The production of sharp stone tools for cutting has long been thought to be one of the features that separated early humans from the rest of the animal world — until now. It turns out wild bearded capuchin monkeys in Brazil produce sharp-edged stone flakes similar to those made by early Stone Age human ancestors. There’s one big difference: the human ancestors made their tools on purpose.

These capuchin monkeys smashed rocks into each other, producing the sharp stone flakes. However, the monkeys didn’t appear to use the flakes as tools, according to a study published today in Nature. The findings create new avenues for exploring how this early form of technology developed, and how intelligent hominins had to be in order to produce sharp flakes for scraping and cutting meat.

Today’s high-tech all began with simple stone tools carved out of rock, but there’s still a lot we don’t know. Which groups, for example, were the earliest toolmakers? The oldest known stone tool, dated to 3.3 million years ago, was discovered last year in Kenya. At that time, no specimens from the group that includes modern humans — the genus Homo — were around; Homos only go back to 2.8 million years ago. So early human ancestors must have had the cognitive skills to purposely hit stones together to create sharp tools.

Today’s study shows that modern primates are able to create similar tools, however unintentionally. "It’s breaking down the boundaries of how unique are early humans," says lead author Tomos Proffitt, a postdoctoral research assistant in primate archaeology at Oxford University. "It opens up interesting possibilities about how stone tool technology might have evolved or might have emerged, and what these tools might look like."

The capuchin monkeys in the study were observed smashing rocks in the Serra da Capivara National Park in Brazil. Capuchin monkeys are known for using stones to crack nuts, dig spiders and roots from underground, and for sexual displays (females throw rocks at males to get their attention). When the monkeys smashed rounded stones against a cliff face, they produced large quantities of sharp-edged flakes. Upon examining 111 of these flakes, the researchers found that about half of them had features associated with tools made by early humans. "You would think that they are indeed made by hominins," Proffitt says. "That’s what’s impressive about the material."

The monkeys, however, didn’t use the flakes, and it’s even unclear why exactly the animals smash the stones against one another. The monkeys were often seen licking the stones after the blows, which could suggest that they’re ingesting the nutritional components in the powdered rocks, Proffitt says.

Only West African chimpanzees were found to unintentionally make stone flakes before, while they were cracking nuts, but those fragments don’t resemble hominids’ stone tools as much, the study authors say. The capuchin monkey’s "artefacts," instead, look legit. "It’s exciting to see that even by chance another species other than our species and our ancestors were able to produce stone tools," says Hélène Roche, director of research at the French National Center for Scientific Research (CNRS), who wasn’t involved in the study. "The difference is in the intention."

While these stone flakes look like early tools, the authors aren’t trying to suggest that tools dated to millions of years ago were mistakenly attributed to early humans. Those tools were found alongside other evidence that unmistakably pointed to hominins, Proffitt says. Instead, the study could inform our understanding of human evolution: how did the tool-making technology that set our ancestors apart develop? If modern primates can make stone tools similar to those made by early humans, how intelligent did hominins have to be to master the technology? And what about the shape of their hands? Palaeoanthropologists are still trying to understand how advanced hominin hands were in order to hold the stones. The only way to answer these questions is by finding more ancient stone tools and fossils that can give us a glimpse into the early days of technology.

"For me, that’s what’s interesting," Proffitt says. "It opens up possibilities."