In an international collaborative effort, Peter Underhill of Stanford University and colleagues have carried out a study of human Y-chromosome variation, by far the most comprehensive and informative ever conducted (Nature Genetics, 01 Nov 2000). Whereas it confirms the 'out-of-Africa' origin of modern humans, it suggests that our most recent common paternal ancestor ('Y-chromosome Adam') would have been about 84,000 years younger than our maternal one ('Mitochondrial Eve'). This suggests that our genetic blueprint evolved as a mosaic in which the modern pieces (of DNA) could have appeared at different times and spread within our population, each with its own pace. The study also suggests that natural selection might have put a specific pressure on the Y chromosome, and may increase understanding of male infertility in our times.
Fossil records have suggested that Homo sapiens of the sapiens variety, to which all of us belong, first appeared in Africa or South-West Asia, about 150,000 years ago, a time when the current brain size is also thought to have been fixed. It is thought that they later left Africa for Asia and Europe where the earliest fossils of our kind are about 50,000 years old. This out-of-Africa hypothesis was confirmed by studies of mitochondrial DNA, a peculiar part of our genome that comes exclusively from the mother. Based on these studies, our most recent common ancestor is thought to be a woman who lived in Africa about 143,000 years ago. Through a detailed analysis of the paternal lineage of more than 1,000 men from 22 different geographic areas, researchers propose that our most recent common ancestor was a man who lived in Africa around 59,000 years ago. How can these studies be reconciled? How could Adam and Eve have ever begotten us if they never met?
Paleontology has always had a problem with dates. However, the thoroughness of the current study as well as 13 years of mitochondrial DNA analysis indicate that fallible interpretations cannot explain the whole discrepancy with the Book of Genesis. Rather, they show that the different chromosomes now in favour among humans were not 'invented' all at once. Approximately 143,000 years ago, among the different mitochondrial DNA sequences floating around in the human population, one remained advantageous for its carriers and started to expand; eventually, all women carrying the other versions of mitochondrial DNA would leave no descendants. In parallel, different versions of the Y-chromosome were dispersed through the population as well, but it took 84,000 more years before one particular version of it started to take over in the human population. In other words, each chromosome evolved to its own drum and each had a common ancestor chromosome of its own, in very distant times. The other chromosomes, the X and the autosomes, first appeared even earlier, up to about 2,000,000 years ago, which is in the times of Homo erectus, before the separation between neanderthalis and sapiens (circa 600,000 years ago). Remarkably, the Y chromosome currently in vogue among modern men started to expand only very recently, suggesting that it conferred some natural advantage to its carriers' fitness. In these days of concern over a possible global decline in human sperm quality, hence in male fertility, the large set of new molecular markers provided by Peter Underhill and colleagues will prove an invaluable resource to medical research.
Dr. Peter A. Underhill
Department of Genetics
Stanford, California, USA
Telephone: +1 (650) 723-5805
Fax: +1 (650) 725-1534
Dr. Peter J. Oefner
Telephone: +1 (650) 812-1926
Fax: +1 (650) 812-1975
News & Views)
Prof. Colin Renfrew
The McDonald Institute for Archaeological Research
Telephone: (44) 1223-333-521
Fax: (44) 1233-333-536
(C) Nature Genetics press release.
Message posted by: Trevor M. D'Souza
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