A new study suggests early humans began populating the rainforest as early as 20,000 years ago on the island of Sri Lanka. Previously, the earliest evidence of consistent rainforest inhabitancy was thought to be around 8,000 years ago.

Really old teeth are the evidence of this early affinity for the rainforest. Researchers analyzed 26 sets of ancient human teeth fossils for carbon and oxygen isotopes. All of them, ranging in age from 20,000 to 3,000 years old, proved to have feasted on a diet provided by the bounty of the rainforest.

"This is the first study to directly test how much early human forest foragers depended on the rainforest for their diet," explained lead author Patrick Roberts, a doctoral student investigating early human adaptations at Oxford's Research Laboratory for Archaeology and the History of Art.

"The results are significant in showing that early humans in Sri Lanka were able to live almost entirely on food found in the rainforest without the need to move into other environments," Roberts said. "Our earliest human ancestors were clearly able to successfully adapt to different extreme environments."

Previous archaeological evidence has suggested humans set foot in rainforests as early as 45,000 years ago, but such research has proven inconclusive as to whether the trips were sustained and specialized, or brief and cursory.

Researchers say Sri Lanka and other densely forested islands in Southeast Asia forced the hands of the earliest humans. Whereas other places provided more open land for exploration and sustenance, people confined to small islands were forced to make due with what was available or move on.

"Our research provides a clear timeline showing the deep level of interaction that early humans had with the rainforest in South Asia," explained Mike Petraglia, co-author and Oxford archaeologist. "We need further research to see if this pattern was also followed in other similar environments in Southeast Asia, Melanesia, Australasia and Africa."

The study was published this week in the journal Science.