Let us suppose the members of a tribe, practising some form of marriage, to spread over an unoccupied continent, they would soon split up into distinct hordes, separated from each other by various barriers, and still more effectually by the incessant wars between all barbarous nations. The hordes would thus be exposed to slightly different conditions and habits of life, and would sooner or later come to differ in some small degree. As soon as this occurred, each isolated tribe would form for itself a slightly different standard of beauty; and then unconscious selection would come into action through the more powerful and leading men preferring certain women to others. Thus the differences between the tribes, at first very slight, would gradually and inevitably be more or less increased. CHARLES DARWIN, THE DESCENT OF MAN
OUT OF AFRICA there are two routes. One is to travel above the northern tip of the Red Sea and through the Sinai desert into the Levant, the lands of the eastern Mediterranean. The other is to cross the Red Sea at its southernmost point, the Bab al-Mandab or Gate of Grief. For the ancestral people in their East African homeland, the vast desert to their north may have been a serious barrier. But the Gate of Grief was almost on their doorstep.
Today these straits are some 12 miles wide. But 50,000 years ago much of the world's ocean water was locked up in the glaciers of the Pleistocene ice age, and the straits would have been much narrower. At that date the water level
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