Brazilian capuchins have been monkeying around with stone tools for hundreds of years, Oxford University scientists have discovered.

The primates have been observed using stones as hammers and anvils to crack open hard foods such as seeds and cashew nuts.

According to the new findings, they have been wielding tools for a long time. Discarded buried stones bearing marks from tool use and cashew nut residue were dated to "at least" 600 to 700 years old.

The evidence is the earliest archaeological proof of monkeys using tools outside Africa.

It raises the possibility that early humans may have been influenced by watching monkeys with stone tools, say the scientists.

Lead researcher Dr Michael Haslam, from Oxford University's School of Archaeology, said: "Until now, the only archaeological record of pre-modern, non-human animal tool use comes from a study of three chimpanzee sites in Cote d'Ivoire in Africa, where tools were dated to between 4,300 and 1,300 years old.

"Here, we have new evidence that suggests monkeys and other primates out of Africa were also using tools for hundreds, possibly thousands of years. This is an exciting, unexplored area of scientific study that may even tell us about the possible influence of monkeys' tool use on human behaviour.

"For example, 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."

The capuchins studied by the researchers live in Serra da Capivara National Park in north-east Brazil.

They were seen to have recognisable "cashew processing sites" where stone tools were left in piles in specific locations, such as at the foot of cashew trees or on tree branches.

The monkeys showed an ability to select the most suitable tools for the job in hand. Stones used as anvils were more than four times heavier than "hammer" stones, and hammers were four times heavier than average natural stones.

Smooth hard quartzite stones were chosen for use as hammers, while flat sandstones were preferred as anvils.

The buried stones were dug from a depth of 0.7 metres close to cashew trees where modern capuchins had been observed frequently using tools.

As well as bearing distinctive marks caused by pounding, the stones were found to be smeared with cashew nut residues.

Carbon dating gave the scientists, whose findings appear in the journal Current Biology, a conservative estimate for the age of the stone tools.