers have felt the discombobulation of changing time zones: being excessively tired during the daytime but struggling to sleep at night.

Jet lag often emerges when people hop several time zones. The majority struggle more when flying east because our body clock is typically just over 24 hours. "It's a little bit harder to shift earlier," Helen Burgess, a professor at the University of Michigan's Sleep and Circadian Research Laboratory, told Live Science.

So what's going on in our bodies when we have jet lag, and is there a way to prevent it?

What is jetlag?

Jet lag is a mismatch between our internal body clock and our time zone.

The term "jet lag" was coined in a 1966 Los Angeles Times article, in which journalist Horace Sutton described the symptoms experienced by long-haul jet-setters as "not unakin to a hangover."

"He said, at the time, that jet lag derives from the simple fact that jets travel so fast they leave your body rhythms behind," Dr. Charles Czeisler, a professor of sleep medicine at Harvard Medical School, told Live Science. Before flights, people traveled overland or via ship so slowly that their circadian clock didn't get out of alignment.

Jet lag is the body's normal response to skipping time zones. "If the circadian system is working correctly, you should have a problem with jet lag," Czeisler said.

But some populations — such as older people, frequent fliers or those with mood or sleep disorders — are more likely to feel symptoms.

Jet lag might seem trivial, but even a small dose can have serious health implications. When the clocks move forward — essentially causing an hour's jet lag — there's an increase in fatal car accidents, Czeisler said. Getting behind the wheel straight after a long-haul flight is dangerous because a lack of sleep is a known cause of car crashes. In fact, road accidents are the leading cause of non-natural death among U.S. citizens traveling abroad.

And shifting circadian rhythms can even "flip people into mania or depression," Czeisler said. According to one study, 186 people over two years were admitted to a psychiatric hospital from Heathrow Airport with jet-lag-related mental health conditions, such as depression and hypomania (a shorter, milder form of mania), while another study has shown that chronic jet lag could increase the risk of neurological disorders.

Minimizing jet lag

The best way to adjust to your new time zone is through light exposure. "Light is the strongest signal that shifts the timing of the clock," Burgess said.

But the timing has to be right: exposure to light at the wrong time can push your clock in the wrong direction, making jet lag last longer.

Around two or three hours before your usual wake-up time, your body clock switches from interpreting light as evening light, making you sleepy, to morning light — which wakes you up.

"Light before the crossover point will shift you later, and light after the crossover point will shift you earlier," Burgess said. This is why napping isn't always the right solution.

If you usually wake up at 6 a.m., your crossover point will be around 3 a.m. This means that if you travel to a time zone six hours ahead and land at 7 a.m., this would be around 1 a.m. for your internal clock, which is still on your original time zone. So, your body interprets all the morning light you take in while landing and going through security as evening light and it wakes you up again, pushing your clock later.

Knowing when to expose yourself to light requires planning. To get light when you need it, go outside, use light boxes, or put on wearable light therapy devices. To avoid light when it will push you in the wrong direction, nap, stay inside, or wear sunglasses or glasses that block blue light within a specific wavelength range (460 to 480 nanometers).

To minimize the effects of jet lag, stay hydrated, avoid alcohol, be mindful of not drinking too much caffeine — "maybe just one cup of coffee" at strategic times, Burgess said, adding that caffeine has a long half life so will stay in your body and make it harder to sleep — and adjust to local mealtimes as soon as possible. Some people may also speak to medical professionals about whether melatonin is suitable for them.

You can prevent jet lag by shifting your body clock to the new time zone before you arrive. "On average, we can only shift the clock earlier by about an hour and a half a day," Burgess said.

For instance, to adjust to a time zone 10 hours ahead, you would need to start shifting your light exposure timings a week before the flight. For example, if you usually sleep between 11 p.m. and 6 a.m., you'd be aiming to shift toward going to bed at 9 a.m. and waking up at 4 p.m — something not feasible for most people.

It works, Burgess said, but when she tried it, she felt disconnected from the world around her because everyone was asleep during her waking hours. "I just felt really, really isolated," she said.

Although it's impractical for most people, you wouldn't experience the symptoms of jetlag because of how gradually you'd be moving your sleep schedule toward the new time zone. "People do not experience jetlag symptoms because everything is being moved slowly together," Burgess said, not "yanked apart" in one flight.

If you don't want to become nocturnal before you leave, nudge yourself in the right direction by shifting your schedule earlier by around an hour each day for just a few days. In essence, if fully switching to the new time zone before you depart is too challenging, nudging closer to it before arrival will still be beneficial. "It can make a huge difference," especially for those flying east, Burgess said. "Take the five or 10 minutes to plan it out, and that's going to really help."

This article is for informational purposes only and is not meant to offer medical advice.

Around 66 million years ago, a gigantic asteroid smashed into Earth and wreaked chaos globally.

Superheated rock from the impact spewed into the air, creating a mushroom cloud that heated Earth's upper atmosphere to a scorching 439 degrees Fahrenheit (226 degrees Celsius). Mile-high tsunami waves rushed through the Gulf of Mexico and disturbed ocean basins half a world away. Fires raged, burning animals and plants to a crisp. Shock waves propagated, blasting everything in their path. And particles from the collision, including sulfur, shot upward, blocking the sun and falling down as acid rain.

In all, 75% of Earth's species went extinct, including the nonavian dinosaurs. So how did some animals ‪—‬ including species of birds, turtles and mammals ‪—‬ survive the catastrophic asteroid collision and its aftermath?

Size, it turns out, played a crucial role. Earth's largest apex predators and herbivores — dinosaurs like Tyrannosaurus rex and Triceratops, and marine reptiles like plesiosaurs and the behemoth Mosasaurus — were doomed from the moment of impact, Kenneth Lacovara, founding executive director of the Edelman Fossil Park and Museum of Rowan University in New Jersey, told Live Science. That's because their enormity meant they were more likely to be harmed during the initial blast, were unable to hide in a safe place during the apocalyptic aftermath, and needed massive amounts of sustenance to survive in a time when food was scarce.

It's not too surprising that size would be linked to survival, Lacovara said. On land, "it's pretty clear in terms of correlation that you have to be small, and you have to be a burrower to get through this event," said Lacovara, who is also a professor of paleontology and geology at Rowan University. Small animals, such as some lizards and mammals, which were no larger than badgers at that time, were more likely to find shelter from the asteroid's immediate and long-term aftereffects. Other small animals that required less food, such as some turtles and fish, were sheltered in the water.

The avian group that led to modern birds survived likely because they were small and had powerful wings and chest muscles that allowed them to fly well and escape dire situations or find new opportunities. Their chicks also grew quickly, meaning they could soon fend for themselves and not overtax their parents.

This led to an overall shift in the average size of Earth's animals. On land, most of the largest surviving animals were about the size of house cats; in the water, the largest survivors were about the size of a "run-of-the-mill shark," Lacovara said.

What the survivors ate

Diet was likely another important factor, said Roger Benson, a curator of dinosaur paleobiology at the American Museum of Natural History in New York City. The plant eaters and those that ate them were especially hard-hit, as the sun was obstructed for up to a decade. Even with size on their side, some smaller creatures, such as certain lizard and turtle species, went extinct because their diets were too reliant on photosynthesizing plants, Benson said.

Aquatic ecosystems were a bit more shielded from the asteroid's initial impact, especially in deeper oceans and freshwater ecosystems. But as photosynthesizing plankton died off due to a lack of sunlight, food systems collapsed and large marine animals starved. Those that consumed dead, organic detritus had a better chance of survival. Some of those resilient marine creatures included the sea sponges, sharks of the Carcharias genus, and mollusks, including the lineage that led to today's chambered nautilus (Nautilus pompilius).

Seed eaters, including birds, and foraging insectivores, such as the tree-dwelling primate Purgatorius janisae, had a better shot at surviving because their food sources of seeds and insects weren't destroyed by extreme temperature changes and a lack of sunlight.

A common theory is that species with more generalist diets had a better chance of surviving the drastic environmental change. For example, Purgatorius coracis, a small mammal, survived the mass extinction thanks to their wide-ranging diet of insects, fruits and seeds. (This trend holds true for modern animals facing climate change: Those with generalist diets, such as crows and raccoons, are expected to do well because they have a wide range of foods to fall back on in case one food source disappears.)

Some species got lucky if their prey were also survivors. A few aquatic turtle species, like Hutchemys rememdium, had an adaptive feeding habit of eating shelled creatures which were living off detritus in aquatic ecosystems. A 2026 study found this adaptation, called durophagy, was linked to comparatively high survival rates during this mass extinction.

Certain behaviors, such as a higher capacity for reproduction or behavioral flexibility in a changing environment, may have enabled survival Benson said.

Mysterious exceptions

Despite current evidence, there are still unknowns surrounding mass extinction survivorship. Recent research indicates we don't know why certain adaptations helped some species but not others.

For example, many present-day species of bivalves feed on microscopic aquatic organisms that rely on the sun to survive. However, among the aquatic bivalves that survived the asteroid and its impacts, reliance on the sun didn't strongly determine survivorship. In another case, night lizards that survived near the impact site are known to have small litters, which goes against the hypothesis that fecundity is advantageous following extinctions. However, these night lizards' slow metabolisms were likely helpful for their survival, and they persist today, the study noted.

And the discovery that a large, terrestrial croc (Tewkensuchus salamanquensis) survived the asteroid strike in what is now Argentina raises questions about how this 660-pound (300 kilogram) species survived. This also raises questions whether the asteroid's impacts weren't as strong in the Southern Hemisphere as they were in the Northern Hemisphere. For example, the wide diversity of plant fossils uncovered in present-day Argentina suggests plants were able to regrow faster at southernmost latitudes.

"We also don't know why mammals emerged from the extinction as the dominant macrofauna," Lacovara said. One theory is that mammals are more resistant to fungal infections than reptiles are, giving mammals a better shot at surviving the "mini age of fungus" that followed the mass extinction.

After all, how evolution proceeded following the mass extinction is a part of humanity's history. The animals that survived ultimately ushered in the age of mammals, including humans, that continues to this day.

What do you know about crocs? Test your knowledge with our crocodile quiz!

When Earth first formed around 4.5 billion years ago, it was a ball of molten rock. Over time, heavier elements, like iron and nickel, sank to the planet's center, forming the Earth's early core.

Today, Earth's core remains an incredibly hot and dense sphere deep inside our planet. It consists of a liquid outer core, which starts at around 1,800 miles (2,900 kilometers) below Earth's surface and extends for 1,400 miles (2,200 kilometers). There is also a solid inner core, which begins at around 3,200 miles (5,150 km) below ground, with a radius of roughly 758 miles (1,220 km).

But just how hot is Earth's core? And how did scientists figure it out, if they can't go that deep underground?

Thanks to a combination of techniques, scientists have estimated that the temperature of the Earth's core is about as hot as the surface of the sun: It reaches around 9,000 to just over 10,000 degrees Fahrenheit (roughly 5,000 to over 5,500 degrees Celsius). This temperature comes from the boundary between the inner and outer core, which is thought to be the hottest part of the core.

However, this temperature was not measured directly. Instead, it is inferred via experiments and theories that scientists have of the composition of the core.

Earth's center is composed primarily of iron, roughly 85%, alloyed with nickel and other lighter elements; this material is in liquid form in the outer core and solid in the inner core. Scientists deduced these properties from a mix of laboratory measurements of iron alloys at high pressures, analyzing the composition of meteorites, and understanding how seismic waves bend or disappear as they travel through the planet's interior.

Because Earth's outer core is made mostly of liquid iron, temperatures in this region must be higher than iron's melting temperature. At the planet's surface, the melting point of pure iron is 2,800 F (1,538 C). But this number doesn't take into account the "enormous pressures" of the deep interior, Quentin Williams, a mineral physicist at the University of California, Santa Cruz, told Live Science. Increasing pressures boost the melting point of iron and most other substances, which explains why the inner core is very hot, but remains solid due to its high pressure.

To determine the melting temperature of iron at astronomical pressures, scientists have conducted a number of experiments to simulate this environment. Some studies have squeezed a piece of iron between two sharpened diamonds (called diamond anvil cells) to generate high pressures while a laser heated the iron to high temperatures. Others have hit pieces of iron with high-velocity projectiles or shock-creating rays to simulate crushing pressures. The results from those experiments were then plotted and extrapolated to the pressures at the boundary of the inner and outer core, which led to the estimates ranging from around 9,000 to just over 10,000 F.

"To some extent, what we know about the Earth['s] core is all an educated guess," Shichun Huang, a geology professor at Sun Yat-sen University in China, told Live Science. Many mechanisms, such as how the solid inner core crystalizes into a solid, are still a mystery.

Hot from the beginning

All of this heat hints at our planet's unique history. When Earth formed, all sorts of material got pulled together, including the iron that makes up the core. That "gravitational potential was converted to heat," Huang said.

Further, scientists think that some time during that formation, a Mars-size object hit our protoplanet and that this force deposited a lot of heat into the interior. Some scientists think that radioactive elements, like potassium, uranium and thorium, also contribute to the planet’s internal heat, although whether these elements are actually present in the deep Earth is debated, Huang said.

A hot core also contributes to Earth's ability to host life. In contrast to other planets, Earth's interior has held onto much of its original, primordial heat.

"We're not really good at planet cooling," Williams said, meaning that Earth has held onto much heat from its original formation, unlike other rocky planets in our solar system. As a result, our planet has properties like plate tectonics, which moves pieces of the Earth's surface, bringing up nutrients and creating diverse habitats for life to evolve and thrive. The iron core that is part liquid also generates Earth's magnetic field, which protects the planet and life on it from dangerous solar winds.

"If you care about life, you should care about the inside of the Earth," Huang said. A blazing hot core in the center of our planet is what allows all of us to survive where we are today.

What is Earth made of? Find out with our inside Earth quiz!

At the Alcor facility in Arizona, more than 150 disembodied heads reportedly lie in cryogenic chambers, preserved in hopes that future medical advances can bring these brains back to life in new bodies. Given that scientists still cannot revive a cryogenically preserved brain, why do patients bother with cryonics at all? Why couldn't these heads just be stitched onto new bodies in the present day, when they're still fresh? In other words, why isn't a brain transplant possible?

Dr. Max Krucoff, an assistant professor of neurosurgery at the Medical College of Wisconsin, would rather describe such a procedure as a body transplant. Unlike a patient who receives a donor heart or liver, transplanting a brain into a patient's body would make them "a completely new human being," he told Live Science. "Your agency, your identity, is contained within your brain."

Semantics aside, Krucoff said this transplant is currently impossible because surgeons cannot yet forge signaling connections between nerves in the central nervous system, which includes the brain and spinal cord. Transplanted peripheral nerves, or the nerves that branch off throughout the body beyond the brain and spinal cord, can eventually communicate with their new neighbors, since these nerve cells can regenerate. There is less evidence that the adult human central nervous system can generate fresh neurons, and if it does, it's to a limited degree. Neurons can form new connections throughout a person's lifetime, which is how learning is encoded, but scientists do not understand this pathway well enough to exploit it for a transplant.

Even a partial brain transplant, such as a cerebellum swap, is out of the question for now. In the case of the cerebellum, the structure is home to millions of specialized neurons called Purkinje cells, each of which receives signals from thousands of other neurons. "The number of connections is exponential," Krucoff said. "That's way beyond our capacity."

Fusing brain and body at the spinal cord would be the simplest theoretical brain transplant, since connections between spinal nerves are more straightforward than neurons in the brain. The surgeon performing this head transplant could connect the skin, muscle, vessels and bones in the neck, and even align the nerves of the spinal cord, "but to get those cells to communicate, we just haven't figured out how to do that yet," Krucoff said.

Has a brain transplant ever been attempted?

Scientists began attempting head transplants on animals with the advent of new blood vessel suturing techniques in the early 1900s. Most of the dogs and monkeys involved in those experiments survived a few days at most, as researchers struggled to create a working vascular system or to manage host rejection of the transplanted head with immunosuppressive agents.

Starting in 1970, Dr. Robert J. White transplanted the heads of monkeys onto new host bodies. The animals could chew and swallow food after surgery, and postsurgical electroencephalogram readouts suggested that their brains were awake, though none survived more than nine days.

Inspired by this work, Italian surgeon Dr. Sergio Canavero laid out his vision for the first human head transplant in 2013, attracting widespread pushback from colleagues on ethical and scientific grounds. Canavero's 2017 announcement that he had performed the first head transplant on a human cadaver was described at the time as "the continuation of a despicable fraud" by New York University bioethicist Arthur Caplan, who pointed out the likelihood of immune rejection and the challenge of linking a brain with all-new nervous inputs.

Can any tissue be transplanted into the central nervous system?

