Human skulls found in Africa with arrows embedded in them and skeletons with horrific trauma are likely to be the earliest evidence of humans at war.
A scientific expedition, led by the University of Cambridge including an Australian researcher, has uncovered the 10,000-year-old skeletal remains in Kenya.
The group of 27 hunter-gatherers were found at Nataruk near Lake Turkana.
Of the 27, 12 skeletons were relatively complete and 10 revealed clear signs of violent death, including extreme blunt-force trauma to the head and broken bones.
Arrowheads were lodged in the skull and thorax of two men.
The arrowheads are made from a naturally occurring volcanic glass not used by people in the region - suggesting the group was attacked by an external invader.
One of the research team, Australian Professor Rainer Grun from Griffith University, said the discovery broadened our knowledge of human behaviour.
"The findings are one of the earliest indications of humankind's propensity for group violence," he said.
"It raises questions about whether the capacity for organised violence is elemental to our nature or a product of circumstances and opportunity."
The expedition findings, published in Nature, say the fossils show warfare has a much deeper history among humans than previously thought.