Were the first humans superpredators?

The first humans were mega-carnivores who took down prey with savvy hunting skills, a controversial new study suggests.

In a new research paper, scientists argue that humans and their close relatives were expert hunters from early on, starting at least 2 million years ago. Not only that, but the earliest human species were superpredators, taking down animals twice as large as any terrestrial creature alive today, said Miki Ben-Dor and Ran Barkai, researchers at Tel Aviv University in Israel, and Raphael Sirtoli, a doctoral student at the University of Minho in Portugal.

"So far, attempts to reconstruct the diet of Stone Age humans were mostly based on comparisons to 20th-century hunter-gatherer societies," Ben-Dor said in a statement. "This comparison is futile, however, because 2 million years ago, hunter-gatherer societies could hunt and consume elephants and other large animals — while today's hunter-gatherers do not have access to such bounty. The entire ecosystem has changed, and conditions cannot be compared."

Related: 15 of the largest animals of their kind on Earth

A limited record 

Fossil evidence from the earliest human ancestors is scarce. But based on archaeological evidence, Ben-Dor told Live Science, it's clear that Homo sapiens and their close relatives ate "anything edible." But how much of their diets comprised plants versus animals is the sticking point. (Another sticking point: When did humans start hunting meat themselves, rather than scavenging it?) Many animals considered omnivorous actually have diets weighted one way or another. Chimpanzees, for example, are technically omnivores, but meat makes up only about 6% of their diets, according to the Jane Goodall Institute of Canada. Dogs and wolves eat mostly meat but sometimes gorge on grains, leading to a debate over whether they should be classified as omnivores or carnivores.

The ancient human species Homo habilis was eating meat at least 2.6 million years ago, Ben-Dor said. Another early human species, Homo erectus, seems to have been a particularly enthusiastic meat eater by 1.8 million years ago; its teeth and gut shrank compared with earlier ancestors — adaptations for digesting meat instead of plants — and it used stone tools capable of butchering meat.

Ben-Dor and Barkai argue in their paper, published March 5 in the American Journal of Physical Anthropology, that meat wasn't just a bonus for these human species and the first Homo sapiens. Instead, the authors believe large animals weighing over 2,200 lbs. (1,000 kilograms) — such as elephants, hippopotamuses and rhinoceroses — made up most of humans' diets. These huge herbivores were much more common — and much larger — in the Pleistocene epoch, starting about 2.5 million years ago, than they are today. .

"Elephants 500,000 years ago could weigh 12 tons, compared to 4 to 6 tons today," Ben-Dor said.

These animals would have been walking buffets of fatty meat, well suited to feeding humans' energy-hungry brains, according to the researchers. The authors argued in another recent paper that hunting large prey might have been what drove human brain evolution.

This idea is controversial, however, and researchers do not agree on how useful a huge influx of meat would have been to hunter-gatherers in the days before refrigeration, nor on how skilled ancient humans would have been at taking down prey that other apex carnivores, like lions, struggle to defeat. 

"There are some archaeologists who'd say, 'Yeah, they hunted elephant once in a while, but that was like a once-in-a-lifetime hunt; that's the thing grandparents would tell their kids stories about,'" said John Hawks, a paleoanthropologist at the University of Wisconsin-Madison who was not involved in the research. "There are others who said 'No, meat from an elephant can last a long time. … Without storage, it's less than you think, but it was a regular part of their subsistence, and it was important to them.'" 

 A fatty diet? 

Eating large, fatty animals would have been a benefit to the earliest humans, Ben-Dor and his colleagues wrote in their paper, because bringing down that many calories in one hunting trip — rather than multiple attempts to stalk smaller prey —— would have freed up time for other pursuits, such as toolmaking and child-rearing. The researchers argue that humans show adaptations for this high-fat, meat-heavy life, ranging from particularly acidic stomach juices (also found in other animals with meat-heavy diets) to small jaws (because meat eaters have to chew less than herbivores that must break down large amounts of fibrous vegetation for the same calories).

Archaeologically speaking, it's difficult to categorize humans and their relatives as one level of predator prior to about 50,000 years ago, Ben-Dor said. That's because the only reliable biochemical way to distinguish whether an animal is a top predator or fits lower on the food chain is a method called stable nitrogen isotope analysis, which requires testing collagen for molecules introduced into the body via the diet. Consumers contain a few percentage points more of the isotope nitrogen-15 than what is found in either the plants or animals they eat, making it possible to determine their level in the food web, also known as their trophic level.

Collagen, the connective tissue found in abundance in bones, doesn't preserve well prior to 50,000 years ago, though. The samples from that era hail from Europe, where cooler temperatures allow for better preservation, and they do indicate that humans were eating large mammals. However, 50,000 years ago in Europe is a far cry from 300,000 years ago in Africa, when and where the first H. sapiens arose, Hawks said.

Adding to the difficulties in determining ancient humans' diets, it's hard to determine precise dates for archaeological materials from the crucial time periods in the middle Pleistocene, when human diets were evolving, Hawks added. 

"This is a time frame when our ability to determine the age of things relies on methods that have about a 100,000-year, sometimes 50,000-year, span of uncertainty about them. … That's a lot of error," Hawks told Live Science. And there are far fewer sites to make inferences from that are older than 100,000 years compared with those younger than 100,000 years, he said. 

Despite the limited evidence from humanity's early evolution, the researchers said there is more work to be done to show whether these human ancestors truly were specialized carnivores. This might include more work on the abundance of animals of different sizes throughout the Pleistocene, explorations of genetic changes over time that would have altered humans' ability to digest different foods and comparisons of trends in prey size over time.

"I feel that we have only scratched the surface, exploring paleobiology's potential to discover our past and present adaptation to consuming meat and animal fat," Ben-Dor said.

Originally published on Live Science.