The creation of primitive stone tools has long been hailed as a landmark in the human odyssey -- but should our species' much-trumpeted smartness take all the credit?

Scientists observing wild-bearded capuchin monkeys in Brazil discovered that when the animals hammered away at stones, they unintentionally created rough flakes similar to those first used as tools by human forerunners.

Unlike hominins -- early humans and their close primate relatives -- the monkeys do not use the shards for tools.

But the discovery, published on Wednesday in the science journal Nature, raises questions about how humans realised stone flakes could be useful.

"These findings challenge previous ideas about the minimum level of cognitive and morphological complexity" needed to produce stone shards, lead author Tomos Proffitt of Oxford University said.

"It raises the possibility that stone flaking may have been invented separately by different species," Proffitt told AFP in an email exchange.

It calls into question whether stone tools emerged as an "intentional step" or simply as the by-product of bashing rocks together, he said.

Co-author Michael Haslam, also of Oxford University, said the findings could have an impact on the story 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.

- Spanner in works -

"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."

Haslam emphasised, though: "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."

An explanatory video showing the capuchin behaviour can be seen https://www.youtube.com/watch?v=j0jqJUF1nOs.

Proffitt also stressed the findings do not cast doubt upon the uniqueness of stone tools.

"The levels of complexity that we see, even with the earliest hominin stone tool technology, is much more than what is observed with the capuchin material," he said.

In fact researchers have been observing the capuchins and their stone-bashing for years, with a first study published in 2007. 

But "nobody was particularly interested in the material remains" of that behaviour, Proffitt told AFP.

His team used archaeological techniques to investigate the primates' activity, which led them to see a nascent similarity with tool flakes.

"It's not surprising that this material has not been looked at before, because people weren't really looking for it," he said.

Source; AFP