Mercader and co-investigators from Germany, UK, the U.S. and Canada report on the findings in the latest edition of PNAS, the Proceedings of the National Academy of Sciences. PNAS is in the top echelon of academic journals internationally.

"It's not clear whether we hominins invented this kind of stone technology, or whether both humans and the great apes inherited it from a common forebear," says Mercader, also a Canada Research Chair in Tropical Archaeology. "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, like some scientists used to claim."

The stone hammers that the team discovered, essentially irregularly shaped rocks about the size of cantaloupes -- with distinctive patterns of wear -- were used to crack the shells of nuts. The research demonstrates conclusively that the artifacts couldn't have been the result of natural erosion or used by humans. The stones are too large for humans to use easily and they also have the starch residue from several nuts known to be staples in the chimpanzee diet, but not the human diet.

Using so-called "percussive technology" to free the edible parts of nuts is more complicated than it sounds. "We know that modern chimpanzee behaviour regarding nut-cracking is socially transmitted and takes up to seven years to learn," Mercader says. "Some of the nuts require a compression force of more than a thousand kilograms to crack. And the idea is to crack the shell but not smash it -- it's not a simple technique."

The discovery suggests that a 'chimpanzee stone age' reaches well back to ancient times. "Chimpanzee material culture has a long prehistory whose deep roots are only beginning to be uncovered," the authors write.

Although it's difficult to prove whether the technology was adopted through imitation, another possibility is convergence -- that is, both humans and great apes arrived at the technique independently.

"We used to think that culture and, above anything else, technology was the exclusive domain of humans, but this is not the case," Mercader says. Previous research that Mercader published in the journal Science in May 2002 has paved the way for the new sub-discipline of chimpanzee archaeology, which combines archaeology, paleo-anthropology and primatology.

Other researchers are excited about the work. "Mercader's paper presents strong archaeological data for the antiquity of nut cracking by chimpanzees and shows that this behaviour developed long before farmers arrived in the area," says Dr. Michael Chazan, a University of Toronto anthropology professor who specializes in Paleolithic archaeology.

"Of course, this article, like most great discoveries, opens as many questions as it answers. Why and how did this group of chimpanzees maintain nut-cracking behaviour while other chimpanzee groups living in locations with the same nuts available did not? It might be that the archaeology of chimpanzees will produce more surprises in the future."

Adds Dr. Alison Brooks, a research associate at the Smithsonian Institution and professor of anthropology at George Washington University: "The first non-human archaeological site of considerable antiquity raises interesting ideas about our common heritage with chimpanzees, as well as about the interpretation of early human sites, especially those in rainforests. The study of starches on the tools is particularly compelling evidence for association with chimps rather than humans. The authors should be congratulated for their research."

The new paper is titled, "4,300-year-old chimpanzee sites and the origins of percussive stone technology." The other authors include Huw Barton (University of Leicester), Jason Gillespie (University of Alberta), Jack Harris (Rutgers University), Steven Kuhn (University of Arizona), Robert Tyler (University of Saskatchewan) and Christophe Boesch (Max Planck Institute for Evolutionary Anthropology).

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