Sunday, June 1, 2008
I had been driven by my own amateurish excercises on haploid genetic timing to begin pondering that the OOA epysode may need to be significatively older than the usual 70-60 milennia BP if we are to account for the the old ages that well studied Y-DNA clades like R1b or O3 are yielding in MRCA estimates.
But I just found some scholarly support and precisely for about the same ages I was pondering:
A Bokyo et al, Assessing the Evolutionary Impact of Amino Acid Mutations in the Human Genome. PLOS Genetics, 2008.
The paper is wider in scope, as it tries to find out how much of autosomal SNP diversity is adaptative and how much is neutral, an important issue on its own. But the part that most got my attention is the section of the Discussion about Inference of Demography. The best fit demographic models (table S1) for Africans and Europeans suggest that the former have been expanding since some 170,000 years BP (6800 generations x 25 years) and that Europeans probably had two expansions with a bottleneck (in between?). The first expansion was c. 130,000 BP and the second c. 14,500 BP. The duration of the bottleneck was of only some 2100 years (84 generations).
There are other two models that are theoretically somewhat fit for Europeans but both are simpler and not as well fit as the one above. They are:
One states that there was just one expansion (no previous bottlenecks) since c. 6500 BP (I'm sure creationists would love this one! - but it can't account for much older Africans, so forget it). From a rationalist viewpoint anyhow it makes not much sense as 6500 BP would be post-Neolithic, suggesting a total replacement of farmers by what? Indo-European hordes? Sumerian erratics? Megalith builders? Hard to make sense of. The corrsponding 4500 BCE date actually doesn't relate with any exapansion: it's rather the end of Neolithic expansion in Europe, save for some marginal areas, that were still hunting and gathering.
The other one suggests a single expansion since c. 22,000 BP (just before the LGM) and a brutal bottleneck of c. 193,000 years. So with this model Europeans would have been in the hiding since the very beginning of Humankind and would have only began expanding precisely in the coldest period of the Ice Age. Again hard to make sense of.
But the complex model makes some sense: expansion since after the LGM, as most agree with and a bottleneck that would not be too long: about half the LGM span. The problem comes with the other expansion date: c. 130,000 BP. It should be an OOA or post OOA expansion, I understand but normally the OOA age is dated to about half that age.
Yet I already mentioned that such an old OOA event is not imposible at all: not just there is presence of modern humans in West Asia and Northern Africa since about that age but also Indian paleolithic shows continuity before and after the Toba event and MP artifacts might be work of H. sapiens in the subcontinent since before 100,000 BP.
The OOA migration could certainly be older and I was already thinking on several grounds of dates that approach that one of c. 130,000 BP (more like 100-110,000 BP?).
Note: thanks to Razib for suggesting that this paper might be important, what got me to read it with some more attention.