Researchers describe a 10,000-year-old Kenyan massacre that led to the deaths of 12 people in a study published in Nature today. Of the bodies that were recovered, 10 show signs of violent injuries; the other two may have belonged to people who were tied up when they died. The authors of the study say the finding is the earliest scientifically dated evidence of violence between groups of ancient hunter-gatherers. But that interpretation is disputed by outside researchers, which means a dozen prehistoric skeletons have just started a scholarly fight.

The first skeleton that the researchers excavated was found in 2012 in Kenya, at a site named Nataruk. It was lying with its face in the lagoon sediments, and had very clear evidence of major traumatic lesions to the head, says Marta Mirazon Lahr, an evolutionary biologist at the University of Cambridge and a co-author of the paper. The second skeleton they found was also lying face down, as was the one with that had a small blade still embedded in its skull. "At that point, we knew we were dealing with an extraordinary finding," she says.

Ultimately, researchers uncovered remains belonging to at least 27 people. Many were broken and in bad shape, but the scientists managed to find 12 skeletons — 10 of which appeared to have died of lethal injuries. The position of their hands of the other two skeletons suggest that they were bound when their owners died. Whatever happened led to the deaths of at least seven men, and possibly five women.

Humans have been waging war for a long time, but anthropologists haven't yet nailed down its origins. For some scholars, warfare started after certain groups of humans stopped moving around and started building permanent settlements. And in some ways that makes sense; the evidence that scientists have for prehistoric warfare has come from settled, sedentary communities. But others aren't so sure. Humans may have a predisposition for violence, which would make the origins of warfare a lot older. After all, chimpanzees are great apes like us, and they've been known to kill members of competing groups for resources like food. That's why today's study is so intriguing. It presents evidence that human warfare is older than some scholars previously thought, Mirazon Lahr says.

In addition, a cemetery burial means that the North Sudan site was a very different case from that of the Nataruk massacre, Mirazon Lahr says. The bodies in North Sudan were buried by their own community, which "presumably lived in the vicinity in a semi-permanent basis, and thus no longer nomadic hunter-gatherers." Because the site and the humans within it are still undated, it’s possible that their deaths are more recent than those at Nataruk. And that would make sense because the humans in North Sudan built a cemetery — an act that’s consistent with the transition toward more settled lifestyles, she says. "So, I stand by what we say in the paper: Nataruk is the earliest scientifically dated case of violence among different groups of hunter-gatherers."

As for Trinkaus' comments about Nature, a spokesperson for the journal told us that each published paper is selected based on scientific significance and undergoes rigorous peer-review. "That in turn may lead to citation impact and media coverage, but Nature editors aren't driven by those considerations, and couldn’t predict them even if they wished to do so."

To find out exactly when these humans died, the scientists used a number of techniques, including radiocarbon dating — a method that uses the properties of carbon to assign dates to organic materials. They estimate that the bodies are between 9,500 and 10,500 years old, which places the massacre in the West Turkana region of Kenya during a time when it was home to a large population of hunter-gatherers.

Mirazon Lahr and her team don't know why these people were murdered, but they posit that the deaths might be the result of a raid by another, competing group that wanted control of their resources. Or the groups of hunter-gatherers may simply have had a very bloody and extremely unfortunate disagreement. Either way, a violent fight between groups of hunter-gatherers 10,000 years ago would be a significant discovery.

"There is a vocal minority of scholars who insist that hunter-gatherers were not prone to warfare or intergroup violence," says Curtis Marean, associate director of the Institute of Human Origins at the University of Arizona who didn't participate in the study. But today's study represents "clear evidence" for conflict between groups of hunter-gatherers, before food production, he says. "It’s a great and important result."

But Annemieke Milks, a prehistoric weaponry researcher at the University College London's Institute of Archaeology, isn't convinced that the scenario put forth by Mirazon Lahr’s team is correct. There may not be enough evidence from the site to conclude that the skeletons belonged to hunter-gatherers, or that they all died at the same time, she says. That's because the combination of projectile injuries to the men, along with blunt force trauma and possible evidence that certain members had their limbs bound suggests a complex series of events — events that could have happened at different times. And because the events at Nataruk occurred during a window of time when the balance between mobile and more settled ways of life were starting to shift, "we have to treat the evidence from the site with caution," she says. "The authors of this paper say the victims at Nataruk were probably mobile foragers, but little can be inferred about the way of life of the group from the archaeological context."

Finally, Milks also told The Verge that the cemetery in North Sudan is "an earlier site" — although she concedes that those humans may have adopted "some kind" of early version of a sedentary lifestyle.

The importance of today's finding may be in dispute, but its rarity isn't. Well-preserved skeletons like those at Nataruk don't come about every day. Because people at the time had no permanent houses and cemeteries, "the chances of finding a group of skeletons of people who died at the same time preserved are extremely small," Mirazon Lahr says. That might explain why massacres such as the one at Nataruk haven't been found before, "leading people to think that warfare was a recent development in human society."

Given the many, many reasons why humans kill each other today, it’s not all that surprising that the same kinds of disputes might have led to the deaths of 12 people 10,000 years ago. But until scientists agree on the context behind these deaths, it'll be hard to know what place Nataruk should occupy in human history.

Video by Miriam Nielsen and Kimberly Mas.