Researchers have observed capuchin monkeys in Brazil making stone flakes similar to those made by early hominins for cutting and scraping.

The findings could complicate the interpretation of artifacts recovered from Stone Age archaeological sites. Until now, archaeologists assumed a large collection of stones with conchoidal fractures and sharp cutting edges were a sure sign of early human activity. That's no longer the case.

"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," Tomos Proffitt, an archaeologist at the University of Oxford, said in a news release. "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."

Monkeys don't appear to create the flakes intentionally for use as tools. Instead, their production is a byproduct of "percussive behavior," the slamming of stones together to get at minerals and bits of lichen. Unfortunately, intentionality isn't easy to identify several hundred thousand years after the fact.

Researchers say the latest findings -- detailed in the journal Nature -- don't discredit the earliest hominin artifacts recovered in Africa. But they do raise new questions about the early evolution of tool-making.

"These findings challenge previous ideas about the minimum level of cognitive and morphological complexity required to produce numerous conchoidal flakes," Proffitt said.

While research may need to reconsider the attributes considered unique to the earliest humans, there remain other ways to differentiate between humans and monkeys.

"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," said archaeologist Michael Haslam.