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...