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Monkeys Making Stone Flakes Cast Doubt on Early Human Theories

Scientists who found piles of sharp-edged stone flakes have for decades used them as evidence that early humans had been at work, making and using them as tools.

But a team working in Brazil has made, well, monkeys out of these theories.

Image: Wild-bearded capuchin monkey in Serra da Capivara National Park, Brazil
Wild-bearded capuchin monkey in Serra da Capivara National Park, Brazil, unintentionally creating fractured flakes and cores. Michael Haslam / Primate Archaeology Group

They've watched and videotaped wild capuchin monkeys smashing rocks and leaving their own piles of stone flakes.

"The production of archaeologically visible cores and flakes is therefore no longer unique to the human lineage," Tomos Proffitt of the School of Archaeology at the University of Oxford and colleagues write in their report, published in Nature.

They aren't sure why the monkeys are doing it, but they lick the rocks afterwards, so they may be feeding on lichens inside the stones.

"The capuchins of Serra da Capivara National Park (SCNP) in Brazil use stone tools in more varied activities than any other known nonhuman primate, including for pounding foods, digging and in sexual displays," the team writes.

"Bearded capuchins and some Japanese macaques are known to pound stones directly against each other, but the SCNP capuchins are the only wild primates that do so for the purpose of damaging those stones."

The monkeys were never seen actually using the sharp flakes they made. Early humans were believed to have used them as knives and scrapers. So as far as Proffitt's team can tell, the monkeys are not displaying human-like, tool-using behavior.

However, the finding does suggest that researchers might be a bit cautious in deciding that the presence of these flakes means that pre-humans were at work at a site.

In 2015, researchers working in Kenya said they'd found 3.3 million-year-old stone flakes and said they were the earliest known human-made tools.

"This does not mean that the earliest archaeological material in East Africa was not made by hominins," Proffitt said ina s tatement.

"It does, however, raise interesting questions about the possible ways this stone tool technology developed before the earliest examples in the archaeological record appeared. It also tells us what this stone tool technology might look like."