Although a surgeon can't switch one brain out for another, stem cell or organoid grafts could one day replenish damaged or diseased brain tissue.

Stem cells programmed to develop into neurons may have better odds of integrating into existing circuitry than mature neurons do, said Ruslan Rust, an assistant professor of research physiology and neuroscience at the University of Southern California Keck School of Medicine.

Ideally, these stem cells could be derived from the patient's own tissue, to lower the odds of immune rejection, but standardized donor cell lines reduce the need for quality control on individual tissue samples. In this way, neurons originating from one person's tissue could populate another person's brain.

Stem cell therapies have been tested in clinical trials for Parkinson's disease, stroke, spinal cord injury and epilepsy, but none of these treatments for these conditions are approved by the U.S. Food and Drug Administration for commercial use.

Rust told Live Science that future research will need to address the potential risks of these techniques, such as incompletely differentiated stem cells forming a tumor after transplant, or cells that do become neurons disrupting signaling pathways.

"The billion dollar question is, how do we make [transplanted cells] the cells that we want them to be, and how do we make sure that they integrate into those local circuitries where we want them to go?" he said.

Stem cells are also used to create lab-grown models of nervous tissues, called organoids, that can be transplanted into the brain. A 2023 study reported that human brain organoids could repair injured rat cortex, though therapies based on organoid transplant are still years away. Rust said such procedures could be more invasive than transplanting individual stem cells, and would need to account for vascular supply to the new tissue.

See how much you know about the most complex organ in the human body with our brain quiz!

It's a plotline that has haunted science fiction for decades: Humans become infected by plant pathogens, causing illness, death or a zombie-like state. But is there any truth to this idea? Can plant germs really cross over to people?

The short, disturbing answer is yes, experts told Live Science, but it's incredibly rare.

Plant pathogens are viruses, bacteria, fungi or other microorganisms that cause disease in plants, usually by infecting cells, reproducing, and spreading within and feeding on plant tissues. This can cause cell death, wilting, peculiar growths and discoloration; interrupt plants' ability to photosynthesize; stunt their growth; and, ultimately, kill them. Fungi cause an estimated 85% of plant diseases overall.

Much of what we know about how the human body functions makes it seem unlikely that these pathogens could invade our bodies, too. "It is extremely rare for a pathogen to jump from a plant to a human or vice versa because our biological architectures are fundamentally different," Dr. Soma Dutta, a physician and medical microbiologist at Apollo Multispeciality Hospitals in Kolkata, India, told Live Science over email.

Plant pathogens have evolved to breach the thick, rigid walls of cellulose or hemicellulose inside plants. By contrast, human cells are protected by lipid membranes, which are lined with receptor proteins that can initiate an immune response. Plant pathogens generally don't have the tools to break through these membranes, Dutta explained.

Besides, for most plant-specific fungi and bacteria, which typically survive in temperatures of up to 77 degrees Fahrenheit (25 degrees Celsius), our much higher bodily average is uncomfortably hot. "The human body is essentially a furnace," Dutta said. "The heat denatures their proteins and prevents them from causing infection."

Deadly pathogens

But there are exceptions. In 2023, Dutta and colleague Ujjwayini Ray, a consultant microbiologist at Apollo Multispeciality Hospitals, Kolkata, published a case report abouta patient who had developed a persistent cough and sore throat, was struggling to swallow, and was not eating, with seemingly no cause. When doctors examined him, they found a pus-filled abscess developing beside his windpipe. An exam revealed spores from Chondrostereum purpureum, a fungus that causes silver leaf disease in plants.

The researchers discovered that the 61-year-old was a plant mycologist, someone who studies the relationship between fungi and plants. "For years, his professional work involved direct, intense contact with decaying wood, mushrooms, and various plant fungi. This provided a clear pathway for inhalation of spores," Dutta explained. She and her colleague hypothesized that the spores had somehow managed to "hide from or resist" phagocytosis, a crucial phase of the human immune response, allowing them to establish a colony and spread. But to Dutta, the fungal infection remains a puzzle, because despite being a cool-weather fungus, it had managed to survive the higher temperatures of the human body.

With a course of antifungal tablets, that patient recovered fully. But that's not always the case; in the rare situations where plant pathogens cross over into humans, they typically affect those with weakened immune systems, which can result in death. Many of these pathogens thrive in warm, moist, stable environments, and they have been detected in catheters and breathing equipment in hospitals, which is how vulnerable patients can become exposed.

For example, the bacterium Pantoea agglomerans causes disease in a range of food crops ‪—‬ like rice, maize and sorghum ‪—‬ and if it gets into the human bloodstream, it can be deadly. Another type of bacteria, Burkholderia, causes rot in onion and rice, and research has documented cases where this germ has infected cystic fibrosis patients, causing pneumonia, blood poisoning and death. An investigation into several strains of B. cepacica, found that the microorganisms could surpass the defensive cilia and mucous lining the airways and get into and spread within the lungs.

Another bacterium, Pseudomonas aeruginosa, causes soft rot in plants like lettuce and potatoes. But it has also been found in hospitals, where it has passed to immunocompromised patients with severe burn wounds, cancer or AIDS, causing infections in their urinary tracts, blood and lungs.

Plant viruses

Plant viruses are generally believed not to pose a risk to humans because of the unique mechanisms they use to infect plants, but scientists have unearthed some compelling clues that we might be vulnerable to the diseases they can trigger. In 2014, Philippe Colson, a professor of medical virology at Aix-Marseille University in France, delved into the strange case of pepper mild mottle virus, a highly contagious plant pathogen that causes pepper plants to shrivel up. Peppers are widely consumed, so Colson and colleagues studied the stool samples of over 400 people and found the RNA of the virus.

Individuals who had the virus in their stools were significantly more likely to have symptoms of disease. "We were able to find a correlation between the presence of this virus in human stools and the symptoms of fever and pruritus [itchy skin]," Colson told Live Science. Although they didn't show a definite causal link ‪—‬ the symptoms may have been triggered by something else in the food ‪—‬ "it made us question whether there could be an interaction with our body, and whether or not it could be pathogenic in some cases," he said.

The study also showed that the affected patients' stools contained antibodies specific to the pepper virus. The virus's ability to withstand the human gut suggests it is highly resilient; in fact, pepper mild mottle virus is so robust that it's been considered for use as an indicator of human fecal pollution in waterways.

Colson is also intrigued by tobacco mosaic virus, which leaves a distinctive pattern on tobacco leaves that causes shriveling, affects their ability to photosynthesize, and stunts their growth. Strikingly, previous research into this virus had "looked at people that had lung cancer, and they found the virus in their biopsies," Colson said. Colson's own research has detected tobacco mosaic virus in several cigarette brands and in the saliva of people who smoked them. This raises tentative questions about whether the virus can travel into human bodies through cigarettes and possibly play a role in smoking-related cancers, Colson said.

For now, research on viral plant pathogen crossover into humans is limited, with the central puzzle being how they might infect humans if they do; plant viruses typically enter plant cells through a hole in the cellulose wall, whereas viruses in humans have to interact with a series of receptors around the cell before they can break in, Colson explained. Yet the important implications of potential plant virus infection in humans makes him "quite confident that this is a field that will expand in the near future."

The research so far shows that it's highly unusual for any plant diseases — fungal, bacterial or viral — to seriously, or fatally, develop in people who don't already have severely compromised immune systems. Crossovers are "extremely rare," Dutta noted, but she believes that our changing planet means plant pathogens deserve continued attention and research.

"The most significant barrier of the human body is temperature, which is being eroded by climate change and global warming," Dutta said. As global temperatures rise, plant pathogens may be finding ways to adapt to these warmer conditions, equipping them to survive in our bodies.

"The final message regarding plant-to-human crossovers is one of evolving vigilance," Dutta said. "While the biological barriers between plant and animal cells remain very strong and not easily breakable, the case of the Chondrostereum purpureum infection serves as a vital reminder that 'rare' does not mean 'impossible.'"

They sleep on our beds, steal our food, and generally rule the house. Between them, cats and dogs make up two-thirds of pet ownership worldwide. But which of the two companion animals are we more closely related to?

The answer depends on how you look at the question.

"From an evolutionary perspective we are equally related to dogs and cats," Mark Springer, a professor emeritus of evolution, ecology and organismal biology at the University of California, Riverside, told Live Science in an email.

Cats, dogs and humans are all mammals. In the mammalian family tree, which maps out how different mammal species are related to one another, both cats and dogs belong to the order Carnivora, while humans are primates. These two groups split from a common ancestor about 90 million to 95 million years ago, Springer said. Meanwhile, cats and dogs split from each other much later, around 55 million years ago.

In terms of common ancestry, "dogs and cats are more closely related to mammals such as pangolins, horses, cows, whales, bats, shrews and moles than they are to humans," Springer said. And "humans are more closely related to colugos [flying lemurs], tree shrews, rabbits, rats, and mice than they are to cats and dogs."

Genetic ties

Another way of deciding which species we are more closely related to is through a genetic lens.

If you measure how much the DNA code has changed over time, humans are about equally related to cats and dogs, William Murphy, a comparative genomicist at Texas A&M University, told Live Science in an email.

However, scientists also compare how DNA strands are organized within chromosomes. Here, a difference emerges.

Murphy explained that the ancestors of modern-day dogs went through extensive chromosome rearrangements over evolutionary time. (Such rearrangements are not unique to dogs; they occur across different animal and plant species, though scientists don't fully understand why some lineages rearrange faster than others.) Cats, on the other hand, retained a genome organization that's closer to ours. "In terms of how genes are arranged within chromosomes, humans and cats are twice as similar to each other as humans are to dogs," he said.

Because the way DNA is organized affects how genes are switched on and off, cats may be a better model than dogs for understanding human gene regulation, Murphy said.

That also makes them useful for studying genetic diseases. For example, polycystic kidney disease occurs in both humans and cats, and treatments developed for cats could help inform therapies for people.

Cats may also provide clues about cancer. A recent study found that cancer-related genes in cats are strikingly similar to those in humans, both in number and variety. One notable example involves a gene called FBXW7, which was mutated in more than half of the feline mammary tumors studied. In humans, mutations in the same gene are linked to worse outcomes in breast cancer

That said, dogs are also used to model and analyze human illness, including Alzheimer’s disease, idiopathic epilepsy, eye disease and heart disease.

Although cats may share more similarities with humans in gene regulation, to date more research has focused on dogs. This may be due in part to the fact that the complete feline genome became available later than the canine genome, as well as historical bias — cats have long been perceived as less cooperative in research settings.

So, which are we more closely related to? From an evolutionary standpoint, it's a tie, but genetically, at least in terms of genome structure, we are closer to cats.

How much of a cat fan are you? Find out by taking our cat quiz!

Over the past two decades, some studies have suggested that human brains are shrinking. But there is also evidence that IQ scores have risen over the past century.

But is it possible for us to get smarter as our brains shrink? Live Science contacted experts to find out.

First, it's important to note that a bigger brain size does not necessarily mean higher intelligence, said Jeremy DeSilva, an anthropology professor at Dartmouth College. Brain size "is only weakly related to measures of intelligence in humans. Albert Einstein's brain, for instance, was quite small, and he was Einstein!" he told Live Science in an email.

In Einstein's case, it appears that extraordinary folding patterns in several of his brain regions may help account for his genius. While there is some debate, studies suggest that there is little or no relationship between our intelligence and brain size in humans.

Have human brains shrunk?

Not all scientists agree on whether human brains have gotten smaller. However, many experts we spoke with said there is evidence for shrinkage over time.

"My research indicates that human brain size declined during the [entire span of the] Holocene by about 10% of its volume or about 150 ml on average," Maciej Henneberg, a professor emeritus of anthropological and comparative anatomy at Adelaide University in Australia, told Live Science in an email. The Holocene is an epoch that followed the last ice age, and spans from 11,700 years ago to the present day. In his research, Henneberg looked at the brain size by analyzing skulls from all over the world. In many cases, he analyzed the skeletons personally.

It's important to remember that Homo sapiens emerged around 300,000 years ago, so a brain shrinkage starting around 11,700 years ago would be a relatively recent development.

Other scientists have reported similar findings. "Our lab thinks that the data currently available show a clear, global trend toward a decrease in brain size in more recent times," said DeSilva, whose research looked at more than 5,000 skulls from people who lived in Europe, Asia, Africa and Australia. Many of the skulls date to the Holocene.

Jeff Stibel, who holds a doctorate in brain science and has published a number of papers on the topic of brain shrinkage, said the "Holocene warming period has coincided with more than a 10% reduction in brain size in modern humans." Stibel took part in DeSilva's research, and then gathered more data by analyzing the brain size of roughly 800 additional skulls from around the world.

However, some scientists disagree. Research by Brian Villmoare, an associate professor of anthropology at the University of Nevada, Las Vegas, and his team found no evidence that human brains are shrinking. "I see no evidence that, once we acquired our modern form, that our brains have changed in any meaningful way," Villmoare said in an email.

Some say more nuance needs to be considered. "Brain size in some human populations decreased over the last 15000 years," John Hawks, an anthropology professor at the University of Wisconsin-Madison, told Live Science in an email. "But there are some complexities."

Hawks noted that datasets of brain size tend to overrepresent men of European ancestry, making it difficult to determine whether there is a global trend among diverse populations.

"Brain size did rebound toward larger sizes during the last 150 years in industrializing countries," Hawks said. "This was likely mostly attributable to nutrition and correlated with body size, but whether body size accounts for the entire effect is not clear from the limited data."

Why are human brains getting smaller?

If our brains have shrunk, then why? Scientists have proposed a few ideas.

One has to do with the introduction of farming.

"During the Holocene, humans gradually introduced food production [such as agriculture and animal husbandry] that allowed them to live in larger communities," Henneberg said. "The brute physical strength required for hunting big game and protecting families against strong predators became less necessary while a smaller body size required less food, so it was favoured by natural selection."

In fact, it's not just our brains; our bodies have also shrunk. "At the end of the Ice Age, male body height was about 1.75 m [5.74 feet], whereas in mid-Holocene agricultural communities it was 1.65 m [5.41 feet]," Henneberg said. "Body mass has declined even more since we see thinner (less robust) human bones." Heights have rebounded recently in some parts of the world, and Hawks said that our brain sizes may be rebounding also.

The warming that occurred after the end of the last ice age also may be a factor. Stibel said two biological principles, known as Bergmann's rule and Allen's rule, state that "bodies and organs tend to become leaner in warmer climates to increase surface area and dissipate heat."

Another possible explanation is that human intelligence itself has changed as we have taken on more specialized jobs and share more information with each other, meaning we don't have to know everything, just a subset of knowledge to keep society going.

DeSilva said his team proposed that a "population increase, the subsequent specialization of people into specific roles in society and the growth of collective intelligence may have contributed to individual brain size decrease as modeled in some eusocial insects like ants and wasps." For instance, "I'm a decent enough anthropologist and anatomist, but you wouldn't want me fixing your car or investing your retirement savings on Wall Street."

Stibel agrees that this is a significant factor. "Some ant species that develop complex social systems see similar reductions in individual brain size as the colony carries more of the cognitive load," he said. "We appear to have done something remarkably similar with culture and technology.

"What our research suggests is that we've undergone a fundamental shift in how cognition works," Stibel added. "Rather than relying solely on individual brainpower, we've become extraordinarily dependent on cultural and technological networks."

Stibel noted that large brains can be a burden, consuming more energy which requires people to eat more food to survive. "Large brains are metabolically expensive, consuming roughly 20% of our resting energy and producing significant heat." During times when food is scarce, such as during an Ice Age, this can result in people with larger brains being more likely to die of starvation.

This doesn't mean humans are smarter or dumber than they were; it just shows our intelligence is different.

"We've likely traded some raw computational capacity for the ability to leverage collective intelligence," Stibel said. "Whether that's a gain or a loss depends entirely on how you define intelligence. And, of course, how stable the cultural and technological systems we now depend on turn out to be in the long run."

See how much you know about the most complex organ in the human body with our brain quiz!

The samurai of Japan are famous for being skilled warriors who followed a code of honor. In popular culture and museums, samurai are often depicted as men, which raises a question: Were any samurai women?

Female samurai existed and there is some evidence that they fought in battle, several experts told Live Science. But how often they fought is a matter of debate, with some scholars calling it very rare and others suggesting it happened more often.

One important point in answering this question is that the samurai were an entire social class, sometimes called the "bushi" class. Anyone born into this class was a "samurai," regardless of whether they fought or practiced any form of martial arts.

"Any woman born in the samurai status group was a 'female samurai' even if she never picked up a weapon, just as any man born into that status group was a samurai, no matter how wimpy/untrained/etc. he may have been," Sean O'Reilly, a professor of Japan studies at Akita International University, told Live Science in an email.

