On the rocky slopes of Brazil's Serra da Capivara National Park, a group of small monkeys called capuchins are pounding rocks together. One grabs a sizable stone, grips it in both hands, and pounds it down like a hammer on another stone. Then the monkey bends down and delicately licks the pulverized quartz left behind. After a few minutes, this hammering shatters the stone, producing a few shards, called "flakes," that look almost exactly like the sharp-edged stone knives and scrapers created by humanity's ancestors millions of years ago.

University of São Paolo zoology researcher Tiago Falótico saw the whole thing and got it on video. This monkey behavior had never been observed before, so he passed it to his colleague Tomos Proffitt at the University of Oxford's Primate Archaeology Group. In a paper for Nature, they describe the astonishing discovery that monkeys are capable of making the equivalent of hominin Stone Age tools, albeit unintentionally.

Previous researchers had seen the capuchins pounding stones but had assumed it was a form of aggression, meant to scare off enemies. But the videos gathered by Proffitt and his colleagues show something very different. "It wasn't an aggressive behavior and seemed instead to be aimed at pulverizing the quartz into dust so that it could be ingested and rubbed onto the skin," Proffitt told Ars via e-mail.

The researchers remain uncertain about why the monkeys lick the stones they've hammered. They may be eating lichen they've dislodged, or maybe they're seeking minerals in the form of powdered silicon from the rock. Regardless of why they do it, what's certain is that the monkeys' inadvertent production of sharp-edged stones, called conchoidally fractured flakes, could change our understanding of how tool use evolved among humans.

Evolutionary biologists have suggested that our ancestors began creating specialized flake tools over 3 million years ago because they underwent some kind of cognitive change. This change would have affected their intelligence as well as their hand coordination, ultimately leading to a diverse toolkit. It's even possible that these early humans created their first flake tools the same way these capuchins did, by smashing rocks together and accidentally making smaller pieces that were ideal for cutting and scraping. Either way, it seemed that this style of tool-making was unique to humans and our ancestors.

But we now know that monkeys can make the same kinds of stone tools early hominins did, despite diverging from us on the evolutionary tree long before we started smacking rocks together in the early Paleolithic. "It raises interesting questions about what minimum level of cognitive complexity as well as how complex a hand needs to be to produce a high number of sharp edged conchoidally fractured flakes," Proffitt told Ars. "We now have evidence of a different species being able to produce flaked stones which share characteristics that we have, up until now, thought to have been unique to hominins."

He's not suggesting that monkeys are about to go Planet of the Apes on us. Instead, this discovery means that we may have to revise our understanding of why human ancestors developed stone tools. Obviously, we are not alone in our ability to shatter stones and create sharp flakes. So something else was at play that set our species on their current path, and we'll need new hypotheses to explain it.

What's certain is that this discovery does not call into question the long history of hominins using flake tools. We have ample evidence going back millions of years that our ancestors created flake tools and used them for butchery, based on telltale marks we've found on bones at ancient campsites. But it might explain 40,000-20,000 year-old stone flakes discovered in the Americas, millennia before we have evidence of human occupation there. It's possible that capuchins were smashing and licking rocks in Brazil long before humans arrived, leaving debris behind that looks confusingly like Stone-Age human tools.

Watching the capuchins using rocks as hammers, however, does "lead to interesting questions about what stone tool technology might have looked like prior to the appearance of this technology in the archaeological record," Proffitt said. Many species may have been producing sharp flakes. But for some reason, only hominins decided to start using them as tools.

Nature, 2016. DOI: 10.1038/nature20112