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Science

Did eating meat make us human? A new study complicates the evolutionary theory

Hello, bias, my old friend.

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The first primate to look like modern humans, Homo sapiens that is, was Homo erectus, an ancient hominin that evolved in Africa about 2 million years ago. H. erectus was taller than the archaic hominins that came before — but they also had longer legs, shorter arms, and a bigger brain. The timing of their emergence coincides, strangely, with an apparent change in the diets of these ancient peoples — specifically, increased consumption of meat, giving rise to a hypothesis: Eating meat made us human. But the connection between carnivory and our crania may be less straightforward than researchers previously thought.

The hypothesis — H. erectus represents a giant evolutionary leap for hominins as a whole, and the concurrent proliferation in meat-eating, so the fossil record suggested, led to a “meat made us human” hypothesis to explain the origins of modern-day humans. Essentially, the theory goes that eating meat fed the bigger brains and bodily changes that gave rise to H. sapiens.

Meat is a nutritionally dense food, and generally much more nutritionally dense than plants. Eating it may have given our ancestors the evolutionary edge. Taller bodies with modern proportions, faster and easier digestion, and of course, bigger brains are, this theory posits, easily chalked up to eating meat. (Of course, not every scientist agreed with this theory.)

But in a new study published this week in the journal Proceedings of the Natural Academy of Sciences, researchers reanalyze old data to discover that the increased evidence of meat-eating at archeological sites may be evidence of a similar uptick in archaeological interest — ie. when you dig up more total fossils from a place and time, you find more evidence — rather than true evidence of a shift in our evolutionary timeline. In essence: The theory that meat made us human may be founded on bias, rather than fact.

Homo erectus had relatively big brains, like our own species.Shutterstock

What the science shows — It is true the ancestors of modern humans did eat meat along with plants. Researchers have found “butchery marks” on excavated animal bones, dents or scratches from where prehistoric hominins likely used stone tools to separate meat from an animal’s skeleton. Butchery marks can be found on 2.6 million year old fossils, so our progenitors were likely eating some meat at least that long ago.

But around the same 2-million-year mark, there seems to be a boom in eating meat. That’s also when H. erectus first shows up in the fossil record. For decades, scientists believed these two shifts were not mere coincidence, but intrinsically linked: A meat-rich diet enabled Homo erectus to evolve its hallmark traits, and in turn, Homo erectus evolved because hominins were eating more meat.

Indeed, this was what Briana Pobiner, a study author and a paleoanthropologist at the Smithsonian Museum of Natural History, believed.

“I have definitely been a ‘meat made us human’ hypothesis purveyor,” she tells Inverse.

But when Pobiner and her team revisited old data from 59 archeological sites in East Africa dating to between 2.6 and 1.2 million years ago, they came up empty.

They analyzed fossils in hundred-year chunks, controlling for the number of samples from each time point using a proxy, and found that evidence of meat-eating only seems to increase with the number of specimens representing each timepoint.

“At times when you have bigger fossil samples, you’re more likely to get evidence for meat-eating,” Pobiner says. In other words, the theory may be corrupted by sample bias — this means when you look for something, you will likely find it.

With this new analysis, the trend becomes far less clear Pobiner says. And after meat-eating appears in the record at 2.6 million years, it fluctuates irregularly across sites.

Why it matters — Pobiner was “totally surprised” by the results. Much of her previous research assumed the theory was correct, too.

Pobiner is quick to point out there is still ample evidence many of our ancestors ate meat and probably contributed to our evolutionary origins. But she adds the story is likely more complex than a single thing spurring modern humans’ development.

“There may be more than one factor involved in these big changes in our evolutionary history,” she says.

Henry Bunn is a paleoanthropologist at the University of Wisconsin-Madison who was not involved in the new research. Bunn agrees with Pobiner on the multi-factored nature of our evolutionary history, but little else.

In his view, meat was never the only driving factor behind the developmental shift.

“No one is saying that meat is the only thing driving the evolution of humankind,” Bunn tells Inverse. But if not meat, he posits, then what else?

The jaws of H. erectus. Not seen: meat.Print Collector/Hulton Archive/Getty Images

Theories of the origin of humans beyond meat include the theory that cooking with fire led to more accessible nutrients, that grandmothers helped to feed offspring and thus made for better diets, and that eating insects rather than large animals all helped humans — and our big brains — arise. But there is no 2-million-year-old fossil evidence to support any of these ideas.

In Bunn’s view, Pobiner and her co-researchers have created a “strawman” by focusing on their estimate of sample intensity. The earliest evidence of meat consumption is found in tiny fragments of bone. But some H. erectus sites have “50 dead animals represented, all in a deposit that’s only four inches thick,” Bunn says. He believes early meat-eating was incidental and random, nothing like the regular and coordinated consumption by H. erectus that shows up in the fossil record at a few sites after 2 million years. And he points out that the new study falsely treats all the sites uniformly when each has a unique set of conditions that determine the quality of fossils.

“The effect of that is to add noise to the system,” he says.

Shara Bailey, a biological anthropologist at New York University who was also not involved in the recent findings agrees meat likely played a critical, but not solo, role in our evolutionary history.

“I do think carnivory plays a role in us becoming human,” Bailey tells Inverse. “It becomes a prevalent pattern later on in the Pleistocene. It probably had more importance in certain time periods and populations than others.”

What’s next — Pobiner admits the research has limitations, including the assumptions they had to make about study sites and sampling intensity to run the analysis. “It's a caveat that bothered all of us.” But she’s eager to engage with all feedback, including critical responses like Bunn’s. Her hope is that the new study gets anthropologists to question their assumptions, rigorously revisit existing data, and collect new fossils.

And she also hopes it gets people outside the field reconsidering how they think about their diets.

“There’s a lot of people that adhere to modern ‘Paleo Diet’ movements that think that meat-eating was the most important dietary change in our evolutionary history,” Pobiner says. But our ancestors were omnivores who ate everything available to them.

“I can’t imagine any of our early human ancestors going, ‘no, I’m not going to eat those tubers. I’m not going to eat those lentils.’ Food is food.”

“I joke sometimes when people say, ‘well, what was the paleo diet?’ It was literally everything. Everything that was edible in the past was on a paleo diet.”

Abstract: The appearance of Homo erectus shortly after 2.0 Ma is widely considered a turning point in human dietary evolution, with increased consumption of animal tissues driving the evolution of larger brain and body size and a reorganization of the gut. An increase in the size and number of zooarchaeological assemblages after the appearance of H. erectus is often offered as a central piece of archaeological evidence for increased carnivory in this species, but this characterization has yet to be subject to detailed scrutiny. Any widespread dietary shift leading to the acquisition of key traits in H. erectus should be persistent in the zooarchaeological record through time and can only be convincingly demonstrated by a broad-scale analysis that transcends individual sites or localities. Here, we present a quantitative synthesis of the zooarchaeological record of eastern Africa from 2.6 to 1.2 Ma. We show that several proxies for the prevalence of hominin carnivory are all strongly related to how well the fossil record has been sampled, which constrains the zooarchaeological visibility of hominin carnivory. When correcting for sampling effort, there is no sustained increase in the amount of evidence for hominin carnivory between 2.6 and 1.2 Ma. Our observations undercut evolutionary narratives linking anatomical and behavioral traits to increased meat consumption in H. erectus, suggesting that other factors are likely responsible for the appearance of its human-like traits.
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