by Picking up a potato-sized rock, wild long-tailed macaques (Macaca fascicularis) smash oil palm nuts on stone anvils in Thailand. As they continue to hammer, sharp flakes sometimes fly from their hammerstones – flakes that are “almost indistinguishable” from stone tools made by early human relatives more than 3 million years agoaccording to a controversial new study. In fact, the researchers argue, the ape flakes are so similar to our ancestral tools that many archaeologists would readily classify them as early stone tools.
The study published today in scientific advancescomplements 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 make stone tools,” says Ignacio de la Torre, an archaeologist at the Spanish National Research Council who was not involved in the work.
However, other archaeologists are not convinced. “Sure, a few flakes with you [ancient] Archaeological sites could have originated from monkeys smashing rocks together and accidentally flaking them,” 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 flakes and anvils – date back to 3.3 million years ago. They were discovered in Lomekwi, Kenya in 2011. Scientists don’t know which early human relative made the flakes or how they used those sharp-edged blades, but they date back to the origin of our species. homo, which formed between 3 million and 2.5 million years ago. Much more is known about Oldowan stone tools, first discovered in the 1930s in the Olduvai Gorge in Tanzania. These range in age from almost 3 million to 1.5 million years and can be 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 been doing so for more than 4000 years. In 2016, scientists showed that Burmese long-tailed macaques on the Thai island of Piak Nam Yai have been using rocks to crack 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-old African sediments, “I would say [they] were definitely man-made,” says Tomos Proffitt, archaeologist at the Max Planck Institute for Evolutionary Anthropology and first author of the study.
Lydia Luncz, a primatologist also at the Max Planck Institute and co-author of the new study, first recognized the rockfall behavior of macaques in 2016. In a palm forest perched on a limestone cliff, she stumbled upon “what looked like a nut-cracking spot,” she said, similar to those left behind by chimpanzees in Ivory Coast.
Luncz set up camera traps and filmed the macaques breaking open palm kernels (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 was studying stone tools, and he urged her, “Are you sure a monkey did that?”
In 2017 and 2021, researchers collected 1,119 pieces of rock debris from 40 nutcrackers on Yao Noi Island in Lobi Bay. Next, they compared the flakes produced by macaques to stone material found at archaeological sites in Africa between 3.3 million and 2 million years ago.
The scientists analyzed the size, shape and other characteristics of each macaque flake and compared it to Oldowan flakes. They found that macaque flakes were smaller and thicker than their Oldowan counterparts, but they “fall within the range of variation” of early human-made flakes, they write. And that points to a problem for archaeologists, says Luncz. “How do we know when we found the first intentionally made stone tools?”
Other researchers disagree with the team’s analysis. Sonia Harmand, an archaeologist also at Stony Brook, says the new study shows only “accidental, accidental detachment of fragments without specific organization or control.” Also, she argues, “You don’t look primarily at the flakes to prove flaking; you look at the cores,” says Hammersteine. Only by analyzing the drill cores, which bear the marks of targeted blows, “can one read the intention, the conscious organization of the move [of flakes].”
The authors of the new study agree that there is one important difference between the macaques’ flakes and those left by early humans: The monkeys don’t intentionally flake off the rocks. Instead, they’re random by-products of smashing nuts. In fact, the animals will drop any hammerstone that flakes off. “It’s broken for them,” says Luncz. “They’re looking for a new one” that’s easy to find between the nearby cobblestones. (By contrast, early hominins wandered for miles in the Olduvai Gorge 2.5 million years ago to collect rocks that were good for kneading.) And the apes don’t need the sharp flakes because they have sharp canines.
Still, the study serves as a warning to archaeologists, says Rick Potts, a paleoanthropologist and stone tool expert at the Smithsonian Institution. “As tiny and unintentional as these flakes are, they are similar to those found at earlier archaeological sites. That means we have to find a way to factor [them] outside at Oldowan sites.” Most importantly, the study shows these macaques have “the ability to smash together rocks that break,” he adds. “It’s the missing link that led to our ability to make tools.”
