Stone Flakes Made by Monkeys Raise Questions About Early Human Tools

The flakes accidentally produced by long-tailed macaques resemble those thought to have been made by early humans

A long-tailed macaque bangs on a source of food with a rock
A long-tailed macaque uses a stone to get at food. The striking of one stone on another accidentally creates stone flakes the monkeys don't use. Lydia V. Luncz

When monkeys use two rocks to smash open a nut, the collision can unintentionally cut a flake off one of the stones. These flakes resemble artifacts thought to be tools made by early humans, researchers reported Friday in the journal Science Advances.

The findings suggest some ancient stone flakes, which scientists think early humans made to use as tools, may have been created by accident. The paper does not mean that every previous discovery of a Stone Age tool is void, researchers say, but it adds something to consider when examining such artifacts.

“I wouldn’t go so far as to say that all of the old material is not intentional,” Tomos Proffitt, a co-author of the new study and an archaeologist at the Max Planck Institute for Evolutionary Anthropology in Germany, tells New Scientist’s Christa Lesté-Lasserre. “But what our study shows is that we can’t be 100 percent certain that every single flake in the early Stone Age archaeological record was intentionally made. There may be a component within that record that’s unintentional.”

Scientists think that early humans began purposefully making these flakes as early as 3.3 million years ago to use as cutting tools. They’ve used signs such as groups of flakes clustered together, as well as particular qualities of the flakes themselves, to suggest they were made intentionally, per the paper. Based on these tools, scientists have made inferences about early humans’ fine motor skills, for example. So, determining whether these stone flakes were made on purpose can change archaeologists’ perception of the past.

“[The study] has ramifications that range from, like, when did the first ever stone tools get made by early humans all the way to, like, when did people begin to move into South America,” Jessica Thompson, a paleoanthropologist at Yale University who wasn’t involved in the research, tells NPR’s Nell Greenfieldboyce.

Some scientists, though, are calling for more research to back up the findings. “Sure, some flakes at [ancient] archaeological sites may come from monkeys bashing rocks together and accidentally making flakes,” Jason Lewis, a paleoanthropologist at Stony Brook University who did not contribute to the research, tells Science’s Virginia Morell. “But that’s a hypothesis that needs to be tested.”

In the new study, the researchers watched long-tailed macaques in Phang Nga Bay in Thailand. When using stones to crack open palm oil nuts, the monkeys produced stone flakes that they just left on the ground. Once they accidentally broke a stone, the monkeys discarded it and found a new one, per New Scientist.

Then, the researchers compared the rock fragments to flakes from archaeological sites in places including Tanzania, Kenya and Ethiopia. These flakes were thought to be made intentionally by early humans between 1.5 and 3.3 million years ago. The researchers found similarities between the two types of flakes, and they say the macaques’ flakes could be misidentified as tools of early humans.

“If we would take the kind of assemblage that we find with the macaques and we would drop them somewhere in East Africa, everybody would think they were definitely made by early hominins,” Lydia Luncz, a study co-author and a primatologist and archaeologist at the Max Planck Institute for Evolutionary Anthropology, tells Scientific American’s Zach Zorich.

Scientists have previously observed other primates, including capuchins and chimpanzees, using stone tools to crack nuts, extract shellfish and dig, according to the paper. But what leaves an archaeological record is when monkeys use rocks as hammers and anvils—that is, a rock that does the smashing and a rock that holds the item getting smashed. Wild capuchins in Brazil have been observed banging two stones together and creating flakes, which has led some researchers to question evidence suggesting humans were in South America 50,000 years ago.

But Sonia Harmand, an archaeologist at Stony Brook University who wasn’t involved in the research, tells Science that “to prove flaking, you don’t look at the flakes primarily; you look at the cores,” or the stones used as hammers, which she says can help determine whether the flakes were produced intentionally. Harmand disagrees with the new study’s findings, because it only shows “random, accidental detachment of fragments without any specific organization or control,” she tells the publication.

Perhaps the bottom line is the importance of interpretation when it comes to archaeological finds. “I think this study is useful because it really brings home the point that people need to do careful behavioral interpretation of their artifacts,” Thomas Plummer, a paleoanthropologist at Queens College, City University of New York, who was not involved in the research, tells Scientific American.

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