Northern Ethiopia was once home to a vast, ancient lake. Saber-toothed cats prowled around it; giant crocodiles swam within. The streams and rivers that fed it—more than 3 million years ago, during the Pliocene—left behind trails of sediment that have now hardened into sandstone.
Deposited within these layers are fossils: some of early hominins, along with the bones of hippos, antelope, and elephants. The anthropologist Jessica Thompson encountered two of these specimens, from an area named Dikika, in 2010.
At the time, she was a visiting researcher at the Institute of Human Origins at Arizona State University. Given no explanation as to their history, she analyzed the bones and found signs of butchery. Percussion marks suggested someone may have accessed the marrow; cut marks hinted that flesh was stripped from bone. To her surprise, the specimens were 3.4 million years old, putting the butcher’s behaviors back 800,000 years earlier than conventional estimates would suggest. That fact got Thompson, now an assistant professor in the department of anthropology at Yale University, thinking there might be more traces of tool use from those early times.
In a wide-ranging review published in February’s issue of Current Anthropology, Thompson joined a team of researchers to weave together several strands of recent evidence and propose a new theory about the transition to large-animal consumption by our ancestors. The prevailing view, supported by a confluence of fossil evidence from sites in Ethiopia, is that the emergence of flaked-tool use and meat consumption led to the cerebral expansion that kick-started human evolution more than 2 million years ago. Thompson and her colleagues disagree: Rather than using sharpened stones to hunt and scrape meat from animals, they suggest, earlier hominins may have first bashed bones to harvest fatty nutrients from marrow and brains.