In addition to using rocks for various purposes, such as cracking open a seed or fruit to extract the edible part, the Bearded or Black-Striped Capuchin (Sapajus libidinosus), a species of wild monkey found in Serra da Capivara National Park in Piaui State, Brazil, makes sharp stone flakes by strongly and repeatedly hammering one rock against another embedded in an outcrop with a clear intent to smash it. Monkeys observed performing this activity then lick and sniff the quartz dust resulting from the fragmentation of the rock.

This behavior by S. libidinosus frequently produces sharp-edged conchoidal flakes with smooth rounded facets resembling the shape of a scallop shell. The flakes resulting from multiple percussions are left where they fall and are not used as tools by the monkeys.

Researchers at the University of Sao Paulo's Psychology Institute (IP-USP) in Brazil, in collaboration with colleagues at the University of Oxford's Archeology School and University College London's Archeology Institute in the UK, analyzed the flakes inadvertently produced by S. libidinosus and found many that were similar to the lithic tools carved from rocks by hominins (human ancestors) during the Paleolithic era.

The research was conducted as part of the Thematic Project "Tool use by wild bearded capuchin monkeys (Sapajus libidinosus)", for which Eduardo Ottoni is principal investigator. The findings were published in the online version of the journal Nature.

"We discovered that some of the stone flakes produced accidentally by these capuchin monkeys were very similar to Oldowan stone tools," said Tiago Falotico, a postdoctoral fellow of IP-USP and one of the authors of the study.

Named for Olduvai Gorge in Tanzania, Africa, where they were first found in the mid-twentieth century, Oldowan tools are among the oldest stone tools ever found, dating as far back as 2.6 m years ago and usually associated with Homo habilis, one of the earliest species of the genus Homo, which includes modern humans.

Falotico and Ottoni have long studied the behavior of capuchin monkeys, specifically the use of tools by S. libidinosus in Serra da Capivara National Park.

They have frequently seen monkeys sniffing and licking the quartz dust left after they pounded the rocks, but cannot be sure why.

"Our observations suggest they may be seeking a dietary source of the trace element silicon or trying to remove lichen from the rocks," Falotico said. He began collecting flakes and cores (rocks reduced by removal of flakes) during his PhD research.

The flakes, together with other material obtained by Falotico and Ottoni over a period of several years from digs conducted for research into primate archeology, were analyzed by Tomos Proffitt, a researcher at the University of Oxford's Archeology School and an expert on Oldowan stone tools, and Michael Haslam, a researcher at University College London's Archeology Institute, both of whom are collaborating on the project.

The results of their analysis showed that the stone flakes unintentionally produced by the capuchin monkeys have identical characteristics and morphology to intentionally produced hominin tools.

Around half of the flakes exhibited conchoidal fracture, which is typically associated with the hominin production of flakes. They had varying degrees of percussive damage across their surfaces, including small impact points surrounded by circular or crescent scars, as well as sharp edges.

"Paleoanthropologists use the characteristics of sharp-edged stone flakes both to distinguish them from naturally broken stones and to attribute them to the hominins that produced flakes with these shapes intentionally in order to use them as tools," Falotico said.

The researchers point out in the Nature article that because these flakes are indistinguishable from many made intentionally by early hominins, the findings open up the possibility that flaked stones could be identified in the paleontological record of extinct apes and monkeys.

"When sharp-edged flakes are found at an archeological site, it's usually easier to attribute them to hominins because the same digs generally also turn up many stone cores and signs of human occupation, such as fires," Falotico said. "However, when they're found in isolation with few or no other archeological artifacts, we should bear in mind that they might not necessarily have been produced by human ancestors."

According to the researchers, other observations have been made of primates hammering rocks and producing fragments similar to flaked tools.

In 2007, for example, a group of researchers from Canada, Germany, the UK and the US found that the West African Chimpanzee (Pan troglodytes verus) used stone tools to crack nuts 4,300 years ago, in the Later Stone Age. The evidence came from the world's only known prehistoric chimpanzee settlement, located in Cote d'Ivoire's Tai National Park.

The fragments these chimpanzees produced by means of stone-on-stone percussion did not have the same characteristics as those made intentionally by hominins, however.

"For example, a chimp would hammer a coconut with a stone and accidentally smash it against another stone placed underneath as an anvil," Ottoni said.

He added that other primates such as the Crab-eating Macaque (Macaca fascicularis) and the Japanese Macaque (Macaca fuscata) are also known to use stone-on-stone percussion, but only capuchins have been observed deploying percussive behavior deliberately.

"The capuchins we studied in Serra da Capivara National Park also use wooden tools, and they use stone tools in more varied activities than any other known non-human primate," Ottoni said.

"Besides using percussive stone tools as hammers to crack open cashew nuts and coconuts, for example, or to smash rocks, as described in the article, they also use twigs to probe nooks and crannies for insects or caterpillars, which they eat."