An analysis of teeth dating back to 20,000 years in Sri Lanka has suggested that humans adapted to living in rainforests much earlier than thought.
The researchers from Oxford University working with a team from Sri Lanka and the University of Bradford analysed the carbon and oxygen isotopes in the teeth of 26 individuals with the oldest dating back to 20,000 years.
They found that nearly all the teeth analysed suggested a diet largely sourced from the rainforest.
The team studied the fossilised teeth of 26 humans ranging from 20,000 to 3,000 years ago.
"This is the first time scientists have investigated ancient human fossils in a tropical forest context to see how our earliest ancestors survived in such a habitat," said study co-author professor Julia Lee-Thorp from Oxford University.
The study showed that early modern humans adapted to living in the rainforests for long periods of time.
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Previously it was thought that humans did not occupy tropical forests for any length of time until 12,000 years after that date, and that the tropical forests were largely "pristine" - human-free environments until the Early Holocene, 8,000 years ago.
Scholars reasoned that compared with more open landscape, humans might have found rainforests too difficult to navigate, with less available food to hunt or catch.
For the new study, all of the teeth were excavated from three archaeological sites in Sri Lanka, which are today surrounded by either dense rainforest or more open terrain.
The analysis of the teeth showed that all of the humans had a diet sourced from slightly open "intermediate rainforest" environments.
Early humans in Sri Lanka were able to live almost entirely on food found in the rainforests without the need to move into other environments.
"Our earliest human ancestors were clearly able to successfully adapt to different extreme environments," added lead study author Patrick Roberts.
The study was published in the journal Science.