THE first human ancestor to walk upright may have evolved in Asia, not Africa, according to a radical new theory.
In a major challenge to the "Out of Africa" theory, some academics believe fossil remains of Homo erectus found in Asia are up to 1.8 million years old - just as ancient as those found in Africa.
Professor Robin Dennell, of Sheffield University, and Professor Wil Roebroeks, of Leiden University in the Netherlands, say changes to the climate which resulted in the forests of Asia drying up to be replaced by a vast savannah grassland may have been the trigger for early humans to become bigger and to stand up on two legs.
It had been thought Homo erectus evolved in Africa and, driven by a new-found wanderlust innate in true humans, set off to colonise the rest of the planet. But the new theory suggests the ape-like australopithecines were found across Africa and Asia, rather than confined to the Rift Valley in East Africa.
Their remains were found in Chad, hundreds of miles from the Rift Valley, and Prof Dennell and some other academics argue that, if australopithecines were so widespread some two million years ago, there would have been no reason for them not to have lived in Asia.
They believe human-like remains discovered in Dmanisi in Georgia, once inaccessible as part of the Soviet Union, could be the "missing link" between australopithecines and Homo erectus and that an "Out of Asia" theory is now rivalling the established orthodoxy.
Prof Roebroeks said: "A famous pre-historian once said the cradle of mankind is a cradle on wheels. A hundred years ago it was in Asia, in Indonesia, and then it was in East Africa. The cradle has stayed for a long time in East Africa, but some think it might be in Asia again.
"We're not saying early humans came out of Asia, we're just saying the 'Out of Africa' story is very weak. We put question marks on a story usually adorned with exclamation marks."
He said what was known as "Out of Africa 2" - Homo sapiens are said to have evolved in Africa about 200,000 years ago and then spread out across the world - was stronger, because there was some genetic evidence for this.
The science writer Marek Kohn told New Scientist magazine the accepted sequence of events for "Out of Africa 1" had begun to unravel.
"Hominins [early human ancestors] clearly did not need long legs to stride out of Africa," he said. "There is no climate change in Africa coinciding with the emergence of the earliest known examples of Homo erectus around 1.8 million years ago. Nor does Homo erectus have any clearly identifiable immediate predecessors. At around the time Homo erectus emerged, selective pressures to evolve would have been greater in Asia than in Africa ... traces of a global cooling pulse starting around 2.5 million years ago have been detected in the soils of China's Loess Plateau.
"This would have brought monsoons and polarised the years into seasons, with summers becoming increasingly arid over subsequent millennia, causing the grasslands to expand.
"The Dmanisi hominins may represent a missing link in the evolution of Homo erectus, responding to climatic pressures. Australopithecines were adapted to open spaces in woodlands, ranging around relatively small areas, living off plants, seeds, small mammals and perhaps carcases. As these open spaces expanded into savannah, the Dmanisi hominins would have faced pressures to evolve more human-like traits, increasing the distances over which they ranged and turning more to animals as a source of food."