Stone flakes made by monkeys cast doubt on ancient human 'tools'

By using stones to break open nuts, monkeys accidentally create sharp-edged flakes that look like the tools believed to have been used by our ancient human relatives.

The finding casts doubt on whether all the stone flakes found in archaeological digs really are the tools of early hominins — and raises the possibility that they might be accidental by-products of hitting things with whole stones, says Lydia Luncz at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

In 2016, Luncz and her colleagues realised that Brazilian capuchins produce stone flakes from the rocks they use to pound food, dig and engage in sexual displays, without necessarily meaning to. The flakes were essentially identical to those found in hominin settlements dating to at least 3 million years ago. It made the team wonder whether the artefacts really reflected any technical planning by those 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 across nut-cracking sites – a surprise, as long-tailed macaques weren’t previously known to break open nuts.

The team set up motion-activated cameras to study the behaviour of the wild macaques. During 100 hours of footage, the team witnessed monkeys accidentally creating flakes as they struck nuts between two stones – serving as a hammerstone and an anvil – and then leaving the broken stones to find new, whole stones.

This is almost exactly what the capuchins did in the earlier study, says Luncz, showing that the flake-making wasn’t a one-off. “This was occurring on the other side of the planet, in a different ecosystem and a different species,” she says. “So it was just so obvious that this is a primate thing. This is a foraging behaviour that we assume also happened in early hominins.”

So far, capuchins, chimpanzees and long-tailed macaques are the only non-human primates known to use stone tools in the wild – and all of these are now proven to incidentally produce flakes that look like ancient hominin tools, she says.

The team then compared 1119 stone flakes from the macaques’ nut-cracking sites with artefacts found at hominin sites in Ethiopia, Kenya and Tanzania. The monkeys’ thin, flat, wide stone flakes – ranging from 1.3 to 7.9 centimetres in length – were “almost indistinguishable” from flakes that were 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 monkeys’ flakes were, on average, smaller and thicker than the hominin flakes, for example – they were nonetheless so similar that they could have replaced up to 70 per cent of the ancient humans’ tools.

The findings could challenge the current understanding of early stone technology, says Proffitt. “I wouldn’t go so far as to say that all of the old material is not intentional,” he says. “But what our study shows is that we can’t be 100 per cent certain that every single flake in the early Stone Age archaeological record was intentionally made. There may be a component within that record that’s unintentional.”

For Zeray Amelseged at the University of Chicago, the study mostly illustrates the gradual progression of cognitive evolution in primates. “Is what we find in the archaeological record just a result of process without intentionality?” he says. “I don’t think we have an answer, but an important point in this paper is that the actions of stone tool-making and stone tool use have a much deeper history in time as well as in the primate world. And that’s what’s becoming clearer.”