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Monkey rock bashing resembles tools made by early human ancestors

Hefting a potato-size rock, wild long-tailed macaques (Macaca fascicularis) in Thailand smash oil palm nuts on stone anvils. As they pound away, sharp flakes sometimes fly off from their hammer stones—flakes that are “almost indistinguishable” from stone tools made by early human relatives more than 3 million years ago, according to a controversial new study. Indeed, the researchers argue, the monkeys’ flakes are so similar to our ancestors’ tools that many archaeologists would classify them as early stone tools without a second thought.

The study, published today in Science Advances, adds to another recent finding that Brazilian white-faced capuchin monkeys also produce stone flakes. Together, they “show that human manipulative and cognitive skills are not necessary to produce stone tools,” says Ignacio de la Torre, an archaeologist at the Spanish National Research Council who was not involved with the work.

Other archaeologists aren’t convinced, however. “Sure, some flakes at [ancient] archaeological sites may come from monkeys bashing rocks together and accidentally making flakes,” says Jason Lewis, a paleoanthropologist at Stony Brook University. “But that’s a hypothesis that needs to be tested.”

The oldest described stone tools—consisting of flaked stones and anvils—date to 3.3 million years ago. They were discovered in Lomekwi, Kenya, in 2011. Scientists do not know which early human relative made the flakes or how they used these sharp-edged blades, but they predate the emergence of our genus, Homo, which arose 3 million to 2.5 million years ago. Much more is known about Oldowan stone tools, first discovered in Olduvai Gorge in Tanzania in the 1930s. These range in age from almost 3 million to 1.5 million years, and are found at sites across the African continent, as well as in Europe and Asia.

Other primates also make and use stone tools. Brazilian capuchin monkeys have used rocks to crack open seeds and nuts for at least 3000 years, and chimpanzees in Ivory Coast have done so for more than 4000 years. In 2016, scientists showed Burmese long-tailed macaques on Thailand’s Piak Nam Yai Island have been using stones to pry open oysters for at least 65 years, spanning two generations or more.

If an archaeologist found the flakes analyzed in the current study in 3-million-year African sediments, “I’d say [they] were definitely made by humans,” says Tomos Proffitt, an archaeologist at the Max Planck Institute for Evolutionary Anthropology and the study’s lead author.

Lydia Luncz, a primatologist also at the Max Planck Institute and a co-author of the new study, first recognized the macaques’ stone-flaking behavior in 2016. In a palm forest perched on top of a limestone cliff, she stumbled over “what looked like a nut-cracking site,” she said, similar to ones she’d seen left behind by Ivory Coast chimpanzees.

Luncz set up camera traps and recorded the macaques breaking open palm nuts (see video, above) with hammer stones. Only after nearly cutting her hand on a flake did she realize the site would be “a big deal for archaeologists.” She showed the flakes to Proffitt, who studies stone tools, and he pressed her: “Are you sure a monkey did this?”

In 2017 and 2021, the researchers collected 1119 pieces of stone debris from 40 nut-cracking sites on Yao Noi Island in Lobi Bay. Next, they compared the macaquemade flakes with stone material dating from 3.3 million to 2 million years ago from archaeological sites in Africa.

The scientists analyzed the size, shape, and other attributes of each macaque flake and compared them with Oldowan flakes. They found the macaque’s flakes were smaller and thicker than their Oldowan counterparts, yet they “fall within the range of variation” of early humanmade flakes, they write. And that points to a problem for archaeologists, Luncz says. “How do we know when we’ve found the first intentionally produced stone tools?”

Other researchers strongly disagree with the team’s analysis. Sonia Harmand, an archaeologist also at Stony Brook, says the new study merely shows “random, accidental detachment of fragments without any specific organization or control.” Furthermore, she argues, “to prove flaking, you don’t look at the flakes primarily; you look at the cores,” meaning the hammer stones. Only by analyzing the cores, which bear the marks of deliberate strikes, “can you read the intention, the deliberate organization of the removals [of flakes].”

The authors of the new study agree that there is an important difference between the macaques’ flakes and those that early humans left behind: The monkeys aren’t flaking the rocks intentionally. Instead, they’re accidental byproducts of nut smashing. Indeed, the animals drop any hammer stone that flakes. “For them, it is broken,” Luncz says. “They look for a new one,” which is easy to find among nearby cobbles. (In contrast, early hominins 2.5 million years ago at Olduvai Gorge trekked for kilometers to collect stones that would be good for knapping.) And the monkeys have no need for the sharp flakes, because they have sharp canines.

Nevertheless, the study serves as a caution to archaeologists, says Rick Potts, a paleoanthropologist and stone tool expert at the Smithsonian Institution. “As tiny and unintentional as these flakes may be, they are similar to those from early archaeological sites. That means we have to find a way to factor [them] out at Oldowan sites.” Most importantly, the study shows these macaques “have the ability to hit stones together that fracture,” he adds. “That’s the missing link that led to our ability to make tools.”

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