The researchers uncovered 206 stone pieces from sites in Ivory Coast that resembled the tools and detritus produced by modern chimp nut-bashing. Dating the surrounding muck via its carbon 14 content revealed that the stones were laid down 4,300 years ago. To make sure that the stones were not formed by natural processes—so-called geofacts—the rocks were mixed in with such naturally formed artifacts and submitted to a blind test by outside experts, which confirmed the damage was not made by nature.
The researchers then determined that the stones must have been chimp tools because of their size: a typical human hammer stone is no longer than 120 millimeters (less than 5 inches) and weighs less than 400 grams (less than one pound). Even the anvils rarely weigh more than 1,000 grams (more than 2 pounds). Yet, these stones averaged roughly 320 millimeters in length (12.6 inches) and 2,030 grams (4.5 pounds) in weight. "They are bigger stones, heavier stones," Mercader says. "You need a bigger hand and a bigger arm to use them." Plus, the researchers tested the stones for food residue, including the five nuts chimps are known to crack and eat as well as 24 varieties of nuts and other starches that humans exploit in the region. "Not only did we find the starch that chimps eat," he adds, "but we also do not find evidence of the ones that humans prefer."
The antiquity of the stones means that chimpanzees have been cracking nuts since long before human farmers reached the region—one explanation for the ability of modern chimps to use hammer stones and anvils to open food. This is no easy feat; modern chimps undergo a seven-year apprenticeship to master the technique. But it turns out this training may be age-old. Mercader, for one, believes that the use of such stone tools may be a technology traceable to a shared ancestor of chimps and humans. "I'd like to see if there is any evidence of stone pieces that could resemble these kinds of technologies at early hominid sites," he says. "But in order to find those, you have to be open to the possibility."