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.
