An ancient mass grave in Kenya reveals a brutal, violent end for a group of humans that lived 10,000 years ago. According to a paper published Wednesday in Nature, this may be our oldest ever evidence of human warfare.

Of a dozen relatively complete skeletons found near Kenya's Lake Turkana (in a group that included at least 27 individuals in total who had not been buried), 10 showed signs of violent, fatal injuries.

"These human remains record the intentional killing of a small band of foragers with no deliberate burial, and provide unique evidence that warfare was part of the repertoire of inter-group relations among some prehistoric hunter-gatherers," lead author Marta Mirazon Lahr of the University of Cambridge said in a statement.

The remains show signs of arrow wounds and blunt force trauma. One female skeleton paints a particularly jarring picture: Her hands, chest and knees all show signs of fracturing from blunt force trauma. The researchers believe her hands were bound when she died. Nearby, another woman – one who was pregnant – died with her hands and feet bound.
Several of the skeletons were found with the obsidian weapons that may have killed them. Because obsidian tools aren't particularly common in archeological sites from this exact time and place, the researchers write that this may be a sign that the killers came from a different neighborhood.

It seems clear that these remains are the result of some kind of massacre – something different from the interpersonal violence that's surely as old as humanity itself. But does that mean humans were engaging in war 10,000 years ago? It depends on your definition of warfare. The remains have pushed back our earliest evidence of group conflict between humans, but is that all it takes to make "war"? Previously, some scientists had argued that it took cities, farms and industry to incite true war. But these new findings suggest that the resources and squabbles of hunter-gatherer society may have caused groups to turn on one another.
"I’ve no doubt it is in our biology to be aggressive and lethal, just as it is to be deeply caring and loving," co-author Robert Foley said in a statement. "A lot of what we understand about human evolutionary biology suggests these are two sides of the same coin."

That's not entirely surprising, given behavior we see elsewhere in the animal kingdom: Lots of animals have one-on-one fights, sometimes even with weapons in hand (or tentacle). But research suggests that chimpanzees, some of our closest relatives, engage in targeted killings to increase their territory and resources. It's not exactly trench warfare, but it's easy to link these chimp behaviors to the events at Lake Turkana. At the time of the massacre, the area would have been a lush lagoon. It would make sense for rival foraging groups to envy the resources accumulated there.

It's also possible that the attack served in part to capture some members of the group, and that the individuals left for dead were those who put up a fight or were too young or old to be of any use. Only six children were found in the mass grave, all of them next to the women, and none of them were teenagers. "Whether [the teenagers] managed to escape, or were taken, we will never know," Miraozn Lahr told Reuters.