New dating of teeth from a cave in western Sumatra, Indonesia, suggests that modern humans were present in tropical southeast Asia earlier than previously thought.
'Using the latest investigative techniques we have been able to show that the teeth are definitely modern human, and they date from at least 63,000 years ago,' says Prof Chris Stringer, one of the Natural History Museum
's human origins experts and co-author on the study.
This early date complicates the current scientific consensus of when modern humans spread from Africa to populate the rest of the world.
'They add to a growing picture that Homo sapiens was spreading through Asia towards Australia before the date of around 60,000 years ago usually assigned to the main Out of Africa dispersal,' says Prof Stringer.
Old findings reassessed
Earlier work on the site and specimens was carried out by two Dutch palaeoanthropologists. Eugène Dubois originally excavated the Lida Ajer cave in the 1880s. Among his findings were two teeth. In the 1940s, Dick Hooijer identified the teeth as coming from modern humans.
However, many researchers have discounted the significance of these findings, as there have been continuing doubts over the dating of the site and the identification of the teeth.
'This cave has been shrouded in doubt since it was first excavated' says Dr Kira Westaway, lead author of the study from Macquarie University, Sydney.
'We employed a range of dating techniques from different institutions to establish a robust chronology that would, after 120 years, finally put an end to the uncertainty associated with the age and significance of these teeth.'
New dating techniques
The international team of researchers performed a barrage of tests on the site and specimens. They dated the human and other mammal teeth found at the site, the sediment around the fossils and the overlying and underlying rock deposits in the cave.
The scientists combined the resulting date ranges given by their tests to estimate that the deposit and fossils were laid down between 73,000 and 63,000 years ago.
The teeth push back the earliest skeletal evidence of modern humans in the islands of southeast Asia by 20,000 years.
Implications for Out of Africa timings
The current consensus among scientists is that modern humans first arose in Africa about 200,000 years ago. They then spread out from Africa around 60,000 years ago to populate the rest of the world.
This timing is supported by genetic studies on modern humans, which estimate when populations outside Africa started to diversify from each other.
However, scientists are building up a range of evidence, such as this current research, suggesting that modern humans may have dispersed earlier than this date.
'Either the genetic calibrations suggesting a late Out of Africa are wrong - or there must have been earlier, but perhaps ultimately unsuccessful, migrations of modern humans spreading far from our African homeland,' says Prof Stringer.
The study also illuminates how adaptable early modern humans were to living in new environments different from their African home, according to co-author Dr Julien Louys, formerly from the Australian National University. Between 73,000 and 63,000 years ago, Sumatra had a similar rainforest ecosystem as it has today.
'Living in a rainforest was not thought to be possible until only the last few thousand years,' says Dr Louys.
'This is because sourcing enough carbohydrates and proteins in dense canopy forests requires sophisticated hunting technology and knowledge that the first humans out of Africa would not have possessed.'
'However, here we have humans making use of such challenging environments as soon as they arrived in Sumatra.'