"...It is somewhat more probable that our earliest progenitors lived on the African continent as elsewhere," wrote Charles Darwin, modestly, in his Descent of Man. So began the "Out of Africa" theory of human origins. The sequencing of the human genome has added enormously to the data supporting this theory. If Homo sapiens did, indeed, begin their ascent in Africa then one would expect that a preponderance of the genetic variation of the species would be found there, and this is exactly what has been observed.
Nearly 100 percent of the genetic variation within the human species can be found in populations indigenous to Africa, whereas populations from other regions have a subset of this variation, consistent with migrations out of Africa.
Several studies have found that genetic variation declines with migratory distance from Africa. Though the details are still controversial, geneticist Spencer Wells argues for two major migrations. One group inhabited Southern India, Southeast Asia, and Australia, while another, representing over 90 percent of non-Africans now living, took a northern route, colonizing Europe, Northern Asia, and finally the Americas.
Aside from variability, one can consider the matrilineal mitochondria line or the patrilineal Y chromosome to get an idea of human origins. Mitochondrial Eve, the mother of us all, is reckoned to have lived two hundred thousand years ago in East Africa. Y-chromosome Adam didn't show up for at least another 50,000 years. The assignment of these bits of DNA to a single progenitor suggests severe genetic bottlenecks in our history.
Darwin supported the idea of a single genetic origin for the human race, but he was opposed vociferously by a number of prominent thinkers, among them Louis Agassiz, who favored a polygenic hypothesis, partly on Biblical (and possible racist) grounds. "We may conclude that when the principle of evolution is generally accepted . . . the dispute between the monogenists and polygenists will die a silent and unobserved death," wrote Darwin. And so, at last, it has.
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