Thursday, April 30, 2009
Just a meditation on starlike haplogroups.
I noticed yesterday that haplogroup H holds almost as much high-level diversity as M does and is clearly superior to N and R in this aspect. All them are starlike in structure, indicating a rapid expansion or demic explosion of some sort but the dimensions of that explosion can be deduced from the number of top-level sublineages. So we have:
M: 35 sublineages
N: 12 sublineages (apparently)
R: 15 sublineages
H: 28 sublineages (plus a good deal of H* still around)
If we take M as reference (div(M)=1), then div(N)=0.34, div(R)=0.43 and div(H)=0.80. That makes H the most directly comparable clade with M in these terms of sudden expansive drive. And I think we are pretty safe directly associating H expansion with the Aurignacian pulse c. 41-40 kya (only U8, or maybe just U8a, would also belong to it but as minority clade).
So what do we know about this Aurignacian pulse? That it was a very fast expansion in Central and Western Europe happening in less than 1000 years. If there were any Neanderthals around, they posed no effective resistance anyhow.
Can we say the same of the M pulse? Probably yes. We don't yet understand well the archaeological circumstances surrounding the M explosion but it must have been something similar to the Aurignacian expansive drive (even greater surely, as it produced 25% more lineages) . Actually no wonder: either if this event corresponds with either the arrival of early Eurasians to an "empty" and fertile Southern Asia or if it corresponds to the recolonization of that area after the Toba catastrophe, both scenarios would imply the fast colonization of a nearly empty land. Whatever the case, M was possessed by very strong dynamics.
What about N and R? They are also macrolineages and also show a notable starlike structure. But they produced about half (or even markedly less in the case of N) the derived diversity of M or H. I understand that this is because, in spite of their intrinsic vigour, they had to compete with the already estabilished M population.
This may explain why relatively few N-derived lineages show a presence in South Asia (probably the homeland of M expansion) and instead they seem to have succeeded in the fringe "frontier" areas (East Asia, Australia and West Asia). As I have discussed somewhere else, the expansion of N seems (maybe excepting Australia, where it is dominant, probably because of founder effect) rather weak and scattered in space and time. Out of the frontier areas, only R (and maybe N9) show some clear dynamism, which actually should be considered in their own terms rather than as part of the N pulse. Of course, the N pulse itself (the N node and its starlike structure) does show some notable dynamism and I speculate that it is related to the colonization of SE Asia, which N would have pioneered maybe.
R is a very interesting case because it did expand very vigorously, notably in the context of South Asia, which was with all likehood already colonized mostly by M descendants. This suggests that they possessed some novel technology or some other effective advantage that allowed them to expand ignoring or overcoming the pre-existent M/N hold of those lands.
Still R is minoritary (albeit diverse) in South Asia and, to a large extent, also in East Asia. Their initial success was only relative and it seems they expanded first through the already transited Indo-Pacific Y (or T) axis. While some eastern lineages (B, F, Q) managed to make themselves some room in East Asia, the really strong expansion in fact happened in West Eurasia.
In this western frontier area, R-derived R0, U and JT seem to have found the conditions to leave their mark and become clearly dominant. And this leads me back to macro-haplogroup H, completing the discursive circle.