humans lived in rainforest
A view from Batadomba-lena rock shelter, the site at which the oldest fossils used in the study were found Patrick Roberts

Humans relied entirely on tropical rainforests to survive 20,000 years ago – 10,000 years earlier than previously believed.

Previously, it was thought early man could not have survived in rainforests alone because they were such dangerous environments – offering little food and being difficult to traverse.

However, more recent studies have increasingly pointed to humans living in the rainforest up to 45,000 years ago, but hard evidence has proved elusive.

Researchers at the University of Oxford have now analysed the fossilised teeth of a human that lived in Sri Lanka 20,000 years ago to find out what their diets consisted of. Findings showed it was primarily foodstuffs found in rainforests, as opposed to plants and animals found in open habitats.

"They could get all of their resources and survive just by living in the rainforest," study author Patrick Roberts told IBTimes UK. "We were maybe expecting a little more mixing or a little more input of open resources ... It seems pretty stable 20,000 years ago – maybe these habitats were just very productive for them. Clearly we were very flexible."

early humans rainforest
Sri Lankan wet zone rainforest, taken near Batadomba-lena rock shelter Patrick Roberts

The study, published in the journal Science, analysed the carbon and oxygen isotopes in the teeth of 26 individuals from between 20,000 and 3,000 years ago. Previously, it had been thought humans did not occupy tropical forests until 12,000 years ago.

Findings shows almost all had diets sourced from the rainforests. Just two showed a diet that would have come from open grassland, and these were the latest samples from the start of the Iron Age, when agriculture developed in the region.

In a statement, Roberts said: "This is the first study to directly test how much early human forest foragers depended on the rainforest for their diet. 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. Our earliest human ancestors were clearly able to successfully adapt to different extreme environments."

Although exact foods could not be identified, he said: "Associated animals at these sites include monkeys, giant squirrels, porcupines, snails, nuts and other starchy plants. There seem to have some kind of technology to be able to kill these monkeys regularly as a reliable source of protein."

Roberts said he now hopes to extend the study further back in time, with older samples showing how early man lived, as well as possibly looking at other regions in the world.