Razib Khan One-stop-shopping for all of my content

April 6, 2011

Africa in 12 ADMIXTURE chunks

Some have asked what the point is in poking around African population structure when Tishkoff et al. and Henn et al. have done such a good job in terms of coverage. First, it is nice to run your own analyses so you can slice & dice to your preference, and not rely on the constrained menu provided by others. There’s value in home cooking; you can flavor to your taste. Second, you never know what data people might leave on your doorstep. I’ve received the genotypes of three Somalis. Nothing too surprising, a touch more Cushitic than the Ethiopians in Behar et al., but interesting nonetheless.

Also, you can see how ADMIXTURE tends to come to weird conclusions in certain circumstances. Below is a K = 12 run ~50,000 SNPs. I’ve included in a few Behar et al. and HGDP populations to the Henn et al. set, as well as pruned a lot of the African groups which seem redundant in terms of information. I’ve added a few geographically informative labels as well.

Observe below that there is a Fulani cluster. I think this is pretty much an artifact. At K = 7 the Fulani have a majority component ...

March 31, 2011

The Bantu völkerwanderung

Filed under: Africa,Archaeogenetics,Bantu Expansion,Genetics,Genomics — Razib Khan @ 2:34 pm


Image Credit: Mark Dingemanse

I recall years ago someone on the blog of Jonathan Edelstein, a soc.history.what-if alum as well, mentioning offhand that archaeologists had “debunked” the idea of the Bantu demographic expansion. Because, unfortunately, much of archaeology consists of ideologically contingent fashion it was certainly plausible to me that archaeologists had “debunked” the expansion of the Bantu peoples. But how to explain the clear linguistic uniformity of the Bantu dialects, from Xhosa of South Africa, up through Angola and Kenya, to Cameroon? One extreme model could be a sort of rapid cultural diffusion, perhaps mediated by a trivial demographic impact. The spread of English exhibits this hybrid dynamic. In some areas (e.g., Australia) there was a substantial, even dominant, English demographic migration coincident with the rise of Anglo culture. In other areas, such as Jamaica, by and large the crystallization of an Anglophone culture arose atop a different demographic substrate, which synthesized with the Anglo institutions (e.g., English language and Protestant religion). The United States could arguably be held up as a in-between case, with an English founding core population, around which there was an ...

August 29, 2010

People, not pots, in Africa

324_1035_F5Last weekend I mentioned a paper, The Genetic Structure and History of Africans and African Americans, which had the best coverage of disparate African populations we’ve seen so far. The map to the left shows the various ancestral population clusters inferred from the samples they had. Really the only failing is that they didn’t have samples from Angola, Zambia, Zimbabwe and Mozambique. Unfortunately, that’s not totally trivial. These are regions which were effected by the Bantu Expansion, with southern Angola in particular still having remnants of Khoisan language speakers which likely attest to the pre-Bantu populations. Luckily for us innovation and scientific ingenuity are such that minor questions can quickly be answered because of how cheap the basic methods have become. A new paper in The European Journal of Human Genetics tackles Mozambique in particular, and discerns a heretofore unknown possible population cluster. A genomic analysis identifies a novel component in the genetic structure of sub-Saharan African populations:

Studies of large sets of single nucleotide polymorphism (SNP) data have proven to be a powerful tool in the analysis of the genetic structure of human populations. In this work, we analyze genotyping data for 2841 SNPs in 12 sub-Saharan African populations, including a previously unsampled region of southeastern Africa (Mozambique). We show that robust results in a world-wide perspective can be obtained when analyzing only 1000 SNPs. Our main results both confirm the results of previous studies, and show new and interesting features in sub-Saharan African genetic complexity. There is a strong differentiation of Nilo-Saharans, much beyond what would be expected by geography. Hunter-gatherer populations (Khoisan and Pygmies) show a clear distinctiveness with very intrinsic Pygmy (and not only Khoisan) genetic features. Populations of the West Africa present an unexpected similarity among them, possibly the result of a population expansion. Finally, we find a strong differentiation of the southeastern Bantu population from Mozambique, which suggests an assimilation of a pre-Bantu substrate by Bantu speakers in the region.

The main value-add of the research were the 279 individuals from Mozambique, who they plugged into previous data sets (e.g., HGDP, HapMap3). It must also be noted that they limited their genetic survey to ~2800 SNPs.This is sufficient for their purposes. Below are the figures of interest from the paper. Note immediately how Mozambique separates out at K = 4 in the first image. The subsequent figures are from PCA. The axes represent components of variation. The last panel shows a PCA plot transposed onto a map. In this case, PC 1 & PC3.t

The first figure is important because it suggests population structure we hadn’t known of in the Bantu Expansion. This doesn’t mean that it should be surprising. With Africa’s current level of genetic variation it seems implausible that the carriers of the Bantu culture would not have assimilated other groups along the wave of advance. In fact, as a cultural movement gains steam through positive feedback loops different societies may become co-opted into them, and spread the culture in their own turn. As an American example, I will give the Irish American Catholic hierarchy’s campaigns against German language parochial school instruction in the 19th century. Old English aside the language of the Irish was originally not English, but by the early 19th century apparently English had already become dominant among the Roman Catholic peasantry of Ireland. So they brought English, not Gaelic, to the United States. Similarly, the spread of Islam in India occurred predominantly under the ageis of Turks and Afghans, not Arabs, while the spread of Islam in Southeast Asia was promoted by South Asian Muslim merchants in their turn. So you have Arab cultural forms in eastern Indonesia thanks to cultural expansions at two removes from the original Arab source (in fact, it could be argued that the Turks and Afghans were Islamicized through a Persian intermediate as well).

But it is the PCA plots which are of more curiosity for me. They note that it is the third component of variation which maps well onto geographic distance. In the paper they say:

This is the PC that is mostly correlated with geography…and the fact that it is the third rather than the first component, as would be expected if isolation by distance was the predominant force shaping genetic diversity…implies that directional population movements (such as the Bantu expansion) and barriers to gene flow (such as that between food producers and hunter gatherers) are more relevant than geographic distance to understand the genetic landscape of sub-Saharan Africa….

There were folk migrations in Africa. They might simply not have been the ones we are aware of, at least in our sparest conceptions. Those folk migrations were very recent, within the last ~2,000 years or so. Which is why the distinctive correlations between language and genes persist, especially on the outer edge of the wave of advance in southern Africa (in contrast, the Pygmies of the Congo have lost their native language, and the western Pygmies are highly admixed with their neighbors).

Citation: A genomic analysis identifies a novel component in the genetic structure of sub-Saharan African populations

Addendum: The life of Shaka may give us a clue as the disturbances which pushed the Bantu ever outward.

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