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They are literally a tough nut to crack. To enjoy tasty cashews you first have to figure out a way to remove the shells, which .

The bearded capuchin monkeys of Brazil may have been up to the task for centuries 鈥� and watching them work could even have taught us how to eat cashew nuts safely.

We know that at least three non-human primates use stone tools: chimpanzees in West Africa, long-tailed macaques in Thailand and bearded capuchins in Brazil. The first ever 鈥減rimate archaeology鈥� dig 鈥� carried out in Ivory Coast and published in 2007 鈥� confirmed that chimpanzees have been living through their Stone Age for at least 4300 years. A similar investigation in Thailand published earlier this year traced back the macaque Stone Age at least 65 years.

Now, at the University of Oxford and his colleagues have done the same feat in the Americas, by finding ancient evidence of the capuchin Stone Age.

The researchers excavated a 35-square-metre area in Brazil鈥檚 Serra da Capivara National Park to a depth of up to 70 centimetres. They uncovered 69 ancient stone tools.

Charcoal deposits left by past natural forest fires meant that the sediments containing the tools could be radiocarbon-dated. The oldest were deposited in the 13th century, more than 200 years before Columbus arrived in the Americas.

It鈥檚 very unlikely that the stone tools were used by humans, says Haslam. 鈥淭here have been many decades of archaeological research in Serra da Capivara National Park so we know precisely what to expect from a human site,鈥� he says. 鈥淗uman sites include pottery, flaked and ground stone implements, and a variety of other potential materials such as cooking places. None of these were present at our site.鈥�

Capuchins pound cashew nuts to remove the irritant-containing shell, uncovering the edible kernel. The capuchins might have maintained this stone tool tradition, unchanged, for about 100 generations, says Haslam.

Monkey Luddites?

This lack of change over hundreds of years suggests monkeys are technologically conservative, unlike humans living in the same region, the team say in their paper.

We shouldn鈥檛 be surprised by that, says at the University of Georgia in Athens. 鈥淭hey have methods that work well for them, and the foods they exploit with these tools have remained the same over this period,鈥� she says. 鈥淲hy should they have modified their technology?鈥�

Even so, Fragaszy welcomes the new work and says it confirms the suspicions of many primatologists. 鈥淚n the New World, as in Africa, humans are not the only primates that have used stone tools for hundreds of years.鈥�

at the Institute of Cognitive Sciences and Technologies in Rome, Italy, is more cautious. Her observations of wild capuchins suggest the cashew nuts do not require much force to break open, so she is not convinced that the apparent impact marks on the unearthed stones are unambiguous evidence that they are ancient capuchin tools.

However, Haslam says the stones were identified as such using several lines of evidence. For instance, they are unusually large compared with other stones in the area 鈥� suggesting they were selected and brought there by monkeys.

Also, the ancient stone tools are almost identical in size and shape to those modern capuchins use to crack cashews (see video below).

We know that humans in this region of Brazil have been eating cashews for about 7000 years, says Haslam. It鈥檚 possible, he says, that the capuchins were cracking open cashews with stones way back then, so the first humans to arrive in the region may actually have learned to eat cashews by watching the monkeys.

鈥淚t鈥檚 speculative, but not out of the question,鈥� says Haslam 鈥� although finding conclusive evidence to support the idea will be difficult. So far 鈥減rimate archaeology鈥� excavations have extended back only about 4300 years.

We do, however, know from more traditional archaeological evidence that humans encountered capuchins deep in prehistory. 鈥淭here are rock art pictures of capuchins at Serra da Capivara,鈥� says Haslam.

But we don鈥檛 know whether humans were carefully watching and copying them. 鈥淪adly none [of the pictures] found so far show the monkeys using tools.鈥�

Read more: Tools maketh the monkey; Human ancestors got a grip on tools 3 million years ago