Wild capuchin monkeys in Brazil deliberately break stones, unintentionally producing flakes similar to the ancient sharp-edged flakes seen in early hominin tools, reports a paper published online in Nature this week. Although it is unclear why the monkeys engage in this behaviour, the results suggest that the production of sharp-edged stones can no longer be considered unique to hominins.
Palaeoanthropologists use the distinctive characteristics of flaked stone tools - such as sharp, cutting edges - to identify and distinguish tools from naturally broken stones at archaeological sites. Previous comparisons between hominin intentional stone flaking and wild primate stone tool use have focused on West African chimpanzees.
Tomos Proffitt and colleagues observed wild bearded capuchin monkeys (Sapajus libidinosus) in the Serra da Capivara National Park in Brazil. The authors collected fragmented stones immediately after capuchins were observed using them, as well as from surface surveys and archaeological sites in the same area. They find that as monkeys repeatedly pound one stone onto another, deliberately crushing the surface of both, they break the stones, unintentionally producing flakes with the characteristics and shape of deliberately produced hominin tools. Although the monkeys were seen to re-use broken hammer-stone parts as fresh hammers, they were not observed using the sharp edges of fractured tools to cut or scrape other objects.
The authors conclude that, in the absence of supporting evidence, such as cut-marked bones, sharp-edged flake production can no longer be solely associated with the intentional production of cutting flakes by our ancestors.