It's unclear how often female samurai fought in battle, however. Women who fought in battle are sometimes called "onna-musha," which translates to "women warriors."

"I must say, as an historian, that onnamusha ‪—‬ female warriors ‪—‬ were probably not as frequent or as militarily significant as most people today believe," O'Reilly said.

Some particularly good evidence for female samurai participating in battle comes from the late 19th century, near the time when the samurai class was abolished, Diana Wright, who was a professor at Western Washington University, wrote in a 2001 article in the journal "War in History."

At that time, Japan was in a civil war as supporters of the Tokugawa Shogunate, which ruled Japan from around 1603 to 1868, battled those who wanted to overthrow the shogunate and return some of the military governor's powers to the emperor. The Boshin War, as it is sometimes called, lasted from January 1868 to June 1869. During that war, there were a number of recorded instances where female samurai, who fought on the side of the shogunate, engaged in battle, Wright noted in her article.

The shogunate forces were led by the Aizu domain (a regional government in northern Japan), and during the siege of the Aizu's capital of Aizu-Wakamatsu, a group of female samurai formed their own unit known as the "Joshigun."

"Although 20 to 30 women are believed to have made up the unit, the names of only 10 are known," Wright wrote. A 22 year-old woman named Nakano Takeko was the unit's unofficial leader.

Armed with only swords and naginatas (pole weapons with curved blades that can both stab and slash), they fought in a battle at Yanagi bridge against a force equipped with rifles, Wright explained. Records indicate that Nakano Takeko killed five or six men with her naginata before she was shot down. Ultimately, the battle ended in defeat and the surviving members of the Joshigun, along with the male troops, had to withdraw to a castle.

During the time of the Tokugawa shogunate, women of the samurai class were required to undergo martial art training with the naginata so they could defend themselves and their families, Wright noted. The amount of training they received varied, with the women of the Aizu domain tending to receive a larger amount.

Remains of female warriors?

A mound located in Numazu, a city in central Japan, may hold the remains of female samurai who fought in battle, some scholars believe. The mound contains human skulls, along with other skeletal bones, and an analysis of the mound's remains was published in Japanese in 1989 in the Journal of Anthropology. The skulls are from about 105 people; all of them were young adults when they died, and about one-third were women. They date to the 16th century, and scientists interpreted them as being the remains of people who were killed in combat, likely in the Battle of Senbonhama (also known as the Battle of Senbon Matsubara), which was fought between the Takeda and Hojo clans.

This mound is "indicative that women of fighting age fought and died in sixteenth century battles," Thomas Conlan, a professor of medieval Japanese history at Princeton University, told Live Science in an email.

However, Karl Friday, a professor emeritus of history at the University of Georgia, said the mound should be regarded with caution, as we can't be sure everyone buried in it actually fought in a battle. It's possible that some of the people buried in the mound were noncombatants who were killed anyway, Friday told Live Science in an email.

Stories and legends of female samurai

A number of stories refer to female samurai fighting in battle. Perhaps the most famous was Tomoe Gozen, who lived during the late 12th century. Stories say she served a lord named Minamoto no Yoshinaka and fought in the Genpei War, which was fought between the Taira and Minamoto clans between about 1180 and 1185, Thomas Lockley, a law professor at Nihon University who has studied and written extensively about the samurai, wrote in a 2022 article in the magazine Medieval World: Culture & Conflict.

One of the chronicles, called "The Tale of the Heike," says that as "a fighter she was a match for a thousand ordinary men, skilled in arms, able to bend the stoutest bow, on horseback or on foot, ever ready with her sword to confront any devil or god that came her way" (translation by Thomas Lockley).

Another famous woman mentioned in stories was Ōhōri Tsuruhime, who lived circa 1526 to 1543. She became the chief priestess of Ōyamazumi Shrine, located on the island of Ōmishima, after her father and brothers were killed while defending the island from a daimyo (a regional governor) named Ōuchi Yoshitaka, Stephen Turnbull, a historian who has written extensively on the samurai, wrote in his book "Samurai Women: 1187-1877" (Osprey Publishing, 2012). Despite being just 16 years old, she took charge of the island's defense force and defended it from the invaders. During her defense, she claimed to have been aided by the shrine's kami (spirit) and has been compared to Joan of Arc, Turnbull noted.

Some of what's said to be Tsuruhime's armor survives today and is displayed at the shrine. Conlan said that it is a suit of 16th century armor that is "tailored to the female anatomy."

However, Friday said we should be cautious when interpreting stories like these. "We do have stories about female warriors, like Tomoe Gozen, Hangaku Gozen, Ohori Tsuruhime, Ueno Tsuruhime, and a few others, but these women are all semi-legendary — especially with regard to their participation in battles," Friday told Live Science in an email.

Regardless of how accurate the stories are, female warriors became famous. "Mythologizing female warriors of yore began in Japan's Kamakura period [circa 1185 to 1333] and intensified in the Edo period [circa 1603 to 1868], with a huge proliferation of woodblock prints showing women holding naginata and so forth," O'Reilly said. Friday said the "very fact that these women became so famous is a pretty good indication of how uncommon female warriors must have been."

Taboos about women and battle

Friday thinks it would have been very rare for female samurai to engage in battle because it was considered taboo.

"One fascinating primer on military conduct, passed down within a branch of the Hōjō family, enjoined against such things as sharing quarters with women for three days prior to battles, allowing pregnant women or women who had recently given birth to touch a warrior's weapons, riding in boats with female passengers while in route to battle, and even allowing women to look upon the backs of officers departing for campaigns!" Friday said.

"The bottom line is that while there almost certainly must have been at least a few cases of women participating in Japanese battles over the course of [the] 8th to 16th centuries … there's absolutely no good evidence to support the conclusion that women warriors were any more common in Japan than they were in medieval France or ancient Sparta, much less that this occurred often enough to justify calling it a practice or even a phenomenon," Friday said.

While the samurai class was effectively abolished during the 1870s, some of the training practices done by female samurai are still carried out today. Eric Shahan, a Japanese translator who specializes in translating martial arts texts, noted that the Yoshin School (a branch of traditional Japanese martial arts) "still practices Naginata in Kimono, reflecting the fact that women may have to suddenly take up arms — and therefore have no time to change into training gear."

Can you identify these historical objects of war? Test your smarts with our weapons of the world quiz!

Whether it's having the "eye of the tiger" or being lionhearted, references to lions and tigers evoke power and determination. That's not surprising given these animals are both large, powerful felines. But they are distinct species. So what do these iconic cats have in common, and how are they different?

To answer this question, let's consider the two species' appearances, evolutionary histories and how they live their lives as kings of their ecosystems.

Appearance and anatomy

The most obvious difference between modern lions (Panthera leo) and tigers (Panthera tigris) is their distinct physical appearances, particularly their coats.

"Just looking at them, you can clearly tell that tigers have stripes and lions don't," Dell Guglielmo, a zookeeper who cares for lions and tigers at Smithsonian's National Zoo, told Live Science. "The lion males have the large huge mane, and the females do not." In contrast, male and female tigers look the same.

A tiger's stripes help the cat blend in among the patterns of shadows and light created by grasses and trees in the dense temperate and tropical forests of Asia where they live. The orange and brown or black stripes stand out to the human eye, but to a tiger's colorblind prey, orange appears green and blends in. The stripes are pigmented on their skin, too. Each tiger's stripe pattern is unique, Guglielmo said.

Lions' tawny coloring also serves as camouflage. Lions blend in with the dry grasses of Africa's open savannas.

On the inside, however, lions and tigers are very similar. "If you look at the skeletons, all cats essentially look the same," Z. Jack Tseng, an associate professor of integrative biology at the University of California, Berkeley, told Live Science. There's an unofficial saying among those who study cat fossils. If "you ask them to describe the cat family fossil record, usually people will say, 'Well, a cat is a cat is a cat,'" Tseng said. Even your pet cat is basically identical to a lion or tiger, skeletally speaking — just proportionally a lot smaller.

Evolutionary differences

Despite their similarities, "lions and tigers are not each other's closest relatives," Tseng said. Tigers are more closely related to snow leopards, while lions are grouped with leopards and jaguars.

Moreover, tigers emerged much earlier as a species than lions. "The oldest putative tiger fossils are around 2 million years old," Tseng said, "whereas lion fossils are probably no more than a million years old, probably younger than that."

When it comes to classifying cats, size matters. Lions and tigers belong to the Panthera genus, along with leopards, jaguars and snow leopards. The last common ancestor of all five species likely emerged at least 5.57 million years ago, a genetic modeling study found. Tseng and his co-authors modeled this age based on genetic sequences and anatomical data, such as characteristics of bones and teeth. No fossil has been found yet, but this ancestor was likely smaller than lions and tigers, Tseng said.

He noted that both species independently evolved to be large. Animals can grow in body size for a variety of reasons, including living in cooler climates, as this gives them a smaller surface area-to-volume ratio, which helps to reduce heat loss. "The fossil record is not detailed enough to say whether lions and tigers got large independently for the same reasons, but they probably followed one or more of these broader trends as they evolved," Tseng said.

Behavior

Lions and tigers behave quite differently in the wild. Lions live in social, matriarchal groups with multiple female relatives and one or two male adult lions who may leave if challenged by another male. That's where the mane can come in handy.

"Cats often go for the neck when they fight and when they kill their prey," Jacob Shanks, a wildlife care manager at San Diego Zoo Wildlife Alliance, told Live Science. "Male lions being social, there's a lot of battling for territory and access to reproduction and other resources, so the manes help protect against neck wounds."

Tigers have far less social interaction than lions do. Tiger cubs remain with their mother for two years before going their separate ways. As adults, tigers generally avoid each other, aside from breeding. When a tiger wants to approach another individual, it makes a puff-like sound, called a chuff, by pushing air through its nose. Tigers are "one of the only cat species that chuff," Guglielmo said. If the second tiger is OK with the first tiger moving closer, it will chuff back. "Lions don't use a chuff; they just use different body language or just walk up and see if it works," Guglielmo said.

Would tigers and lions ever meet in the wild? That's not likely, as they live in completely different parts of the world. Even in the fossil record, there is no overlap in their range, which prevents the two species from competing and breeding, Tseng said.

However, humans have artificially bred lions and tigers in captivity, creating a hybrid called a liger. "Hybridization doesn't exist in the wild," Shanks said. "I've never seen a case of wild lion-tiger hybridization." Making hybrids doesn't benefit either species, he added.

A key similarity between lions and tigers are the threats they face. Tigers are considered endangered, and lions are vulnerable, according to the International Union for Conservation of Nature. Both big cats face threats from human conflict, habitat loss and climate change. "They need active conservation to protect and to help improve if we're going to not have them slip away," Shanks said.

Think you know about lions and tigers? Test your smarts with our big cat quiz!

Most cat owners are familiar with the funny head shake their pets do, often after eating, drinking or enjoying a nice cheek scratch ​​‪—‬ they flick their head back and forth a mile a minute. Dogs and other mammals are known to show similar behaviors.

So why do our furry friends shake like this, and does it mean something is wrong?

Usually, this head shaking is totally normal, experts told Live Science. However, the reasons for it can vary a bit between cats and dogs.

Why cats shake their heads

Cats shake their heads when the nerve endings on their heads are stimulated, Sarah Crowley, an anthrozoologist at the University of Exeter in England, told Live Science.

"The physiological driver for it is stimulation of the very sensitive hairs and nerve endings around the top of the head, particularly around the ears," she said.

These nerve endings can be stimulated by a satisfying scratch, prompting the kitty to shake it off. "If a caregiver has touched a sensitive area" ‪—‬ for example, around the ears or whiskers — "this may result in a head shake," said Amy Miele, a clinical animal behavior lecturer at The Royal (Dick) School of Veterinary Studies at the University of Edinburgh in Scotland.

This rapid swinging motion is very effective at dislodging obstructions, such as food or drink, from the cat's ears or whiskers. It can also help to resettle their fur, Crowley added.

"Often the head shake is followed by grooming of the face [or] ears," Miele told Live Science in an email.

The rapid head toss can also indicate annoyance. It's often a response to having their ear blown on — not something you should do on purpose, as they don't like it.

Thankfully, this erratic side-to-side motion doesn't usually cause dizziness. "Animals (including humans) are more likely to make themselves dizzy by spinning in circles," Miele said.

The full-body shake

Cats aren't the only animal with these silly shakes. "Lots of mammals have this head shake reflex to something in or around their ears," Crowley said. "You see mice and rats do it when they've been eating and drinking."

In dogs, breeds with upright ears, like German shepherds, are more likely to shake their heads than floppy-eared pups such as King Charles spaniels because it's easier to get something (like a grass seed or insect) stuck in an open ear.

Dog owners might recognize other types of shakes: the predatory shake (when your pooch tries to playfully "kill" its toys by whipping its head from side to side while its "prey" is in its mouth) and the full-body shake, sometimes called the "wet dog shake." "It starts at the head and then rolls down the whole body and ends at the tail," Crowley said. This is the kind of post-bath shake that soaks everything in your bathroom before you've had a chance to dry Fido with a fluffy towel.

The full-body shake can also indicate a transition between places or behavioral states ‪—‬ for example, if the pup just experienced something stressful or exciting. "This 'shake off' is a way of helping the animal to relieve tension and reset emotionally and physically," Miele said.

"It could be after quite an intense play session with another dog that was really fun," Crowley added. "You often see both dogs will finish playing, and then they'll both shake, and then they'll go do something else."

When to be concerned about your pet shaking its head

Although head shaking can be normal, if your pet starts doing it more often, in unusual circumstances, or while scratching or tilting their head, it might indicate a medical issue, such as an infection, parasite, irritation, neurological problem or something lodged in their ear.

"It's painful and uncomfortable, and that's the only thing that they can do, apart from stick their foot in it," Crowley said.

If you're concerned about your pet's head shaking, consult your vet. "If ear infections are left untreated, then this can result in hearing loss and risks the infection spreading beyond the tympanic membrane (ear drum) and into the middle ear, causing problems with balance and resulting in the animal becoming very unwell," Miele said.

For the most part, though, head shaking is just an adorably normal behavior that helps your pet stay clean and comfortable.

This article is for informational purposes only and is not meant to offer medical advice.

How much do you know about cats? Test your knowledge with our cat quiz!

Some stars and constellations are ever-present in the night sky; Polaris, for example, has served as a steady guiding light for millennia in the Northern Hemisphere.

But if you look closely at the sky night after night, you may notice that some stars move east to west from dusk to dawn, as well as change positions from season to season. For instance, Orion disappears from view for part of the year. So why do some constellations stay visible year-round, while others disappear and reappear depending on the season?

It boils down to Earth's seasonal movements, experts told Live Science. "A lot of what we can and cannot see comes down to the fact that we're on a rotating globe," Michael Brown, an observational astronomer at Monash University in Australia, told Live Science.

To make sense of this, it's helpful to note that when we look at the night sky, we're viewing only a section of the universe. "When the Earth turns to face away from the sun, that's when we have night time," Valerie Rapson, an astronomer at the State University of New York at Oneonta, told Live Science. "And whatever stars are in that direction … those are the stars we would see."

In addition, which constellations you'll see depends on where you are on the globe. Stargazers in the Northern Hemisphere mostly see stars and constellations above the equator, whereas Southern Hemisphere observers mostly see stars and constellations south of the equator.

As our tilted Earth revolves around the sun over a span of a year, different parts of space come into view while others shift away. In the Northern Hemisphere, for example, Orion's Belt ‪—‬ what looks like a line of three bright and blinking stars in the Orion constellation ‪—‬ is most clearly seen in the winter. But in the summer, as Earth orbits to the other side of the sun, Orion is in the part of space not visible from the Northern Hemisphere because it is obscured by daylight.

Visible yearlong

Some constellations don't disappear with the seasons. These are called circumpolar constellations, which are positioned near the celestial poles. These constellations are always visible, but will spin and change positions throughout the year, Rapson explained.

For example, in the Northern Hemisphere, there's Polaris, the North Star, which is almost right above the North Pole. The Big Dipper, a familiar series of stars known as an asterism, in the constellation Ursa Major, spins in Northern Hemisphere skies year-round.

The Southern Hemisphere, where Brown is based, has fewer of these circumpolar constellations. "The south celestial pole [is] a blank bit of sky," he said, although southern skies do feature the Southern Cross year-round, a constellation of four bright stars.

In total, the International Astronomical Union recognizes 88 constellations in the night sky, and astronomers have mapped these constellations' regular motions using celestial coordinates. Today, there are interactive digital star charts ‪—‬ like Stellarium ‪—‬ that help stargazers navigate and find particular constellations based on the time of the year.

