27 people massacred in 'earliest evidence of warfare' dating back 10,000 years
Showing no mercy, the killers clubbed and stabbed to death men, women and children alike, leaving their bodies strewn along the shore of a lagoon.
At least some of the victims, including a woman in the last stages of pregnancy, had their hands bound as their heads, knees and limbs were smashed and torsos pierced.
It sounds like a horrific scene from a present day or medieval atrocity in the Middle East or Europe. But this massacre, in which 27 individuals died, took place some 10,000 years ago, when humans lived as nomadic hunter-gatherers.
Experts believe it is the earliest evidence that war was being waged by prehistoric people thousands of years before the bloody business of large-scale planned violence to settle disputes was thought to have originated.
The scene of the slaughter was Nataruk, a remote site 19 miles west of Lake Turkana in Kenya which at the time it occurred 9,500 to 10,500 years ago supported a large population of hunter-gatherers.
Scientists found the partial remains of 27 people, including at least eight women and six children. Ten skeletons showed clear signs of violent death, including crushed skulls and faces, broken hands, knees and ribs, arrow wounds, and stone projectile tips lodged in skulls and chests.
Some of the unburied bodies had fallen into the lagoon, which has long-since dried, their bones preserved in sediment.
The evidence, described in the journal Nature, suggests the victims may have been members of an extended family who were attacked and killed by a rival group.
Lead researcher Dr Marta Mirazon Lahr, from Cambridge University's Leverhulme Centre for the Study of Human Evolution, said: "The deaths at Nataruk are testimony to the antiquity of inter-group violence and war.
"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."
The site was first unearthed in 2012, but it has taken years for scientists to date the remains accurately using radiocarbon and other techniques.
Among the signs of injury discovered were five cases of extreme force to the head that may have been inflicted by a wooden club.
Two of three projectile points found in two bodies - one embedded in the victim's skull - were made from obsidian, a black volcanic rock that can be worked to a razor sharpness.
This was evidence the rival groups came from different locations. Obsidian is rare in other late Stone Age sites from the same region.
Most horrific of all, the remains of a six-to-nine month old foetus were recovered from the abdominal cavity of a woman victim. She was discovered in an unusual sitting position, suggesting that her hands and feet had been bound, and her knees were broken.
The origins of war are controversial, and most experts believe it developed with the emergence of agriculture and settled communities around 6,000 years ago.
Co-author Professor Robert Foley, also from the Leverhulme Centre, said: "I've no doubt it is in our biology to be aggressive and lethal, just as it is to be deeply caring and loving.
"A lot of what we understand about human evolutionary biology suggests these are two sides of the same coin."