Fossilised remains of men, women, and children showing clear signs of being violently massacred have been discovered in Kenya, suggesting that humans have been engaging in warfare for at least 10,000 years. The discovery is believed to be the earliest scientifically dated sign of human conflict.
That's important, because scientists still aren't sure how war and organised violence - which, sadly, are now very much part of our lives - began in our species. Were they triggered by something deep in our evolutionary history, or were they a symptom of the sense of ownership that came with agriculture?
The new evidence comes in the form of 27 partial skeletons of a hunter-gatherer tribe, which appear to have all been brutally killed and left unburied.
A team from the University of Cambridge in the UK found evidence that the individuals - including women and children - had been subjected to injuries such as extreme blunt-force trauma, broken bones, and arrow lesions to the neck. Stone projectile tips were lodged into the skull and thorax of two men.
Four skeletons were also found in positions that indicated that their hands had been bound, including a woman in the late stages of pregnancy.
"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," said lead researcher Marta Mirazón Lahr.
The remains were discovered at a site called Nataruk, which is 30 km west of Lake Turkana. Thanks to radiocarbon dating, the time of death has been put somewhere between 9,500 and 10,500 years ago.
At that time, Nataruk, which his now scrubland, would have been a fertile, wooded region on the shores of a lagoon, and the tribe's enviable location may well have been the cause of the conflict. There was also evidence of property at the site, which suggests the Nataruk people were storing food.
"The Nataruk massacre may have resulted from an attempt to seize resources - territory, women, children, food stored in pots - whose value was similar to those of later food-producing agricultural societies, among whom violent attacks on settlements became part of life," said Mirazón Lahr.
"This would extend the history of the same underlying socio-economic conditions that characterise other instances of early warfare: a more settled, materially richer way of life."
But it's also hard to rule out that Nataruk wasn't simply what the researchers call "a standard antagonistic response to an encounter between two social groups at the time".
In other words, how do we know the two groups didn't simply come across each other and try to take one or the other's resources - something that isn't classified as warfare, and was common between hunter-gatherers?
What makes this scenario unlikely is that in those antagonistic attacks, usually only the men were killed, with women and children being taken by the winning group. What makes this discovery unique is that few individuals, if any, appear to have been spared.
Of the 27 individuals analysed, 21 were adults, including eight males, eight females, and five remains that couldn't be verified. The bones of six children were found near to the remains of four of the adult women.
The researchers haven't been able to find much evidence regarding who killed the Nataruk people, but they did discover two arrow or spear tips made of obsidian - a black volcanic rock that wasn't used by other late Stone Age tribes in the region. "Which may suggest that the two groups confronted at Nataruk had different home ranges," said Mirazón Lahr.
Unfortunately, we'll probably never know exactly what happened at the Nataruk massacre, or why these individuals were killed. But it's not the first time that signs of warfare have been found in our early evolutionary history - research in 2014 found that chimpanzees also engage in coordinated attacks.
"I've no doubt it is in our biology to be aggressive and lethal, just as it is to be deeply caring and loving," said Robert Foley, co-author of the paper published in Nature. "A lot of what we understand about human evolutionary biology suggests these are two sides of the same coin."