Expert: John Shea - 9/30/2008

Question
John - Regarding ‘Out of Africa’. Can it be explained why African populations retain aspects of the prognathous jaw of our presumed hominid ancestors, yet human strains whcih are the end-product of extended evolutionary periods outside Africa, do not? Does this feature argue against a subsequent return to Africa of migratory strains of people and a remixing of the gene pool. I see the advantage of losing dark skin pigmentation in northern climates (I presume early African polulations, our distant ancestors, were dark skinned), but what advantage might have brought about the flattening of the face in 'out-of-Africa populations?
(for ref. Zavier already rejected this question saying no homework questions please. It may look that way, but I have been retired for many years and school is a blurry, patchy memory).
Thanks

Answer
Tony
It may be a mistake to assume alveolar prognathism, which is pronounced among some NE Africans and some early Europeans, was a shared feature of all H.sapiens populations who dispersed out of Africa.  This character varies widely among both recent and ancient Africans (see Khoisan south Africans, not very prognathic all).  This African prognathism thing also gets exaggerated in a lot of the older phys. anthro. literature.
It is possible the populations who dispersed out of Africa into S. Asia were non-prognathic or that sexual selection of one kind or another made prognathism less common among these small dispersing populations.
The only "evidence" for a back-to-africa dispersal at the moment is some close similarities among SW Asian and N. African Mediterranean populations purportedly "dating" back to 30-40 Kya.  There are some similarities between Upper Paleolithic stone tool assemblages deposited in both regions around that time, but the African assemblages are unreliably dated, so one can infer migration in either direction. I put "dating" in quotes because I think there is a kind of circular logic going on with archaeology being used to date genetic divergences that are in turn cited as evidence for a biological cause for variation in the archaeological record.
Bottom line: Human populations densities in equatorial Africa are among the highest in the world.  Same is true, even if one looks just at hunter-gatherers.  An Eurasian human population dispersing back to Africa would have been "swimming against the tide" of expanding African human populations.
Cheers,
John Shea

PS: If you want a good recent book on this subject Steven Oppenheimer's book, The Real Eve, is pretty good and up-to-date.