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Matt Ridley is the author of provocative books on evolution, genetics and society. His books have sold over a million copies, been translated into thirty languages, and have won several awards.

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Out of Africa, but when?

My latest Mind and Matter column for the Wall Street Journal is about the exodus from Africa, either 125,000 years ago or 65,000 years ago.

Everybody is African in origin. Barring a smattering of genes from Neanderthals and other archaic Asian forms, all our ancestors lived in the continent of Africa until 150,000 years ago. Some time after that, say the genes, one group of Africans somehow became so good at exploiting their environment that they (we!) expanded across all of Africa and began to spill out of the continent into Asia and Europe, invading new ecological niches and driving their competitors extinct.

There is plenty of dispute about what gave these people such an advantage-language, some other form of mental ingenuity, or the collective knowledge that comes from exchange and specialization-but there is also disagreement about when the exodus began. For a long time, scientists had assumed a gradual expansion of African people through Sinai into both Europe and Asia. Then, bizarrely, it became clear from both genetics and archaeology that Europe was peopled later (after 40,000 years ago) than Australia (before 50,000 years ago).

Meanwhile, the geneticists were beginning to insist that many Africans and all non-Africans shared closely related DNA sequences that originated only after about 70,000-60,000 years ago in Africa. So a new idea was born, sometimes called the "beachcomber express," in which the first ex-Africans were seashore dwellers who spread rapidly around the coast of the Indian Ocean, showing an unexpected skill at seafaring to reach Australia across a strait that was at least 40 miles wide. The fact that the long-isolated Andaman islanders have genes that diverged from other Asians about 60,000 years ago fits this notion of sudden seaside peopling.

Sea levels were 150 feet lower then, because the cold had locked up so much moisture in northern ice-caps, so not only were most Indonesian islands linked by land, but the Persian Gulf was dry and, crucially, the southern end of the Red Sea was a narrow strait. Recent work by Prof. Geoffrey Bailey and colleagues from York University in Britain has shown that the gap was often less than 2½ miles wide for up to 60 miles. People would not have needed to move through Sinai and the inhospitable Arabian desert to reach the Indian Ocean shoreline. They could raft or swim across a narrow marine canal.

The story grew more complicated last year when a team led by Hans Peter Uerpmann of the University of Tübingen in Germany described a set of stone tools found under a rock overhang in eastern Arabia, dating from 125,000 years ago. The tools were comparable to those made by east Africans around the same time. This was when Arabia was wetter than today, but the Red Sea crossing was wider.

So maybe Arabia was colonized early and there was a long pause before the Beachcomber Express set off for southeast Asia? If so, the genetics of Arabians should show convergence on an ancient ancestor of more than 125,000 years ago. They don't: Recent research suggests a common ancestor only 60,000 years ago.

Two ways out of the impasse come to mind. One is that the Arabian settlers of 125,000 years ago died out and were replaced by a new exodus from Africa. The second is that there may have been back-migration into Africa to muddy the genetic water. Complicating the issue is the volcanic eruption of Toba, in Sumatra, around 74,000 years ago, which injected so much sulfurous dust into the high atmosphere that it caused prolonged droughts that might have come close to wiping out many human populations.

Prof. Bailey reckons the answer to these riddles lies beneath the waters of the Red Sea, where ancient coastlines, teeming with undisturbed archaeology, remain to be explored.