Wobbling Earth

These stars and constellations, however, won't look this way forever. Earth's spin has a slight wobble. This means that over thousands of years, constellations and their positions and movements in the skies will change slightly, at least from Earth's perspective. For example, the timings of sky's zodiac constellations, constellations found along the equator, are now shifted from what they were many millennia ago due to the Earth's wobble.

The wobble also affects the North Star. Polaris, for instance, wasn't always the star in this position. In fact, in 2500 B.C., the spot was held by Thuban, a star in the constellation Draco. The north star of the future will change, too. "Five thousand years from now, we will have a totally different North Star, or maybe no technical north star at all, at least not one that lines up perfectly," Rapson said.

Understanding this nuance and getting the correct timing of when constellations are above the horizon is important for astronomy. Brown, who studies the evolution of galaxies, noted one constellation of interest.

"Sagittarius is the center of our galaxy," he said. It's visible primarily in the winter in the Southern Hemisphere (summer in the Northern Hemisphere), and it's also right next to the Milky Way's supermassive black hole. "So people who study the Milky Way's black hole, they study at the particular times of year when [the black hole] is not blocked by the sun," Brown said.

Learning how the stars change with the seasons, therefore, is an important part of discoveries related to this mysterious celestial object and many others.

"Sometimes what initially seems a little bit confusing can become clearer when we shift our perspective a little bit," Brown said. By understanding the night sky and deciphering Earth's movements in relation to the stars, we can understand more about our position and wonders in the universe.

Editor's note: This story was updated at 2:44 p.m. EDT on May 5 to note that stargazers in the Northern Hemisphere mostly, but not exclusively, see stars above the equator. Similarly, those in the Southern Hemisphere mostly see stars south of the equator.

Can you name all the animals, objects and mythological figures hiding in the night sky? Find out with our constellations quiz!

Many chickens meet their end with a swift blow to the neck. But there's lore that chickens can run around headless, and there were even news reports of a chicken nicknamed Miracle Mike who reportedly lived 18 months after a farmer tried ‪—‬ and failed ‪—‬ to kill it by cutting off its head.

So can chickens really survive without their heads?

The reality is that they can't live long that way ‪—‬ less than a minute, experts told Live Science.

After decapitation, chickens commonly flap their wings and move their legs, said Dr. Marcie Logsdon, a veterinarian in the Exotics and Wildlife Department at Washington State University's Veterinary Teaching Hospital in Pullman, Washington. However, "I think the actual running around is fairly uncommon," Logsdon said. "It's usually just going to be strong muscle contractions of both the wings and the legs, and that's a very common thing," she said. Those movements typically last a minute or less, she added.

However, the answer to whether a chicken is alive or dead in the seconds following decapitation depends on how death is defined. In the case of a decapitated chicken, brain death occurs first, while cardiac death happens a few seconds later. So, in those moments after brain death and before cardiac death, one could view the chicken as either alive or dead, depending on the definition of death used.

Brain death is a state of unconsciousness in which the entire brain is permanently damaged and an individual cannot breathe on their own. Brain electrical activity in chickens stops within 30 seconds after cervical dislocation (breaking the neck), according to a 2019 study published in the journal Animals. Decapitation includes breaking the neck, so according to the framework of brain death or loss of brain activity, chickens survive a few seconds after decapitation.

"That is not to say that the animals are consciously aware of anything happening for those seconds, but there is residual electrical activity occurring," Andrew Iwaniuk, a comparative neuroscientist at the University of Lethbridge in Alberta, Canada, who specializes in bird brains, told Live Science.

Cardiac death, which happens when the heart permanently stops beating, tends to occur a few seconds after brain death, Iwaniuk said. "The time difference would be in the order of less than 10 seconds," he said.

As a result of these different definitions of death, Logsdon considers a chicken's post-decapitation movements "post-mortem reflexes," whereas Iwaniuk views the chicken as alive during those last stirrings.

Chickens move after decapitation because "there'll be some residual neural activity in the spinal cord," Iwaniuk said. Continued breathing is also due to residual neural activity, he said. The heart muscles, on the other hand, can continue to contract and release without neural input — until they run out of energy and oxygen, he said.

Further, the brain normally sends signals telling the muscles to relax when they aren't needed, Logsdon said. Decapitating a chicken stops those signals, and when that happens, "we sometimes get some jerking," Logsdon said. "That does seem to be very exaggerated in chickens."

Miracle Mike

Something very different happened in the case of Miracle Mike. In September 1945, the BBC reported in 2015, Wisconsin farmer Lloyd Olsen decapitated a group of chickens to take to market, but one of them didn't die. For 18 months, the rooster, who came to be known as Miracle Mike, toured the U.S. in sideshows. Olsen and his wife fed the rooster through its esophagus and cleared its airway to try to keep it from choking, but it did eventually choke to death in 1947, after the Olsens misplaced the throat-clearing syringe, the BBC reported.

This story may seem to illustrate that a chicken can live without its head. But the reality is that there's only one widely known case of a chicken living without some of its head. Instead of beheading Mike by cutting straight across the neck, Logsdon explained, the farmer "cut off a chunk of the brain and essentially most of the face."

Mike's decapitation left him with the back of his brain as well as one ear, the BBC noted. He likely retained the brainstem, located at the back of the brain, which controls basic physiological functions like breathing and heart-rate regulation, Iwaniuk said. Mike probably also had his cerebellum, which helps coordinate movements, Logsdon said.

"It's probably why he was able to actually stand up and walk, as opposed to just running around and flailing," Logsdon said.

How much do you know about our feathered friends? Test your knowledge with our bird quiz!

Legend has it that King Arthur and the Knights of the Round Table were busy heroes, with tales of them slaying a giant, organizing a successful search for the Holy Grail, and ruling a kingdom from a city called Camelot.

Some stories say Arthur was conceived or born at Tintagel, a site in Cornwall, England, that flourished between the fifth and seventh centuries. While many stories of King Arthur are likely false or greatly exaggerated, there is one question that scholars have long debated: Did King Arthur really exist?

There is currently no consensus, although each side lays out their arguments in interviews, scholarly papers and books, Live Science found.

A work of fiction?

Some scholars believe that Arthur is fictional.

The "king" was conceptualized in the ninth century, Nicholas Higham, a professor emeritus of early medieval history at the University of Manchester in the U.K., told Live Science in an email. He added that the earliest evidence for King Arthur is from a text called "Historia Brittonum" (Latin for "History of the Britons") that was written in Wales around A.D. 829, possibly by a monk named Nennius.

The book refers to King Arthur not as a king but as a war leader who defended Britain against Saxon invaders around A.D. 500.

The text "displays obvious signs of having been stitched together, apparently in Latin, from a variety of conflicts noted in earlier literature," Higham said. However, the earlier literature that the ninth-century writer draws from makes no mention of Arthur, and it seems that the writer invented him.

"The Arthurian tradition rests on what must be judged a ninth-century fiction, therefore, an extraordinarily successful one needless to say, but a fiction nonetheless," Higham wrote in his book "King Arthur: The Making of the Legend" (Yale University Press, 2018).

During the early ninth century, Anglo-Saxon kingdoms controlled much of Britain and were encroaching into Wales. The ninth-century writer would have wanted to create a character who could successfully fight them, Higham said.

We "can be reasonably confident that the Arthur with whom we are familiar was made up by one imaginative clerk early in the ninth century as the last of a string of courageous British war-leaders through whom he was seeking to deliver a vision of British success in warfare against foreign interlopers," Higham wrote in his book.

Other scholars agree that King Arthur was not a real person.

"Personally I don't think Arthur existed, as he is not named in any early source material" and isn't mentioned until the ninth century, Helen Fulton, a professor of medieval languages and literature at the University of Bristol, told Live Science in an email.

That era in Britain had no shortage of rulers, she noted. "Clearly there were British kings and war-leaders who emerged from the Roman occupation of Britain and fought with each other and with the incoming Saxons," Fulton said.

A real man?

But some scholars argue that King Arthur was, in fact, real. For evidence, some researchers have turned to "Annales Cambriae" (Latin for "Annals of Wales"), a series of texts that records historical events in Wales and other parts of the region. An analysis of two of these annals that discuss Arthur suggests that these passages were originally composed during the sixth century, Bernard Mees, a researcher of history at Monash University in Australia who did the analysis, wrote in his book "King Arthur and the Languages of Britain" (Bloomsbury, 2025).

While the earliest surviving copy of the "Annales Cambriae" dates to around 1100, Mees noted that some of the language used in the annals about Arthur is anachronistic, reflecting spelling that was used in the sixth century, after the Roman Empire had collapsed in Britain. This suggests that the annals mentioning King Arthur were composed during the sixth century and that Arthur actually existed, Mees wrote in his book.

The real-life Arthur would have been a king or a prince, he said. "The earliest records don't specifically call Arthur a king, but it's difficult to see what else he would have been," Mees told Live Science in an email.

Ken Dark, an archaeology professor at the University of Cambridge, told Live Science that "in all probability, a historical Arthur did exist, but we can't absolutely say that he did."

Of the two Arthur annals in "Annales Cambriae," the second could potentially be accurate, Dark said. That annal dates to 537 and discusses Arthur and Medraut (also known as Mordred), who was possibly Arthur's son or nephew. The annal reports that both died in the Battle of Camlann. In some Arthurian stories, Mordred kills Arthur, although the annal doesn't say this.

The annal also mentions there was plague in Britain and Ireland at the time. We know from other historical texts and archaeological remains that an epidemic — possibly the bubonic plague — swept through the Mediterranean region in 536, and it could have made its way to Britain and Ireland by 537, Dark said. Additionally, the annal is fairly brief, similar to other annals, and has no obvious legendary material.

Interestingly, between the mid-sixth and mid-seventh centuries, there was a relatively high number of royal family members named Arthur in Britain and Ireland, Dark noted. This suggests that the kings who named them "were basing their name on a famous Arthur," Dark said. This famous Arthur could have been a real person who was a war leader, although we can't be certain.

If Arthur were real, he would have been quite a different person from the Arthur in the stories, Dark said, adding that people like Lancelot, Guinevere and the Knights of the Round Table would have been fictional characters added later.

"Nobody claims that any figure of the fifth, sixth or even seventh centuries would have been anything like the Arthur of legends," said Dark, who is writing a book called "Tyrants and Traders: Tintagel, Arthur and the Lost Kings" (Bloomsbury, 2026), which is set to come out later this year.

Mary Bateman, an English lecturer at the University of Bristol who has studied the Arthurian stories extensively, said Arthur can be both fictional and real at the same time.

Arthur is either a figure or figures "of history that have picked up a lot of myths along the way," Bateman told Live Science in an email, "or else was originally a figure of myth [that] acquired new stories/narrative threads etc from historical figures."

Whether you're watching a spectacular lunar eclipse or studying lunar features through a telescope, there are plenty of reasons to gaze at the moon.

But does the moon look the same from everywhere on Earth?

The short answer is no, astronomers told Live Science. Not only does the moon's appearance change from place to place, but it even seems to swivel in some locations from moonrise to moonset.

"How we see the Moon and Stars is all a matter of perspective," Pamela Gay, a senior scientist at the Planetary Science Institute, a U.S.-based nonprofit that investigates solar system exploration, told Live Science in an email.

From the North to South poles

If you were to look at a full moon from Earth's North Pole, you'd see the iconic Tycho crater, with its splayed ejecta rays at the bottom of the moon's face. However, from the Amundsen-Scott South Pole Station, situated exactly at our planet's South Pole, Tycho crater would appear at the moon's top.

Less-dramatic changes in the moon's orientation appear from temperate regions. A diagram collated by the Lunar and Planetary Institute shows that the moon's orientation in Wellington, New Zealand, is 97.1 degrees counterclockwise to that visible in Los Angeles. This angle depends on the difference in latitude between the locations.

In other words, while someone in Illinois might see an upright "man on the moon," for an observer in Sydney, the "moon is now a bunny jumping downward," Gay said.

This happens because your orientation with respect to space-based objects changes as you travel between latitudes. Observers on opposite ends of Earth look at the moon from opposite vantage points.

Still, all places on Earth see more or less the same face of the moon. That's because the moon completes one rotation on its axis in exactly the same time it takes to orbit Earth once ‪—‬ a phenomenon called synchronous rotation (though librations, or wobbles, cause the view to slightly vary).

From crescent to boat

The moon's phases during each lunar cycle — the 29.5-day cycle from full moon to new moon and back — also differ between the Northern and Southern hemispheres.

"This is a result of how the local horizon plane … is oriented relative to the positions of the Earth, moon and sun," Catherine Miller, observatory specialist at Mittelman Observatory at Middlebury College in Vermont, told Live Science by email. Far from the equator, she said, the boundary between the moon's unlit and lit regions is aligned nearly vertically, so the moon's phases progress horizontally.

But while Northern Hemisphere observers watch the moon grow and shrink from right to left, the opposite occurs for the Southern Hemisphere, as per the Lunar Planetary Institute. This difference, Miller said, again stems from the different perspectives from the hemispheres. That's why unicode calendar symbols (the kind you see in your phone's emoji list) for the first- and third-quarter moons — which have been designed from a Northern Hemisphere viewpoint — appear inaccurate for Southern Hemisphere observers, according to a 2017 Unicode Technical Committee document.

Things get even more interesting at the equator. The moon, seen at the time it rises, expands vertically rather than sideways as it heads toward the full-moon phase. This means the crescent moon often looks like a boat. However, across most of Earth's surface, the moon's phases change from being more vertical to more horizontal (and vice versa) over different seasons, according to the book "Astronomy for All Ages" (Globe Pequot Publishing, 2000).

Changes through the night

In many places, the orientation of the moon's face appears to rotate about its center as it travels across the sky on a given night, Miller said. For example, at the equator, it looks like the "Moon's face can rotate by roughly 180 degrees in an evening," she said.

This is because the moon's orbit is nearly aligned with the orbital plane that Earth takes around the sun. Consequently, it frequently passes nearly over the zenith (the point in the sky directly above an observer) at the equator, according to a 2025 article in the Journal of the British Astronomical Association.

When the moon rises from the east at the equator, it will set to the west after having crossed the zenith. At moonrise, you will be facing due east, but to see the moon at moonset, you will have to rotate your body by 180 degrees. So, it's not the moon that has twisted, but people watching the moon who are turning their bodies to follow the moon's path. "It's all about how [the moon] follows that arc," Gay said.

This phenomenon is not true at higher latitudes, where the moon does not pass over the zenith, so you do not have to turn by a full 180 degrees to see it.

This apparent "rolling" of the moon decreases as you move away from the equator, toward the poles. The farther the moon is away from the zenith, the less it appears to twist between moonrise and moonset.

So, the next time you travel, take a peek at the moon. You might just be stunned.

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Scientists have identified more than 3,000 species of bacteria living in the human gut. We know they play a role in digestion and immune function. But can they also influence the kinds of food we crave?

In a 2014 study in the journal BioEssays, researchers proposed that gut microbes might manipulate the eating behavior of their hosts by generating cravings for foods the bacteria thrive on, or even causing discomfort until the host eats what benefits them.

"We don't always see eye to eye with our microbes," Dr. Joe Alcock, co-author of the study and a professor in the Department of Emergency Medicine at the University of New Mexico, told Live Science.

Salmonella Typhimurium is one example. It hijacks the chemical signals between the gut and brain to keep its host eating through infection.

"Typically, when you have a GI [gastrointestinal] infection, you stop eating," Alcock said. "And Salmonella [Typhimurium] seems to actually impair that … so that animals continue to eat and continue to produce infectious particles in their poop that go on to infect other animals."

However, this was a theoretical paper — it proposed mechanisms by which microbes could manipulate cravings, but hadn't demonstrated that they do. The pathways it proposed — including things like altering taste receptors and hijacking the vagus nerve — were plausible, but unconfirmed, especially in the context of everyday food cravings.

How the microbiome may influence food choices

In 2022, researchers tested this hypothesis. In their study, Kevin Kohl, an associate professor of biology who focuses on how interactions with microbes impact the physiology, ecology and evolution of animal hosts at the University of Pittsburgh, and Brian Trevelline, a microbiologist and postdoctoral fellow at Cornell University, transplanted microbiomes from wild rodents with different diets — carnivore, herbivore and omnivore — into germ-free mice, and then measured what they ate.

"I perhaps naively thought that the carnivore-inoculated mice were going to eat the high-protein diet," Kohl told Live Science. "That's not what we saw."

Instead, the mice with herbivore microbiomes preferred protein, while the mice with carnivore microbiomes preferred carbs. But a key finding held: Different microbiomes led to significantly different food choices.

But how? Gut bacteria can produce many of the same neurotransmitters the brain uses to regulate appetite, including serotonin, which signals to the brain when you've had enough to eat. In fact, roughly 90% of the body's serotonin is produced in the gut, not in the brain, and research has shown that gut bacteria play a direct role in that production.

