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Ancient west Eurasian ancestry in southern and eastern Africa 2013
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[QUOTE]Originally posted by the lioness,: [QB] http://www.pdf.investintech.com/preview/55bad1b8-03d7-11e3-8242-003048d80846/index.html Ancient west Eurasian ancestry in southern and eastern Africa 2013 excerpts We then applied our method for dating multiple admixture events to the eastern African populationsin these data (Supplementary Figure 24-38). Pagani et al. [2012] previously dated the earliest admixture events in Ethiopia to around 3,000 years ago, but with considerable variation between populations. We find evidence for multiple episodes of population mixture in eastern Africa; most populations have evidence for an early admixture event that we date to around 80-110 generations (2,400-3,300 years) ago (Figure 4). As insouthern Africa, the west Eurasian ancestry is present in the early admixture event. The dates we estimatein eastern Africa are almost uniformly older than the dates we estimate in southern Africa (Figure 4). One potential concern regarding this conclusion is that the southern and eastern African populations displayed in Figure 4 were genotyped on different genotyping arrays; however, this pattern remains when using onlypopulations typed on the same array (Supplementary Figure 39). [b]Back-to-Africa gene flow in eastern Africa. A major open question concerns the initial sourceof the west Eurasian ancestry in eastern Africa. The estimated mean time of gene flow in eastern Africa isaround 3,000 years ago, and the amount of gene flow must have been quite extensive, as the west Eurasianancestry proportions reach 40-50% in some Ethiopian populations (Table 1 and Pagani et al. [2012]).[/b] Archaeological records from this region are sparse, so [b]Pagani et al. [2012] speculate that this admixture is related to the Biblical account of the Kingdom of Sheba.[/b] However, archaeological evidence is not completely absent. During this time period, architecture in [b]the Ethiopian culture of D’mt has an “unmistakable South Arabian appearance in many details” [Munro-Hay, 1991],[/b] though there is some debate as to whether these patterns can be attributed to large movements of people versus elite-driven cultural practices [Mitchell, 2005;Munro-Hay, 1991]. Additionally, linguistic evidence suggests that this time period was when Ethiosemitic languages were introduced to Africa, presumably from southern Arabia [Kitchen et al., 2009]. It is perhaps not a coincidence that[b] the highest levels of west Eurasian ancestry in eastern Africa are found in the Amharaand Tygray,[/b] who speak Ethiosemitic languages and live in what was previously the territory of D’mt andthe later kingdom of Aksum.The hypothesis that west Eurasian ancestry entered eastern Africa through Arabia must be reconciled. with the observation that the best modern proxies for this ancestry are often found in southern Europerather than the Middle East (Supplementary Table 4). This observation can be interpreted in the context of ancient DNA work in Europe, which has shown that, approximately 5,000 years ago,[b] people genetically closely related to modern southern Europeans were present as far north as Scandinavia [/b][Keller et al., 2012;Skoglund et al., 2012].[b] We thus find it plausible that the people living in the Middle East today are not representative of the people who were living the Middle East 3,000 years ago.[/b] Indeed, even in historical times,there have been extensive population movements from and to the Middle East [Davies, 1997; Kennedy, 2008].West Eurasian ancestry in southern Africa. A second question is: which population or populations introduced west Eurasian ancestry into southern Africa? The best genetic proxy for this ancestry that we have found is the west Eurasian ancestry in eastern Africa (Figure 5). The most parsimonious explanation for this observation is that west Eurasian ancestry entered southern Africa indirectly via eastern Africa (though the alternative scenario of direct contact with an unsampled west Eurasian populationcannot be formally excluded; however, there is no archaeological, historical, or linguistic evidence of suchcontact). The relevant eastern African population may no longer exist. However, such a migration has beensuggested based on shared Y chromosome haplotypes [Cruciani et al., 2002; Henn et al., 2008] and sharedalleles/haplotypes associated with lactase-persistence [Coelho et al., 2009; Schlebusch et al., 2012] between the two regions. Furthermore, based on a synthesis of archaeological, genetic, climatological and linguistic data Güldemann [2008] hypothesized that the Khoe-Kwadi languages in southern Africa were brought to th eregion by immigrating pastoralists from eastern Africa. Our observation of elevated west Eurasian ancestry in Khoe-Kwadi groups in general (Table 1) is consistent with this hypothesis.Alternative historical scenarios. We note that we have interpreted admixture signals in terms of large-scale movements of people. An alternative frame for interpreting these results might instead proposean isolation-by-distance model in which populations primarily remain in a single location but individuals choose mates from within some relatively small radius. In principle, this sort of model could introduce west Eurasian ancestry into southern Africa via a “diffusion-like” process. Two observations argue against this possibility. First, the gene flow we observe is asymmetric: while some eastern African populations haveup to 50% west Eurasian ancestry, levels of sub-Saharan African ancestry in the Middle East and Europeare considerably lower than this (maximum of 15% [Moorjani et al., 2011]) and do not appear to consist of ancestry related to the Khoisan. Second,[b] the signal of west Eurasian ancestry is present in southern Africa but absent from central Africa, despite the fact that central Africa is geographically closer to the putative source of the ancestry.[/b]These geographically-specific and asymmetric dispersal patterns are mostparsimoniously explained by migration from west Eurasia into eastern Africa, and then from eastern tosouthern Africa. Conclusions. {b]Based on these analyses, we can propose a model for the spread of west Eurasian ancestry insouthern and eastern Africa as follows: first, a large-scale movement of people from west Eurasia into Ethiopiaaround 3,000 years ago (perhaps from southern Arabia and associated with the D’mt kingdom and the arrivalof Ethiosemitic languages) resulted in the dispersal of west Eurasian ancestry throughout eastern Africa.This was then followed by a migration of an admixed population (perhaps pastoralists related to speakers of Khoe-Kwadi languages) from eastern Africa to southern Africa, with admixture occurring approximately1,500 years ago.[/b] Advances in genotyping DNA from archaeological samples may allow aspects of this modelto be directly tested [URL=http://www.ephotobay.com/share/picture-24-88.html] [IMG]http://www.ephotobay.com/image/picture-24-88.png[/IMG][/URL] Table 1: Estimates of the proportion of west Eurasian ancestry in southern and easternAfrican populations. We estimated the percentage of west Eurasian ancestry in each southernand eastern African population as well as the Mandenka from western Africa (Methods). Shown arethese estimates for each population. Populations are sorted according to the estimated proportionof west Eurasian ancestry, and rows of southern African populations are shaded. Standard errorson all estimates ranged from 0.3% to 1.1%, with an average of 0.7 __________________________________________________ [URL=http://www.ephotobay.com/share/picture-23-95.html] [IMG]http://www.ephotobay.com/image/picture-23-95.png[/IMG][/URL] Table 2: Estimates of the proportion of Khoisan, putative eastern African, and putativeBantu-related ancestry in southern African populations, ordered by the amount ofputative eastern African ancestry. The Nama were excluded from this analysis because oftheir recent European ancestry. Additionally shown is the proportion of west Eurasian ancestryin each population as estimated by the linear model (these proportions are slightly different fromthose in Table 1). *The admixture proportions of the Ju|hoan North were fixed in this analysis [/QB][/QUOTE]
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