Human relatives used tools to butcher and likely eat each other 1.45 million years ago: study

View of the hominin tibia and magnified area that shows cut marks. Scale = 4 cm. (JenniferClark/Smithsonian National Museum of Natural History)

Nine cut marks on a fossilized shin bone are the oldest clear evidence of human relatives using stone tools to butcher and likely eat each other, according to a new study.

"Based on the location of the cut marks, the butcher(s) were most likely defleshing the leg – cutting meat off of it, ” lead author and paleoanthropologist Briana Pobiner told CTVNews.ca. "The best explanation for that is that the leg was butchered in order to eat the meat from it. This is the earliest solid evidence of hominins defleshing other hominins."

The word hominin refers to humans, our evolutionary ancestors and closely-related species. As a research scientist at the Smithsonian's National Museum of Natural History in Washington, D.C., Pobiner's work focuses on the evolution of the human diet, including meat eating and cannibalism in hominins.

Pobiner was combing through fossils at a museum in Nairobi, Kenya, looking for clues about the prehistoric predators that ate humans' ancient relatives. When she first picked up part of a 1.45-million-year-old hominin tibia, or shin bone, she used a magnifying lens to examine it for bite marks, but instead found cuts that looked like evidence of butchery.

Pobiner made moulds of the bone and sent them to co-author Michael Pante at Colorado State University. Without knowing Pobiner's theory, Pante used 3D scans to compare them against a database of 898 different tooth, butchery and trample marks that were created in earlier experiments. Nine of the 11 marks were identified as clearly coming from stone tools. The other two were likely from one of three extinct species of sabre-tooth cat that prowled the area in the early Pleistocene age.

While the cut marks themselves don't prove that human ancestors ate each other, Pobiner believes this is the most likely scenario. The cuts, she notes, are located where the calf muscle attaches to the bone, and they are also all orientated the same way, as if a stone age butcher held the bone with one hand while the other hand used a sharp tool to remove the flesh.

"We interpret this butchery as nutritional, rather than related to ritual or ceremony,” Pobiner said. “I have studied hundreds of fossil animal bones from this time and place in northern Kenya (Koobi Fora) with similar cut marks to the ones on this hominin fossil, and those are all interpreted as the result of butchering animals to eat them."

Researchers can't say if this is evidence of cannibalism, which refers to a specific species eating its own kind. At least three hominin species could have been present in the region at the time, and it is unknown if they all made and used stone tools. The shin bone itself also doesn't provide enough information to assign a specific species. As for the big cat's bite marks, it's unclear if they were made before or after our ancient relative’s death.

"Presumably this individual died, either naturally or from predation, and then their lower leg was both nibbled on by a big cat and butchered by another hominin," Pobiner said. "Anything more than that would be pure speculation."

The study was published Monday in the peer-reviewed journal Scientific Reports.

Pobiner says evidence of cannibalism isn't uncommon in fossils from humans and our relatives. The use of stone tools dates back approximately 2.9 million years, she adds, while consistent evidence of death rituals only occur long after the fossil's date. Some argue that a possibly older skull from South Africa shows similar marks, but those findings have been contested.

Pobiner says her grisly discovery also points to the value of scouring museum collections for new research.

"I like to emphasize that this finding demonstrates that not all paleoanthropology discoveries are made in the field," she said. "Museum collections are valuable treasures worth re-studying with new questions, tools and analytical techniques!"