Newsletter of the Association for the Study of Language In Prehistory
Issue 26 · Spring 1996
Maybe Not Out of Africa?
Alvah Hicks reports L.B.Jorde et al, 1995. "Origins and Affinities of Modern Humans: A Comparison of Mitochondrial and Nuclear Genetic Data". AM.J.HUMAN GENETICS 57: 523-528 (includes T.Jenkins, AR Rogers & 7 others). This is not simply quibbling but has substance. Alvah summarizes: "An evolutionary tree based on mtDNA displays deep African branches, indicating greater genetic diversity for African populations. This finding, which is consistent with previous mtDNA analyses, has been interpreted as evidence for an African origin of modern humans. Both sets of nuclear polymorphisms, as well as a third set of trinucleotide polymorphisms, are highly consistent with one another but fail to show deep branches for African populations. These results, which represent the first direct comparison of mtDNA and nuclear genetic data in major continental populations, undermine the genetic evidence for an African origin of modern humans. ..."
"Long branch lengths are seen for most of the African populations. This pattern has been observed in most other mtDNA analyses and has been a major component of the argument for an African origin of modern humans [references deleted -HF]. The non-African populations have comparatively short branch lengths and the nodes separating these populations are very close to one other. . . ."
"The HVS-2 data analyzed here show a similar departure from neutrality in Asians and Europeans, although it is not statistically significant. These departures may reflect the action of natural selection, or they could be the results of past population expansions (Rogers & Harpen-ding 1992). Since there is no recombination in the mitochondrial genome, natural selection on a coding gene will exert a substantial genetic 'hitchhiking' effect, even on polymorphisms in the non-coding D loop. It is thus possible that the differences seen here in mtDNA and nuclear DNA may be produced by natural selection rather than population history."
"Increased mtDNA diversity in Africans has been a linchpin of the argument that modern humans originated in Africa and then replaced existing archaic populations on other continents. Proponents of this view argue that since Africa is more diverse genetically, its population must be older (Stoneking 1993). However, diversity can be strongly affected by events in a population's history, such as the timing of major bottlenecks, and therefore does not necessarily reflect a population's age (Rogers & Jorde 1995). Our findings further compromise the diversity argument by showing that nuclear DNA trees lack the deep branches (and thus the excess genetic diversity) observed in mtDNA trees. These results do not disprove the African replacement hypothesis. However, they weaken the genetic evidence in its favor." (End of Jorde etal.)
In another summary Alvah gives this lively & quotable thought: "Eve is from Kansas!" The reader is referred back to Tishkoff et al who basically refute or at least rebut Jorde et al.
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(c) 1996 Association for the Study of Language In Prehistory