Fossil evidence suggests early humans adapted to life in tropical rainforests thousands of years earlier than previously thought, researchers say.

The findings address a long-standing debate about exactly when our early ancestors moved into rainforests, with many experts suggesting such habitats would have been inhospitable to early hunter-gatherers.

However, analysis of fossilized teeth discovered at a number of sites in Sri Lanka, dating back as far as 20,000 years, revealed evidence of a diet consisting of rainforest animals and plants, researchers have reported in the journal Science.

Previously, it was been believed tropical forests were mostly pristine, human-free environments until the Early Holocene period around 8,000 years ago.

Before now, scientists have been unable to find any direct evidence of human life in rainforests before then.

However, the new evidence pushes that date back considerably, the researchers say.

"Humans have been manipulating and living within dynamic rainforest environments for at least 20,000 years and probably even longer," says University of Oxford archaeologist Patrick Roberts.

"The lifestyle, as we can see, was dedicated rainforest subsistence," Roberts, who specializes in the study of early human adaptation, added.

The fossil teeth evidence means early humans in Sri Lanka lived almost entirely on food sourced in the rainforest without needing to move into other environments, the researchers said.

Rainforests present undeniable challenges to humans, including dense vegetation making travel difficult, small, agile and often arboreal prey animals that are difficult to catch and plants and fruits that often can be poisonous, they point out.

"However, it is clear that hunting and gathering communities in Sri Lanka figured out how to adapt to such settings," says Mike Petraglia, another Oxford archaeologist.

The study of the teeth showed early humans in Sri Lanka hunted monkeys, mouse deer, giant squirrels, porcupines as well as other mammals.

The also ate freshwater snails, nuts and starch-rich rainforest plants, the researchers said.

"These results further the picture that our species was incredibly adaptive, and it is arguably this that made us the first species to expand across the diversity of the globe's ecologies and environments," says Roberts.

In the study, the researchers form Oxford's Research Laboratory for Archaeology collaborated with scientists from the University of Bradford, also in Great Britain, and a team of scientists in Sri Lanka.

While the study has put the movement of early humans in rainforests at least 20,000 years ago, there is some evidence from previous archaeological work that they might have moved into Sri Lankan forests as early as almost 40,000 years ago, although more research will be needed to confirm that, the researchers said.

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