THE STUDY “4,300-Year-Old Chimpanzee Sites and the Origins of Percussive Stone Technology” by Julio Mercader et al., published in the February 27, 2007, issue of Proceedings of the National Academy of Sciences.
THE MOTIVE Jane Goodall publicized tool use among chimps in the 1960s, but the first written record of it comes much earlier, from a 17th-century Jesuit priest in Sierra Leone who described how a chimp with palm nuts “and with a stone in its hand breaks the nuts and eats them.” How much further back does this habit stretch, and how did chimps acquire the skill? Did they learn tool use from humans, invent it themselves, or did both humans and chimps inherit the trick from a common ancestor who lived more than 5 million years ago? A team of archaeologists led by Julio Mercader of the University of Calgary is seeking answers in what appears to be a collection of stone hammers from a chimp-occupied rain forest site in Ivory Coast, West Africa. Dated at 4,300 years old, the rocks—which bear nut starch grains and the hallmarks of chimp hammer use—suggest that the chimpanzee practice of using tools to crack nuts stretches back millennia, if not further.
THE METHODS Back in 2001, Mercader and primatologist Christophe Boesch of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, collected and analyzed a more recent stash of hammer stones from a chimp-foraging spot in Ivory Coast’s Taï forest. Boesch and his wife, Hedwige, had spent two decades there watching the chimps bashing granite rocks on wooden anvils to crack hard nuts produced by a tree called Panda oleosa. The sharp-edged rocks, the team discovered, were pitted from repeated pounding on the nuts.
Mercader and Boesch decided to track the chimp nut-cracking habit back in time by digging into the sandy, muddy rain forest soil nearby for evidence of older tools. At one site, named Noulo, they turned up 206 tool-like pieces of stone, mostly quartz and granite, whose varying ages—from 230 to 4,300 years old—they estimated by carbon-dating burned charcoal buried in the soil alongside. But were these rocks really used as tools?
To find out, Mercader took them back to his lab in Calgary and cleaned the dirt off the rocks by submerging them in water and beaming them with ultrasonic waves. He then asked two experts in prehistoric stone technology, Jack Harris of Rutgers University and Steven Kuhn of the University of Arizona in Tucson, to assess whether the stones from the Taï forest had been formed by random geologic forces—battered by river currents, say—or had been deliberately shaped by thrusting or flaking. The observers agreed that 35 stone specimens were created by hammering; 28 showed evidence of flaking—an activity typical of early humans—in which slivers are chipped off a stone core for use as sharp blades.
Chimps have never been known to flake rocks intentionally to fashion tools, but Mercader believes that way back during a “chimpanzee Stone Age,” they did crack nuts with the stones found close to the Noulo site. The cavity-pocked, cantaloupe-size rocks look similar to the modern stones Mercader found earlier in the Taï forest. Weighing between 8 and 15 pounds, the tools were too cumbersome for hominids, whose early hammer stones usually weigh less than a pound.