In the mouse study, the team found that the mice that received the herbivore microbiome had significantly more tryptophan — a building block of serotonin — in their blood. Previous research has shown that higher serotonin levels suppress carbohydrate cravings in particular, which may explain why those mice shifted toward a high-protein diet.

"That might be at least one potential avenue in which the microbiome is affecting diet, appetite and dietary preferences," Trevelline said.

The findings also raise the possibility that the relationship runs both ways. If your microbiome shapes your cravings, and your diet shapes your microbiome, small changes in what you eat could shift the cycle over time.

"I could totally see some feedback cycles where shifts in the microbiome either perpetuate behaviors or bring about different cravings," Kohl said.

However, Kohl and Trevelline's study was in mice. "Food choice is really tricky and totally different in humans," Kohl said. "It's influenced by culture, society, economics, learned behaviors, associations." In other words, a lot of other factors affect our dietary choices.

Still, one recent research paper has started to connect these findings to human health. In a 2025 study published in the journal Nature Microbiology, researchers found that a gut bacterium called Bacteroides vulgatus can suppress sugar cravings in mice by producing a metabolite that triggers the production of glucagon-like peptide-1 (GLP-1), the same hormone targeted by drugs like Ozempic. People with type 2 diabetes also had lower levels of this bacterium, the researchers found.

But Kohl cautioned against giving your microbes too much credit for your choices. "Free will still exists," he said. "The microbes are not driving our choices. But these cravings, low-grade feelings about food — those come from our internal nutritional state" — things like amino acids and other compounds circulating in the body — "which we know is influenced by the microbiome."

Between roughly 2000 and 1500 B.C., the Minoan civilization flourished on Crete and nearby islands, building palaces decorated with frescoes, engaging in athletic activities such as bull leaping, and creating written scripts that experts have never been able to decipher. One of the most important cities they constructed was at Knossos, on the northern coast of Crete, and it contained a palace the size of two football fields.

Around 1500 B.C. their written scripts stopped being used and Minoan palaces show evidence of decline and destruction. So how did this civilization end?

Scholars have proposed a wide variety of explanations, including a volcanic eruption, earthquakes, climate change, the disruption of trade routes, and conflict with the Mycenaeans, a group from the Greek mainland. However, the civilization may have had a less-dramatic ending, experts told Live Science.

What was the Minoan civilization?

The Minoan civilization got its name from the British archaeologist Sir Arthur Evans, who excavated the remains of Knossos in the early 20th century. Evans named the civilization after King Minos who, according to ancient legends, ruled Crete and constructed a labyrinth where a minotaur was kept. The Minoans flourished during the "Bronze Age" (3300 to 1200 B.C.), and they are known for their palaces decorated with marine motifs such as dolphins.

To figure out what happened to the Minoans, scholars need to define exactly what Minoan civilization was and determine how different it was from the Mycenaean civilization that thrived on Crete after 1500 B.C. The Mycenaeans were based on the Greek mainland, and they boasted a warrior-elite society that inspired Homeric epics as well as a religion with deities similar to the later Olympic gods. The Mycenaeans are sometimes considered to be the earliest Greeks.

"[What] do we mean by Minoan or Mycenaean civilization, and what do we mean by 'end'?" said Guy Middleton, a visiting fellow at Newcastle University who specializes in the archaeology of Late Bronze Age Greece, the Aegean and the eastern Mediterranean.

"What archaeologists mean by Minoan and [Mycenaean] is sets of material culture — archaeological cultures — not a people, and not ethnic groups," Middleton told Live Science in an email. "Any person could adopt a particular material culture, and would then look like a Minoan or Mycenaean person."

For instance, the lavish burial of a high-ranking warrior at the site of Pylos on mainland Greece dates to around 1500 B.C. and has artifacts with Minoan designs. But the warrior was buried on the Greek mainland, which is thought to be where the Mycenaeans originated. "Was he a Mycenaean or a Minoan? These are modern distinctions. Who knows how he thought of himself," Middleton said.

A time of change

One thing that changed after 1500 B.C. was language. Whereas the Minoans used two undeciphered scripts, known as Linear A and Cretan hieroglyphs, the Mycenaeans used a text called Linear B, which encoded the Greek language, said Philip Betancourt, a professor emeritus of prehistoric Aegean art history and archaeology at Temple University in Philadelphia.

"If the gradual disappearance of the Minoan language is used as the sign for the culture, it was gradually lost after an invasion by Greek-speakers gradually changed the complexion of the culture," Betancourt told Live Science in an email. This linguistic change, which "did not affect all of the island at the same time, happened around the middle of the second millennium [B.C.]," Betancourt said.

Although Middleton thinks a takeover by the Greek-speaking Mycenaeans is plausible, he proposed another possibility. The change in culture "can also be seen as an internal Cretan [development] — since not all Cretans were identical," he said. "Just as Mycenaeans adopted elements of Minoan culture into their own, for their own reasons, so Minoans may have adopted elements of Mainland culture."

However, Nanno Marinatos, a professor emerita of classics and Mediterranean studies at the University of Illinois Chicago, said there was no invasion. "Minoans had all the technology to avoid threats," she told Live Science in an email, noting that they had a navy that could stop any invaders.

Marinatos thinks that a major climatic event could have contributed to the Minoans' decline. The eruption of Thera, a volcano on an island in the Aegean Sea 70 miles (110 kilometers) north of Crete, around 1500 B.C. may have caused considerable damage to the Minoans by destroying ships and disrupting trade networks. This catastrophe may have played a significant role in the decline of their civilization.

Did the Minoan civilization ever end?

Another possibility is that the Minoan civilization never had a formal end.

The "simple reply is that like most ancient complex societies, it simply evolved into one with a later modern definition," Betancourt said. "Genetic studies show that the Minoan genes are still around. The descendants still live in Crete and elsewhere."

Middleton agreed that there was "no sudden end to Minoan Crete, just a series of changes over a long period of time." He noted that Minoan deities continued to be worshipped for centuries after 1500 B.C.

"To some extent, the way we divide up history, geographically and chronologically, makes us think in terms of 'ends," Middleton said. "But what we really have is constant and normal interaction and change."

Over 100 million people, and more than 30% of adults, suffer from some type of allergy in the United States, and that number is on the rise. But where are these allergies coming from? Do we inherit them, or do they result from environmental factors?

The answer involves a little of both, said Dr. Derek Chu, an allergy specialist at McMaster University in Ontario. Allergies occur when the immune system mistakes a harmless substance, such as pollen or a certain type of food, for a dangerous substance and attempts to get it out of the body. Once the body reacts this way toward an allergen, it will repeat the allergic response for every new exposure, unless there is a successful intervention.

"The body goes awry and is trained to do the wrong thing," Chu told Live Science.

According to Leah Kottyan, an immunologist at Cincinnati Children's Hospital, there are several main types of allergies. They include allergic dermatitis, which manifests through skin conditions such as rashes and hives; allergic asthma, which involves airway inflammation and the overproduction of mucus; allergic rhinitis, which results in sneezing and a stuffy nose; and food allergies, which can trigger a whole-body immune response. All of these responses could be driven by the same allergen, Kottyan noted. Moreover, people who have one type of allergic response are more likely to have others and more likely to be allergic to more than one thing.

According to Kottyan, there is almost certainly a genetic component to the development of allergies. Independent studies comparing the prevalence of allergies in identical and fraternal twins found that the identical twins were far more likely to exhibit similar allergies than the fraternal twins, indicating that their genetic makeup played a role. In one study, the identical twins had an average of 95% similarity between four different types of allergies, whereas fraternal twins had an average of around 37%.

However, the link between genetics and allergies is complicated. Allergic responses can be traced to mutations in hundreds of genes. One of the most studied such genes, known as filaggrin or FLG, affects the development of the skin's moisture barrier, Kottyan said. Mutations in FLG cause the moisture barrier to not act as it should. This increases a person's risk of developing eczema, allergic dermatitis and other types of allergies.

FLG mutations can put the skin in a compromised state, which can increase the skin's susceptibility in getting cuts and scrapes and from developing dry and cracked skin. When this compromised skin comes into contact with an allergen, the immune system is more likely to become sensitized improperly to that allergen, Kottyan said. This can cause an allergy to develop.

"Literally, when there's food on the baby's skin, the food is coming in through the damaged skin barrier, and the child is getting sensitized to that food through the skin," Kottyan told Live Science.

Environmental factors

Although allergic reactions have a genetic component, environmental factors — including exposure to different allergens — play a big role in how someone develops allergies.

"It doesn't track one to one," Chu told Live Science. In a recent study Chu co-authored, scientists investigated nongenetic risk factors for allergies, including the baby's method of delivery, early exposure to antibiotics, and when they were introduced to solid food.

Moreover, even people who inherit a tendency to develop allergies may not be allergic to the same things as their parents. Rather, whether the baby becomes allergic to peanuts, for example, will likely be determined by their early exposure to that allergen. There is now a FDA-approved immunotherapy treatment for children that centers on daily exposure to peanut proteins over a long period of time.

"If the immune system is exposed inconsistently, then it's not learning to tolerate what it's being exposed to, and instead it can easily end up misbehaving, leading to allergic reactions," Chu said.

The microbiome also plays a big role in the development of allergies. Scientists have found that early exposure to a wide variety of environmental microbes results in robust and healthy gut and skin microbiomes that are more tolerant of potential allergens. Without this diversity, or with a microbiome consisting of higher percentages of certain microbes, it is more likely that a person will develop conditions like eczema and food allergies.

A similar theory suggests that the eradication of many common parasites, and the more sterile environment many of us now live in, may have played a role in the uptick in allergy cases in the United States. With no parasites and fewer viruses and bad bacteria to go after, our immune systems may be developing too many allergy-fighting cells instead of the bacteria and virus-fighting ones.

Researchers like Chu and Kottyan are continuing to pinpoint the risk factors that can cause someone to develop allergies and test new preventive measures to help children grow into allergy-free adults. For now, Kottyan said, the best steps parents can take to prevent allergies in their children is to expose them to common allergens early and often and to take good care of their children's skin, especially in areas that are prone to dryness and eczema.

"Allergy genetics are not deterministic; they aren't going to say you absolutely are going to get a disease," Kottyan told Live Science. "It's not a life sentence."

This article is for informational purposes only and is not meant to offer medical or dietary advice.

The sun has been Earth's constant companion ever since our planet emerged. But if the sun were to suddenly disappear, what would happen to our home planet?

To understand the fate of a sunless Earth, it's important to know how both arose. The sun formed about 4.6 billion years ago, when a massive spinning cloud of gas and dust collapsed in on itself and condensed, creating the biggest object in what would become our solar system and eventually reaching a temperature of 27 million degrees Fahrenheit (15 million degrees Celsius) at its core.

Much of the remaining material nearby then clumped up to form Earth and the other rocky planets, including Mercury, Venus and Mars, as well as moons and asteroids. Since its formation, Earth has been heavily reliant on its star. The sun's gravitational pull keeps our planet in orbit in the "Goldilocks zone," the just-right distance from its star where it's not too hot or too cold for water to exist as a liquid on a planet's surface. The sun also drives photosynthesis and water cycles, and it provides sunlight and heat, which influence our climate. Plus, the sun's ultraviolet light helps our bodies make vitamin D, which is needed for healthy bones and teeth.

If the sun suddenly vanished, Earth and the vast majority of life would be in dire straits. It would start "a ticking time bomb on the survival of every living thing on earth that relies on photosynthesis, which is the vast majority of surface life and all of humanity," Timothy Cronin, an associate professor of atmospheric science at MIT, told Live Science over email.

For at least 8 minutes, 20 seconds, no one would know the sun went missing ‪—‬ that's how long it takes light from the sun to reach Earth. During that time, "we'd almost certainly have no idea that anything had happened," Cronin said.

Then, the real trouble would begin.

After the sun's eight-minute swan song, there would be "a sudden blackout," Cronin said. Without sunlight, artificial lighting from electricity, oil or gas would be the main ways we could still generate light, along with fire, bioluminescence and fluorescence. We'd lose track of day and night. The moon, which reflects the sun's light, would go completely dark, although distant stars in the sky would still be visible. And without the sun's mass and gravity keeping the planets and other celestial bodies in orbit, "all the planets would fly off in the direction of their current travel," Cronin said.

But humanity would have more immediate problems than flying off into interstellar space. No sunlight would mean crucial processes, such as growing food, would become much more complicated.

Photosynthetic organisms would be done for, Michael Summers, a professor of planetary sciences and astronomy at George Mason University in Virginia, told Live Science. Most plants that weren't grown under artificial lighting would quickly suffer. And while some "might stay dormant for weeks to months, like they do in the wintertime, eventually all photosynthetic organisms would die."

Fungi, meanwhile, feed on living or dead matter, and "there would be a great deal of dead material available," Summers said. So fungi likely wouldn't die from a lack of food, but from the cold.

Cold planet

It wouldn't take long for frigid temperatures to change the Earth as we know it.

At first, Earth would cool by an average of roughly 36 F (20 C) every 24-hour period, Summers said. "That plunges almost the whole world into subfreezing temperatures within just two to three days," although as it got colder, the temperature change per day would decrease, he said. Small ponds might freeze over within a week, whereas lakes might take weeks or months. The oceans could persist "for many years, maybe decades," and in certain places, like "the deepest parts of the oceans where you have volcanoes, they might stay liquid for potentially as long as the volcanoes last," Summers said. "And that could be billions of years."

To understand how cold Earth would ultimately get, let's consider Pluto. Right now, Pluto is "about 40 times as far from the sun as Earth is, and the temperature there now is about minus 400 degrees Fahrenheit [minus 240 C]," Summers said. "Once you eject the Earth out of our solar system, it's going to get much further away than Pluto very quickly."

But Earth's temperature wouldn't reach absolute zero, thanks to the Big Bang that happened around 13.8 billion years ago. Even "the lowest temperatures in the universe are limited by heat that's left over from the Big Bang," Summers said. "Take any object very far away from a star and let it cool for a million years," and it will still remain a few degrees above absolute zero. The temperature of the leftover radiation known as the cosmic microwave background is about minus 454 F (minus 270 C), whereas absolute zero is slightly chillier at about minus 459 F (minus 273 C).

At an ultracold temperature, human civilization and most of life would almost certainly collapse. "It's conceivable that people could survive underground in caves, sustained by geothermal or nuclear energy, with plants grown under artificial lighting," Cronin said, "but this would be an extinction event to make all others look puny."

What would survive?

One thing that might survive? Near-microscopic animals called tardigrades, also known as water bears. "Ugly little critters," Summers said, but "hard to kill." They can be zapped with radiation or immersed in certain types of alcohol and still survive; perhaps hitting them with a hammer would kill them, he suggested. "Otherwise, they're pretty much one of the hardiest forms of life on Earth."

Likewise, bacteria that don't require photosynthesis, such as types that live around deep ocean vents, would likely survive. That's because certain microbes, including some bacteria and archaea, use chemosynthesis, as opposed to photosynthesis, to "live off of chemical bonds in rocks and minerals," Summers added.

Fortunately for humanity, there is no reason to believe the sun will vanish in the blink of an eye. Over time, however, the sun will die. It will continue to create heat and light for another 5 billion years or so, but once its fuel runs out, it will expand into a red giant, swallowing Mercury and Venus and perhaps Earth. Regardless, humans likely won't last that long; the sun's gradual increase in brightness is expected to vaporize Earth's oceans in a little over a billion years from now.

While those impacts may be a long way away, Summers said it's important to consider the potential outcomes. When "we understand more about stars and how they can change over time, on short timescales and on long timescales, we understand the universe better."

Sun quiz: How well do you know our home star?

One of the best-known side effects of smoking cannabis is "the munchies," or an intense increase in appetite. One minute someone might feel full, and the next they appear ravenous.

This effect, known scientifically as cannabis-induced hyperphagia, is powerful enough that for regular marijuana users, it can even cause weight gain, a seven-year analysis found.

So what's really going on? Why can cannabis make people want to eat even when they're already full?

The short answer is that tetrahydrocannabinol, or THC, affects the body's endocannabinoid system. This network, found in both the brain and body, is made of naturally produced signaling molecules, called endocannabinoids, and their corresponding receptors. It helps regulate mood, stress, pain, memory, immune responses, and even appetite.

Endocannabinoids act "like a 'dimmer switch,' fine-tuning neural activity to maintain balance, or homeostasis," Ryan McLaughlin, co-director of the Cannabis Research Center and associate director of graduate studies in neuroscience at Washington State University, told Live Science in an email.

The body produces endocannabinoids because the brain needs tight control over how strongly circuits activate, especially when responding to challenges like hunger, McLaughlin explained. Any disruption in this system is "linked to conditions like anxiety, depression and chronic pain, making this system highly relevant to mental and physical health," he noted.

