© Jeffrey Phillips
When did a human-like mind first emerge, setting its owner on a path distinct to that of other apes?

We paleoanthropologists have long looked to tool use as the marker - particularly the appearance of a cutting tool known as a flake.

It now seems we were wrong.

Recent research published in Nature by a team led by Tomos Proffitt at the University of Oxford shows that capuchin monkeys regularly produce sharp-edged flakes indistinguishable from those made by early hominins.

Could these South American simians be taking the same first steps that eventually delivered the spanner, wheel and smartphone? As it turns out, no. The flakes are produced by accident when the monkeys smash rocks together. Nonetheless, the capuchins have thrown a spanner in the works for archaeologists.

Since the flakes they make are not tools at all, we can no longer assume the flakes found in the archaeological record are tools either.

We know that monkeys can make tools of other kinds, of course. Ever since British primatologist Jane Goodall's pioneering work in the 1960s, we have known our chimpanzee cousins use tools to shell nuts and to fish for termites.

Nor is tool use confined to primates. Other mammals, birds, snails, octopuses and even insects all turn out to be tool wielders. In fact, back in the 19th century an American husband and wife team, Elizabeth and George Peckham, first documented tool use outside human beings. They observed wasps hammering dirt with pebbles to build their burrows.

Nevertheless, the one tool we've never seen in any animal's kit is the flake. One of archaeology's most famous couples, Louis and Mary Leakey, first found flakes in the Olduvai Gorge in Tanzania. The artefacts are associated with Homo habilis, an early human ancestor who lived close to 2 million years ago. H. habilis made the flakes, it was believed, by selecting a piece of rock - called a core - and using a stone hammer to strike off a thin wedge. The resultant edge, sharp as surgical steel, enabled H. habilis to butcher animals. Telltale cut marks on ancient bones attested to their use as ancient tools.

Archaeologists argued that making a flake required sophisticated mental machinery such as the ability to plan and an understanding of the physical properties of raw materials. This was coupled with uniquely human hand-eye co-ordination that, for instance, allows us to thread cotton through the eye of a needle.

Flake making was also thought to be associated with the beginnings of language, since to develop such a sophisticated technology implied individuals who could communicate and collaborate, pass on knowledge and create culture.

Now it seems that flakes per se may not represent what we thought they did. Capuchins pound rocks together to crack them open and lick the powdered quartz, probably to access dietary minerals. The process sends flakes flying in every direction. But the monkeys don't use the flakes as tools; they just leave them lying about.

So what these clever monkeys show us is that, if we find ancient flakes, we can no longer assume they were a tool made by a human ancestor.

The discovery of flakes at the Lomekwi archeological site in Kenya, which dates to 3.3 million years ago, led researchers to propose in 2015 that early humans appeared about 700,000 years earlier than previously thought. Now, however, without other evidence, such as cut marks on bones, we can no longer assume the flakes are evidence of a human presence.

One thing is clear: the capuchins have forced us to set the bar higher. A flake alone is not enough. The hunt now begins to find a new kind of artefact that is quintessentially human in its style of manufacture and use as a tool. Perhaps something like the hand axe that we see with Homo erectus much later, 1.6 million years ago.

It is a very exciting time to be an archaeologist.