In the video above, there is something fascinating happening: a group of Brazilian capuchin monkeys are busily hammering rocks together, often creating sharp-edged stone flakes that spin off in all directions. The presence of those rock flakes has caused a stir in scientific circles. The reason is that over the decades, scientists have often found similar rock chips and attributed their existence to early human tool use. Rocks, when they are split a certain way, make handy cutting tools, ideal for scraping meat off of a dead animal or breaking a bone.

Indeed, scientists believe that tool use goes back some 3.3 million years, and that before early humans, they were created and used by a hominid ancestor like Australopithecus. It wasn’t thought that monkeys could make these stone chips as well. But paleoanthropologist Tomos Proffitt from Oxford University and his team released a paper in Nature that called this part of human history into question.

It is important to note that Proffitt did not witness the monkeys using the rock chips as tools; the monkeys are simply banging rocks together and the flakes are the result. But the study has cast doubt on whether some of the stone flakes found over the years might actually have been the work of less evolved and less intelligent monkeys.