And because THC has phytocannabinoids that target the same endocannabinoid system, understanding it helps scientists sort out cannabis's potential benefits and risks.

THC "hijacks" our appetite circuit

Of the 140 or more types of phytocannabinoids that cannabis plants produce, THC is one of the best known. "THC works by mimicking endocannabinoids and binding primarily to CB1 receptors in the brain," McLaughlin said. Those CB1 receptors show up heavily in brain areas tied to craving and reward, including the hypothalamus, which helps coordinate hunger signals.

When a person is not smoking cannabis, the body's endocannabinoids are usually released briefly and locally, nudging the system toward hunger in a controlled way. But when someone uses cannabis, "THC activates these receptors more broadly and for a longer period of time," McLaughlin said. "In that sense, it 'hijacks' a system that normally fine-tunes neural activity."

That "hijack" is what causes the munchies, as THC turns up the brain's interest in food to more extreme levels for longer than normal.

"When THC activates CB1 receptors in these areas, it enhances the motivational value of food, essentially tricking your brain into thinking it is in an acute fasting state," McLaughlin said.

The munchies isn't about food type

That fasting state helps explain why the munchies don't always look like a dessert binge. In a 2025 study co-led by McLaughlin and others at Washington State University and the University of Calgary, 82 adults ages 21 to 62 were randomly assigned to vape either 20 milligrams of cannabis, 40 milligrams of cannabis, or a cannabis placebo. People who vaped cannabis ate significantly more than those in the placebo group, but it wasn't just junk food.

"Beef jerky was one of the No. 1 things intoxicated people gravitated toward, which I don't understand. Honestly, I would have thought chocolate, chips, Rice Krispies treats — things like that," study co-author Carrie Cuttler, a WSU psychology professor, said in a statement. According to the researchers, water was also among the popular consumption choices.

To further test whether weed affected taste preferences, the researchers also ran tandem experiments in rats at the University of Calgary. The animals could access different foods but had to work for it by pulling a lever.

Sober rats that were already full tended to quit the task. But after ingesting THC, they behaved differently. "You get them stoned again, and even though they're now full and they've eaten, they go right back as if they're starving," study co-author Matthew Hill, a professor of medicine at the University of Calgary, explained in the statement.

Like their human counterparts, the rats were not picky. The researchers expected more interest in carb-rich foods because these foods trigger a release in dopamine, a neurotransmitter that induces a feeling of pleasure, but that's not what they saw. "It just seemed to be any food," Hill said.

Other things can trigger the munchies

While cannabis use is known to trigger the munchies, other factors that affect the endocannabinoid system can create the same effect.

"Sleep deprivation, stress, intense exercise, or just the sight or smell of food can strongly activate appetite circuits in the brain," McLaughlin said.

However, THC takes things a step further. "What is unique about THC is that it enhances both hunger-regulating regions and reward circuits [in the brain], which together can amplify the biological drive to eat as well as the pleasure of eating," McLaughlin added.

Given that THC and other phytocannabinoids are still highly regulated, challenges in understanding the munchies and other effects caused by these substances remain. "One big misconception is that the 'munchies' happens to everyone, every time," McLaughlin noted. In reality, people's responses vary depending on the THC dose, the person's tolerance, the way the drug is ingested and a person's individual bodily makeup.

"Much of what we know comes from self-report rather than tightly controlled laboratory studies measuring actual caloric intake, hormones, and neural activity," McLaughlin said. "But as cannabis laws evolve and research access improves, I think that there will be more attention paid to understanding this phenomenon."

This article is for informational purposes only and is not meant to offer medical or dietary advice.

It's easy to think of pupils as simple holes that widen in the dark and shrink in the light. But if you look across the animal kingdom, you'll see a variety of pupil shapes: vertical slits in cats and snakes, horizontal rectangles in goats and horses, and W-shaped crescents in cuttlefish, for instance. The shape of an animal's pupil can reveal a lot about how that animal sees and what it needs to survive.

In a perfect optical system, the shape of the pupil shouldn't matter much. "In an ideal world, the way optics is generally taught, the pupil's sort of irrelevant because all the light should be coming to an exact point anyway," said Jenny Read, a visual neuroscientist at Newcastle University in the U.K.

But real eyes are imperfect — light coming through the pupil creates diffraction and defocus that different pupil shapes resolve in different ways. "It actually turns out to be really complicated," Read said.

One important factor is depth of field, Marty Banks, a professor emeritus of optometry at the University of California, Berkeley, told Live Science. His 2015 study in the journal Science Advances was the first to systematically explain why the orientation of an animal's pupil matters for survival.

Depth of field is most easily illustrated with a camera lens, where the camera's aperture acts as its pupil. A narrow aperture creates an image where things are in focus both far away from and near to the camera. A wide aperture puts one object in focus and blurs everything in the foreground and background. But when a pupil isn't perfectly round, it can produce more variation in focus, and some animals' eyes take advantage of that.

Ambush predators, like cats and snakes, have forward-facing eyes that judge distance by comparing the slight differences between their two views — a process called stereopsis. Because the eyes are side by side, the differences show up most clearly along vertical edges, which means those edges need to be sharp.

"How do you make sure they're sharp? Well, you [narrow] the pupil, and you increase the depth of field," Banks said. "But they only need to do it for the vertical contours. So it's really clever to stop the pupil down horizontally and leave it wide open vertically, because for the other contours where stereopsis is not useful, like for horizontal contour, now they can use blur to estimate the distance."

That's why the pupils of ambush predators are shaped like slits: Shrinking the pupil horizontally helps with stereopsis, while widening it vertically helps them estimate depth from blur.

However, this works best for predators close to the ground. Larger predators, like lions and tigers, tend to have round pupils because they look at the ground at a steeper angle that reduces the advantage of blur to estimate distance.

Prey animals have different priorities for survival, and the shapes of their pupils reflect that.

"For prey animals, it's more about field of view than it is about image sharpness," Banks said. "They need to be able to see panoramically along the ground because most of the predators that are going to approach them are going to be on the ground."

In contrast, the eyes of prey animals like goats, sheep and horses tend to be positioned on the sides of their head, and their pupils are shaped like a bar ‪—‬ wide horizontally and short vertically. That wide shape lets in more light from the horizontal plane in front of them and behind them to help them scan their surroundings, while the short vertical opening sharpens horizontal contours — basically the opposite of what predators' eyes do.

But there's a problem: A prey animal that's lowering its head to graze would turn its horizontal pupils sideways and ruin its ability to scan the horizon. However, these animals have evolved a surprising solution, Banks discovered: As the animal's head changes orientation, its eyes rotate in its sockets to compensate.

"They've developed this ability to move their eyes in opposite directions on the two sides of the head to keep the pupil parallel to the ground," Banks said.

There are other animals with even stranger pupil shapes, especially in the sea. Cuttlefish, for example, have W-shaped pupils. Scientists still don't have definitive answers for why.

"Some people have argued that it makes them less visible to other animals," Banks said. "I'm not sure I believe that a W somehow would be harder to see than a circle."

Other theories suggest that these pupil shapes might help reduce light from above to minimize scattering and improve contrast. One early theory proposed that the shape may help with color perception — cuttlefish have only one photopigment, which should mean they see only in black and white, despite their dazzling colors and knack for camouflage.

Banks' study focused on land animals, and he acknowledged that many aquatic pupils remain unexplained. Read suspects the unknowns go even further. "It makes you think what other abilities may be out there in animal eyes that we just have no ideas about," she said.

Animal quiz: Test yourself on these fun animal trivia questions

Humans are the only species with a chin — a feature absent from even our closest relatives. Indeed, it's such a unique anatomical quirk that it's one of the main traits anthropologists use to identify Homo sapiens remains in the fossil record.

Yet, for such a defining feature, we know surprisingly little about its evolutionary purpose. So why are we the only species with a chin?

This question is hard to answer because experts haven't agreed on a single definition of a chin. While some researchers have argued that animals like elephants and manatees have chin-like protrusions, they’re not the same T-shaped structures that protrude beyond our own bottom teeth. As a result, some scientists have moved away from thinking of the chin as a single trait, instead referring to it as the collective result of interactions between many different parts of our head and jaw.

"So much about the chin is complicated," said Scott A. Williams, an evolutionary morphologist at New York University. "It cannot be quantified by a single metric but is rather composed of a constellation of morphological features."

A better understanding of the chin's function, in turn, could help scientists craft a definition. Experts have proposed several possible purposes for the chin.

Some have suggested that as we evolved smaller teeth, the chin appeared to reinforce our lower jaw and keep our teeth from breaking as we chewed. Others believe the chin may be linked to yet another unique human trait — our capacity for speech — with the chin providing an anchor point for our tongue muscles. And still others say the variation in how pronounced our chins are offers a hint that it could be linked to sexual selection.

Noreen von Cramon-Taubadel, an evolutionary morphologist at the University at Buffalo in New York, set out to winnow that list by determining whether the chin could have evolved by random chance or if evolution has been acting upon it directly.

To do so, von Cramon-Taubadel and her team studied dozens of traits linked to head and mandible size, including nine traits associated with the chin. Then, using an evolutionary tree of 15 hominoids — a group that includes humans, their fossil ancestors, gorillas, chimpanzees, orangutans and gibbons — they looked at whether those traits have changed more or less over time compared to random chance. Either result would suggest a role for natural selection in the evolution of the lower jaw.

Compared with other species, "the human cranium is more different from our ancestors' than we would expect given how much time has passed," she said. However, only three of the nine chin-specific traits appeared to be under direct selection.

Together, the team's results, published in the journal PLOS One, suggest the chin may be what's known as a spandrel — a term borrowed from architecture to describe a feature that is a side effect of something else. Coined by evolutionary biologists Stephen Jay Gould and Richard Lewontin in 1979, the concept of a spandrel was introduced to argue against the view that every feature must serve a specific, evolved purpose.

"Instead, it appears that structurally, we have to have a chin, but not because the chin evolved to have a particular function," von Cramon-Taubadel told Live Science. "More and more studies are showing that things that we used to think were terribly important in terms of differences between humans and other apes actually could evolve just by random drift and gene flow."

Von Cramon-Taubadel said the group's findings appear to be more strongly influenced by known major landmarks in human evolution, including when we started walking upright and growing larger brains.

Despite these takeaways, von Cramon-Taubadel and Williams agree that the question is far from settled. It's unknown, for example, when traits like speech first appeared, so it's difficult to link them to chin evolution. While Williams accepts that the chin may not have evolved for a specific purpose, that doesn't make it arbitrary.

"It is still one of the defining features of our lineage that is present in some form in every human living on the planet today," he said.

Human skeleton quiz: What do you know about the bones in your body?

Around 4,000 years ago, one of the world's oldest civilizations emerged: The Indus Valley Civilization, flourishing in what is now Pakistan, western India, eastern Iran and parts of Afghanistan. In addition to building sizable cities, its people created a written script that consists of hundreds of signs that remain undeciphered.

The signs, sometimes called Harappan script, vary, with some looking like a diamond with a square in its corner; a U with three "fingers" at each end, and an oval with an asterisk-like shape inside it.

Most of the surviving texts are inscribed on nonperishable materials, like clay and stone. So will this undeciphered script ever be deciphered? Could recent advances in artificial intelligence (AI) help with decipherment?

The undeciphered script

The Indus Valley Civilization flourished between roughly 2600 and 1900 B.C. Thousands of artifacts containing the script survive to the present day, Michael Philip Oakes, a researcher in computational linguistics at the University of Wolverhampton in the U.K, wrote in a paper published in the Journal of Quantitative Linguistics.

The surviving texts tend to be very short, with an average of five signs per text, Oakes noted. There is no known bilingual text recorded in the Indus Valley script and a known text to aid with decipherment — in other words, Indus Valley Script doesn't have its own Rosetta Stone. It's also uncertain which language the script encodes, and some scholars have argued that it may not encode a language at all, suggesting that they may function more like emblems that convey a person or entity.

Exactly how many signs the script contains is a matter of debate, but they number in the hundreds, Oakes said.

Experts have a mix of ideas about whether the script will ever be deciphered. Even if it is decoded, the texts' short lengths and scholars' wide differences of opinions may make it hard for any decipherment to be widely recognized.

While some experts think that AI could help decipher the language, researchers will likely have to guide the AI for a full decoding, the experts said.

Is it partially deciphered already?

Steve Bonta, an independent researcher who holds a doctorate in linguistics and has studied the script extensively, said part of the work may already be done.

"I think that the Indus Valley Script is already partially deciphered, but that recognition of that fact is severely lagging," Bonta told Live Science in an email. Bonta said he showed "back in the '90s that certain signs and canonical sign fields must be indicative of notations of assets, expressed in various weights." However, many scholars do not recognize the decipherments as accurate.

Bonta said his claims of partially deciphering the script are far from alone. Prior to the mid-90s, "claims of decipherment were published fairly regularly," Bonta said. None of these claims has gained widespread acceptance, with one problem being that the shortness of the surviving texts makes it difficult to prove the accuracy of any decipherment.

"Most of the Indus inscriptions are brief and highly repetitive, which makes the task of reproducible decipherment very difficult," Bonta said.

Turning to AI

AI is useful for decipherment attempts and may help researchers generate lists of possible sign values. However, in the end, human researchers will still need to take the lead. AI "is an extension of human intellect and intuition, albeit an extraordinarily powerful one," Bonta said.

Peter Revesz, a professor of computing at the University of Nebraska-Lincoln who is an expert in computational linguistics and has studied the Indus Valley script extensively, believes the script will be deciphered and that AI may play a significant role. Revesz's team has used data mining and statistical analysis to help determine which Indus Valley script signs are likely to have similar meanings.

The "Indus Valley Script surely will be solved one way or another, and AI can help, but it needs to be guided by a good research design," Revesz said in an email.

Rajesh Rao, a computer science professor at the University of Washington in Seattle who has co-written several papers on the Indus script, is less optimistic about a full decipherment but said AI will be useful. Back in the 2000s, with the more primitive AI that was available at that time, his team determined that the script has a statistical pattern that suggests that it encodes a language.

However, even with AI, a full decipherment seems unlikely with the existing texts, according to Rao. The chances "are not very high," Rao told Live Science, noting that a partial decipherment may be possible. "We may be able to reconstruct their number system," Rao said.

Rao said that the number system is already partly understood because some inscriptions have tally marks (vertical lines) that are thought to represent numbers. These are located next to symbols that likely represent objects. Additionally, archaeological data indicates that people used a system of standardized weights that involved ratios of 1, 2, 4, 8, 16, 32 and 64. By using the tally marks and weight system, it may be possible to determine which numbers are recorded on the inscriptions.

To decipher the entire script, Rao thinks archaeologists will need to uncover more texts. There are many Indus Valley Civilization sites that are largely unexcavated, and he hopes future excavations may yield lengthier texts or ones that feature the Indus Valley script alongside a known language.

Wherever there's light and an object, there's a shadow. By blocking sunlight, Earth casts a shadow, too.

But can you see Earth's shadow? Astronomers told Live Science that yes, you can see it cast on several objects. In fact, under the right conditions, you may even spot Earth's shadow every day.

Shadow on the moon

Because the sun is an extended source of light (meaning it isn't a single point of light), Earth's shadow has three parts: a dark central umbra, a lighter peripheral penumbra, and an antumbra that appears at faraway distances where overlapping penumbral regions replace the umbra.

Earth's shadow is the most prominent during total lunar eclipses. During a lunar eclipse, the full moon passes through Earth's shadow. The moon enters Earth's penumbra, and then its umbra, before passing through the penumbra again. Distinguishing Earth's penumbra is difficult, since the moon dims only a bit. In contrast, the umbra, which can be up to 2.7 times the lunar diameter, is much easier to see.

However, when the moon is entirely ensconced in Earth's umbra, it typically isn't fully black. Most times, it's a faint to deep red. That's because our planet's atmosphere scatters sunlight and then bends these reddened beams into the umbra.

"In a way we are seeing sunrises and sunsets on Earth reflected off the Moon!" Emily Rice, an astronomy professor at Macaulay Honors College of the City University of New York told Live Science in an email.

The shadow can also tell us about Earth's atmosphere. "The cloudier and dustier the Earth's atmosphere at the time, the more red the Moon will appear," she noted.

In comparison, seeing a blackened moon is possible when the moon straddles Earth's umbral-penumbral boundary. This stems from a contrast effect, since the portion of the moon in the penumbra is 500 times brighter than in the umbra, astrophysicist Fred Espenak wrote on EclipseWise.

Earth's shadow is also visible in partial lunar eclipses, when only part of the moon passes through the umbra.

