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Monkey-Made “Tools” Cast Doubt on High Intelligence in Early Hominids

Photo credit: Sakurai Midori, CC BY-SA 2.1 JP , via Wikimedia Commons.

A new paper in Science Advances, titled “Wild macaques challenge the origin of intentional tool production,” is making headlines. Macaque monkeys from Lobi Bay, Thailand, have been observed “unintentionally” producing stone tools that are “almost indistinguishable” from some very old stone artifacts typically attributed to our supposed evolutionary ancestors. Gizmodo reports, “Uh-Oh: Monkeys Make Stone Flakes That Look a Lot Like Human Tools.” According to The Hill, “New study on monkeys using stone tools raises questions about evolution.” The reason for the “Uh-Oh” and the “questions” is that if living monkeys can produce tools resembling those supposedly produced by our intelligent hominid ancestors, then many tools currently ascribed to our purported ancestors may be no such thing. From Gizmodo

Researchers studying macaques in one of Thailand’s national parks made a surprising discovery: While cracking nuts, the monkeys make stone flakes that look an awful lot like the flakes that scientists have attributed to ancient human. The finding means that some of the material held up as the earliest evidence of hominin tool use may have actually come from monkeys and not our ancestors.

The Hill elaborates: 

The Thai monkeys produced stone artifacts “indistinguishable from what we see at the beginning of the [human] archeological record — what we see as the onset of being human,” said Lydia Luncz of the Max Planck Institute for Evolutionary Anthropology, a co-author on the study. 

The monkeys — long-tailed macaques — seem to have made their artifacts by accident, not by design. But in many ways, that only makes the finding more disruptive. 


Ancient humans used knapping to break apart rocks to create an incredibly flexible set of tools — the earliest forms of which cannot be distinguished from the ones macaques made by accident.

That points to a possibility that could throw a wrench into the established narrative, Luncz said: that “all the conoidal flakes we find in the archaeological record — deemed to be intentionally made — could be unintentional byproducts.”

Doubts About Our “Ancestors”

Similarly Inverse reports, “Because the macaques created stone flakes by accident, Luncz and colleagues wondered if some ancient primates may have done the same — creating objects that researchers today might interpret as intentional tools.” It continues:

For the new study, the researchers compared more than 1,000 stone flakes made by macaques to ancient flakes dating back 1.3 to 3.3 million years ago. Samples came from several excavated sites in Tanzania, Kenya, and Ethiopia.

Looking at the shape, size, and markings on each flake, the researchers determined that there were very few physical differences between the samples from today and those made by human ancestors from millions of years ago.

That leads the researchers to believe it’s possible that some ancient stone flakes may have been misinterpreted as intentionally-made tools, since the macaques make such similar ones without even trying.

“Given these similarities, it may be that some flakes and flaked stones from Plio-Pleistocene contexts are derived as a by-product of percussive behaviors and may be easily misidentified as intentional products,” they write.

“All of the Same Characteristics”

Just how similar are these “tools” to the stone tools we find in the archaeological record? The technical paper reports “morphological overlap between unintentionally produced (macaque) material and those interpreted as intentional (Oldowan and Lomekwian),” and concludes, “These results show that between 30 and 70% of an Oldowan assemblage can be replaced by macaque flakes before significantly changing the central tendency of the morphological and technological parameters of the original assemblage.” A press release from the authors of the technical paper states:

[M]any of these artefacts bear all of the same characteristics that are commonly used to identify intentionally made stone tools in some of the earliest archaeological sites in East Africa.

This evidence has apparently not been well received, as The Hill reports:

That idea remains controversial in the field, however. 

“You will not believe the fights we had to fight,” Luncz said.  

Even calling the macaque-produced stone flakes “artifacts” was controversial because some scientists felt it implied an overlap between tool use by Homo sapiens and other primates that wasn’t justified.

‘Artifacts,’ after all, shares its root with ‘art’ and ‘artifice’ — words that suggest intention, planning and humanity. 

“People were not happy with monkeys being able to create those artifacts,” she added. “And somewhere in the records of macaque and early hominid tools, there must be a difference. But right now, the diagnostic criteria we’re using can’t find one.”

A Word of Caution

But the paper offers a potential way forward:

Our results suggest that larger, more elongated flakes, with high cutting edge–to–mass ratios and more platform preparation are attributes associated with intentional flake production. However, measures such as external platform angle and platform dimensions, flake scar frequency, flake scar directionality and cortical coverage on flakes, as well as exploitation strategies and number of exploited surfaces on flaked pieces do not differ between nut cracking and intentionally produced assemblages.

But the paper offers a caution at the end, noting that previously used criteria also turned out to wrongly infer “intentional” activity:

Previous studies have relied on the aforementioned co-occurring attributes to infer intentional hominin tool production. Building on previous studies of primate flaked stone assemblages, this study shows that these criteria now occur repeatedly within multiple phylogenetically and geographically diverse nonhuman primate lineages.

This study is a potential gamechanger. As they conclude, “The results of this study demonstrate that a fundamental reassessment of how we define and identify this uniquely hominin behavior in the archaeological record is still needed.” If current criteria for inferring intelligent tool production can be wrong, it raises the specter that future studies could show monkeys and apes are capable of “unintentionally” producing other complex tools still ascribed to our supposed hominid ancestors. This could mean that a sacred evolutionary icon — apelike species acting smart and making tools — could potentially be axed (pun intended!).