Why don't humans have gills?

Roughly 375 million years ago, a funny-looking fish named Tiktaalik ventured ashore using novel adaptations: lobed fins to propel itself to "walk" on land and air sacs in its throat to breathe oxygen from the air. Tiktaalik, which also had gills, is the earliest known common ancestor of tetrapods, or four-legged animals.

Over hundreds of millions of years, tetrapods evolved into countless species, including Homo sapiens. So, if humans evolved from fish, why don't we have gills?

Part of the answer is practical: Gills need to stay wet in order to work, which isn't ideal for animals that don't live underwater. Gills have a large surface area and thousands of tiny blood vessels, giving oxygen easy access to the bloodstream. As water rushes over the gills, oxygen diffuses in and carbon dioxide diffuses out, said Chris Organ, an evolutionary biologist at Montana State University. If land animals had gills, they would quickly dry up, making it an inefficient way to breathe. Our human lungs, on the other hand, are good at taking oxygen from the air and into our bloodstream via gas exchange.

But lungs were already around long before the sea-to-land transition. "Lungs are actually surprisingly primitive in evolution," Neil Shubin, an evolutionary biologist at the University of Chicago who was part of the team that discovered the Tiktaalik fossil in 2004, told Live Science. When our fishy ancestors still lived underwater, they already had lungs in addition to gills.

"The fish doesn't come onto land and say, 'I need lungs; I'm going to evolve lungs,'" Organ told Live Science. Only fish with existing lungs were able to encroach onto land and survive. If a fish without lungs tried to live on land, it would die. "It's about having these traits that evolved for other reasons that then allow this animal to exploit this new environment," he added.

Related: Can fish and other marine animals drown?

Similarly, scientists think our fish ancestors evolved arms to move around on the ocean floor, Organ said, which later came in handy for finding food and moving around on land. That's where natural selection came in: Because those arm-like structures were beneficial on land, animals evolved longer limbs and hands over the next several million years. The same thing likely happened with lungs. Soft tissues like lungs don't fossilize well, so scientists aren't sure exactly how human lungs evolved, Organ said. But existing evidence suggests early lungs first evolved into the simple lungs of lizards and then into the subdivided lungs characteristic of mammals. Notably, mammals evolved to have a diaphragm — the muscle that regulates our breathing — perhaps as far back as 300 million years ago.

Conversely, structures that are no longer beneficial often go away. Over time, gills shrunk and became limited to juveniles until they ultimately disappeared entirely in full-time land animals in the Carboniferous period about 315 million years ago, Organ said. That was around the time when the first reptiles and the ancestors of the first birds and mammals started to evolve.

It might seem odd that primitive fish had lungs at all. While gills are good at extracting oxygen from the water, they don't always provide large amounts of oxygen, especially for bigger animals that need more of it. Seasonal changes can affect the amount of oxygen in the water, too, Shubin noted. For example, if there are lots of dead leaves in the water, those will take up oxygen. So, air sacs — primitive lungs — allowed fish to gulp air above the surface to supplement their oxygen intake. Modern-day lungfish, which have been around for more than 400 million years, have this same capability, which helps explain why moving landward was even possible.

But we haven't lost those early gills entirely. Human embryos have a fishy physical trait: Tiny folds called pharyngeal arches resemble gills, but we don't use them to breathe. And while they aren't exactly gills, they are definitely a relict of early gills, kind of like an ancient recipe that is now making something different than before, Shubin said. Throughout embryonic development, those arches become parts of the jaw, throat and ears. "Every creature that has a head passes through a pharyngeal arch stage," Shubin said. In other words, an animal's head cannot form without pharyngeal arches.

Aquatic, gill-breathing species have these arches in embryonic development, too. The only difference is that the arches develop into real gills, along with the bones, muscles, nerves and arteries surrounding them, Shubin said.

We can thank our inner fish for the lungs we now use to breathe, without awkward gill slits getting in the way.