When it comes to the origins of warfare, there are two main schools of thought. One school believes the propensity for violence is embedded deep in human nature; the other believes that it arose in response to the need to protect property, a symptom of humans’ move from nomadic hunting and gathering towards a more settled lifestyle. Those who support the latter idea argue that the Paleolithic era was generally a peaceful time, with little evidence of organized violence between hunter-gatherer groups.

But new research complicates that second argument: In a study published today in the journal Nature, a team of anthropologists describe a prehistoric mass grave whose inhabitants died a violent death, evidence that small-scale warfare was alive and well even among hunter-gatherer communities.  

The site, discovered in Nataruk, Kenya, in 2012, housed the remains of 27 people, including 12 intact skeletons. Based on carbon dating of the bones and surrounding sediment, the researchers estimate that the deaths took place somewhere between 9,500 and 10,500 years ago.

Ten of the bodies had signs of injuries that were likely lethal: Five showed evidence of blunt-force head trauma, and five or six (one was ambiguous) had markings consistent with arrow wounds around the head and neck. Several of the skeletons also had fractured hands, knees, and ribs, and some were found in positions suggesting that their arms and legs had been bound.

The researchers also discovered a few arrows at the site made of obsidian, a volcanic rock that wasn’t native to the area, indicating that whoever had attacked had traveled far from home to do so.

The Nataruk of today isn’t a particularly exciting place in terms of natural resources, but 10 millennia ago, it would have been prime real estate. Lake Turkana, currently around 18 miles away, once extended past its present shoreline—meaning the gravesite, at the time of its inhabitants’ deaths, was in a fertile, marshy area, one that was likely a popular spot for hunter-gatherer groups. Pottery found around the area indicates that some groups may have set up storage systems for food and other items, suggesting some degree of settlement.

The massacre may have resulted from an attempt to seize resources that had been stockpiled, said Marta Mizaron Lahn, an anthropologist at Cambridge University’s Leverhulme Center for Human Evolutionary Studies and one of the paper’s lead authors, in a statement.

On the other hand, she added, “Nataruk may simply be evidence of a standard antagonistic response to an encounter between two social groups at that time.”

In other words: The bodies prove that prehistoric groups warred with one another, but the reason why is as ambiguous as ever.