By using rocks to crack open nuts, monkeys inadvertently create sharp-edged flakes that look like the tools thought to have been used by our ancient human relatives.
The find casts doubt on whether all the stone flakes found in archaeological digs are really early hominin tools – and raises the possibility that they are accidental by-products of hitting things with whole stones, says Lydia Luncz of the Max Planck Institute Institute for Evolutionary Anthropology in Leipzig, Germany.
In 2016, Luncz and her colleagues found that Brazilian capuchins produce stone flakes from the rocks, which they use to crush food, dig, and engage in sexual displays without necessarily wanting to. The flakes were essentially identical to those found in hominin settlements at least 3 million years ago. The team wondered if the artifacts really reflected engineering by these early humans.
Since then, Luncz and her colleagues have been studying tool use in long-tailed macaques (Macaca fascicularis) on the islands of Phang Nga Bay in Thailand. In the forests there, Luncz stumbled upon nutcracker sites – a surprise since long-tailed macaques were previously not known for cracking open nuts.
The team set up motion-activated cameras to study the wild macaques’ behavior. During 100 hours of footage, the team watched monkeys accidentally create flakes when they smashed nuts between two rocks – which served as a hammer stone and anvil – and then left the broken rocks to find new, whole rocks.
That’s almost exactly what the capuchins did in the earlier study, says Luncz, showing that the flake-making was not an isolated incident. “It happened on the other side of the planet, in a different ecosystem and in a different species,” she says. “So it was just so obvious that this is a primate thing. This is a foraging behavior that we believe also occurred in early hominins.”
So far, capuchins, chimpanzees and long-tailed macaques are the only nonhuman primates known to use stone tools in the wild — and all of them have now been shown to produce flakes that look like ancient hominin tools, she says.
The team then compared 1,119 stone flakes from macaque nutcracker sites to artifacts found at hominin sites in Ethiopia, Kenya and Tanzania. The apes’ thin, flat, broad stone flakes — ranging in length from 1.3 to 7.9 centimeters — were “almost indistinguishable” from flakes associated with ancient humans up to 3.3 million years ago, says Tomos Proffitt, another member of the research team at the Max Planck Institute for Evolutionary Anthropology.
While there were a few different trends—the ape flakes, for example, were on average smaller and thicker than the hominin flakes—they were still so similar that they could have replaced up to 70 percent of the tools used by primitive humans.
The results could challenge current understanding of early stone technology, says Proffitt. “I wouldn’t go so far as to say that all the old material is unintentional,” he says. “But what our study shows is that we cannot be 100 percent certain that every single flake in the early Stone Age archaeological record was intentionally made. There may be a component in this dataset that is unintended.”
For Zeray Amelseged of the University of Chicago, the study primarily illustrates the gradual progression of cognitive evolution in primates. “Is what we find in the archaeological record just the result of a process without intentionality?” he says. “I don’t think we have an answer to that, but an important point to make in this article is that the acts of stone tool making and stone tool use have a much deeper history both in time and in the primate world. And that is becoming increasingly clear.”