These Tool-Wielding Monkeys Shed Light on a Huge Evolutionary Mystery

The first tools made by our early Stone Age ancestors were simple, but monumental. Hammerstones, handaxes, and sharpened flakes were their implements of choice — all carved from rock and used for tasks like hunting and foraging.

Creating those modest tools made it possible for our ancient human relatives to exploit their environments in new ways, ultimately leading them to an evolutionary path that set them apart from other species.

While scientists have dug up swaths of Stone Age tools from sites in Africa, Europe, and Asia, questions remain about how those objects were crafted. One way to glean insight is to watch how our evolutionary cousins — today’s primates — make their own tools from stone.

Researchers turned to macaques, a genus of Old World monkeys, to shed light on what our ancestors may have been doing millions of years ago. It could also help researchers determine which stone artifacts were made on purpose, and which happened by accident.

The results were published in Science Advances on Friday.

Smashing stones

The endangered long-tailed macaques of Thailand’s Lobi Bay have been frequent subjects in scientific research. Lydia Luncz, a primatologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, tells Inverse that she’s been studying their behavior for about nine years.

“From hundreds of primate species, only just a handful use stone tools,” Luncz, a co-author of the new study, says. These macaques are one of the select few, making them ideal candidates for understanding how simple stone tools are used.

The macaques forage for nuts and shellfish, breaking them open by smashing repeatedly with a hand-held stone. Similar tools in the archaeological record, known as hammerstones, were probably used the same way by ancient human ancestors.

But for the new study, Luncz and colleagues were more interested in what happened when the hammerstones and the stone anvils beneath them broke. In the process of smashing, the macaque’s tools would sometimes chip and break, leaving behind small pieces.

Those pieces look similar to another ancient tool, known as a stone flake. Researchers believe that ancient humans crafted these flakes to cut meat, due to their sharp properties. But for macaques, a stone flake is basically useless.

“They go for oysters and sea snails and stuff like that — that's where they get their meat from,” Luncz says. “But they don't need sharp cutting flakes to do so; they need a percussive tool. They need a pounding tool.”

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.

Striking resemblance

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.

However, Luncz cautioned that the results do not mean that scientists studying ancient tools need to throw everything they know out the window.

“We're just saying we might need different criteria to tell apart … the difference between intentionally made stone artifacts and accidental byproducts,” she explains.

When looking at individual ancient flakes, Luncz says it’s easier to confuse them with the ones made by today’s macaques. But when looking at an entire archaeological site, it becomes more clear if the individuals who once lived there were intentionally crafting tools.

Pieces of the past

Clues beyond the physical appearance of a flake help researchers determine if a stone artifact was a tool, or just a byproduct of some other process.

Paleoanthropologist Jason Lewis of Stony Brook University in New York tells Inverse that there are many cues beyond how a flake was broken that can help piece together if it was used intentionally. Lewis was not involved with the new study, but works extensively on one of the archaeological sites — called Lomekwi 3 — included in the study.

For example, “we use other factors about how far certain rocks were moved from where they would have naturally or originally occurred on the landscape,” Lewis explains. Since certain types of rocks have properties that make better tools, an early human ancestor may have collected rocks from one area and moved them to another place for certain tasks.

Searching for that evidence can help determine if there was any thought or intentionality behind the creation of an artifact.

As for the new study, Lewis says the findings are important because studying the behavior of modern-day primates gives us a better grasp on some of the biggest questions in the history of our species.

“Understanding and identifying the distinctions between when our ancestors or other primates used stone tools, versus made stone tools, is one of the key issues in understanding the origins and evolution of behavior that's specific to us, or very important in our own evolution,” Lewis explains.

Having evidence of macaque tool use from such a broad region makes the study important as well. But he says it doesn’t seem likely that the results of this study would change how archaeologists make assessments of ancient tools.

“I would say that the other early Stone Age archaeologists that I know and that I work with are not at all dubious about our ability to detect whether flakes are coming from an accidental percussive process versus were intentionally [shaped] for use as tools,” Lewis says.

It makes sense, he says, that stone flakes accidentally made by macaques would look a lot like the ones made intentionally by our ancient ancestors.

“As we move back in time, it's absolutely logical and expected that as our toolmaking was more primitive in the past; it's gonna look a heck of a lot like what other primates end up doing when they accidentally break stones,” Lewis explains.

But it’s the full context of where artifacts are found and what they’re made of that helps researchers understand if someone made them on purpose.