Macaques use stones as hammers to smash foods such as shellfish and nuts.
When monkeys in Thailand used stones as hammers and anvils to help them crack nuts, they often accidentally created sharp stone chips that looked like stone cutting tools made by early humans.
In this surprising finding, published in the journal Science Advances, archaeologists wondered if they needed to reconsider their assumptions about some of the stone tools produced by early human ancestors more than a million years ago.
Jessica Thompson, a paleoanthropologist at Yale University, said: "You have a group of non-human primates who are creating something that looks a lot like what we want to attribute exclusively to humans and the behavior of human ancestors. t in the team that conducted this new study.
She noted that the manufacture of sharp knives in stone, dating back 3.3 million years, has long been seen as a key technological innovation in human history, which encompasses a series of assumptions about evolution that unique human characteristics.
But now, Thompson said, archaeologists will have to work on the problem, trying to figure out whether the sharp stone chips were made intentionally or unintentionally.
"Its impact ranges from when early humans made the first stone tools to when people started migrating into South America," she said.
Scientists used to think that making and using tools was entirely a human activity, but they now know that using tools is actually not uncommon in animals.
Still, the use of stone tools by primates is very rare.
A small number of chimpanzees in West Africa are known to use rocks as hammerstones, although they do not leave many flakes, probably because of the type of stone they use.
Brazil's capuchins have been shown to smash seeds and nuts with stones – they have apparently done this for hundreds of years and have left their own archaeological record.
That's why some researchers have recently questioned some of Brazil's earliest evidence about when humans may have entered the continent, saying ancient sites 50,000 years ago may have been created by monkeys rather than humans.
For unknown reasons, capuchins also sometimes deliberately knock rocks together to break them (they also sometimes lick or smell gravel).
Scientists reported in 2016 that this activity produced a large number of flakes with sharp edges that looked like deliberately made stone tools – although those monkeys in Brazil never used the broken flakes as tools.
Some of the researchers involved in the study are now turning their attention to wild long-tailed macaques in Thailand. These monkeys often use stones as anvils and hammers to crack open the kernels of oil palm trees.
"They're a little bit larger than peanuts and they're very tough," says Tomos Proffitt of the Max Planck Institute for Evolutionary Anthropology. "They put the oil palm fruit on an anvil and use hammerstones with one or both hands."
When monkeys repeatedly try to knock nuts, they sometimes miss and instead knock two stones together. This creates rubble blocks that collect around the anvil.
"These tools and these fragments look very similar to some of the things we saw in the early archaeological record," Proffitt said.
David Braun, an archaeologist at George Washington University, said he went into the forest and saw hundreds of artifacts scattered on the ground "and knew that no human was doing that," which actually made him "a little uneasy."
Anvils and hammerstones used by long-tailed macaques to crush nuts
If archaeologists like him had found these tools in an excavation a million years ago, he said, "we'd make a diagnosis like: 'Oh, they're making flakes to cut things.'" But they're not."
No one saw what these monkeys did with these flakes - apparently they had nothing to cut. "Once the flakes land on the floor, it stays there," says Proffitt.
He and his colleagues analyzed more than 1,000 stones associated with monkeys, which they called "the most extensive dataset of non-human primate impact stone chips and stone chips to date."
When they compared these stones to stone tool collections or combinations from ancient human ancestral sites in Tanzania, Kenya, and Ethiopia, they found many similarities and overlaps.
There are ways to distinguish between stone tools made specifically for cutting, such as the presence of animal bones with cut marks, or additional modifications made to make the tool more fancy, or evidence that the stone was imported from elsewhere specifically to make the tool.
In addition, archaeologists can look at the core rock that was hit to produce the flakes to see if there are patterns that indicate that toolmakers understand fracture patterns and are exploiting them.
Still, Braun said, a person could throw "quite a few" of macaque-produced flakes into an excavation of early human artifacts, but no one would notice.
"Were the assemblages we see in the fossil record made by monkeys? Probably not," Braun said.
But he thinks archaeologists must now seriously consider that some or even many of the sharp flakes they see at human sites may have formed unintentionally.
"There's a good chance that some of the records that we think are associated with producing sharp edges may actually be a shock technique," he said.
In particular, Thompson thinks the study could add to the debate about the nature of an archaeological site in Kenya dating back 3.3 million years.
The site has very primitive-looking stone tools, probably the oldest ever discovered. They are so old that they may have been made by species older than the earliest humans in the genus Hominida.
Emma Finestone, a stone tool expert at the Cleveland Museum of Natural History, said it's interesting to keep this new study in mind when considering the first use of stone tools in human history.
"Could it start out because the percussion behavior became more prominent, and then the flakes appeared as a by-product of the tapping?" She said. "Perhaps this is a clue to the original origin of stone tools."
Braun said chimpanzees and other primates with sharp canine teeth don't need knives because they can tear almost anything they want with their teeth.
While wild primates have not been observed using cutting tools, captive primates can be trained to do so, and an untrained captive orangutan has been observed to spontaneously cut things with sharp stones.
Braun said that during human evolution, teeth shrink as the brain grows, and if humans are to use large prey as food resources, sharp cutting tools become a necessity.
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He said there is a growing awareness that various primates accidentally make stone chips, suggesting that when something needed to be cut, early human ancestors may have had plenty of tools at their fingertips if needed.
"Of course they would have produced them, or could have produced them," he said, "much earlier than they actually needed them." ”