(7 of 7)
The big question now: How does the apparent quick exit from Africa affect one of the most heated debates in the field of human evolution? On one side are anthropologists who hold to the "out of Africa" theory -- the idea that Homo sapiens first arose only in Africa. Their opponents champion the "multiregional hypothesis" -- the notion that modern humans evolved in several parts of the world.
Swisher and his colleagues believe that their discovery bolsters the out-of- Africa side. If African and Asian H. erectus were separate for almost a million years, the reasoning goes, they could have evolved into two separate species. But it would be virtually impossible for those isolated groups to evolve into one species, H. sapiens. Swisher thinks the Asian H. erectus died off and H. sapiens came from Africa separately.
Not necessarily, says Australia's Thorne, a leading multiregionalist, who offers another interpretation. Whenever H. erectus left Africa, the result would have been the same: populations did not evolve in isolation but in concert, trading genetic material by interbreeding with neighboring groups. "Today," says Thorne, "human genes flow between Johannesburg and Beijing and between Paris and Melbourne. Apart from interruptions from ice ages, they have probably been doing this through the entire span of Homo sapiens' evolution."
Counters Christopher Stringer of Britain's Natural History Museum: "If we look at the fossil record for the last half-million years, Africa is the only region that has continuity of evolution from primitive to modern humans." The oldest confirmed fossils from modern humans, Stringer points out, are from Africa and the Middle East, up to 120,000 years B.P., and the first modern Europeans and Asians don't show up before 40,000 years B.P.
But what about the new report of the 200,000-year-old human skull in China? Stringer thinks that claim won't stand up to close scrutiny. If it does, he and his colleagues will have a lot of explaining to do.
This, after all, is the arena of human evolution, where no theory dies without a fight and no bit of new evidence is ever interpreted the same way by opposing camps. The next big discovery could tilt the scales toward the multiregional hypothesis, or confirm the out-of-Africa theory, or possibly lend weight to a third idea, discounted by most -- but not all -- scientists: that H. erectus emerged somewhere outside Africa and returned to colonize the continent that spawned its ancestors.
The next fossil find could even point to an unknown branch of the human family tree, perhaps another dead end or maybe another intermediate ancestor. The only certainty in this data-poor, imagination-rich, endlessly fascinating field is that there are plenty of surprises left to come.