By Dani Cooper.
Evidence of a massacre about 10,000 years ago has been unearthed in Africa, pushing back the known record of inter-group warfare.
Skeletal remains of 27 individuals, including at least eight women and six children, were found at Nataruk, about 30 kilometres west of Lake Turkana in Kenya.
Twelve of the fossilised remains were in an almost-complete state and revealed the victims had died a violent death, with signs of severe blows to the head and face, broken bones and arrow wounds.
Four individuals were found in a position that suggested their hands had been tied. One of these was a woman in the late stages of pregnancy - foetal bones were found in her abdominal cavity.
The remarkable discovery, led by researchers from Cambridge University's Leverhulme Centre for Human Evolutionary Studies, is outlined in today's Nature journal.
Co-author Professor Rainer Grun, from Australia's Griffith University, who played a key role in dating the remains, said that until now, the fossil evidence for war suggested it coincided with the start of agricultural practices.
"As soon as people stayed in one place, there is lots of evidence of war in all societies," he said, pointing to Syria, Egypt and the Mayan empire as examples.
"The question is whether this [warfare] happened with hunter-gatherers.
"There has been evidence of people killing each other before but it is singular. So certainly this is one of the earliest examples where you see a whole group wiped out."
The remains were found partly or completely exposed in 2012, preserved in sediment of a lagoon that has long since dried.
Radiocarbon and uranium dating techniques used on the skeletons and sediment and shell samples, place the attack as happening between 9500 and 10,500 years ago.
'Unusual' massacre included women and children
The total number of people who died at the site is unknown, because only those that were partly exposed were excavated. However, of the 27 people recorded, 21 were adults — eight males and eight females and five unknown — and all except one of the children were aged under six years.
Professor Grun said it was unusual to find evidence that women and children had been killed.
"Quite often you find men and juvenile males are killed, but here there seems to have been no distinction between males and females — they just wiped out the whole group."
The archaeologists found three artefacts embedded in two of the bodies, identified as the possible remains of arrows or spear tips.
Lead researcher Dr Marta Mirazon Lahr, from Cambridge University, said one of the artefacts was made from obsidian, a volcanic glass, which was rare among Stone Age sites around Lake Turkana.
This suggested the two groups that confronted each other at Nataruk had different home ranges.
"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," she said.
While the reasons for the attack could not be known, Dr Mirazon Lahr said the presence of pottery artefacts nearby suggested the storage of 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 Dr Mirazon Lahr.