'Cashew nuts are native to this area of Brazil, and it is possible that the first humans to arrive here learned about this unknown food through watching the monkeys and their primate cashew processing industry,' said Michael Haslam from Oxford.
Humans might have learned how to eat cashews by observing monkeys, according to a new study which found evidence that Brazilian capuchins have been using stone tools to crack open the nuts for at least 700 years.
Researchers from Oxford University in the UK and University of Sao Paulo in Brazil observed groups of modern capuchins at Serra da Capivara National Park in northeast Brazil and combined this with archaeological data from the same site.
They watched wild capuchins use stones as hand-held hammers and anvils to pound open hard foods such as seeds and cashew nuts, with young monkeys learning from older ones how to do the same.
The capuchins created what researchers describe as 'recognisable cashew processing sites', leaving stone tools in piles at specific places like the base of cashew trees or on tree branches after use.
They found that capuchins picked their favourite tools from stones lying around, selecting those most suitable for the task.
Stones used as anvils were over four times heavier than hammer stones, and hammers four times heavier than average natural stones, researchers said.
The capuchins also chose particular materials, using smooth, hard quartzite stones as hammers, while flat sandstones became anvils, they said.
Using archaeological methods, researchers excavated a total of 69 stones to see if this tool technology had developed at all over time.
They dug to a depth of 0.7 metres at a site close to cashew trees where they had seen modern capuchins frequently using their stone tools.
They identified the tools from inspecting the size and shape of the stones, as well as the distinctive damage on the stone surface caused by capuchin pounding.
Through mass spectrometry, researchers were able to confirm that dark-coloured residues on the tools were specifically from cashew nuts.
They also carbon dated small pieces of charcoal discovered with the stones to establish the oldest were at least 600 to 700 years old - meaning the tools predate the arrival of Europeans in the New World.
Researchers estimate that around 100 generations of capuchins have used this tradition of stone tools. They compared tools used by modern capuchins with the oldest excavated examples, finding they were similar in terms of weight and materials chosen.
This apparent lack of change over hundreds of years suggests monkeys are 'conservative', preferring not to change the technology used, unlike humans living in the same region.
"Cashew nuts are native to this area of Brazil, and it is possible that the first humans to arrive here learned about this unknown food through watching the monkeys and their primate cashew processing industry," said Michael Haslam from Oxford.
The findings were published in the journal Current Biology.