September 24, 2005

Out of Africa vs. Multiregionalism, the debate that will not end

I was recently glancing through the excellent Human Evolutionary Genetics, and one of the opinion boxes in the book was titled "Modern Human origins - why it's time to move on". In it, Robert Foley and Marta Mirazon Lahr pronounce the victory of the out of Africa model and the death of multiregionalism:
The 'out of Africa' model of human evolution has basically proved to be empirically sound, and the field is now (at last!) moving on.
On the other hand, Erik Trinkaus, on a recent article on modern human emergence seems to reject the pure Out of Africa model as well as the regional continuity model:
Versions of the assimilation model have remained contenders for the interpretation of modern human phylogenetic emergence, if frequently overshadowed by the more polarized regional continuity (with gene flow) and (out of Africa with) replacement scenarios. The last two interpretations are finally intellectually dead. Both are contradicted by available evidence, and it is time for the discussion to move on. Yet, despite the general acceptance of some form of the assimilation model, issues remain.
So, perhaps we should move on, but where? Most anthropologists and geneticists today may reject the multiregional model, and accept that most recent human ancestry is derived from Africa, yet the existence and extent of non-African ancestry in modern humans is a matter of great controversy.

For example Michael Hammer and colleagues have recently published a paper which explicitly rejects the African-ness of a particular haplotype on the X chromosome. A paper on the non-admixture between moderns and Neanderthals, but also a paper on a 3-million year old polymorphism in Europeans which may have been introduced into the European gene pool by Neanderthals. Not to mention of lice speaking of modern human-erectus hybridization in Asia, a 2 million year old non-African polymorphism in Asians, 1.1 million year polymorphism in North Africa and the Middle East, and research which suggests that the fact that Africans have more ancestral alleles than non-Africans should not be interpreted as evidence of an African origin of humanity (all of them here).

Henry Harpending and Vinayak Eswaran have written a letter in the latest issue of Science, in which he takes issue with another article on ancient Out-of-Africa migrations and their Orang Asli descendants:
For example, nuclear loci rarely, if ever, show the low coalescence times (~200,000 years) seen in mtDNA, nor do they show strictly African roots. Indeed, there is now growing evidence of strictly non-African polymorphisms that date to before the birth of modern humans (1-5).
Vincent Macaulay and the other authors of the paper reply:
In cases where autosomal loci do have the necessary resolution, they suggest the replacement model (6-8). The discordant population-size estimates referred to by Harpending and Eswaran are likely more apparent than real, since these long-term values are usually obtained with the multiregional stipulation of random mating and constant population size. The analysis of overly simplistic models with methods that throw away what little information there is in most of these loci throws up straw men, such as the apparent lack of "strong signals of expansion" in some autosomal loci (9).
It is becoming awfully hard to keep up with the debate, especially since the experts themselves interpret the evidence in completely different ways. Should we despair of the ability of genetics to throw any light on our species' origin, and go back on discovering and measuring skulls? The African mitochondrial Eve discovery seemed to tilt the balance towards the Out of Africa hypothesis (first proposed forcefully by W.W. Howells), but our optimism that genetics would succeed where palaeoanthropology had failed may have been premature. Almost twenty years later, the discussion seems to have barely just begun!

What do you think?

Which model of human origins do you accept?
Out-of-Africa with Assimilation
Multiregional evolution

Free polls from
Read also, the Multiregional Stipulation Society by John Hawks.

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