Monkeys are making stone tools thought to be unique to humans

Stone flake production might not be all it is cracked up to be.

Courtesy of Michael Haslam/Primate Archaeology Group
A wild-bearded capuchin monkey bangs rocks in Serra da Capivara National Park, Brazil, unintentionally creating fractured flakes and cores.

Stone flakes excite archaeologists hunting for early human artifacts. The razor-sharp edges that suggest deliberate creation point to someone with skills. Generally, that someone is human.

But stone flake production might not be so special, according to a paper published Wednesday in the journal Nature. Capuchin monkeys can make stone flakes too.

"It's an incredibly interesting behavior," says study lead author Tomos Proffitt, an archaeologist at the University of Oxford. "Another species is making sharp, conchoidally fractured flakes, an artifact that we only ever thought is unique to hominins."

But here's the catch: In the monkeys, the flake production seems to be the unintentional result of another behavior.

The researchers observed the capuchins of Serra da Capivara National Park in Brazil using one stone, a hammer stone, to hit quartz cobbles stuck in a large rock conglomerate wall. As the monkeys hit the quartz repeatedly, flakes broke off the hammer stone. 

The monkeys seem to pay the flakes no mind. Instead the researchers see them focusing on the quartz dust produced by smashing the rocks, which the capuchins lick – perhaps to ingest the minerals in the rocks.

"In terms of a technique of making flakes, this is a very similar technique to what has been hypothesized was the technique to make the very earliest archaeological flakes," Dr. Proffitt tells The Christian Science Monitor. In that technique, called passive hammer knapping, the tool-maker hits a stationary stone with a hammer stone, breaking flakes off the stationary stone and keeping the hammer stone intact.

Despite the similarities, Proffitt says that this doesn't really change the human archaeological record, as even the most ancient of those stones flakes were unearthed in association with hominins.

Some of the monkeys' individual stone flakes could be slipped into an early hominin assemblage of stone tools, and nobody would be the wiser, Dietrich Stout, a professor of paleolithic archaeology at Emory University who was not part of the study, tells the Monitor. But the larger context provides the key differences, he says.

When stone knappers intentionally produce stone flakes, they work the stone to produce as many flakes as possible, Dr. Stout explains. That results in many more flakes for each core (the remaining rock). So among the 2.6 million-year-old artifacts associated with early humans, he says, archaeologists find many more flakes to each core than the capuchins create.

The superficial similarities between the capuchin stone flakes and the archaeological ones is likely due to the way rocks fracture, says John Shea, a paleoanthropologist and professor of paleolithic archaeology at Stony Brook University who was not part of the study. So simply by hitting rocks together, these flakes are bound to be produced.

"They're producing objects that are visually similar to the most distinctive component of human stone tool technology," Dr. Shea tells the Monitor, adding that the similarities are outweighed by the fundamental difference: "Humans use the flakes to cut things, capuchins don't."

But probing why the monkeys are performing this behavior could yield the most interesting insights, says Stout. If researchers can sort out the conditions under which primates will break stones and what that behavior will look like, they'll gain more clues to how tool-use and tool-making behavior emerged.

Until researchers can figure out why the capuchin monkeys are banging rocks together, the impact of this study is simply that "this behavior that we don't understand can produce things that look like this other behavior, making tools for cutting meat and that sort of thing," Stout says.

For archaeologists, he says, this study serves as a caution to not "jump the gun if you find five flakes and one core. You may not be able to know what it was for."

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