Our most recent estimated dates correlate with
sub-Saharan admixture in North Africa, which is continuous during the last few centuries (from the 13th century to the 20th century, see cluster L in fig. 5), as previously suggested by historical records (Newman 1995) and genetic data (Harich et al. 2010; Henn et al. 2012). However, it is noteworthy that
very precise dates are found in some cases in the 17th century in western clusters (see cluster K and M). The admixture dates in the 17th century could be the consequence of
the trans-Saharan slave trade that resulted from the Ottoman rule in North Africa and the arrival of the Crown of Castile and the Portuguese Kingdom to the West African seaports in the 16th century. The Iberian presence, driven by the search of a workforce in their recent settled Atlantic territories, modified the political and socioeconomic structure of Western Africa. This also intensified traffic through trans-Saharan routes to North Africa after the emergence of the sugar industry in this region and the Atlantic territories (Newman 1995; Oliver and Atmore 2001; Da Mosto 2003). Comparison of inferred ancestry proportions between the autosomes and X chromosome in Cluster M is indicative of sex-biased admixture with an overabundance of males with Middle Eastern (Syrian-like) ancestry and females with sub-Saharan African (Yoruba-like) ancestry. Moreover, we infer a lower proportion of sub-Saharan ancestry older than previously described in all admixture events dated from the first century B.C., which could be attributed to more ancient slave trade during the Roman or Islamic periods, such as the servile Haratin population of Nilo-Saharan origin in Berber groups such as the Sanhadja and Zenata (Newman 1995). Caution is warranted, however, as there are serious difficulties in reliably estimating the proportions contributed by each source population in the admixture events, mainly because the lack of a proper ancestral North African population. In our analyses, we have considered the population from Tunisia Chenini as the best proxy, but genetic drift in Chenini samples due to isolation and interbreeding might substantially underestimate the contribution of the autochthonous ancestral groups in extant North African populations.
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