A monkey picks up a potato-sized rock in his tiny hands, raises it above his head and smashes it down with all his might on another stone embedded in the ground. As the creature enthusiastically bashes away, over and over, flakes fly off the rock he is wielding. They are sharp enough to cut meat or plant material, but the monkey does not pay much attention to the flakes, save to place one on the embedded rock and attempt to smash it, too. Still, he has unintentionally produced artifacts that look for all the world like stone tools found at some human archaeological sites.

The monkey is a wild capuchin in northeastern Brazil's Serra da Capivara National Park, where these animals have long been known to use rocks for a wide range of activities, from cracking open nuts and digging for roots to catching the attention of potential mates. Other nonhuman primates, including West African chimpanzees, also use rocks as tools in the wild. But the Serra da Capivara capuchins are the only ones that scientists have seen banging rocks together to break them—an activity previously thought to be exclusive to members of the human family. Humans do it to create sharp-edged tools for cutting things. The capuchins, in contrast, have never been seen using the flakes they make; they just lick the surface of the embedded stone, perhaps in pursuit of mineral dust.

Capuchin artifacts resembling those made by humans (above) could necessitate reanalysis of other enigmatic stones. Of particular concern are those found at the archaeological sites of Pedra Furada in Brazil, located near the monkeys' home. To read more about the controversy, visit www.ScientificAmerican.com/monkey-tools.

Source: From “Wild Monkeys Flake Stone Tools,” by Tomos Proffitt et al., in Nature, Vol. 539; November 3, 2016

Now a new study has examined the capuchin-produced stone flakes, and it turns out that the chips meet criteria used to distinguish human tools from naturally broken rocks. The findings, published in fall 2016 in Nature, could fuel debate over controversial archaeological sites. The discovery also raises questions about what differentiates humans from other primates and how our lineage started fashioning implements from stone.

Tomos Proffitt of the University of Oxford and a group of his colleagues watched the capuchins select rocks to use as hammers and subsequently strike them against cobbles. The researchers retrieved the fragmented stones and also collected other such artifacts found in excavations within the surrounding area—just as they would if they were excavating a human archaeological site. They then analyzed this collection of 111 capuchin artifacts, examining their shapes and sizes, as well as the nature of the scars left on the rocks by all the bashing.

Remarkably, the team found that the capuchin artifacts exhibit distinctive scoop-shaped, or “conchoidal,” flaking and sharp edges and that the monkeys often removed multiple flakes from a single rock—all hallmarks of man-made stone tools. (The authors note that stone fragments produced during chimpanzee nut cracking, in contrast, lack most of the diagnostic criteria, as do flakes produced by captive bonobos that have been taught to knap.)

Experts have previously linked such characteristics to the emergence of humanlike hands and coordination and to shifts in human cognition. But the fact that monkeys produced rocks with these same traits demands a different evolutionary explanation. And if modern-day monkeys modify rocks in this way, it is possible that extinct monkeys and apes did, too, leaving behind archaeological assemblages of their own. Archaeologists thus need to refine the criteria they use to identify stone tools intentionally produced by members of the human family, Proffitt and his colleagues argue.

“Many people are going to be disturbed that these tools can be made by capuchins,” says archaeologist Sonia Harmand of Stony Brook University, who was not involved in the new research. According to Harmand, the monkey artifacts would not look out of place at East African sites containing tools made by human ancestors in one of the earliest technological traditions: the Oldowan, which dates back to 2.6 million years ago at the site of Gona in Ethiopia. The capuchin flakes resemble the simplest examples of Oldowan technology. But other Oldowan stone tools exhibit considerably more sophistication and planning, she says. The monkey artifacts also diverge from the oldest known stone tools in the world: 3.3-million-year-old implements that Harmand and her team excavated from the site of Lomekwi in Kenya. The Lomekwi tools are far larger and are made of basalt and phonolite—rocks that are denser than the quartz and quartzite rocks the capuchins use.

Some experts wonder whether the capuchins' flakes could spark doubts that members of the human lineage made the oldest stone tools. Although researchers have attributed the tools to human ancestors, the sites lack diagnostic fossils to establish the connection. “We have no clue” who created the material at Lomekwi and Gona, says archaeologist Wil Roebroeks of Leiden University in the Netherlands. Hélène Roche of Paris West University Nanterre La Défense disagrees, writing in a commentary accompanying the Nature paper that the capuchin findings should not raise suspicions about who produced the early stone tools found in Africa. Archaeologists have studied hundreds of those sites, she notes—and many of them contain contextual clues, including cut-marked bones that show how tools were used, as well as fossils that indicate human ancestors made them.

Although the capuchin discovery demonstrates that nonhuman species can accidentally produce fragments of rock that look just like human-crafted cutting tools, that does not mean the man-made tools are not special, Harmand cautions. Even if human ancestors started creating flakes unintentionally like the capuchins do, there was something that made them realize they could put them to use and even make new tools to suit their purposes. Moreover, human technology evolved from the comparatively simple tools seen at Lomekwi and at Oldowan sites to hand axes with carefully shaped cutting edges a million years later and eventually to the elaborate machinery we have today. Why did technology fail to evolve to the same degree in chimps and monkeys? Harmand asks. Why did humans alone take it to such an extreme?

Proffitt is eager to determine how long capuchins have been using rocks this way. Other evidence demonstrates that they have been using the cobbles to crack open nuts for at least 600 years. And chimpanzee stone tools from the Ivory Coast in West Africa date back to 4,300 years ago. Beyond that, “we have no evidence of what ancient monkeys or great apes were doing,” Harmand observes—which leaves plenty of room for more surprises in the future.