Daily delight

But you don't have to wait for an eclipse to see Earth's shadow. Immediately before sunrise and just after sunset, our planet's silhouette — cast into space — becomes visible in the sky near the horizon in the direction opposite the sun.

"During the day, the clear sky is bright because direct sunlight illuminates the gas molecules and particles of our atmosphere, which can scatter some of this sunlight," Raymond L. Lee, adjunct professor in the Mathematics and Science Division at the U.S. Naval Academy, told Live Science in an email. "As the sun sets or rises, the [roughly] spherical earth blocks the lowest solar rays from illuminating the atmosphere directly, thus casting a curved shadow on the corresponding lowest parts of the atmosphere that are opposite the sun."

However, the exact features constituting Earth's shadow are debated. A 2017 study in the journal Applied Optics says the shadow comprises an upper dark-blue band and a lower brown band. But Giovanni di Giovanni, an amateur astronomer who has a master's degree in physics from the University of L'Aquila, in Italy, said neither is the real shadow. Instead, our planet's shadow is "a thin, very dark band, but with little contrast compared to the other bands above it," he told Live Science in an email.

In any case, the shadow is present when the sun is just above the horizon until it's about 4 degrees below it. The planet's silhouette persists for about 15 minutes, before it disappears with the rising sun or merges with the darkening night sky.

Seeing Earth's shadow requires a clear, dust-free sky at high elevation. "Very few observers would notice it … only from the tops of high mountains, and perhaps with less difficulty from an airplane," di Giovanni said.

Earth's shadow on other objects

Spotting Earth's shadow on other objects is possible but trickier. If the International Space Station passes overhead on a cloudless evening at sunset, a pair of stargazing binoculars can help you see it dim as it passes into Earth's shadow.

Additionally, telescopes can show how geostationary satellites "vanish" in Earth's shadow. Such satellites orbit Earth directly above the equator, at the same rate as the planet rotates, making them seem fixed in the sky. According to satellite operator Space Norway, geostationary satellites are mostly bathed in sunlight. But that changes for approximately 21 days around every equinox. According to the Australian Space Weather Forecasting Centre, during these "eclipse seasons," the satellites slip into Earth's shadow for several minutes each night.

Earth's shadow even appears on passing asteroids. For instance, astronomer Gianluca Massi, founder of The Virtual Telescope Project, wrote in a blog post that one of their telescopes recorded the house-size asteroid 2016 VA dimming and disappearing as it passed through Earth's shadow for 10 minutes, 50 seconds at a distance of 74,520 miles (120,000 kilometers).

But there's a limit to how far Earth's shadow extends. "The [Earth's] umbra is a very long cone shape that extends about 1.4 million km (870,000 mi) away from Earth," Rice said. So, Earth's shadow won't reach Mars, which, at its closest, is 34 million miles (55 million km) away. Still, with lots of objects that pass through Earth's shadow, there are many ways to keep seeing — and marvelling — at our planet's silhouette.

Solar system quiz: How well do you know our cosmic neighborhood?

Lizards, birds and fish often sport vivid colors, from neon pink to deep violet, but most mammals are fairly drab. So why don't mammals match the vibrant hues of other animals?

A number of factors culminate in the browns, blacks and whites that make up most mammalian coats. The first has to do with color expression. Matthew Shawkey, an evolutionary biologist at Ghent University in Belgium, explained that animals generally express color in two main ways: through pigments and through structures. Pigments exist within the skin and coat of the animal itself and reflect and absorb light to create certain colors. Structural coloration, on the other hand, involves nanoscale shapes and patterns on top of skin, feathers or scales that can distort light to produce bright, iridescent colors.

Animals can use one method, or sometimes both, to express color. According to Shawkey, however, mammals don't really use either. Of the many color-producing pigments — such as carotenoids, porphyrins and pterins — mammals have just one type: melanin. The presence of that one pigment generates all of the colors seen in mammals, Shawkey said, and its absence creates the white regions seen in animals like zebras and pandas.

Moreover, the composition of the hairs that make up mammal fur limits the structural colors mammals can display. Hair is not a complex structure like feathers, scales and skin are, so it's not surprising that it cannot produce the nanoscale patterns necessary for structural color, Shawkey noted.

For example, mandrills (Mandrillus sphinx), which break the drab-mammal rule with their bright red and blue, have those colors only on spots without fur. Sloths, which sometimes have green patches, get this coloration from algae that grows on their fur, not from pigments or structures on the hairs themselves.

Evolution of color

So why don't most mammals have the tools to create vibrant hues? One hypothesis is that when mammals first evolved, dinosaurs were the apex predators and mammals were prey. To avoid being eaten, mammals spent more than 100 million years as primarily nocturnal animals (and most remain so today).

Those millions of years had a big impact on mammals' appearance. In a 2025 study co-authored by Shawkey and published in the journal Science, a research team compared pigment-storing structures called melanosomes in modern mammals to preserved melanosomes found in six Jurassic and Cretaceous-age mammal fossils. They found that all of the mammal fossils were some shade of brown or gray.

Because these prehistoric animals were living primarily in the dark, darker colors would have helped mammals avoid predators. "Any bright color would have been selected against," Shawkey told Live Science.

In the 66 million years since the nonavian dinosaurs went extinct, mammal diversity has exploded to over 6,000 species. Now, there are mammal species, both nocturnal and diurnal, that have no natural predators. However, mammals have remained mostly brown, gray and black.

This could be due to most mammals' continued lack of color vision, said Ted Stankowich, a behavioral evolutionary ecologist at California State University, Long Beach. Researchers speculate that mammals sacrificed some color vision in order to gain better night vision during the age of the dinosaurs. Most mammals still have dichromatic vision, meaning they only have two of the three types of cones that help the eye perceive color. Dichromats can't see colors such as red, orange, turquoise and purple, and generally can't see colors with as much saturation as trichromats, which have all three types of cones.

The purposes animals primarily use color for — attracting mates and other communications within their species, blending in with camouflage, and signaling to predators that they are poisonous or otherwise dangerous — don't work when their partner or predator can't see the colors they're using. Some mammals have actually used this lack of color vision to their advantage. For example, although tigers look orange to our trichromatic eyes, they look green to their mammalian prey, making them perfectly camouflaged amongst the grass when on the hunt.

Instead of using vibrant colors, Stankowich said, many mammals use patterns and contrasting colors, such as black and white or brown and yellow, to signal to each other. Skunks and polecats, for example, use black and white spots and stripes to let predators know they have a stinky trick up their sleeve. The African wild dog, known for its unique patterning, has a distinctly white tail that researchers think is used for signaling while on the hunt. The Indian giant squirrel, known for its high-contrast black, reddish brown and orange-yellow patterning, may use this as camouflage against various kinds of predators.

Because mammals have adopted new ways of color signaling, there might not be much of a reason for them to regain color vision. (The few mammals with trichromatic vision — primates, including humans and some monkeys — evolved color vision for very specific reasons.) Stankowich noted that the few mammals that display bright blues and reds, such as baboons, golden snub-nosed monkeys and mandrills, are also among the mammals with the best color vision.

Fluorescence and iridescence

Recent studies have highlighted some other exceptions. For example, many mammals fluoresce under ultraviolet light, which the human eye cannot detect but some other mammals can. Moreover, Jessica Dobson, an evolutionary biologist at Ghent University, and colleagues have discovered iridescence in a handful of mammal species not previously known to have this shimmery feature.

"It was a light-bulb moment," Dobson said of this iridescence discovery, which happened when she opened a museum drawer and sunlight hit the preserved pelts of several tropical rat species at just the right angle. Dobson isn't sure whether these iridescent colors serve any evolutionary purpose, but she said it is nonetheless exciting to know that there are still mammalian color mysteries to unlock.

"When you start looking, mammals are more colorful than we give them credit for," Dobson said.

Animal quiz: Test yourself on these fun animal trivia questions

​​We tend to think of metals as hard, strong and resistant to high temperatures — just look at iron, aluminum and steel. While this is generally true, there's one key exception: mercury. With a melting point of minus 37.9 degrees Fahrenheit (minus 38.8 degrees Celsius), mercury is one of only two elements that are liquid at room temperature. (The other is bromine, which is not a metal.)

But why is mercury so different from its fellow metals?

The answer comes down to mercury's position in the periodic table and its knock-on effect on how the metal bonds together.

Melting point is directly correlated with bond strength — "the stronger the bonds, the more energy, in the form of heat, is required to break them," Zoe Ashbridge, a senior lecturer in chemistry for the U.K. Ministry of Defence, told Live Science.

Atoms of mercury, like atoms of all other metals, bind together through metallic bonding — a lattice of positively charged metal particles known as ions, is surrounded by a sea of delocalized (freed) electrons, and electrostatic attraction between these oppositely charged particles acts as the glue that holds the metal together. This structure explains many of the other signature properties of metals, such as electrical conductivity, as the electrons can move freely through the material, and mouldability, as the layers of positive particles can slide over one another to adopt a new shape, lubricated by the free electrons. But it is specifically the strength of the electrostatic attraction that governs the melting point.

The availability of outer electrons to create this delocalized sea is therefore a key factor. "The more positive the metal center is and the more delocalized valence electrons on the outside, the greater the attraction is, and generally this tracks from left to right in the periodic table," Ashbridge explained.

As a group 12 metal, mercury theoretically has 12 outer electrons it could contribute to metallic bonding. "However, all of those electrons are in "filled subshells," she added. "When they are full, that makes them more stable and less likely to delocalize, and this makes mercury particularly reluctant to share its electrons, even with other mercury atoms."

Yet this filled-subshell effect isn't big enough to explain mercury's unusually low melting point. The strength of metallic bonding — and, therefore, the melting point — also decreases from the top to the bottom of the periodic table, as the atoms get larger. But extrapolating from these established trends, mercury should still have a melting point of around 266 F (130 C), which would make it solid at room temperature.

So what causes this giant disparity?

Mercury's liquid state results almost entirely from relativistic effects, said Peter Schwerdtfeger, a quantum physicist at Massey University in New Zealand. Toward the bottom of the periodic table, the electrons in the heaviest elements experience such strong attraction to the atomic nucleus that they move close to the speed of light. At this point, they no longer obey the laws of classical physics, and the resulting quantum phenomena — known as relativistic effects — lead to surprising physical properties. How these manifest depends on the element.

"Relativistic effects become extremely important for the group 11 and group 12 elements, where gold and mercury are," he told Live Science. Consequently, the weird physical properties arising from these quantum effects are most observable in these elements. Gold has an extremely unusual yellowish hue and mercury is a liquid at room temperature.

"They show us a so-called maximum of relativistic effects, and the outer shell of these atoms contract as a result. It's enormous. For mercury, it's about 20%," Schwerdtfeger said. In chemistry terms, this relativity-induced contraction is most easily explained by once again considering mercury's electron configuration.

The full 4f subshell, which contains the electrons associated with the rare earth, or lanthanide elements, is extremely poor at shielding the other electrons from the nuclear charge. This means the outermost electrons are held much closer to the nucleus than usual — a phenomenon called lanthanide contraction. These contracted electrons move close to the speed of light and therefore experience relativistic effects.

"This increases their mass, and when they have an increased mass due to this high speed, it pulls those electrons even closer to the nucleus," Ashbridge said. Consequently, the relativistic effects reduce the availability of the electrons to contribute to metallic bonding, thus lowering the melting point of the metal below room temperature.

At a quantum mechanical level, though, this qualitative explanation is extremely challenging to back up with calculations.

"The Schrödinger equation" — which usually describes the possible positions of particles such as electrons — "doesn't fulfill the relativity principle of Albert Einstein," Schwerdtfeger explained. As a result, this equation doesn't work for high-speed particles such as the electrons in mercury. Scientists must instead turn to the significantly more complicated Dirac equation, making any simulations extremely computationally demanding.

Eventually, though, advances in computing enabled Schwerdtfeger to devise a model that could accurately simulate mercury melting and provide a quantum theoretical explanation for the anomalous melting point.

"Using what we call density functional theory, we were able to establish that the melting point is lowered by over 200 degrees Celsius [360 F] by the relativistic effects," he said. These quantum contributions dominate, so while periodic trends predict a low melting point for mercury, the relativistic effects make the element a liquid at room temperature.

Periodic table of elements quiz: How many elements can you name in 10 minutes?

​​Antarctica, which is nearly four times the size of the United States, is almost entirely covered by a miles-thick layer of ice.

But the South Pole hasn't always been frozen. So when was the last time Antarctica was ice-free?

This ice cap formed relatively recently in geological terms, experts told Live Science. "I think most people would say 34 million years ago was when the ice sheet first formed in Antarctica," said Eric Wolff, a paleoclimatologist at the University of Cambridge. "[Previously] most of it would have been like northern Canada today — tundra and coniferous forest."

Global temperatures are a key factor influencing the extent of ice coverage. Around 50 million years ago, the world was about 25 degrees Fahrenheit (14 degrees Celsius) warmer than it is today, but temperatures steadily decreased over the following 16 million years. By 34 million years ago — a time period known as the Eocene-Oligocene boundary — the climate was 14.4 F (8 C) warmer than it is today.

But what triggered this temperature drop, and was that all it took for the ice sheets to form?

Related: Which is colder: The North or South Pole?

"There are two factors, and probably both were in play," Wolff told Live Science. "One of them is a change in the carbon dioxide concentration of the atmosphere, and the other is the movements of the continents and, in particular, the opening up of the Drake Passage," the strait between South America and Antarctica that connects the South Atlantic with the South Pacific.

The more carbon dioxide that's in the atmosphere, the more heat is trapped and the warmer the planet is.

From about 60 million to 50 million years ago, the carbon dioxide concentration in Earth's atmosphere was really high — somewhere around 1,000 to 2,000 parts per million, or between 2.5 to 5 times today's levels, said Tina van de Flierdt, a geochemist at Imperial College London.

"But we know that the CO2 in the atmosphere came down across that Eocene-Oligocene boundary," she told Live Science. This decrease in atmospheric CO2 would have been accompanied by a cooling of the global climate, she added, probably tipping Earth over a threshold and allowing ice sheets to form.

However, there was also likely localized cooling on the Antarctic continent due to plate tectonics, Wolff said. Around this time, South America and Antarctica finally separated, opening up what's now the Drake Passage.

"This led to what we call a circumpolar current — water going right around Antarctica in a circle," Wolff said. "This isolates Antarctica from the rest of the world and makes it much harder for warm air masses to get across the Southern Ocean and, therefore, makes Antarctica colder."

Plate tectonics also directly influenced carbon dioxide levels, he added. Rock weathering and volcanic activity are both part of the carbon cycle, so over thousands of years, geological processes can shift the balance of gases in the atmosphere.

Although some uncertainty remains, researchers are fairly confident about this transition 34 million years ago thanks to the chemical signatures in rock sediments. Oxygen atoms exist in two forms: oxygen-16 (common oxygen) and oxygen-18 (heavy oxygen). Continental ice contains a higher proportion of the lighter oxygen-16, meaning the oceans — and, therefore, the shells of small sea creatures — contain a higher percentage of oxygen-18 when ice sheets are bigger.

"By looking at the oxygen isotopes in the carbonate shells of small sea creatures in ocean sediments, you see a jump around 34 million years ago, which people take as being because the [lighter] oxygen isotope is going onto the continent of Antarctica," Wolff explained.

As for whether Antarctica could ever be ice-free again, "It's definitely possible, van de Flierdt said. "Planet Earth has done it before. Planet Earth could do it again." While it's unlikely that human activity will lead to the complete melting of the ice sheet, it's important we do everything possible to limit the loss of ice from the Antarctic now, she added. "It's in our hands to avoid the worst-case scenario," van de Flierdt said.

Editor's note: This story was originally published on Sept. 8, 2024.

Elevators have a strange way of messing with your sense of gravity. The moment an elevator lurches upward, you feel it in your feet. For a second, the floor presses harder than usual. When the elevator slows, that pressure eases, leaving you briefly lighter.

If you stand on a scale inside an elevator going up, the number jumps. When it slows to a stop, the number dips. On the way down, the opposite happens.

So what's really going on? Do you weigh more when the elevator goes up or when it goes down?

The short answer is that you can feel heaviest at two points: when the elevator starts moving up (accelerating upward) and when it's slowing down at the very end of a downward trip (decelerating downward). But the explanation depends on what "weight" actually means and what your body can feel.

"The word 'weight' in physics has different meanings," Miguel Morales, a physics professor at the University of Washington in Seattle, told Live Science. In physics, weight can refer to at least three related ideas: your mass (how much matter you're made of), the gravitational force pulling on you, or how hard the scale beneath you is pushing up, Morales explained.

"When you're just standing still, those can all be the same thing," Morales said. "But as soon as the elevator starts to speed up or slow down, you get three different answers. It's just physics."

