Capuchins have been observed in the wild smashing rocks together and creating razor-sharp flakes of rock just like those thought to have been used as knives by early humans.
Among the earliest forms of tools used by the ancestors of modern humans, a stone split to form a razor-sharp cutting edge has been used by palaeontologists investigating our evolution for decades.
But monkeys, it appears, have thrown “a bit of a spanner in the works”, according to academics at Oxford University and elsewhere.
For they have been observed in the wild in Brazil pounding rocks together and, apparently accidentally, creating almost identical stone blades.
The researchers who made the discovery said this disproved the idea that such activity was limited to humans and their ancestors, raising “interesting questions” about scientists’ understanding of our past.
The earliest known stone tools were found at the Lomekwi site in West Turkana, Kenya, and have been dated to 3.3 million years ago.
However one of the lead researchers behind that discovery told The Independent they were confident the tools they found were not the work of monkeys, partly because of the presence of cut marks on bones found at the site.
In the new study, described in a paper in the journal Nature, the scientists said they had observed wild-bearded capuchin monkeys in Serra da Capivara National Park in Brazil smashing quartzite cobble stones onto other embedded in a cliff face.
It is thought they do this to extract minerals such as silicon or remove lichen from them and have no interest in the sharp flakes of rock produced as a byproduct.
They reported that no monkey attempted to use the flakes to cut or scrap something.
Lead author Dr Tomos Proffitt, an Oxford University archaeologist, said: “This does not mean that the earliest archaeological material in East Africa was not made by hominins.
“It does, however, raise interesting questions about the possible ways this stone tool technology developed before the earliest examples in the archaeological record appeared.
“It also tells us what this stone tool technology might look like. There are important questions too about the uniqueness of early hominin behaviour.”
His co-author, Dr Michael Haslam, leader of the Primate Archaeology at Oxford, said understanding technology used by early forms of humans “helps shape our view of human evolution”.
“The emergence of sharp-edged stone tools that were fashioned and hammered to create a cutting tool was a big part of that story,” he said.
“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.”
But he added that even though making the flakes might not be “unique” to humans “the manner in which they used them is still very different to what the monkeys seem capable of”.
Professor Sonia Harmand, lead author of a paper in Nature last year about the discovery of 3.3 million-year-old tools, said she was “very excited” by the “fantastic” study.
But she stressed it did not affect their findings.
“The first important thing to say is that these monkeys are not interested at all by the flakes that are accidentally produced – this is a very, very important thing to say,” said Professor Harmand, of Stony Brook University in the US.
“We have bones associated with the tools, we have cut marks on some bones and we have micro-wear analysis showing they were used. So there is nothing to worry about.”
However she said it was certainly something for scientists to bear in mind if they chance upon a sharp stone.
“If you see things that look like broken stones and tools,” Professor Harmand said, “you have to ask yourself if there were monkeys around.”