New fossil evidence suggests that humans adapted to living in tropical rainforests thousands of years earlier than previously thought, according to a recent study.

Scientists have long debated when exactly it was that we started calling these lush forests our home, with some arguing that the terrain was too difficult for early hunter-gatherers to navigate or hunt and catch food. However, an analysis of fossilized teeth discovered at a number of sites in Sri Lanka, dating back as far as 20,000 years ago, revealed evidence of a diet consisting of rainforest animals and plants, researchers reported in the journal Science.

Previously, it was believed that tropical forests were largely pristine, human-free environments until the Early Holocene period around 8,000 years ago. But now, these findings suggest otherwise.

"Humans have been manipulating and living within dynamic rainforest environments for at least 20,000 years and probably even longer," University of Oxford archaeologist Patrick Roberts, who led the study, told Reuters. "The lifestyle, as we can see, was dedicated rainforest subsistence."

Compared to more open landscape, dense rainforests present a number of challenges to humans, including thick vegetation that makes travel difficult, small, agile prey that are hard to catch, and plants and fruits that are often poisonous. And yet despite these conditions, it appears that people in Sri Lanka found a way to cope.

According to the researchers, these people hunted monkeys, giant squirrels, mouse deer, porcupines and other mammals, as well as freshwater and forest snails, while also eating nuts and starchy rainforest plants.

At least, that's based on an analysis of the carbon and oxygen isotopes in the teeth of 26 individuals from the area - dating back from 20,000 to 3,000 years ago. Only two of the teeth indicated a diet found in open grassland, though these fossils are the most recent, from around the time of the Iron Age when agriculture first developed in the region. The rest, however, show that our ancestors were tougher than we thought.

"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 added in a statement. "Our earliest human ancestors were clearly able to successfully adapt to different extreme environments."

While this study provides the earliest direct evidence of humans relying on rainforest resources, prior evidence suggests people may have taken to Sri Lanka's rainforests as early as 45,000 years ago. Though whether these inhabitants stayed there for the long term or were simply passing through remains to be seen.

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