They look innocuous: a set of small cuts, scratches and dents made in some animal bones. But these marks, recovered from Dikika, Ethiopia, have launched a controversial debate about nothing less than when human ancestors started using tools. It’s an argument that shows no signs of abating.

The debate revolves around when and how the marks were made. Shannon McPherron, who led the team that discovered the bones, thinks that they’re the handiwork of stone tools, wielded by prehistoric butchers at least 3.39 million years ago. That predates the evolution of modern humans, meaning that the tool-users probably belonged to one of our ancestral species, Australopithecus afarensis (such as the famous ‘Lucy’). The timing also pushed back the