KEY POINTS:
Chimpanzees have a stone-age cultural history that dates back thousands of years to when their ancestors used simple stone tools to crack open edible nuts.
Scientists working in West Africa have found stone hammers used by chimpanzees more than 4300 years ago, undermining the idea that tool use is unique to human culture.
Wild chimpanzees - man's closest living relative - have been documented using various tools throughout Africa but this is the first evidence of ancient culture among chimps.
The simple stone hammers were found in the Tai rainforest of Ivory Coast. A study has shown that rock surfaces contain traces of food that only chimps would have eaten.
Scientists believe the discovery suggests that use of tools probably evolved in the common ancestor of humans and chimps, and has been passed down as culturally acquired behaviour, according to Dr Julio Mercader of the University of Calgary in Canada. "We used to think that culture and, above anything else, technology was the exclusive domain of humans, but this is not the case," he said.
There are three possible explanations for tool use among chimps. They may have developed use of tools by imitating forest-dwelling humans, independently of humans or from the common ancestor.
But Dr Mercader said: "There weren't any farmers living in this region 4300 years ago, so it is unlikely that chimpanzees picked it up by imitating villagers, like some scientists used to claim."
The stone hammers discovered in Ivory Coast display the classic signs of "percussive technology" used to strike the hard shells of edible nuts. Signs of wear on the stones could not be the result of natural erosion or human activity, says a study in the Proceedings of the National Academy of Sciences.
The rocks are too large to be used as human tools. Chemical analysis shows they have traces of the starchy material found only in nuts eaten by chimps rather than the food crops eaten by people.
"This discovery speaks of true prehistoric great ape behaviour that predates the onset of agriculture in this part of Africa," the scientists say in the study.
The findings suggest that tool use among chimps still living in the area has been culturally transmitted down more than 200 generations, the scientists say.
Dr Mercader said modern chimps use rocks to open nuts in a manner that is technically difficult for younger, inexperienced chimps to learn from their elders. "We know that modern chimpanzee behaviour regarding nut-cracking is socially transmitted and takes up to seven years to learn. .
"Some of the nuts require a compression force of more than a 1000kg to crack and the idea is to crack the shell but not smash it - it's not a simple technique."
Professor Alison Brooks of the Smithsonian Institution in Washington DC said that this first archaeological site for non-human tools raised intriguing ideas about our common heritage with chimps. "The study of starches on the tools is particularly compelling evidence for association with chimps rather than humans."
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