On the banks of an ancient lagoon in Kenya, researchers have found evidence of a massacre that occurred 10,000 years ago.
The skeletal remains of 12 adult victims discovered in Nataruk, near Lake Turkana, tell a grim story of merciless violence by one group of hunter-gatherers against another -- the oldest known example of its kind in human history.
Ten of the skeletons show clear signs of violent trauma, including club and arrow wounds to the head, ribs, knees, hands and neck, according to a paper published this week in the journal Nature.
One skeleton was found with what appears to be an obsidian arrow still lodged in its skull; another had an arrow in its chest.
Two of the skeletons -- those of an older man and a woman who was at least six months pregnant -- showed no evidence of lethal injury. But researchers say it is possible they suffered violent deaths as well.
Fatal arrow wounds to the abdomen, for example, don't necessarily leave skeletal lesions.
Marta Mirazon Lahr, a human evolutionary biologist at the University of Cambridge who helped uncover the grisly scene, said it suggests warlike behavior goes back further in human history than was previously thought.
"Most scholars have considered that warfare emerged as a result of ownership of land, farming and more complex political systems," she said. "Our findings show that this hypothesis is incorrect, and that intergroup conflict had a much longer history."
Evidence of warfare is well preserved among settled, sedentary communities -- either among themselves, or between them and hunter-gatherers they may have encountered. But until now, there was no archaeological record of armed conflict between early nomadic hunter-gatherer groups, Lahr said.
There have been signs of human violence that predate the Nataruk find, however. For example, human remains discovered at the Qadan graveyard at Jebel Sahaba, Sudan, go back 12,000 to 14,000 years ago and show clear signs of violence. But the fact that the victims were buried suggests they were part of a more sedentary society.
There have also been isolated examples of violent trauma not far from Nataruk, but whether that was a result of intergroup fighting is unclear.
"That is what is unique and important about this paper," said Kim Hill, an anthropologist at Arizona State University who was not involved in the study. "There is little doubt that this represents intergroup warfare."
The prehistoric carnage reported in the paper occurred about a mile from the shores of Lake Turkana, which at the time provided a fertile environment for many plants and animals. The fossil record suggests the area was brimming with life, including elephants, hippos, rhinos, zebras, warthogs, gazelles and millions of fish -- as well as lions, hyenas and wild dogs.
"The edge of the lake must have been an amazing place to live -- but also dangerous," Lahr said. "We have also found several fragments of human fossils at other sites with evidence of having been eaten by carnivores."
The researchers believe the victims of the massacre were a small traveling band of hunter-gatherers who stopped by a lagoon to hunt or fish.
The position of the skeletons suggests that the victims' hands were bound at the time of their death. Since their killers wielded weapons that are not associated with hunting and fishing, the researchers believe the attack was planned.
It is impossible to know how big the group of assailants might have been, but Lahr said it was probably larger than that of the victims.
"The most important thing in determining whether it is worth attacking or not is simply the numbers," she said.
There is no evidence that graves were dug for the victims, and the haphazard position of their bodies suggests they were not moved after their deaths.
Despite the amount of forensic evidence uncovered at the site, it already seems clear that the discovery will not put to rest the raging debate among anthropologists over whether hunter-gatherers engaged in warfare.
Douglas Fry, an anthropologist at the University of Alabama at Birmingham, suggested the abundant resources near Lake Turkana made it possible for the hunter-gatherers who lived there to enjoy some aspects of a sedentary existence, including food storage and more social complexity. These, he argues, lead to warlike tendencies.
"My suspicion is that the finds described by Dr. Lahr and her colleagues reflect the increased intergroup conflicts that become possible -- even likely -- during a pre-agricultural revolution that ushered in a host of social changes," said Fry, who wasn't involved in the Nature study.
Hill saw the paper as offering further evidence that warfare among hunter-gatherer societies probably occurred quite frequently as a way to gain resources.
"In short, theory and common sense suggests that people will kill when they can get away with it and when there is something to gain," he said. "Hence, intergroup violence has probably been part of Hominin natural history for a long time.
"Nataruk is extraordinary for having preserved what was probably not an unusual event in the lives of hunter-gatherers at the time," she said. "But we should also not forget that humans, uniquely in the animal world, are also capable of extraordinary acts of altruism, compassion and caring. Clearly both are part of our nature."