Your mass never changes, no matter what the elevator does. Gravity near Earth's surface also stays essentially the same between the bottom and top of a building. What does change is the third definition: how hard the scale pushes upward. That upward push is what a scale actually measures.

Looking at gravity

This distinction reveals something counterintuitive: "You can't feel gravity. You never could," Jason Barnes, a physics professor at the University of Idaho, told Live Science.

Barnes pointed to astronauts aboard the International Space Station. "The actual gravity of the Earth up there is almost the exact same as here," he said. "But they don't feel it."

That's not because gravity disappears in orbit. At the station's altitude (about 250 miles, or 400 kilometers, above our planet), Earth's gravitational pull is still about 90% as strong as it is at the surface. The difference is that astronauts and the space station are in continuous free fall toward Earth.

The station is moving sideways at more than 17,000 mph (27,300 km/h). As it falls, Earth curves away beneath it. Instead of hitting the ground, it keeps missing it. The result is a constant fall around the planet.

Because the astronauts and the space station are falling together at the same rate, the floor never needs to push up on them. And that upward push is what we actually feel as weight (also called the normal force).

On Earth, the ground constantly prevents you from falling by pushing upward against you. In orbit, there's no such push. The astronauts are still under the influence of gravity, but nothing is stopping them from falling. Without the floor pressing upward, they feel weightless.

Why do elevators make you feel heavier or lighter?

An elevator briefly changes how hard the floor pushes back on you. When the elevator starts rising, it must accelerate you upward, too. "To start going up, that's when you feel heavier," Barnes said. "The elevator pushes back harder than normal in order to accelerate you upward."

In a typical building elevator, that extra acceleration might be about 1 meter per second squared. That is roughly one-tenth of Earth's gravity. For someone who normally weighs 150 pounds (68 kilograms), that would briefly add about 10% to the scale reading. Instead of 150 pounds, the scale might show around 165 pounds (75 kg).

Morales described the same effect from the scale's perspective. "The force of gravity hasn't changed at all," he said. "But now, in order for you to be speeding up, something's got to be pushing you harder than gravity. And so your weight on the scale will go up."

Once the elevator reaches a steady speed, the acceleration stops. Gravity and the upward push balance again, and the scale returns to its normal reading, even though you're still moving.

At the top, when the elevator slows to a stop, the opposite happens. Even though you're still moving upward, the elevator must accelerate downward slightly to slow you down.

The force of gravity hasn't changed. But because the elevator is now accelerating downward, the floor doesn't need to push up as hard to control your motion. With less upward push (normal force), the scale reading drops.

"You kind of feel yourself get a little light," Morales said.

The same pattern repeats on the way down. When the elevator accelerates downward, you feel lighter because the floor pushes up less than usual. But as it approaches the bottom and slows to a stop, the acceleration flips upward again, making you feel heavy again.

This everyday experience turns out to be connected to one of the most important ideas in modern physics.

"It is an effect that Einstein first noted when he was developing general relativity," Barnes said. That insight, known as the equivalence principle, helped Einstein understand gravity not as a force but as a consequence of acceleration and the curvature of space-time itself.

In 1957, an amateur archaeologist working at a Native American site in Maine discovered a perplexing treasure: a 900-year-old silver Norse coin that dated to the late Viking Age (A.D. 793 to 1066).

The artifact, sometimes called the "Maine Penny," is now in the Maine State Museum. Its discovery has raised a number of questions — mainly, how did it get there, and does its presence in Maine mean the Viking reached the Pine Tree State?

The Vikings were expert seafarers with outposts from Ukraine to Canada, so it's conceivable that they could have reached Maine. The Vikings and their descendants had colonies in Greenland that existed from the late 10th century to the 15th century. They also had an outpost at L'Anse aux Meadows on the northern tip of Newfoundland, Canada, that was in use during the 11th century.

Norse sagas tell of Viking voyages to North America, including trips to a place they called "Vinland" (which means "wine land"), where grapes supposedly grew. While grapes do grow in Maine, archaeologists haven't found any Viking Age settlements or artifacts there, except for the penny. But many experts, through scholarly journal articles and books, have weighed in on the issue.

What is the Maine Penny?

The coin, which researchers generally agree is authentic, was minted during the reign of Olaf III, who was king of Norway from about 1066 to 1093, Gordon Campbell, a professor emeritus of Renaissance studies at the University of Leicester in the U.K., wrote in his book "Norse America: The Story of a Founding Myth" (Oxford University Press, 2021). In 1979, the Norwegian numismatist Kolbjorn Skaare dated the coin to sometime between 1065 to 1080, Campbell noted.

The coin is in poor shape and some of its engravings are hard to see. One side of the coin has a cross with what appears to be a circle surrounding it. The other side is badly damaged, with only a few visible lines. The lines might have once formed a human figure that depicted Olaf III.

The coin was found in the coastal town of Brooklin at what is now known as the Goddard site, a Native American trading center that was in use during the late 12th and early 13th centuries, Campbell wrote. The coin has a puncture mark through it, so it could have been used as a pendant. It's also worn and chipped, suggesting it may have passed through many hands over a long period before ending up in Maine.

How did the coin get to Maine?

The scholars that Live Science interviewed tended to think the coin reached Maine through Native American trade networks.

"The position of the Maine State Museum is that the coin reached the Goddard site through down-the-line trade," Andrew Beaupré, curator of archaeological collections at the Maine State Museum, told Live Science in an email. "Goddard has been determined to be a Native American trading center. The Norse coin is not the only artifact that has been traced to the Canadian Maritime sub-arctic," a region that stretches from the Labrador Sea to the Northwest Territories. Other finds at Goddard from this northern area include Indigenous stone tools "that have been traced to Newfoundland/Labrador," he said.

There "is currently no valid archaeological evidence that Norse peoples visited or settled Maine," Beaupré said. However, he noted that the Vikings could have sailed down the coast of Maine from Newfoundland in the 11th century.

Svein Gullbekk, a professor in the Museum of Cultural History at the University of Oslo who has studied the coin extensively, agreed that the likeliest explanation is that it arrived in Maine as Indigenous Americans traded with one another.

"I believe this 11th century Norwegian coin provides solid evidence for cultural and economic contacts between Native Americans and Inuits and Norse people," Gullbekk told Live Science in an email. In "my view, I believe it followed Native American routes, presumably used as a piece of [jewelry], rather than a monetary item."

Joel Anderson, an associate professor of history at the University of Maine, has a similar view. "I think that the 'Native American trade route' hypothesis is the most plausible," Anderson told Live Science in an email. He's not aware of any other evidence for Vikings in Maine. "It's not out of the realm of possibility," he said, "but the available evidence does not support such a conclusion."

Viking quiz: How much do you know about these seaborne raiders, traders and explorers?

When you picture a desert, you probably imagine a vast, empty landscape far from any water. But surprisingly, some of the driest places on Earth lie right beside the ocean. Both the Atacama, in Chile, and the Namib, in southern Africa, stretch along coastlines. So how did these extreme deserts form in places bordered by so much water?

There are three main factors that allow deserts form next to oceans, David Kreamer, a hydrologist at the University of Nevada, Las Vegas, told Live Science: how air moves vertically, how air moves horizontally, and how mountain ranges interact with air moisture.

If you look at a world map, you'll notice that most deserts sit above or below the equator. That's because the equator receives the most direct sunlight and causes the air to warm and rise. As the warm air rises, it creates a low-pressure system — a region where atmospheric pressure is lower than its surrounding area, Kreamer explained. Any moisture in the air cools and condenses into clouds and rain. That's why the regions along the equator are home to lush forests, like the Amazon.

That rising air spreads outward and sinks between 20 and 40 degrees north and south of the equator, and suppresses cloud formation — which explains why there are so many deserts along the subtropical belt, such as the Sahara and the Kalahari.

Then, there is the horizontal movement of air across the planet. Near the equator, trade winds blow from east to west. These winds tend to drop moisture on the eastern sides of continents, leaving their western sides drier. In the case of the Namib, for example, when it does rain, that rain doesn't fall in the desert itself but rather in the mountains to the east, said Abi Stone, a physical geographer at the University of Manchester in England.

Cold ocean currents also play a role. The air that's being blown across the cold current cools on contact with it and picks up some of its moisture, and because of the coldness, the air becomes quite stable. "We kind of envisage packages of air, in some ways, like a balloon, because they don't totally mix, but the balloon skin is really flexible, and they can expand and contract," Stone told Live Science. "The cold air won't tend to do much of that expansion." Without any convection, the pack of air becomes trapped, unable to rise. "But what it can do is hold some moisture, and at the low level, that can get blown on land, and you end up with quite foggy environments at the western part of those coastal deserts," Stone said.

The presence of mountains impacts the dryness of these deserts as well. When moist air is forced over a mountain range, it cools and drops rain on the windward side, Kreamer explained. By the time the air descends on the leeward side, much of its moisture is gone, creating a rain shadow, or an area by the mountains that gets reduced rain. For instance, Seattle, which is located on the western side of the Cascade Mountains, gets an average of 39.3 inches (99.8 centimeters) of rain a year, while Yakima, located on the eastern side of the Cascades, gets an average of 8 inches (20.3 cm) of rain annually.

In the case of the Atacama, Kreamer said, "the wind that comes in South America drops a lot of rain on the east side over the Amazon, and then it hits the Andes. The Andes sap more water off the wind and then right along the coast of South America on the west side, where Chile is," leaving the Atacama exceptionally dry.

These factors give coastal deserts unique characteristics that aren't found in other deserts. They tend to have cooler and more stable climates than inland deserts do, and they're home to plants and animals that have evolved special traits to capture moisture. In the Namib, for example, some beetles harvest water by pointing their butts toward the foggy air.

"People have studied what that surface looks like to make more effective fog nets," Stone said. "There are some amazing creatures."

The formation of polar deserts, like most of Antarctica and the northernmost parts of the Arctic, is driven by many of the same factors as warm coastal deserts. The temperature also plays a role, since the air is so cold in these parts of the world that it can't hold moisture. "In the case of Antarctica, the strong winds and ocean current around the continent is effective at blocking weather systems traveling onto the continent," Stone said.

Equator quiz: Can you name the 13 countries that sit on Earth's central line?

After death, a series of natural processes begin within the human body. The corpse cools, pales and stiffens before it begins to decompose. The stiffening of the corpse is called rigor mortis. It's viewed as macabre, creepy and unsettling, and it's been used as a plot device in crime shows, a jump scare in horror films, and a clue in movie mysteries.

But what, exactly, is this process, and why does it happen to nearly all bodies?

Rigor mortis is a natural process that occurs at a cellular level. It begins immediately after death as the body runs out of adenosine triphosphate, the energy molecule commonly known as ATP. "You can essentially think of [ATP] as your fundamental source of energy for muscle contraction," said Dr. Michelle Jorden, chief medical examiner of Santa Clara County and president of the National Association of Medical Examiners. "The ATP molecule is what's needed for us to move."

After death, the body stops producing ATP, and its supply of the energy molecule is depleted within a couple of hours, on average, Jorden said.

The body needs ATP not only to move but also to relax. As the body's supply of ATP runs out, the muscle filaments — proteins bound together within muscle cells — begin to stick together.

One misconception is that the body snaps into rigor instantly. While rigor mortis starts immediately in all muscles following death, in most cases it can take several hours before it becomes visible to the naked eye as filaments stick together.

Between two and six hours after death, stiffness first appears in the body's smallest muscles in the face. After six to 12 hours, it progresses through larger muscles in the hands, arms and neck and chest. Finally, the body's largest muscles in the lower body stiffen — full-body rigor mortis happens between 12 and 24 hours after death, completing this sequence known as the "march of rigor."

Another misconception about rigor mortis is that it's permanent — that, once stiffened, the body will stay that way forever. "It's a transitional phase, not a permanent state," said Shawn'te Harvell, a licensed funeral director in New York and New Jersey and president of the National Funeral Directors & Morticians Association.

Rigor mortis lasts around 24 to 48 hours and then disappears once decomposition begins, when "muscles become flaccid again as the decomposition breaks down protein structures," he said. The body becomes flaccid in the same order it stiffened — starting with the face and hands, down the neck and torso, and then the legs.

This timeline is a guideline, Harvell said, and the process is unique for each body. Factors such as temperature, age and activity level at the time of death; overall health and body composition; and medications taken during life affect how quickly or slowly the body goes through rigor mortis before decomposition sets in. "No two cases follow exactly the same timeline or intense pattern," he said.

Jorden, a practicing forensic pathologist and neuropathologist, said that determining the timing of rigor mortis can be "a very valuable tool" for assessing the scene of a death.

"[It] can actually provide clues if the body has been manipulated or moved," she said. What's more, if the timing of rigor mortis doesn't quite line up with what they believe the circumstances of death were, it may be a clue that "causes us to pause and ask more questions."

Planets go through different life stages: They form, evolve and eventually meet an end. But the timelines for these processes differ widely between Earth-like planets and worlds that orbit less-powerful stars.

So, how long do most planets last?

First, it's important to understand the life cycles of planets.

"Planets start off as microscopic dust grains in disks orbiting young stars, and eventually grow by a huge amount through a series of collisions," Sean Raymond, an astrophysicist at the University of Bordeaux in France, told Live Science in an email.

Gas giants, like Jupiter and Saturn, start off as massive rocky and icy cores, and then capture gas from the disk to become giants. Rocky planets like Earth undergo a late phase of giant collisions with other growing planets and smaller objects after the gas disk from the sun had dissipated, Raymond said. However, there is still some debate among scientists about the order in which planets formed.

Defining the "end" of a planet, however, is more complicated. "You could say a planet lasts until it's destroyed," Matthew Reinhold, a planetary scientist at Stanford University, told Live Science. Or, you could define a planet's ending to be when it no longer operates under the same conditions. "You could say, 'This was a world that had these conditions at some point, but now it has changed and has these very different conditions," Reinhold said. "Because I prefer those previous conditions; I consider this planet as having ended.'"

Let's take Earth as an example. Like many others, our planet's lifespan is tied to the evolution of the sun. The sun currently creates heat and light through nuclear fusion at its core — a process in which hydrogen transforms into helium. In about 5 billion years, the sun will run out of hydrogen, at which point it will expand into a red giant and eventually collapse into itself.

"Our Earth will 'die' in multiple ways," Raymond said. "First, the slowly-brightening Sun will make conditions on the Earth unlivable by vaporizing the oceans. Then, Earth may be swallowed by the Sun when it becomes a red giant. Finally, Earth (if it's still around) will be tossed into interstellar space."

According to these calculations, Earth's total lifespan will be about 9.5 billion years.

Earth probably won't live as long as most planets, he noted. That's because, unlike the sun, which is a yellow dwarf star, most stars are red dwarfs that are smaller and cooler than our sun, and they burn fuel much more slowly. "They can last for trillions of years," Reinhold said.

In that case, it might not be the death of the star, but rather an internal process that leads to these planets' demise.

In his work, Reinhold has modeled what might happen to a hypothetical habitable planet orbiting a red dwarf star. Active geology, such as plate tectonics, is considered crucial for habitability because it allows nutrients to move between the planet's mantle and surface and drives the carbon-silicate cycle.

"We want a planet that can stabilize its climate," Reinhold said, and the carbon-silicate cycle is Earth's natural thermostat.

Reinhold found that mantle convection will last somewhere between 30 billion and 90 billion years, while mantle melting might last somewhere between 16 billion and 23 billion years. Although these number ranges are too large to be meaningful, Reinhold said, they suggest that any Earth-like planets orbiting a red dwarf will die of an internal process long before their stars get close to the ends of their lives. And even on the shortest timelines, most rocky planets orbiting red dwarfs will maintain their conditions for billions of years.

Bigger stars have much shorter lifespans, because they use up their nuclear fuel more quickly. So the fate of an inner planet orbiting an A-type white star, for instance, would be tied up to the star's lifespan of 100 million to 1 billion years.

It's also possible for gas giants to lose their atmospheres due to the intense light from their star, Reinhold said, becoming rocky planets. This process depends on how close a planet is to its star, how much energetic radiation the star emits, and how strong a planet's gravity is. "The stronger their gravity, the better they are at holding onto their atmospheres, and the more radiation they get from their star, the more intense the stripping power," Reinhold explained. Depending on those factors, this can take anywhere from millions to billions of years.

The end of the universe

Even when a planet's conditions change over time, the rock itself still exists. But over large timescales, there are different possibilities for its fate as the probability of rare events increases. It may collide with another planet, or be kicked out of its orbit.

"In all of this mayhem over quadrillions of years, the planets that have been kicked away from their stars will be kicked out of the galaxy to wander for eternity in the void," Reinhold said. "What actually seals [its] fate really comes down to the nature of the end of the universe," said Reinhold.

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