Some monkeys accidentally make stone flakes that resemble ancient hominid tools

Monkeys in southern Thailand use rocks to pound open oil palm nuts, inadvertently shattering stone pieces off their makeshift nutcrackers. These flakes resemble some sharp-edged stone tools presumed to have been created on purpose by ancient hominids, researchers say.

Thailand’s long-tailed macaques (Macaca fascicularis) produce shards that could easily be mistaken for stone flakes previously found at 17 East African hominid sites dating from about 3.3 million to 1.56 million years ago, say archaeologist Tomos Proffitt and colleagues. The finding suggests that ancient hominids may sometimes have created the stone flakes by accident while using rocks to smash nuts, bones or other objects, the scientists report March 10 in Science Advances.

Previous research has already shown that rock-wielding capuchin monkeys in Brazil unwittingly produce hominid-like stone flakes.

Observations of rock bashing by these two monkey species undermine a long-standing assumption that hominids must have intentionally made certain ancient stone flakes, including some of the earliest known examples of tools, Proffitt says. It’s time to reevaluate how such determinations are made, he contends.

Proffitt’s group identified 219 complete and fragmented stone flakes at 40 macaque nut-cracking sites on the island where the monkeys live. The team also found rocks showing damage consistent with having been used either as pounding implements or pounding platforms.

Some differences do exist between macaque and hominid stone flakes, says Proffitt, of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. For instance, many macaque flakes display battering damage on only one side, versus frequent two-sided damage on hominid artifacts.

Such clues may help archaeologists develop guidelines for estimating whether ancient hominids made stone flakes on purpose or by accident, Proffitt suspects.