All About Eve:
Source: Scientific American 262 (April 1990): 22-.
A controversial theory tracing part of the genetic heritage of every human being to a woman who lived in Africa less than 200,000 years ago has drawn new support--and new opposition. The support comes from an ambitious worldwide genetic survey; the opposition is based on fossil evidence challenging the accuracy of the molecular clock on which the theory is based.
Molecular clocks are derived from mutations accumulated in several lines known to have diverged from a common ancestral stock. The molecular evolutional calibrates the clock by estimating the long-term mutation rate in cases where the time of divergence is known from the fossil record; the clock can then be applied to hitherto undated divergences.
In the mid-1980's a group led by Allan C. Wilson of the University of California at Berkeley based such a clock on the DNA encoding the genes of mitochondria, the organelles in the cell's cytoplasm that transduce energy. Because the mitochondrial genes are inherited only from the mother and therefore are not scrambled by sex, as the more numerous genes of the nucleus are, each occurrence of an all-male generation in a family puts an end of a mitochondrial lineage. Such extinctions eventually winnowed out the heritage of all but one woman, the so-called Mitochondrial Eve.
Wilson's group estimates the mitochondrial DNA (mtDNA) mutation rate at between 2 and 4 percent per million years, from which they date Eve to between 100,000 and 200,000 years ago. The workers constructed a tree of genetic differences among populations that represents non-Africans as twigs on an African branch, indicating that Eve lived in Africa. This determination has now been confirmed in a much wider genetic survey done by Linda Vigilant and other students of Wilson. "We think these results clinch the African origin of mtDNA," Wilson says.
Some paleontologists take issue with the assumptions behind the molecular clock, in part because it suggests that Eve's tribe spread recently and quickly throughout the world without interbreeding with indigenous hominids. In a paper presented at the February meeting of the American Association for the Advancement of Science, Milford H. Wolpoff of the University of Michigan contends such total replacement has no parallel in historical examples of conquest, no matter how great the technological superiority of the conquerors. It would imply, he says, that the African upstarts constituted a new species--modern Homo sapiens. That is impossible, he maintains, because fossil evidence shows modern human racial distinctions had already become evident hundreds of thousands of years earlier.
Wolpoff and two colleagues in Australia and China came to their conclusions by studying dental and facial traits of human fossils in Europe and Asia. Wolpoff says they analyzed only traits that could not have evolved in adaptation to local environments and that must therefore have been produced by genetic drift. Wilson dismisses this argument, suggesting that obscure mechanisms of natural selection might have caused the newcomers' resemblance to their unrelated predecessors. He is also critical of Wolpoff's data. "He is dealing with a handful of traits, and we have nearly 100," Wilson says.
Many paleontologists have resented the encroachment of molecular biologists on their bailiwick. Wilson and his co-worker Vincent M Sarich crossed that border in 1967, when their original molecular clock--based on the proteins albumin and hemoglobin--implied that humans, chimpanzees and gorillas had diverged from a common ancestor only about three to five million years ago, more than 10 million years later than had previously been supposed. The bold finding forced paleontologists to reexamine their bones and revise their date of divergence considerably.
Wolpoff discounts that success. "Even a broken clock is right twice a day," he says. "The molecular biologists were going to straighten out our fossils for us, but this time it's been turned on its headd: our fossils show that the Eve hypothesis does not hold up."
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