Germanic peoples during the Stone Age. Hand colored wood engraving, published ca. 1880.

Stick and Stones

Orangutans could reveal how ancient humans made the first stone tools

This was a critical skill for human development — do apes have these skills, too?

ZU_09/DigitalVision Vectors/Getty Images

The first stone tools that ancient humans made were deceptively simple. At least 2.6 million years ago, our ancestors learned to strike stones and break off sharp flakes that could function as knives. This innovation unlocked the door to foods and other resources that transformed our development. How early Homo sapiens learned this is not well understood, but our living primate cousins, orangutans, can help illuminate the past — but with one vital difference.

We know that orangutans make and use tools — they’ve been observed before using sticks for fishing out bugs from bark, for example. But stone tools like the kind ancient humans used are a level-up. So researchers asked: Can orangutans learn to make and use stone tools, just like archaic peoples once did?

In a new paper published in the journal PLOS One, the researchers describe the results of several experiments designed to try and tease out whether apes innately held this knowledge. Answering this question could help us better understand our own species’ critical periods of development, many millennia later.

The experiment — These researchers sought to answer whether or not orangutans could work out that a stone could make tools and if they then used the tools as intended. Researchers have spotted orangutans using sticks to extract seeds from fruit and scrape insects from a hole in a tree in the wild. But they’re not known to turn stones into blades.

In the first experiment, the researchers studied two young male orangutans living at the Kristiansand Zoo in Norway. They put two puzzle boxes with a tasty piece of fruit, a concrete hammer, and a round stone core in the orangutans’ rooms. In theory, if the apes struck the edge of the core with the hammer, then flakes would break off the core that would be sharp enough to cut through the puzzle box and give them access to the snack.

A young male orangutan, Loui, tests out a stone core against the floor.Motes-Rodrigo et al., 2022, PLOS ONE, CC-BY 4.0

In a second experiment, the researchers studied two groups of female orangutans at the Twycross Zoo in the U.K. A social species like our own, orangutans are known to mimic the behaviors of other members of their group. In this part of the study, the researchers wanted to see if the apes could learn to strike the core and use the sharp flakes if someone taught them how to do it.

Alba Motes Rodrigo, a study co-author and postdoctoral researcher at the University of Lausanne in Switzerland, played teacher. She showed some of the orangutans the link between the flakes and the core by placing the fragments on top of the core. She traded some flakes for snacks with the apes, too.

Then, with a separate set of three apes, she showed the apes how to knock the edge off the stone core with a hammer and create flakes. Of the three apes, two were mother orangutans with offspring, and the third was a young female — an age distinction that ended up correlating with how well they caught on to the task at hand.

The discovery — The young male orangutans quickly figured out the concrete hammer did a great job smashing things. They banged it on the walls and floors of the test room, hitting everything but the stone core. One of the males even loosened the core from its fixed platform and smashed it against the ground — which did shake loose a few flakes, albeit not in the manner the researchers had intended.

Once they had the flakes, they didn’t use them to cut the boxes. So the researchers gave the apes an additional tool: A sharp, human-made shard of flint. That’s when things started to click for these apes: One of the orangutans, Loui, used the shard to cut open the puzzle box.

After bashing the core on the ground, Loui knocked off several flakes from the stone but didn’t manage to use them to cut the box.Motes-Rodrigo et al., 2022, PLOS ONE, CC-BY 4.0

Unlike a human, Loui held the tool in his mouth to slice the box open.

“Orangutans in the wild and in captivity engage often in oral tool use,” Motes Rodrigo explains.

“It may be because as they are mostly arboreal in their natural habitats, their hands are often busy, so they use their mouths.”

What about the three females in the U.K.? The youngest ape, named Molly, did manage to strike the stone core with a hammer — but only after Motes Rodrigo showed her how to do it 27 times. Molly didn’t break off any flakes. She made some concrete pieces break off the hammer, possibly because she smacked it enthusiastically on the floor, core platform, and the walls.

Motes Rodrigo showed the older orangutans how to hit the stone core with the hammer 45 times in total — 18 times more than Molly — yet they had no luck picking up the skill. They didn’t swing the hammer even once.

“I think it is interesting that the two orangutans that performed better in the tasks were relatively young,” Motes Rodrigo tells Inverse.

“There could be an age effect on exploration tendencies meaning that younger orangutans are more curious of new objects.”

The question — Though the researchers observed both tool making and tool use in two separate experiments, none of the apes could fully connect the dots.

“The [orangutans] never made their own tools,” Motes Rodrigo says.

“Either they detached stone pieces which they didn’t use, or they used tools I had given them. But never the full sequence.”

Molly strikes the stone core with a concrete hammer.Motes-Rodrigo et al., 2022, PLOS ONE, CC-BY 4.0

That still leaves the big question: how did early humans learn to associate stone cores with flakes and eventually shape those flakes into sharp tools?

The answer isn’t yet clear, but these results offer insight into primate — and potentially early human — behavior.

What’s next — This study is interesting because it shows orangutans can smack stone cores, use flint to cut into things, and perform other skills critical for both making and using tools to their advantage.

That’s the big takeaway for Motes Rodrigo: She and her team witnessed stone tool use in the orangutans, and this is the first time researchers have recorded apes using a stone shard as a blade. Motes Rodrigo says that the fact they took to the shard as a tool quickly hints that it might not be an uncommon tendency among these primates.

“These behaviors might be more widespread among primates than we previously thought,” Motes Rodrigo adds.

It is possible that using shards as cutting tools was a common skill among the last ancestral species shared by humans and primates. This archaic species existed before the two groups split and Homo sapiens evolved.

For now, that’s just a hypothesis. But suppose other ape species show that they can work stone tools like orangutans. In that case, it could help researchers understand how ancient humans developed this skill — and when they did it — in our evolutionary history.