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The debate on modern human origins has often focused on the relationship …

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- The Context of Human Genetic Evolution

Molecular biology has revolutionized the study of human evolution. The importance of fossils as the primary source of information about our past has been steadily undermined as it has become possible to infer detailed aspects of recent human history from the distribution and frequency of genes found around the world today. To some extent, a fusion of paleontological and genetic approaches came about last year with the extraction and sequencing of ancient DNA from an extinct hominid, the Neanderthal type specimen (Krings et al. 1997), but this is likely to remain a rarity. Ancient DNA, however, has been able to confirm that humans and Neanderthals belonged to different populations over the last quarter of a million years and that Neanderthals did become effectively extinct.

These results fit in with an increasingly consistent interpretation of human evolutionary genetics. Compared with chimpanzees and other apes, the human population is relatively lacking in genetic diversity (Ruvolo et al. 1993); such genetic variation as does exist occurs primarily within populations rather than between (Relethford and Harpending 1994); African populations are more diverse genetically than those found anywhere else in the world (Vigilant et al. 1991; Cavalli-Sforza et al. 1994; Watson et al. 1997); and for the most part, non-African patterns of genetic variation can be treated as a subset of African ones. The chronological and demographic context of the processes of diversification has been strongly disputed, but several genetic systems indicate that living populations derive from a relatively small population (effective population size between 5000 and 50,000 individuals) (Rogers and Harpending 1992; Harpending et al. 1993; Nei and Takahata 1993; Harpending 1994). That size represents a bottleneck in the hominid lineage dating back no more than 200,000 years (Cann et al. 1987; Stoneking et al. 1992), with evidence of demographic expansion occurring in the last 70,000 years (Rogers 1995).

This emerging consensus has been interpreted in evolutionary terms as evidence for a recent African origin (i.e., ~200,000 years ago) for modern humans, followed by a dispersal out of Africa, with little or no interbreeding with other populations of hominid. Many anomalies and further details remain to be sorted out. The geographical clades of the mtDNA have been questioned (Maddison 1991; Maddison et al. 1992), and it has been suggested that much of the genetic evidence is consistent with a number of different models (Templeton 1992, 1993). Furthermore, tree building and mismatch techniques, the primary means of turning genetic information into evolutionary history, have been subject to dispute (Templeton 1992, 1993). Finally, estimates of the coalescence time of human beta-globin genes (~800,000 years) have also been used to argue against a recent African origin for modern humans (Fullerton et al. 1997), but they are not, in fact, inconsistent with the recent-origin model, as the coalescence time for a neutral autosomal locus would be expected to be roughly four times that of a mitochondrial gene, and selection may also play a confounding role.

Such disputes, though, should perhaps not distract from the general pattern: Genetics supports a single recent African origin for modern humans. This consensus leads to two important questions: First, what is the relationship between this evidence and the fossil evidence that it has been claimed, supports the so-called multiregional model? And more broadly, what can paleoanthropology contribute to a field where the bulk of the resources is being put into genetics? The answer to the first question is that it has been clear for a long time that the morphological evidence does not support a multiregional view. However, this has not percolated through to the genetic community. This apparent lack of communication between the two disciplines can be used to help provide an answer to the second question: that paleobiology provides a different sort of evidence, one that focuses specifically on the context in which human genetic evolution occurred. These themes will be developed below.

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