Since I am now the father of a son my Y chromosomal haplogroup, R1a1a, has replicated itself one more time. That’s not a big deal seeing as it is probably the most widespread Eurasian paternal lineage. But why this particular lineage has the distribution it does is interesting and complex. In the early 2000s Spencer Wells published The Eurasian Heartland: A continental perspective on Y-chromosome diversity, which seemed to suggest that its expansion was due to that of the Indo-Europeans. Others have argued for an earlier diversification, due to divergences between European and Asian branches of R1a1a. A new paper puts these debates into deeper historical perspective, and nicely sums up where we are with uniparental lineages. Improved phylogenetic resolution and rapid diversification of Y-chromosome haplogroup K-M526 in Southeast Asia, with relevant bits:
These haploid systems are subject to stronger genetic drift and large evolutionary variance, and may not render accurate signals of population processes by themselves. Yet, the considerable geographic structure at these loci suggests that current patterns of variation may be informative of past population processes. With the implicit assumption that groups dispersing in the Pleistocene were small and experienced strong and long-lasting bottlenecks, patterns of mtDNA and NRY variation have been deemed useful as starting points to formulate hypotheses about human demographic history.
Note the caution. Also, I would take a slight issue with the last quoted sentence: the reason that haploid lineages were useful starting points had less to do with their utility in reconstructing the history of populations and more to do with technical constraints. Mitochondrial DNA is famously copious in comparison to nuclear DNA, while the nonrecombining nature of uniparental lineages makes them very amenable to transparent tree building. Rather than starting points, mtDNA and Y chromosomal lineages should be seen as informative supplements. With the rise of dense SNP chip technologies, and now whole genome sequencing, that’s they’re becoming.
This particular paper is interesting insofar as it traces back the emergence of the ancestor of R1a1a, and more generally R, among the panoply of non-African haplogroups. You can see on the map above that the light blue is basically absent from eastern Eurasian. Those are the R and Q lineages.The diversity of lineages in southeast Asia is very suggestive. Usually where lineages diversify they’ve been around for a while. Areas with lower diversity have often been settled later, and gone through diversity decreasing bottlenecks and such. The authors conclude:
In sum, our results support the hypothesis of a Southeast Asian/Oceanian center for the diversification of Oceanian K-haplogroup lineages and underscore the potential importance of Southeast Asia as a source of genetic variation for Eurasian populations.
The K group in question being the broader linage of which R is just a subset. I’m not sure that I buy the specific model here, but do note that it seems that the ~20,000 year old Siberian remain seems to have been of haplogroup R. And in the above paper they state “Interestingly, ancient DNA evidence suggests that haplogroup R1b – the current dominant lineage in western Europe –
did not reach high frequencies until after the European Neolithic period as given in Lacan et al 26,27 and Pinhasi et al. 28” In other words the results above would have seemed really strange a few years ago, as one would have previously though R was an ancient western Eurasian lineage just by its distribution. And it is today clearly a western Eurasian lineage, but did it always have to be so?
Recall that Y lineages are subject to “stronger genetic drift and large evolutionary variance.” The people today who tend to be carriers of the R lineages don’t exhibit any strong connection to the peoples of southeast Asia. But this one lineage may have risen in frequency among elite males at some point in a particular Eurasian population which came to be dominant. Rapid spread of males and constant intermarriage would dilute the whole genome signal quite rapidly, but the Y chromosomal lineage can maintain itself in the face of this because it does not mix. It replaces or is replaced. Chance may have increased the frequency of the eastern R lineage, but eventually it hitchhiked with destiny.
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