The debate over whether or not eating meat really did “make us human” just became more complicated.
It’s understood that the frequent eating of meat separates humans from other primates, but the exact role it played in early human evolution is getting a fresh challenge from a study published Monday in the journal Proceedings of the National Academy of Sciences.
The findings add nuance to the “meat made us human” hypothesis and may be of interest to modern people who base their dieting decisions on the idea early humans were especially reliant on meat, according to the research team.
Between 2 million and 3 million years ago, early human ancestors began to make stone tools and used them to butcher animals. Scientists can infer meat eating from the butchery marks found on bones — the intentional slicing and scraping made with sharp-edged tools. It’s uncontested that this rise in tool use and change in diet greatly influenced the trajectory of humans.
What is debated, however, are the details around this phenomena, and whether or not past behavior really explains if humans need to eat meat.
The new paper hinges on the argument that quintessential human traits, like larger brains, first appeared in the human ancestor Homo erectus, and that these traits are linked to a dietary shift toward increased meat eating. But the study authors claim their analysis of published data can’t demonstrate an increase in evidence for meat eating after the emergence of the Homo erectus, challenging the “meat made us human” viewpoint.
“It’s clear that eating meat has been important for many groups of humans throughout much of human history and prehistory,” said lead author W. Andrew Barr, an assistant professor at George Washington University. “But the idea that there was a sudden evolutionary event where meat eating went from being relatively unimportant to being so central that it drove the evolution of key human traits just doesn’t shake out in our analysis of the published evidence.”
Research on the evolution of meat eating by humans has typically focused “on very well preserved sites at a few well-known research areas,” he said. Barr and his colleagues examined data relating to 59 of these sites in eastern Africa, representing human activity dating between 2.6 million and 1.2 million years ago. Evidence of eating meat included the number of sites with animal bones with cut marks made by tools and the total amount of this category of bones.
“We used the number of paleontological sites and the number of species preserved at those sites as a barometer for how much fossil-preservation potential there is in a given time period, and then we used that background level of sampling to contextualize the amount of cut-mark evidence preserved in the same period,” he said.
This analysis suggests that the abundance of bones with cut marks doesn’t necessarily represent a rise in meat eating alongside the Homo erectus, but instead is a result of a sampling bias — a focus on excavating samples from sites associated with this ancestor and time period. This study, which controls for differences in sampling intensity, shows there was a persistent increase in the amount of meat-eating evidence after Homo erectus appears, according to the study authors.
However, the conclusion of this study is challenged by competing theories and criticism of the methodology. For example, the focus on brain size as a defining feature of Homo erectus is “perhaps a little misplaced,” Fred Spoor, a professor of evolutionary anatomy at the University College London who was not involved in this study, said in an email. Scientists are increasingly discovering overlap in brain size among Homo erectus, Homo habilis and Homo rudolfensis. It is only later Homo erectus that had a notably larger brain size.
Homo erectus does have a reduction in molar size compared to other species, which is linked to chewing capacity and diet, Spoor said. This change is thought to reflect a shift from eating mostly a plant-based diet toward a mixed diet — one with more meat.
Tim White, the co-director of the Human Evolution Research Center at the University of California, Berkeley, said in an email that the available data compiled in the study is “inadequate” to test whether or not “Homo erectus behavior and anatomy depended on an increased amount of meat in the diet.”
White, who was not involved in this study, said it is difficult to come to a conclusion with the information available: For example, it is not possible to definitively say which Homo species was responsible for the stone tools and bone modifications because of overlaps in the fossil record, and the methodology used to discern which markings on bones were created by humans changes across the studies included in the overall analysis.
Brain size is also not exclusively linked to meat eating across carnivores, White said. A 2021 study published in the journal Evolutionary Biology argues that “the largest brained mammals achieved large relative brain sizes by highly divergent paths.” Baboons, for their part, tend to have bigger brains if they live in larger social groups.
The study, and the conversation surrounding its robustness, are illustrative of the challenges in definitively proving broad trends in human evolution with the information available. Barr and his colleagues write that other theories beyond meat eating could explain why certain anatomical and behavioral traits linked to modern humans emerged, including using fire to cook, which may have increased nutrient availability. These are enticing theories, but they also lack evidence, according to the study team.
As scientists continue to gather evidence and add nuance to the “meat made us human” hypothesis, modern people will have to grapple with the decision over whether or not to continue to eat meat. Meat may have played an essential role in human evolution, but processed meat is quite unlike what our ancestors once ate.