Twenty-four years ago, Briana Pobiner reached into the north Kenyan soil and put her hands on bones that had last been touched 1.5 million years ago. Pobiner, a paleoanthropologist, was digging up ancient animal bones and searching for cuts and dents, signs that they had been butchered by our early ancestors trying to get at the fatty, calorie-rich bone marrow hidden within. “You are reaching through a window in time,” says Pobiner, who is now at the Smithsonian Institution in Washington, DC. “The creature who butchered this animal is not quite like you, but you’re uncovering this direct evidence of behavior. It’s really exciting.”
That moment sparked Pobiner’s lasting interest in how the diets of our ancestors shaped their evolution and eventually the emergence of our own species, Homo sapiens. Meat, in particular, seems to have played a crucial role. Our more distant ancestors mostly ate plants, and had short legs and small brains similar in size to a chimpanzee’s. But around 2 million years ago, a new species emerged with decidedly humanlike features. Homo erectus had a larger brain, smaller gut, and limbs proportioned similarly to those of modern humans. And fossils from around the same time, like those excavated by Pobiner in Kenya, show that someone was butchering animals to separate lean meat from the bone and dig out the marrow. For decades, paleontologists have theorized that the evolution of humanlike features and meat eating are strongly connected.
“The explanation has been that meat-eating allowed this: We got a lot more nutrition, and these concentrated sources facilitated these changes,” Pobiner says. Large brains are phenomenal energy hogs—even at rest, a human brain consumes about 20 percent of the body’s energy. But a switch to a diet full of calorie-rich meat meant an excess of energy that could be directed to supporting larger, more complex brains. And if prehumans hunted their food, that would explain a shift toward longer limbs that were more efficient for stalking prey over great distances. Meat made us human, the conventional wisdom said. And Pobiner agreed.
But in April 2020, Pobiner got a call that made her rethink that hypothesis. The call was from Andrew Barr, a paleontologist at George Washington University in Washington, DC, who wasn’t totally convinced about the link between Homo erectus and meat-eating. He wanted to use the fossil record to check whether there really was evidence that human ancestors were eating more meat around the time Homo erectus evolved, or whether it simply appeared that way because we hadn’t been looking hard enough. Pobiner thought this sounded like an intriguing project: “I love the idea of questioning conventional wisdom, even if it’s conventional wisdom that I buy into.”
The researchers were unable to travel to Kenya for fieldwork because of the pandemic, so instead they analyzed data from nine major research areas in eastern Africa that cover millions of years of human evolution. They used different metrics to assess how well-researched each time period was, and how many bones with butchery marks were found in each site. In a new paper in the journal Proceedings of the National Academy of Sciences (PNAS), Barr and Pobiner now argue that the link between meat-eating and human evolution might be less certain than previously thought. The apparent increase in butchered bones after the appearance of Homo erectus, they conclude, is actually a sampling bias. More paleontologists went looking for bones at dig sites from this era—and as a result, they found more of them.
This doesn’t rule out a link between meat-eating and evolutionary change, but it does suggest that the story might be a little more complicated. “If we want to say how common a behavior was, then we need some way to control for the fact that at some points in time and at some places we’ve looked harder for that behavior than we have at other points,” says Barr. Because sites with well-preserved animal bones are relatively rare, paleontologists often sample them over and over again. But Barr and Pobiner’s study found that other sites that date from between 1.9 and 2.6 million years ago—the era during which Homo Erectus evolved—have been relatively under-studied. “We are drawn to places that preserve fossils because they’re the raw material of our science. So we keep going back to these same places,” Barr says.
For Barr, the new study’s results point to a gap in the paleontological record that needs to be filled in. It might be that other factors were responsible for the evolution of humanlike traits, or it might be that there was a big increase in meat-eating in an earlier period that we just haven’t been able to see yet. “At some point there is no evidence for butchery, and at some point there’s a lot of evidence. And something had to happen in between,” says Jessica Thompson, an anthropologist at Yale University.
