Homo erectus - a distant ancestor of modern humans - migrated from Africa to Java without the help of sophisticated tools, Carl Swisher, from the Institute of Human Origins near San Francisco, said.
New dating techniques revealed that an archaeological site in Java where H. erectus bones were found is twice as old as once thought, he said. The research will be published in Science tomorrow.
The journal says the date of 1.8 million years ago 'moves H. erectus out of Africa almost one million years before they were supposed to hit the road in the accepted version'.
H. erectus was supposed to have invented a primitive stone axe, but Dr Swisher's findings suggest the species of hominid got out of Africa before this invention.
'We're suggesting that Homo erectus got out of Africa, expanding into different environments for totally different reasons. It was probably necessary to exploit some environmental change,' he said.
Bernard Wood, an anthropologist at Liverpool University, said the findings meant the whole pattern of human evolution was much more complicated than once thought. He has suggested human evolution could have happened in more than one place.