“I was floored and shocked and went, ‘No way,’” said Briana Pobiner, lead author of the study and a paleoanthropologist at the National Museum of Natural History in Washington. She recalled approaching others at the National Museums of Kenya in Nairobi where she made the discovery, saying “Come here. Come look at this. Am I crazy?”

To be certain that the cut marks resulted from cannibalism, Pobiner said, “You have to know who is doing the eating and who is being eaten, and in this case we know neither.” Cannibalism requires that both the consumer and consumed be of the same species.

Around 1.5 million years ago, at least three species of hominin existed in the region where the fossil was found: Homo erectus, Homo habilis and Paranthropus boisei. Pobiner said that to determine the species in question, experts would need more of the skeleton than the single bone.

Cannibalism is not unusual in the animal kingdom. More than 1,300 animal species feed on their own kind, including some primates. The earliest evidence of cannibalism among hominins dates back 800,000 years, and was discovered at the Atapuerca archaeological site in northern Spain.

The closer the practice gets to Homo sapiens, the more complex and uncomfortable the questions it raises.

“This behavior connects us to our animal nature and reminds us that we are just one among millions of living beings that have existed throughout evolution,” said Antonio Rodríguez-Hidalgo, a postdoctoral researcher at the Catalan Institute of Human Paleontology and Social Evolution who did not participate in the study, but who took part in a recent workshop on human cannibalism in prehistory entitled “Feast or Famine.”

“On a more unsettling note,” he continued in an email, “cannibalism in Homo sapiens carries deeper philosophical implications. It raises questions about love versus hate, family versus enemy, war cannibalism versus mortuary cannibalism, and feast versus famine.”

Making an impression

The fossilized bone examined by Pobiner was discovered by the famed British paleoanthropologist Mary Leakey, but at the time the markings were not noted as possible signs of butchery. Nor were they noted as such by subsequent researchers who have examined the left tibia over the last half-century.

Pobiner believes researchers who examined the bone did not notice the marks because they were not looking for signs of butchery. In recent years, it has become more common to re-examine previously discovered fossils, she said.

The bone was just one of 199 hominin fossils, all between 1.5 and 2 million years old, that Pobiner examined in July 2017, but it was the only one on which she found cut marks. The marks were the same color as the rest of the bone, indicating that they were made before the bone became fossilized, she said.

While Pobiner detected the marks with a simple handheld magnifying glass, they were later analyzed using more complex technology. She took one long impression of the bone using the kind of molding clay dentists use to take impressions of teeth and check bite marks when installing crowns.

She forwarded the impression to one of her co-authors on the study, Michael Pante, of Colorado State University, telling him nothing about what it was taken from. Over the course of months, Pante used the impression to make 3D computer models of the marks, which were all between 1 and 5 millimeters long. The models were compared with a database of 898 individual tooth, butchery and trample marks that had been created through controlled experiments.

Pante determined that 9 of the 11 were cut marks; the other two, tooth marks, were probably made by a lion-like animal.

“Unfortunately, identifying tool type or raw material from a cut mark is difficult and prone to error,” Pante said by email, “so we chose not to include this comparison.” He said additional research will be needed before the marks can be reliably linked to a specific kind of tool.

No stone tools were found with the bone, though Pobiner said tools have been discovered at various dig sites, including one about 15 miles away.

Far from done

Because the cut marks and tooth marks do not overlap, the story of what precisely happened is unclear. Did the hominin scavenge remains from an individual that was first killed by a lion, or did the hominin do the initial killing and the lion the scavenging?

“It seems a bit unusual for a large feline like a lion to scavenge the remains of a [hominin] that has already had its deeper muscles exploited,” Rodríguez-Hidalgo said. “What would be left for the cats to scavenge? Only the marrow, but large cats are not known for their bone-cracking abilities, and the tibia appears to be intact. So, this scenario doesn’t seem very plausible.”

The fact that only one of the 199 fossilized bones that Pobiner examined contained cut marks suggests to her that it was unlikely hominins of this period ate one another as a regular staple of their diet. Eating other hominins was more likely a response to scarcity of other food. The hominin diet 1.5 million years ago included ancient antelopes, zebras, rhinos, hippos ― “anything you can get your hands on,” as Pobiner likes to say.

James Cole, principal lecturer in archaeology at University of Brighton in England, called the new research “a really interesting and amazing discovery,” which demonstrates the value of going back to fossil collections held in museums.

“Some of the best discoveries have already been found, but perhaps not fully recognized yet,” Cole said. “The evidence here shows that we are far from done in our understanding of our hominin ancestors and the complex and fascinating lives they lived.”