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.