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DNA from 4,500-year-old Ethiopian reveals surprise about ancestry of Africans
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[QUOTE]Originally posted by Troll Patrol # Ish Gebor: [QB] [b]S9. f4 ratio analysis shows that Mota has no component of West Eurasian admixture[/b] We used f4 ratio analysis (63) to formally assess the extent of back-migration to Africa by West Eurasians, using the same logic as adopted by Pickrell et al (58) . We quantified the West Eurasian component in Africa, using Yoruba as our non-admixed African reference, with the ratio f4(Han, Orcadian; X, Druze) / f4 (Han, Orcadian; Yoruba, Druze), where X is a contemporary African population or Mota. However, since Druze has a small level of West African ancestry, this f4 ratio is biased and does not show the desired fraction of West Eurasian component. To correct for this, we define λYoruba,Druze as the fraction of Druze-like (i.e. West Eurasian) ancestry population X, and F as the fraction of Yoruba-like (i.e. West African) ancestry in Druze (estimated in other studies to be F=0.05) (64). We can then write the following equation: f4 (Han, Orcadian; X, Druze) / f4 (Han, Orcadian; Yoruba ,Druze) = (1 - λYoruba,Druze – F) / (1 – F) (2) and solve for λYoruba,Druze for each population X. All statistics were computed with the F4Ratio program from the ADMIXTOOLS package. We first checked that subsetting to the SNPs available for Mota did not affect estimates for contemporary African populations (Table S5), which are in line with those estimated by Pickrell et al (58) using all available positions (Pearson Correlation r=0.9998). Mota does not show any evidence of a West Eurasian component, with a λYoruba,Druze value that is negative (-8.7%, ± 2.2%). This contrasts in particular with the Ari, their closest contemporary relatives, which show large West Eurasian components (17.8%±1.0% and 14.9%±1.2% for Ari Cultivator and Ari Blacksmith, respectively). We confirmed that such a difference is not due to a comparison of a single individual to population estimates by recomputing the f4 ratio for each individual belonging to an Ethiopian population in our dataset (Fig. S6). The absence of a West Eurasian component in Mota supports the dating of the backflow into Africa, which, at ~3.5kya, is younger than our ancient genome (dated to 4.5 kya). Given that Mota predates the backflow, it potentially provides a better unadmixed African reference than contemporary Yoruba. Thus, we recomputed the extent of the West Eurasian component in contemporary African populations using Mota, λMota,Druze, instead of Yoruba in our f4 ratio. By using this better reference, we estimated West Eurasian admixture to be significantly larger than previously estimated, with an additional 6-9% of the genome of contemporary African populations being of Eurasian origin (Fig. S6, and Table S5). Importantly, this analysis shows that the West Eurasian component can be found also in West Africa (Fig. S6), albeit at lower levels than in Eastern Africa. Importantly, a sizeable West Eurasian component is also found in the Yoruba and Mbuti, which are often used a representative of an unadmixed African population. [/QB][/QUOTE]
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