Genetics & Genealogy

Genes Reveal Adam Came Out Of Africa

by Tom Wilkie, Science Editor [The Independent: Thursday, 23 November 1995]

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Adam was black and lived in Africa 50,000 to 100,000 years ago, according to scientists who have detected traces of his genes in the DNA of modern man.

Taken together with earlier results on the evolution of women, the studies carried out by geneticists in the United States and at Cambridge University indicate that the whole of humanity may be descended from a small tribe of about 10,000 people, some of whom migrated out of Africa within the past 100,000 years or so.

Humans split off from their common ancestor with the chimpanzees and gorillas much earlier - between 4 million and 6 million years ago. But although these archaic forms of early humans were the first to spread out of Africa and populate much of the globe, it now appears they left no descendants.

Instead, a small group of anatomically modern humans evolved in Africa, splitting off from the archaic line. The descendants of this group, some of whom started migrating out of Africa one hundred millennia ago, have inherited the earth.

Men inherit from their fathers a special strip of DNA, known as the Y-chromosome. Several years ago, Peter Goodfellow, professor of genetics at Cambridge University, found the genetic "switch" on the Y-chromosome which makes embryos develop into males rather than females. His research student, Simon Whitfield, realised that because the Y-chromosome's genes are not shuffled as happens to other chromosomes, it would carry information about the lineages of humanity.

Mr Whitfield compared the rate at which Y-chromosomes from different human populations acquired mutations with the mutations in a chimpanzee's Y-chromosome. Assuming that they both mutated at the same rate, and knowing from archaeological evidence how long ago chimps and humans diverged, this set the molecular "clock" by which he judged how quickly the modern populations of humans have diverged.

He found less divergence between males than other researchers had found in earlier studies of the DNA passed down solely through the maternal line. He cannot explain the difference in population structure.

Although most molecular data tends to point towards an African origin, Mr Whitfield said, he warned that "we can't be too confident about the dates for the last common ancestor". It may be possible to disprove the competing idea that modern humans evolved at the same time from the archaic populations, but "out of Africa is the most parsimonious hypothesis".

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