Tools of the chimps reveal our primate cousins' Stone-Age skills

Chimpanzees have a cultural history that dates back thousands of years to the Stone Age, when their ancestors were using simple stone tools to crack open edible nuts.

Scientists working in west Africa have found stones used by chimpanzees as hammers more than 4,300 years ago in a discovery that further undermines the idea that tool use is unique to human culture.

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Wild chimpanzees - man's closest living relative - have been documented as using various tools throughout Africa but this is the first time evidence has emerged of ancient culture among chimps. The simple stone hammers were found in the Tai rainforest of the Ivory Coast and they contain traces of food that chimps, rather than humans, 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 the generations as culturally acquired behaviour, according to 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. The first is that they developed it by imitating forest-dwelling humans, the second is that they developed it independently of humans and the third is that they inherited the practice from the same common ancestor that chimps share with humans.

"It's not clear whether we hominins [ancient and modern humans] invented this kind of stone technology, or whether both humans and the great apes inherited it from a common forebear," Dr Mercader said.

"There weren't any farmers living in this region 4,300 years ago, so it is unlikely that chimpanzees picked it up by imitating villagers, as some scientists used to claim."

The stone hammers discovered at the archaeological site in Ivory Coast display the classic signs of "percussive technology" used to strike the hard shells of edible nuts.

Furthermore, the signs of wear on the stones could not be the result of either 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 and 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.

The findings suggest that tool use among the modern chimps still living in the area has been culturally transmitted down more than 200 generations of chimps.