Monkeys have been observed producing sharp stone flakes that closely resemble the earliest known tools made by our ancient relatives, proving that this ability is not uniquely human.

Previously, modifying stones to create razor-edged fragments was thought to be an activity confined to hominins, the family including early humans and their more primitive cousins. The latest observations re-write this view, showing that monkeys unintentionally produce almost identical artefacts simply by smashing stones together.

The findings put archaeologists on alert that they can no longer assume that stone flakes they discover are linked to the deliberate crafting of tools by early humans as their brains became more sophisticated.

Tomos Proffitt, an archaeologist at the University of Oxford and the study’s lead author, said: “At a very fundamental level - if you’re looking at a very simple flake - if you had a capuchin flake and a human flake they would be the same. It raises really important questions about what level of cognitive complexity is required to produce a sophisticated cutting tool.”

Unlike early humans, the flakes produced by the capuchins were the unintentional byproduct of hammering stones - an activity that the monkeys pursued decisively, but the purpose of which was not clear. Originally scientists thought the behaviour was a flamboyant display of aggression in response to an intruder, but after more extensive observations the monkeys appeared to be seeking out the quartz dust produced by smashing the rocks, possibly because it has a nutritional benefit.

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“We’re not altogether certain of why they’re doing it,” said Proffitt.

He and colleagues made a series of observations of the wild-bearded capuchin monkeys who live in Brazil’s Serra da Capivara National Park, as they visited a rocky outcrop and selected rounded quartz hammer stones. The monkeys typically pounded their chosen stone on embedded quartz cobbles and then licked the quartz dust that this produced.

They made no attempts to use the sharp fragments and showed no interest in them, according to the report published on Wednesday in the journal Nature.



Michael Haslam, a co-author also from the University of Oxford, said the emergence of sharp-edged tools has been viewed as a turning point in human history. “The fact that we have discovered monkeys can produce the same result does throw a bit of a spanner in the works in our thinking on evolutionary behaviour and how we attribute such artefacts,” he said. “While humans are not unique in making this technology, the manner in which they used them is still very different to what the monkeys seem capable of.”

The earliest examples of flaked rocks, reported last year from a site in Kenya, date back to 3.3m years ago, long before the evolution of anatomically modern humans. However, scientists said the latest findings do not cast doubt on this discovery or other significant sites, because in each case the sites were either accompanied by fossil evidence of hominins, cut-marked bones or signs that the tools had been further remodelled after being splintered off the rock.

“I don’t think this discovery is going to throw anything into doubt,” said Matt Pope, an archaeologist at University College London, who was not involved in the work.

But the findings could be relevant to future discoveries of even earlier, unrefined stone flake collections. “This is putting us on notice that if we find knapped or struck stone tools in the archeological record, they’re not necessarily cutting tools,” he said.