There is a somewhat interesting new paper that finds some gender bias irregularities in the prehistory of our species:
They argue that there is a “female bias” (greater female transmitted diversity) in Africans and a “male bias” in non-Africans (Eurasians hereafter).
I broadly agree. But I think it needs a qualification: Africans in particular must be always studied with their whole structure, so any study that ignores the branches best represented by Khoisan and Pygmies is itself biased (it applies to many other papers, not just this one).
Similarly Eurasians are studied only in the two standard HapMap samples, whose representativeness for all non-Africans is more than questionable (South Asian and Negrito/Papuan/Australian Aboriginal, as well as Native American samples are also needed).
I’ll explain here the bias in my own terms (all the necessary simplifications should be taken with the lassitude they deserve):
- Southern branch (Khoisan-plus): no bias
- Main branch (all others): no bias (yet)
- Y-DNA B’CDEF, aka Y(xA)
- mtDNA L1″6
Second bifurcation (main group):
- Western branch (Pygmy-plus): no bias
- Main branch (all others):no bias (yet)
- Y-DNA CDEF, aka Y(xA,B)
- mtDNA L2″6
Third bifurcation (main group):
- Major African branch: female bias
- Y-DNA DE (E)
- mtDNA L2″6 (again, all sublineages in Africa)
- Eurasian branch: no clear bias (possible male bias)
- Y-DNA CF and DE (D)
- mtDNA M and N (< L3 < L3’4 < L3’4’6 < L2’3’4’6 < L2″6)
I understand that what is apparent in this third bifurcation, where there is a concentration of a particular male lineage, E (this is called female bias). In parallel there is a possible (but rather unclear, also rather mild a signal in the paper, see fig. 4).
It is very intriguing but hard to explain how this extinction of all male branch lineages between CDEF and DE (or rather E) in Africa happened, because there are nothing less than 16 coding region mutations between L2″6 and L3, what implies a lengthy period (but see PS below), and four clear successive branches in the line to L3 (L5, L2, L6 and L4).
We can maybe discard L5, L6 and L4 because they are small but L2 is not small at all, so there must have been some structure already in Central/East Africa in this period within the L2″6 population.
Even if we totally ignore the Eurasian branches, there is something odd with that African male-only bottleneck.
The authors explain this by polygyny, while Dienekes
rejects this and offers in turn the same explanation by another name:
… an alternative explanation is that the higher female/male ratio in Africans is due to the fact that they are descended from a relatively small number of males who overwhelmed the pre-existing African gene pool.
Hmmm, how is that different from polygyny?
I’m not really sure, sincerely, but what about the L2″6 population coalescing in a relatively small area (roughly Chad-Sudan-Ethiopia in my reconstruction) and this allowing for a case of male-biased drift that did probably have some component of that polygyny/overwhelming macho element in it, along with drift/founder effects?
PS – While there are 16 mutations between L2″6 and L3, most of them are upstream of the L2’3’4’6 node (i.e. after tiny lineage L5 branched out). Between L2’3’4’6 and L3 there are “only” 7 coding region mutational steps, less than half, the 16 mentioned above, and therefore also less than half the time. The branch leading to L5 can surely be safely ignored for this purpose, it is L2, L4 and the internal diversity of L3 which really cause the contradiction: the female gender bias.