The nutcracker tools of the Thai monkeys look confusingly like those of early humans

Monkeys are among the many animals known to use tools, so it’s no surprise that long-tailed macaques crack nuts with stones in Phang Nga National Park, Thailand. Sometimes these “hammer stones” break. The products look very similar to tools found in Africa and attributed to early hominins such as Australopithecus that scientists call them “partially indistinguishable.”

The fact that many animals use tools came as a great shock to anthropologists who had defined humans as tool users. Stone tool bearers are a restricted club, but even some fish qualify, and monkeys find stones handy in many places. At least human stone tools are distinctive, broken off from larger rocks into shapes best suited to the task.

At least that’s what we thought until Dr. Tomos Proffitt of the Max Planck Institute for Evolutionary Anthropology and co-authors had to go in and put a wrench to work—or should we say a wrench? In a new study, they report that when the long-tailed macaques hit nuts with stones, the stones can break and the products can look very familiar.

“The fact that these macaques use stone tools to process nuts is not surprising as they also use tools to access various shellfish,” Proffitt said in a statement. “What’s interesting is that in doing so, they inadvertently create their own extensive archaeological records, some of which are indistinguishable from some hominin artifacts.”

All of these sharp-edged stones were accidentally made by monkeys, but they look like those thought to have been carved by early hominins

All of these sharp-edged stones were accidentally made by monkeys, but they look like those thought to have been carved by early hominins. Photo credit: Proffitt et al., 2023

The discovery in some ways complicates the study of early humans, since now any anthropologist who finds such a set of broken stones must wonder which branch of the primate family made it. As the paper states, “Intentionally manufactured sharp-edged stone flakes and pieces of flake are our prime evidence of the advent of technology in our lineage.” Now, in at least some cases, that evidence needs to be reexamined. “In Plio-Pleistocene contexts, the stones from Lobi Bay would likely be interpreted as evidence of deliberate flake production and use of anvils and hammerstones for various subsistence tasks,” the paper states.

On the other hand, this discovery could provide important insights into how humans came to make such tools in the first place.

“The fact that these artifacts can be produced by nut-cracking has implications for the range of behaviors we associate with sharp-edged flakes in the archaeological record,” said co-lead author Dr. Jonathan Reeves. Stones that look like this are usually interpreted as cutting tools. Maybe instead of slaughtering animals or attacking our enemies 2001style, the early toolmakers used stones to break nuts and then noticed how well the products fitted their hands.

“The identification of core and flake technology in the archaeological record was used to infer the level of cognitive complexity,” the paper points out. “To suggest that hominins were able to select rock types with specific material properties, understand aspects of fracture mechanics, and exhibit precision and coordination in motor skills.” However, perhaps we were influenced by a desire to see these things in our ancestors .

Adding to the confusion is the fact that the macaques leave so many broken rocks around a nut-rich site. The discovery of multiple flakes in one location was once considered a key feature of early hominin toolmaking.

The find also raises questions about the future of the macaques. Last month, a newspaper reported that the manufacture of stone tools produces feedback that alters the toolmaker’s brain, suggesting that the introduction of tools may have accelerated the expansion of the human brain. If so, could we see a similar increase in monkey intelligence? However, we don’t know how long the macaques have been cracking nuts in this way – it may have been a long time since the brains grew.

The study is published in Science Advances.

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