Monkeys observed breaking flakes off stones like prehistoric humans
Wild monkeys have been observed deliberately breaking flakes off stones in much the same way as prehistoric humans.
The big difference is that our ancestors realised the flakes could be used as scraping and cutting implements.
In contrast, for the capuchin monkeys the flaking appears to be merely the by-product of attempts to extract minerals or lichen from the stones.
But the discovery could shed light on how early humans and their forbears first hit on the secret to making stone tools.
Lead scientist Dr Tomos Proffitt, from Oxford University's School of Archaeology, said: "Within the last decade, studies have shown that the use and intentional production of sharp-edged flakes are not necessarily linked to early humans (the genus Homo) who are our direct relatives, but instead were used and produced by a wider range of hominins.
"However, this study goes one step further in showing that modern primates can produce archaeologically identifiable flakes and cores with features that we thought were unique to hominins.
"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."
The bearded capuchins were studied at Serra da Capivara National Park in Brazil.
Individual monkeys could be seen hammering quartzite cobbles onto other stones embedded in a cliff face, which were crushed and dislodged.
In the process, the hand-held "hammer" stones became unintentionally fractured.
The monkeys also picked up broken hammer stones to use as new "hammers".
The team admitted it was unclear why the animals exhibited this activity, but suggested the capuchins may be trying to extract powdered silicon, an essential trace nutrient.
Alternatively they might be attempting to remove lichen for some unknown medicinal purpose, said the scientists writing in the journal Nature.
At no point was any monkey observed using the flakes as cutting or scraping tools.
Co-author Michael Haslam, from Oxford University, said: "Our understanding of the new technologies adopted by our early ancestors 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.
"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.
"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."