John Goodrum
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« on: September 12, 2002, 11:25:49 PM » |
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I hope posting these excerpts doesn't constitute vile nefariousness.
"A much more extensive follow up to the Brown study was published in 1987 by Rebecca L. Cann, Mark Stoneking, and Allan C. Wilson. They examined the mtDNA of 147 individuals from five populations - African, Asian, Caucasian, Australian, and New Guinean - using RFLP analysis with 12 restriction enzymes. The authors could thus survey 467 sites in each of the 147 DNAs without sequencing, in total about 11 percent of the 16,569 nucleotide pairs encompassing the human mtDNA molecules. Of the 467 sites, 195 showed variation among the tested persons, revealing the existence of 134 distinct types of mtDNA among 147 tested molecules. A maximum parsimony tree based on these data consisted of two main branches, one bearing only African mtDNA samples, and the other samples from all five populations, including some African individuals. The authors concluded that all the mtDNA diversity they had uncovered stemmed from one woman postulated to have lived about 200,000 years ago, probably in Africa, and that all the populations examined, with the exception of the African population, had multiple origins, implying that each area was colonized repeatedly.
None of these conclusions were new; all had been stated explicitly or were implicit front the reports published earlier by Nei and Roychoudhury. Not even the application of mtDNA to the study of human evolution was unprecedented; in addition to the study by Brown, researchers from several other laboratories had published the results of similar studies prior to 1987. Nevertheless, all earlier studies were squarely ignored by journalists, while the paper by Cann and her colleagues triggered a media frenzy, the likes of which are usually reserved for prominent murder trials. Why this sudden journalistic entrancement?
Publication of the report in a top-publicity, high-fashion journal (Nature) and the growing sensationalism of science reporting were probably two major contributory factors. Another was the skillful advertising and the attractive gift-wrapping of the product. Allan C. Wilson, in whose laboratory the study was carried out, was not only a brilliant scientist, but also a gifted salesman, who knew how to choose a “sexy” topic and how to market it. The key slogan in marketing this particular product was contained in a sentence printed in the opening paragraph of the report: “All these mitochondrial DNAs stem from one woman who is postulated to have lived about 200,000 years ago, probably in Africa.” The article was accompanied by an editorial entitled “Out of the garden of Eden” that began with the sentence: “A paper by R. L. Cann, M. Stoneking, and A. C. Wilson on page 31 of this issue reports that Eve was alive, well and probably living in Africa around 200,000 years ago.” And with this opening salvo the circus performance featuring Eve in the leading role took off. Without a moment’s hesitation, the major newspapers, magazines, and TV channels snatched up the story and wiggled their hips to its music. Eve was in the headlines, Eve entered the textbooks, Eve became a marketing article in science.
There is nothing wrong in using a catchy metaphor to describe the results of a scientific inquiry, as long as it does not mislead. The Eve metaphor, however, is grossly misleading, for it invokes the false image of Homo sapiens stemming from a single “mother of us all”, as the headlines proclaimed. “We all share a common mother, our mitochondrial Eve” one textbook asserted soon after publication of the report by Cann and her coworkers. Metaphors such as this are bound to be taken literally. Indeed, the story was understood by many (some scientists included) as demonstrating the existence of a single woman 200,000 years ago from whom entire humankind is derived in the same sense as you and your siblings are derived from your mother.
This, however, is not what the data indicate. All mutations differentiating mtDNA molecules of persons alive today can theoretically be tracked down to a single ancestral molecule in which the first of these mutations occurred. This molecule was borne by a single woman who, however, was contemporaneous with several thousand other females, just as there is one female among the three billion women alive today who bears the mtDNA molecule that will be ancestral to all mtDNA molecules in the future. Any polymorphism present in the contemporary human population, whether borne by mitochondrial or nuclear DNA molecules, can theoretically be traced back to an ancestral DNA segment in which the first mutation arose. Indeed, if the mtDNA molecules do not recombine and are passed on strictly through the maternal lineage, as is generally believed, variation in the whole molecule can be traced back to a single woman. Tracking the various nuclear DNA molecules back to their common ancestor is more complicated because these molecules do recombine and pass through both female and male lines of descent. Nevertheless, it can be achieved using short DNA segments, commonly a single gene, which are not likely to have recombined during the period since their derivation from a common ancestor. The variants now found in the human population in the various nuclear genes therefore all have their single ancestors, male or female, thousands of Adams and Eves, each gene with a different ancestor in a different past generation. There must have been as many Adams and Eves as there are genes - close to 40,000 - each living at a different time and in a different place. Obviously, the Adam and Eve metaphor is not only misleading, but also meaningless, not to mention the fact that it is culturally chauvinistic, since to a good proportion of humankind, these two names mean nothing whatsoever."
Klein & Takahata, _Where Do We Come From? The Molecular Evidence for Human Descent_, Springer-Verlag 2002. p278-279.
JG
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