Abstract
BackgroundThe main unequivocal conclusion after three decades of phylogeographic mtDNA studies is the African origin of all extant modern humans. In addition, a southern coastal route has been argued for to explain the Eurasian colonization of these African pioneers. Based on the age of macrohaplogroup L3, from which all maternal Eurasian and the majority of African lineages originated, the out-of-Africa event has been dated around 60-70 kya. On the opposite side, we have proposed a northern route through Central Asia across the Levant for that expansion and, consistent with the fossil record, we have dated it around 125 kya. To help bridge differences between the molecular and fossil record ages, in this article we assess the possibility that mtDNA macrohaplogroup L3 matured in Eurasia and returned to Africa as basal L3 lineages around 70 kya.ResultsThe coalescence ages of all Eurasian (M,N) and African (L3 ) lineages, both around 71 kya, are not significantly different. The oldest M and N Eurasian clades are found in southeastern Asia instead near of Africa as expected by the southern route hypothesis. The split of the Y-chromosome composite DE haplogroup is very similar to the age of mtDNA L3. An Eurasian origin and back migration to Africa has been proposed for the African Y-chromosome haplogroup E. Inside Africa, frequency distributions of maternal L3 and paternal E lineages are positively correlated. This correlation is not fully explained by geographic or ethnic affinities. This correlation rather seems to be the result of a joint and global replacement of the old autochthonous male and female African lineages by the new Eurasian incomers.ConclusionsThese results are congruent with a model proposing an out-of-Africa migration into Asia, following a northern route, of early anatomically modern humans carrying pre-L3 mtDNA lineages around 125 kya, subsequent diversification of pre-L3 into the basal lineages of L3, a return to Africa of Eurasian fully modern humans around 70 kya carrying the basal L3 lineages and the subsequent diversification of Eurasian-remaining L3 lineages into the M and N lineages in the outside-of-Africa context, and a second Eurasian global expansion by 60 kya, most probably, out of southeast Asia. Climatic conditions and the presence of Neanderthals and other hominins might have played significant roles in these human movements. Moreover, recent studies based on ancient DNA and whole-genome sequencing are also compatible with this hypothesis.
Highlights
The main unequivocal conclusion after three decades of phylogeographic mtDNA studies is the African origin of all extant modern humans
Phylogeographic analysis In this study, we focus on the earliest periods of the out-of-Africa spread of modern humans and the likely return to Africa of the carriers of primary mtDNA L3 and Y-chromosome E lineages
Saudi Arabia has been identified as an important receptor of mtDNA Eurasian lineages, as well as those of African origin
Summary
The main unequivocal conclusion after three decades of phylogeographic mtDNA studies is the African origin of all extant modern humans. Estimates based on a molecular clock depend on the mutation rate employed [8, 9] These temporal windows for the exit of modern humans from Africa conflict with fossil, archaeological and ancient DNA data on the timing of the initial migration of anatomically modern humans (AMHs) out of Africa. Several archaeological studies uncovered Middle Stone Age (MSA) lithic assemblages, dated at approximately 125-75 kya, in different regions of the Arabian Peninsula, presenting affinities with northeastern African assemblages of the same period [12,13,14] These findings suggest that African AMHS may have extended their geographic range to eastern and northern Arabia, as well as south Asia, as early as 125 kya, long before the time frame of the migration that shaped the modern global genetic pool as suggested by molecular data
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