In 2014, Michael Haslam wedged between two boulders in northeast Brazil and filmed some monkeys. Oblivious to the voyeur, the monkeys—bearded capuchins—began smashing stones together. They lifted small cobbles into the air and brought these down upon a rock face, like a hammer upon an anvil. In the process, the hammer stones would often shatter.

After the monkeys had gone, Haslam picked up some of these broken fragments—and was amazed. Many had sharp edges, and looked remarkably like human tools.

For millions of years, humans and our ancestors have been using one stone to hammer flakes away from another, leaving behind smoothly curving surfaces with sharp edges or tips. Haslam knew such tools well. He had studied them in Australia, Honduras and India, and even lectures on their evolution at the University of Oxford. “And here were monkey by-products, which I’d seen being made, that had all the hallmarks of basic flaked tools,” he says.

Let’s be clear: The monkeys are not intentionally making knives. They never use the sharpened stones to slice or cut. “They have thousands of sharp stones lying around, and the most we’ve seen a monkey do with them is put one in its mouth,” says Haslam.

Still, their inadvertent creations suggest that our own stone technology may have had similar accidental beginnings. “We need to look again at the criteria used for identifying simple stone flakes as being solely the product of the human lineage, or of human planning, or of human need for cutting edges,” says Haslam. “We’ve now doubled our known number of stone-flaking primate lineages, from one to two.”

Chimps, long-tailed macaques, and other species of primates will use stones to hammer their way into nuts and shellfish. But the bearded capuchins of Brazil’s Serra da Capivara National Park are exceptionally skilled wielders of stone. For over a decade, primatologists Tiago Falotico and Eduardo Ottoni at the University of Sao Paolo have observed these particular capuchins using stones to smash pebbles, enlarge holes in trees, and crack seeds and fruits.

The capuchins are sensitive to the physics of their tools, choosing heavy stones to use as hammers and even heavier ones for anvils. They are dextrous, adjusting the force of each blow to the condition of the nuts they are hitting. They are picky, keeping favorite tools in piles at specific sites. And they’ve been at this for a while: Recently, Haslam’s team showed that these monkeys have been using stones to crack open cashew nuts for at least 700 years.

They also smash stones against other stones. Their motives for doing so are still unclear. They lick and sniff the stones between strikes, so they might be breaking them to get at some hidden nutrient—silicon perhaps, or something in the lichens growing on the rocks.

Regardless, the rocks themselves are eventually discarded. Haslam’s team recovered dozens of these, and their colleague Tomos Proffitt, who has worked on ancient hominid tools from East Africa, classified and described the specimens. These unintentionally flaked stones were, to his eye, indistinguishable from some of the deliberately flaked ones that he had studied before.

In the future, Haslam hopes to show both the capuchin and hominin tools to other experts to see if they can tell the difference. But for now, he says, “If we found these stones with no monkeys or other stone-using animals around, the simplest explanation would be that humans made them.”

This does not spell trouble for archaeologists: capuchins, being New World monkeys, did not make any of the well-studied stone tools from the Olduvai or other parts of Africa. Nor did any other African monkey. As Hélène Roche from Paris-Nanterre University writes in a related commentary,Our knowledge about stone knapping in early-human archaeological sites has a solid foundation, [representing] decades of continuous research in the African continent.” They’ve typically been found with hominid fossils, as well as debris that are absent from the capuchin sites, like cut bones, evidence of fire, and more.

Comparing hominid tools to the capuchin stones is “comparing apples and oranges,” says Sileshi Senaw, who has studied the 2.6-million-year-old stone tools of Gona, Ethiopia. Unlike the monkeys, our ancestors intentionally broke stones to create edges for cutting meat; we know that because fossil bones found at the same sites still bear the marks of these rocky blades.

“So, at first glance, the capuchin flakes don’t require us to change our thinking at all,” says Haslam. “But, what our findings do show is that there are previously unimagined ways for primates to make sharp-edged stones.”

We see tools with sharp edges and we think that they were crafted to cut and slice, because that’s what we’d use them for. Assuming deliberation, we’ve also linked the appearance of such tools to the increase in our ancestors’ mental abilities. But Haslam’s study shows that they be totally unintentional and accidental offshoot of their attempts to smash rocks together, by an animal whose brain could fit inside two shot glasses. “Maybe the earliest hominins were also making such tools unintentionally, as a byproduct of an activity unrelated to cutting anything,” he says.

Sonia Harmand from Stony Brook University agrees. Last year, she, Roche, and others described the oldest stone tools ever discovered, from deposits in Lomekwi, Kenya. With an age of 3.3 million years, such tools predate our genus Homo, and were probably made by some smaller-brained hominin. “Such flakes could have been produced by accident during rock-pounding in the past, but only hominins realized their usefulness and went on to make them deliberately,” says Harmand. And only we improved the tools, making them better and more sophisticated with time.

“We can ask why,” says Harmand. “Why did our ancestors learn to make such sophisticated stone tools, and so long ago, while chimps and monkeys never got beyond a stage? Why hominins alone have taken tool use to such an extreme?”

And why haven’t the capuchins learned to use the sharp tools that they inadvertently make? Perhaps it’s because their teeth are already great at cutting up their prey, obviating the need for any technological help. Maybe it’s that the stone-smashing sites are far away from their foraging grounds. “One of the most important characteristics of a useful tool is that you have it when you need it, as anyone who has left a phone charger at home will confirm,” says Haslam. “If we start seeing the monkeys carrying flakes away from the cliffs, in anticipation of future use, that’s when I think things might get interesting.”

He also thinks that archaeologists should spend more time looking for the works of ancient monkeys and apes. “[We should] consider what other non-human-primates in Africa and elsewhere may have been up to for the past tens of millions of years,” he says. “There is no reason why stone flakes may not be littered throughout primate history, at unknown places and times.”