< Stone flakes made by modern monkeys trigger big questions about early humans

ARI SHAPIRO, HOST:

Only humans use stone tools. That's what scientists used to think, anyway. Now it's clear that other animals do this, too. And it turns out when some monkeys crack open nuts by bashing them with stones, they accidentally create something that looks a lot like stone knives made by early humans. As NPR's Nell Greenfieldboyce reports, that raises all kinds of questions.

NELL GREENFIELDBOYCE, BYLINE: The monkeys are long-tailed macaques that live in Thailand. Tomos Proffitt studies them. He says these monkeys typically use a large stone embedded in the ground as a kind of anvil to help them crack open nuts from oil palm trees.

TOMOS PROFFITT: They're a little bit bigger than peanuts, and they can be quite hard. So they put the oil palm nut on the anvil and use a hammerstone, you know, in one or both hands.

(SOUNDBITE OF NUTS CRACKING)

GREENFIELDBOYCE: The monkeys whack the hard nuts over and over again. This is captured on videos. Proffitt is an archaeologist at the Max Planck Institute for Evolutionary Anthropology.

PROFFITT: We noticed that at the sites that they were cracking these nuts, there was a lot of - you know, a lot of the tools, a lot of the hammerstones, the anvils. But as well as that, there was a lot of broken pieces.

GREENFIELDBOYCE: Broken pieces of rock from when the monkeys would miss the nut and just hit the anvil with the hammer. And the thing is these broken pieces of rock look a lot like the stone tools found in archaeological sites associated with early humans who used sharp flakes of rock as knives.

DAVID BRAUN: So it was actually really kind of somewhat disturbing to me to walk into the forest and to just see, you know, sort of hundreds of artifacts sitting on the ground and to know that there are no humans doing this.

GREENFIELDBOYCE: David Braun is with George Washington University. He says the monkeys just ignore the sharp flakes. They don't cut anything. But if archaeologists like him came across this kind of stuff at a human site that dates back over a million years...

BRAUN: We would have diagnosed this as, oh, they're making flakes to cut up things, but they're not.

GREENFIELDBOYCE: To see just how similar and different the monkey artifacts are from human ones, he and his colleagues compared them with collections of stone tools, or assemblages, from ancient human ancestral sites in Tanzania, Kenya and Ethiopia. The results are in the journal Science Advances. Braun says there's a lot of overlap and similarities, but even though he says you could throw a bunch of monkey stones into a human archaeological site and no one would notice...

BRAUN: Are the assemblages we see in the fossil record made by monkeys? Probably not.

GREENFIELDBOYCE: He says there are ways to know if a stone tool was made deliberately, like the presence of animal bones with cut marks or extra modifications to the stones. But he thinks it's important to realize that at least some of what's at these human sites could have been produced accidentally. Jessica Thompson is a paleoanthropologist with Yale University who wasn't on the research team.

JESSICA THOMPSON: A lot of these kind of very deep conceptions about what it is to be human is wrapped up in intentional stone tool making, not just stone tool use but making, making something that's new.

GREENFIELDBOYCE: She says this study could add to the debate over the nature of one archaeological site in Kenya. It dates back to 3.3 million years ago, and it has what look like very primitive stone tools that would be the oldest ever found. Nell Greenfieldboyce, NPR News.

(SOUNDBITE OF SAVANNAH BLEU AND THE AUDIBLES SONG, "NOT THE SAME")

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