In the post yesterday I reported what was generally known about the Horn of Africa, that its populations seem to lie between those of Sub-Saharan African and Eurasia genetically. This is totally reasonable as a function of geography, but there are also suggestions that this is not simply a function of isolation by distance (i.e., populations at position 0.5 on the interval 0.0 to 1.0 would presumably exhibit equal affinities in both directions due to gene flow). For example, you observe the almost total lack of “Bantu” genetic influence on the Semitic and Cushitic populations of the Horn of Africa, and the lack of Eurasian influence in groups to the south and west of the Horn except to some extent the Masai.
Tacking horizontally in terms of discipline, over the past few generations there has been a veritable cottage industry making the case for the recent origin of many ethno-linguistic populations through a process of cultural self-creation. Clearly there are many cases of this, some of them studied in depth by anthropologists (e.g., the shift from Dinka to Nuer identity). But there has been an unfortunate tendency to over-generalize in this direction. In some ways this is peculiar insofar as these models presuppose the infinite plasticity of culture without observing the sharp and strong norms which those very same phenomenon can enforce. The genetic isolation of non-Muslims in the Middle East after the rise of Islam seems rather well validated by the evidence from genomics. The norms of both Muslims and non-Muslims strongly biased them toward endogamy, and nature of Islamic hegemony and domination was such that Muslims were the ones who were likely to have cosmopolitan affinities with the “Islamic international.” In contrast, non-Muslim minorities began a long process of involution after the Islamic Arab conquests, only disrupted in the past century by emigration and to a lesser extent emancipation.
So back to the Horn of Africa. The vast majority of the people of the Horn of Africa speak an Afro-Asiatic language. Arabic and Hebrew are the most famous members of this group, but it is a very broad classification, ranging from the dialects of the Berbers in the Maghreb all the way to ancient Akkaddian. There are two large subfamilies of particular note and interest here: Semitic and Cushitic. The map above shows the distribution within the Horn of Africa. One can “quick & dirty” summarize the pattern here by observing that Semitic languages in Ethiopia tend to be concentrated in the north-central Christian highlands, while Cushitic is found everywhere else. Additionally, there is the confluence between religion and ethnicity, as there are Cushitic Muslims (Somalis, Afar, etc.) and Cushitic Christians (many Oromo, etc.). From what I can gather many Cushitic social and political elites have had a tendency toward assimilating into an Amhara Semitic identity (Haile Selassie’s mother was a Muslim Oromo). We could therefore generate a possible model where Semitic langauges arrived late to Ethiopia and spread through elite emulation, so the difference between Semitic and Cushitic peoples should be marginal in the genomic dimension (such as the marginal differences between Hausa and Yoruba in Nigeria). Or, we could posit that the Semitic element is distinctive from a pre-existent Cushitic substratum.
To make a long story short by running more ADMIXTURE with a Horn of Africa centered data set I have discerned that one can actually differentiate Cushitic and Semitic elements in the Horn and tentatively identify them with different ancestral components. First, the technical details….
In light of my last post I had to take note when Dienekes today pointed to this new paper in the American Journal of Physical Anthropology, Population history of the Red Sea—genetic exchanges between the Arabian Peninsula and East Africa signaled in the mitochondrial DNA HV1 haplogroup. The authors looked at the relationship of mitochondrial genomes, with a particular emphasis upon Yemen and the Horn of Africa. This sort of genetic data is useful because these mtDNA lineages are passed from mother to daughter to daughter to daughter, and so forth, and are not subject to the confounding effects of recombination. They present the opportunity to generate nice clear trees based on distinct mutational “steps” which define ancestral to descendant relationships. Additionally, using neutral assumptions mtDNA allows one to utilize molecular clock methods to infer the time until the last common ancestor of any two given lineages relatively easily. This is useful when you want to know when a mtDNA haplgroup underwent an expansion at some point in the past (and therefore presumably can serve as a maker for the people who carried those lineages and their past demographic dynamics).
What did they find? Here’s the abstract:
Archaeological studies have revealed cultural connections between the two sides of the Red Sea dating to prehistory. The issue has still not been properly addressed, however, by archaeogenetics. We focus our attention here on the mitochondrial haplogroup HV1 that is present in both the Arabian Peninsula and East Africa. The internal variation of 38 complete mitochondrial DNA sequences (20 of them presented here for the first time) affiliated into this haplogroup testify to its emergence during the late glacial maximum, most probably in the Near East, with subsequent dispersion via population expansions when climatic conditions improved. Detailed phylogeography of HV1 sequences shows that more recent demographic upheavals likely contributed to their spread from West Arabia to East Africa, a finding concordant with archaeological records suggesting intensive maritime trade in the Red Sea from the sixth millennium BC onwards. Closer genetic exchanges are apparent between the Horn of Africa and Yemen, while Egyptian HV1 haplotypes seem to be more similar to the Near Eastern ones.
Much of this is totally concordant with the results we’ve generated from the autosomal genome. Though the autosomal genome is much more difficult when it comes to implementing many of the tricks & techniques of phylogeography outlined above, it does offer up a much more robust and thorough picture of genetic relationships between contemporary populations. Instead of a a distinct and unique line of paternal or maternal ancestry, thousands of autosomal SNPs can allow one t o get a better picture of the nature of the total genome, and the full distribution of ancestors.
The map to the left shows the spatial gradients of the broader haplogroup under consideration, HV1. But what about the branches? Below is an illustration of the phylogenetic network of branches of HV1, with pie-charts denoting the regional weights of a given lineage:
Maju pointed me to a new paper on the genetics of Sudanese today. My interest was piqued, then not so much when I looked more closely. Genetic variation and population structure among Sudanese populations as indicated by the 15 Identifiler STR loci:
There is substantial ethnic, cultural and linguistic diversity among the people living in east Africa, Sudan and the Nile Valley. The region around the Nile Valley has a long history of succession of different groups, coupled with demographic and migration events, potentially leading to genetic structure among humans in the region.
We report the genotypes of the 15 Identifiler microsatellite markers for 498 individuals from 18 Sudanese populations representing different ethnic and linguistic groups. The combined power of exclusion (PE) was 0.9999981, and the combined match probability was 1 in 7.4 1017. The genotype data from the Sudanese populations was combined with previously published genotype data from Egypt, Somalia and the Karamoja population from Uganda. The Somali population was found to be genetically distinct from the other northeast African populations. Individuals from northern Sudan clustered together with those from Egypt, and individuals from southern Sudan clustered with those from the Karamoja population. The similarity of the Nubian and Egyptian populations suggest that migration, potentially bidirectional, occurred along the Nile river Valley, which is consistent with the historical evidence for long-term interactions between Egypt and Nubia.
We show that despite the levels of population structure in Sudan, standard forensic summary statistics are robust tools for personal identification and parentage analysis in Sudan. Although some patterns of population structure can be revealed with 15 microsatellites, a much larger set of genetic markers is needed to detect fine-scale population structure in east Africa and the Nile Valley.
The upside: nearly 500 individuals from a huge range of ethnic groups in Sudan. This is the level of population coverage you’d want. Most of the ethnic groups cover the sample size range from 10 to 50. The downside: only 15 microsatellite markers. About the same number as in the study which I critiqued earlier this week. This is just not a huge number. The authors did try very hard to prune the marker set to be ancestrally informative on this scale, but I think it’s pretty obvious that there are major shortcomings in their analysis. 15 STRs is probably useful for inter-continental genetic variation, but not for intra-continental differences. The paper is open access so you can read the whole thing, but I want to highlight a speculation which they offer based on their results: