Joy Lian

Most paleoanthropologists think that the jump start for human evolution occurred three million years ago when ancestors of humans — called hominids — smashed rocks into crude knives to help them hunt for meat.

But according to a new hypothesis put forward by a team of researchers at Yale, the University of Chicago, University of Oxford and Arizona State University, most paleoanthropologists have it wrong. It was not meat that early hominids were after — it was fat.

According to the study, the team of researchers combined data and expertise from a wide swath of anthropology-related fields to come to their conclusion, and in doing so, contradicted the long-held assumption that early hominids used tools in search of meat on the plains of ancient Africa. The paper was published in the journal Current Anthropology on Feb. 5.

“I think it’s going to be a landmark and a benchmark paper that’s going to be cited for years to come,” said senior author and University of Chicago organismal biology professor Zeresenay Alemseged.

According to Yale anthropology professor Jessica Thompson, hominids needed only a hard rock to break open animal bones and reach the fat stored inside, a method called percussion. Compared to the prevailing scholarly hypothesis that sophisticated stone tool–use was dedicated to meat scavenging, Thompson said, percussion is a smaller intellectual leap and is therefore more plausible.

“It’s a much simpler transition than arguing that you would have to take a rock, hit another rock, make a sharp tool, then take that sharp tool and go find meat and then cut the meat off, and the meat’s probably rotten anyway,” she said. “Percussion is a much simpler explanation.”

In addition to the fact that it is a high-calorie food, bone marrow fat is hard for other animals to reach. The only animals on the African savannah that could break into large bones were hyenas and hominids equipped with rocks, Thompson said.

Also, while meat was easy to find — dead carcasses were only a walk away — it attracted dangerous carnivores, was harder to digest and was less nutritionally valuable than fat. Because of this, hominids turned toward fat to feed their brains, according to Thompson.

“If we were eating the same thing as a zebra, or a wildebeest, we simply wouldn’t have been able to afford the energy surplus required to grow this very large, expensive brain,” she said.

According to Thompson, the study also pushes back on paleoanthropologists’ comparisons to chimpanzees to support their meat-eating conclusion. She said that since hominids walked on two legs, did not have large canines and could not move through trees like chimpanzees — it does not make sense to assume that hominids hunted small prey for their meat.

To support her team’s hypothesis, Thompson personified an early hominid and his or her thought process.

“Hey, I’m cracking nuts, and now I’m finding myself out in open landscapes more often. I’m encountering these carcasses that are left over from carnivore meals, and most carnivores can’t access the marrow that’s inside the bone. I’m holding a rock. I know how to use this rock, so now I’m just going to apply the same technology to this different kind of food,” she illustrated.

Since this was a synthetic study — meaning no new data was collected — the team of researchers is prepared to test its hypothesis through evidence-based archaeological digs and primate observations. Alemseged plans to travel to Ethiopia or Mozambique to look for clues that support — or disprove — his team’s claims.

“As much as we’re excited by the findings that are being reported and being debated in the paper, we’re also excited to actually test the very hypothesis that we’re putting forward,” Alemseged said.

For other primatologists and paleoanthropologists who want to verify the findings, the team of researchers included a table of instructions for future studies to follow. According to Thompson, while it is “unusual” for a paper to have a table, it is necessary so that researchers can change the way they collect their data.

For example, instead of trying to collect large bones or sharp rocks, Thompson encourages field study researchers to seek small rocks and bone fragments because they can have evidence for percussion.

“We also would look for pounding tools rather than looking for sharp-edged stones. We would start trying to train ourselves to be more attentive to the rocks that don’t look obviously modified and systematically examine those microscopically for traces that they might have been used as battering products,” she said.

According to Arizona State University’s Institute of Human Origins, the most famous early hominid, named Lucy the Australopithecus, was three and a half feet tall when she died 3.18 million years ago.

Matt Kristoffersen | matthew.kristoffersen@yale.edu