Monkey archaeology: Ancient evidence of tool use found
Primate archaeology is a new and unusual-sounding field, but it has revealed ancient evidence of some clever and dextrous monkey culture.
Researchers from Oxford University, working in Brazil, found ancient "nut-cracking tools" - 700-year-old stone hammers that capuchin monkeys used to open cashew nuts.
This is the earliest evidence yet of monkey tool use outside Africa.
The findings are published in the journal Current Biology.
One of the Oxford team, primatologist Dr Lydia Luncz, said the find was a "window back into the past".
"Our efforts to look back into the past have been very human-focused," she told BBC News. "So we don't know much about how tool use has evolved in these [other primate] species."
As observations first revealed about a decade ago, capuchin monkeys use stone hammers and anvils to break into the cashew nuts - placing a cashew on a large stone anvil and hitting with a hammer.
The monkeys, Dr Luncz explained, bring these stones to the cashew nut trees. And that behaviour enabled the archaeologists to work out where they should dig for ancient tools.
The liquid inside the nutshell, Dr Luncz explained, discolours the outside of the stone, allowing the researchers to identify the stone tools, and even to test them chemically to confirm what they were used for.
One question the discovery may prompt is whether early human behaviour was influenced by observations of monkeys using stones as tools.
Dr Catherine Hobaiter from the University of St Andrews, who studies primate social behaviour, said that both the tool use findings and the new field of primate archaeology were "fascinating".
"These days if we find a 'new' tool we really shouldn't assume that it was made by humans or our homo ancestors anymore," she told BBC News.
"Either you don't have to be smart to make a tool, or we've been underestimating other species by a long way."
According to this new evidence, the capuchins were using stone tools before European settlers arrived in the New World. But Dr Michael Haslam of the University of Oxford, who led the study, said there was much more to find.
"We definitely don't think we have the oldest activity," he told BBC News.
"We only began this project in the last few years and we have to imagine we're in the same situation as human archaeology was when they started to find the first evidence of stone tools. That [evidence] now goes back more than three million years in Africa.
"So we think we're at the tip of the iceberg."