Prehistoric chimps used rocks to crack nuts, archeologists working in the Ivory Coast have found.
They say their discovery challenges the notion that early humans were the first to use stone tools.
"It is not clear whether we hominins invented this kind of stone technology, or whether both humans and the great apes inherited from a common forebear," says the University of Calgary's Julio Mercader, one of the few archeologists in the world to specialize in chimps and other great apes.
He is part of an international team that published its findings in this week's edition of the Proceedings of the National Academy of Sciences.
Modern chimps have been observed using the same rough stone hammers to get at nuts, and Dr. Mercader and his colleagues wondered if they had learned the behaviour from humans, or developed it on their own.
So University of Calgary researcher and his collaborators from the U.S., the U.K, Germany and Saskatchewan conducted an archeological dig on an ancient chimpanzee site in the Taï rain forest in Ivory Coast, in west Africa.
They found 4,300 years old rocks the size of cantaloupes, with distinctive patterns of wear and traces of old nut starch on them. Proof, says Dr. Mercader, that ancients chimps were using technology. They were similar to the kinds of nut cracking rocks the modern chimps use.
Humans had not settled in that part of Africa 4,300 years ago, the scientists say. When they did start farming in the area, they ate different kinds of nuts then the ones found on the stone tools that were found by the team.
This means, Dr. Mercader says, that chimps had their own stone age, he says, one that started at least 4,300 years ago, and possibly much earlier.
The chimps that lived in the Ivory Coast back then may have discovered how to bash nuts on their own, or learned how to do it from older chimps - a skill that was passed on from generation to generation.
The first animal to have used a rock as a nut cracker may have been an ancestor to both chimps and humans, says Dr. Mercader.
There is also a possibility that humans and chimps developed the ability independently.
He estimates that chimps and humans diverged roughly seven million years ago, although recent research suggests the final split may have come between 5.4 and 6.3 million years ago.
Archeologists and anthropologists know humans were using sophisticated stone tools at least 2.6 million years ago, but don't have a clear picture of how tool use evolved before then.