When it comes to killing and eating other creatures, chimpanzees—our closest relatives—have nothing on us. Animal flesh makes up much more of the average human’s diet than a chimp’s.
Many scientists have long suggested our blood lust ramped up about 2 million years ago, based on the number of butchery marks found at ancient archaeological sites. The spike in calories from meat, the story goes, allowed one of our early ancestors, Homo erectus, to grow bigger bodies and brains.
But a new study argues the evidence behind this hypothesis is statistically flawed because it fails to account for the fact that researchers have focused most of their time and attention on later sites. As a result of this unequal “sampling effort” over time at different sites, the authors say, it’s impossible to know how big a role meat eating played in human evolution.
Even before the study, many experts suspected the link between carnivory and bigger brains and bodies in early humans might be complex, says Rachel Carmody, an evolutionary biologist at Harvard University who wasn’t involved in the work. The new results, though, “take the important step of demonstrating empirically that controlling for sampling effort actually changes the interpretation.”
To conduct the study, W. Andrew Barr, a paleoanthropologist at George Washington University, and colleagues reviewed previously reported data on the appearance of butchery marks at nine archaeological hotbeds of early human activity across eastern Africa spanning 2.6 million to 1.2 million years ago. As expected, the scientist found an increase in the number of cutmarks on animal bones beginning about 2 million years ago. However, the researchers noticed that archaeologists tended to find more cutmarks at the sites that have received the most research attention. In other words, the more time and effort researchers poured into a site, the more likely they were to discover evidence of meat eating.
Each of these sites contains multiple layers of sediments; the deeper the layers, the older the troves of artifacts in them. Layers that are between 2.5 million and 2 million years old simply aren’t as exposed—and therefore aren’t as easily studied—as later sediments, Barr explains, which means those earlier sites have received much less attention than later sites. The more time and energy researchers devote to a dig, the more artifacts they find. That can make comparing excavated evidence—such as butchery marks on bones—from different sites a tricky statistical proposition.
Researchers can control for these differences in sampling effort by calculating how many cutmarks they would expect to find given a site’s research intensity relative to other sites. Barr and colleagues ran the numbers for these sites and found that evidence for meat eating remained essentially constant between 2.6 million and 1.2 million years ago, they report today in the Proceedings of the National Academy of Sciences.
“I’m always sort of inherently skeptical of these tidy evolutionary narratives,” Barr says. “More often than not, when you really dig deep on these things, the evidence isn’t as neat as it appears at first blush.”
He argues that, instead of an uptick in meat consumption, other previously proposed explanations deserve attention. For example, the establishment of cooking, which makes food easier to digest, may have fueled more calorie intake—and thus bigger brains. Evolving humans may have also obtained more calories as their groups became more socially complex, with extended family members hunting and gathering additional food for the family—part of an idea known as the “grandmother hypothesis.”
Christian Tryon, a paleoanthropologist at the University of Connecticut, Storrs, says he generally agrees with study’s interpretation of data from East Africa. But he points out that H. erectus spread far and wide, migrating throughout Africa and the Middle East and to parts of southern Europe, East Asia, and Southeast Asia. “Within the world that they’ve sampled, I think [the study] is really powerful, but I do worry it’s a little bit of a stretch to talk about this for the [entire] species.”
Shara Bailey, a paleoanthropologist at New York University, says she wasn’t particularly surprised by the paper’s findings, because she and others have long suspected the “meat made us human” hypothesis was overly simplistic. Still, she says, it’s good to see researchers digging into the statistical underpinnings of the field’s conventional wisdom.