New study on monkeys using stone tools raises questions about evolution

Monkeys in modern-day Thai forests create stone artifacts uncannily similar to those crafted by early humans — challenging the established narrative of human cultural evolution. 

A new study published on Friday in Science Advances suggests the possibility that a critical hallmark of human tool use happened by accident — potentially blurring the line between tool use by early humans and our primate relatives. 

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 coauthor 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. 

Tool use in nonhuman primates is nothing new. Long-tailed macaques — the small, mischievous and social primates often seen in Southeast Asian cities and temple complexes — use stones to break through shells and get at the meat inside. 

This use can be surprisingly sophisticated. Macaques foraging on beaches choose out long, narrow and heavy stones — what anthropologists call an ‘axe hammer’ — to pop open oyster shells. 

Such narrow stones are perfect for breaking open the brittle shells, which wider rocks risk smashing into sharp fragments — endangering the incautious monkey who tries to stick its face into the jagged hole.

A long-tailed macaque eats a biscuit on the World Wildlife Day at a forest nearby Lhoknga beach in Indonesia's Aceh province on March 3, 2023. (CHAIDEER MAHYUDDIN/AFP via Getty Images)

The Planck group found the first evidence of macaques adapting this seafood-foraging use of stone tools to another food: nuts.

In particular, the monkeys targeted the hard, oil-rich nuts of African oil palms — introduced as a cash crop across the region.

In an abandoned oil palm plantation on a national park site, the monkeys would create nut-cracking ‘stations’ beneath the feral trees.

There they break open the palm fruit’s oil-rich pit between hand-wielded hammer rocks and a thick, flat stone that functions as an anvil. 

Camera traps showed that when the nutcracking monkeys miss a strike, the two stones bang together.

That collision sometimes strikes a flake off of one of the rocks — something very similar to the toolmaking process archeologists call “knapping.”

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."

In many ways, the Science paper lays the groundwork for a more intuitive story of human evolution than the idea that stone flakes — and the human cultural flowering they enabled — sprung forth by deliberate invention.

That narrative requires a lot of additional steps, Luncz said. It presupposes axe-swinging early humans with brains big enough to plan their “extraction” of the perfect flakes from rocks and hand-object movement sophisticated enough to deliver it.

By contrast, the Planck team’s findings suggest another possibility — that the evolution of human tool use could have been more fitful and staggered.

In one possible scenario, ancient humans — like modern macaques — could have first produced stone flakes as a byproduct as they bashed apart bones, nuts or shellfish with rocks.

Then, far later — perhaps alongside some kill where they had used rocks to hammer open bones to get at the marrow within — early humans may have turned to these razor-sharp flakes, which would once have been discarded as trash, to begin cutting up meat.

Or, as Luncz put it: “An accidental stone breakage could have led us down the evolutionary trajectory of making stone tools.”

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.

A long-tailed macaque eats a biscuit on the World Wildlife Day at a forest nearby Lhoknga beach in Indonesia's Aceh province on March 3, 2023. (CHAIDEER MAHYUDDIN/AFP via Getty Images)

‘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.”

The Planck study was controversial in part because it brushed against broader, entrenched debates over nothing less than what it means to be human.

In particular, there is a longstanding debate over whether animal social learning can be described using a word as loaded, venerable and human-inflected as “culture.”

Luncz was careful about using that word. But she noted that “nut cracking in primates is socially transmitted — a monkey in isolation doesn’t learn it. It’s our material culture that we use to recreate our history.”

The questions that studies like this explore are central to human identity, Luncz said. “Why are we the way we are? How did we evolve to become this crazy successful monkey that occupies the whole planet?”

“Tool use plays an enormous role in this. We’re so successful at it that we’re destroying our planet — and that all started with a stone tool.”

In a bitter irony, macaques’ very social ingenuity and flexibility — a hallmark of primates — endangers attempts to preserve and learn from them. 

As their habitats have been cleared in Asia’s rapid urban and agricultural expansion — with forests cleared for suburbs and cash crops like the ubiquitous oil palm — macaque populations have plummeted. 

Last year, the species was listed as “endangered” on the canonical IUCN Red List after a population collapse on a scale “we’ve never seen in the primate world,” Luncz said.

That collapse is mainly invisible: as their habitats have vanished, many macaques have taken refuge in cities and public parks, where they are a familiar and often confrontational presence.

“People aren’t aware that they are an endangered species,” Luncz said. Urban macaques are “always in their face, always there and in the way. They break into houses, steal tourists’ sunglasses, and bite children.”

In many Asian cities, a push to conserve these primate relatives is greeted with responses similar to an American or European proposing “to conserve pigeons.”

But as the species’ wild, forest-dwelling populations break down — and their social memory with it — our ability to learn about our own deep origins is also slipping away.

“The chance we have now to compare our history and living primates is a very fast-closing window,” Luncz said. She added that without far more aggressive conservation, we will only have the mute record of ancient archeology to rely on.