Dennell and Roebroeks get support for their proposal from other experts.
"I think this is an interesting and constructively provocative paper," said Chris Stringer, a researcher in the department of palaeontology at London's Natural History Museum.
"Evidence of humans in the Caucasus [region of Asia], China, and Java more than 1.6 million years ago implies either a very rapid spread from Africa after about 1.8 millions years ago, or that such populations were established outside Africa earlier than present evidence suggests," he said.
"I certainly think we should keep an open mind about the big picture."
The earliest tools found in Asia are routinely attributed to Homo erectus, a species known to have come from Africa.
H. ergasteran African species that many experts believe gave rise to H. erectusis assumed to have been the only primate capable of migrating out of Africa.
Experts cite its body formlong limbs, humanlike proportions, and a brain capable of figuring out how to hunt for meatas evidence that it was the only species suited to life in prehistoric Asian terrain.
This might be a persuasive argumentexcept for the fact that australopithecines, an older form of humanlike primates, had colonized the African savannah by 3.5 million years ago.
Similar grasslands extended across Asia at the time, suggesting that australopithecines could have survived quite well in the region, the authors say.
What's more, fossil evidence for H. ergaster in Asia in the early Pleistocene is weak.
No one yet knows where H. floresiensis first came from, but it may turn out that the diminutive species has its origins in Asia.
Stringer, of the Natural History Museum, sees this as a possibility.
"The unresolved status of the intriguing Flores finds attributed to H. floresiensis leaves open the possibility that this species is the end result and last survivor of an ancient migration of very primitive humans, or even prehumans, that formerly existed more widely across Asia."
So when did early humans first leave Africa? Could they have left as early as 2.6 million years ago, as soon as they started making stone tools?
"Hominins could easily have left Africa two million years ago," Dennell said. "After all, they certainly didn't need big brains or bodies to do so."
Maybe, he concluded, "the Dmanisi [Georgia] hominins are an extremely primitive version of H. erectus that is the ancestor of the H. erectus populations in both Java and those in East Africa.
"In other words, we might be looking at [human migration] 'out of Asia,' and not 'out of Africa.'"
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