Thompson isn’t totally convinced that this new paper does undercut the “meat made us human” hypothesis. Her reservation has to do with the way the authors of the PNAS paper assessed how well different time periods had been researched. The authors estimated this by looking at how many different mammal species exist in the fossil record for certain periods of time. They reasoned that if paleontologists have spent a lot of time digging up sites from a certain era, we’ll have more mammal species in the fossil record for that period. They then used this metric to evaluate whether sites with evidence of butchered bones came from prehistoric periods that were well-studied or not.
But Thompson points out that this “species richness” metric may not be the best way to measure whether paleontologists have searched hard enough for butchered bone fragments. Not every ancient site is explored in the same way, she says. Paleoanthropologists—who study the lives of ancient humans—might search really hard for butchered bone fragments at a particular site, even if this time period hasn’t been well-studied by paleontologists who are looking for other kinds of fossils. And, she points out, the conventional wisdom may be right: If researchers haven’t been able to find much evidence of butchery marks on bones before the emergence of Homo erectus, it’s not necessarily because they weren’t looking hard enough. It might really be because there just aren’t as many examples of butchery from that time period.
Ultimately Thompson agrees that the only way to know for sure—or at least as sure as anyone can be when talking about fossils from millions of years ago—is to look in more detail at those time periods for which we have relatively little data. “What it’s revealed to me is that we have a serious problem with sampling,” she says. “The takeaway is that we need to get into those deposits that date between 2.6 and 1.9 million years ago. We need to find out what’s going on.”
Even if these new findings don’t overturn the meat hypothesis altogether, there still might be more to the story of human evolution during this era. There are all kinds of things that we don’t know about how Homo erectus behaved, says Stephen Merritt, an anthropologist at the University of Alabama at Birmingham, who studies the evolution of meat-eating. Did they scavenge meat or hunt prey? How did individuals learn how to butcher animals? Once they’d butchered an antelope, did they share that meat with other members of their species, or (like other apes) did they mostly keep their food to themselves?
Although these other behaviors are much harder to find evidence of, they might have played an important role in our evolution. One alternative theory to explain the rise of some humanlike traits is the “grandmother hypothesis”: the idea that as climate change reduced our ancestors’ access to easy-to-eat plants such as fruit, older females became particularly important, as they had the knowledge to break open nuts and dig up hard-to-find tubers. They could then share this food with children, allowing those children to move on from breastfeeding more quickly and freeing up their mothers to have their next baby sooner. This might explain why humans evolved to live relatively long lives past menopause. But like any theory of evolution, it’s only based on a few fleeting glimpses of a long-faded picture.
Human evolution might boil down to a lot more than what Homo erectus had for dinner, but this focus on our ancestors’ diets still has a lot of sway today. Enthusiasts of the paleo diet shun processed foods in favor of meat and raw plants, arguing that it’s healthier for us to eat the same kind of diets as early humans. (Some eschew cooked meat altogether, even though evidence for using fire to cook food dates back hundreds of thousands of years.) Jordan Peterson and his daughter famously opted for a diet of only beef, salt, and water, much to the dismay of nutrition experts. The high-fat, low-carbohydrate keto diet is also often framed as a return to the diet of our ancestors, but studies suggest that ancient human meals might have been a lot less meat-heavy than modern fad diets suggest.
To some, an origin story of humanity that’s rooted deeply in carnivory seems to point toward some long-lost masculine ideal that humans owe their very existence to their lust for blood and meat. In reality, the emerging evidence is a little more complex than that. Meat-eating may have evolved alongside a host of other behaviors that unleashed the power of our larger brains and set us down the path to complex language and societies. “Maybe meat made us human not just because we were eating it, but because of the social stuff we were doing around it,” says Merritt. “Rather than asking ‘did meat make us human?’ I would like to know how meat made us human.”