A history of male migration in and out of the Green Sahara

Open access research highlight A history of male migration in and out of the Green Sahara, by Yali Xue, Genome Biology (2018) 19:30, on the recent paper by D’Atanasio et al.

Insights from the Green Saharan Y-chromosomal findings (emphasis mine):

It is widely accepted that sub-Saharan Y chromosomes are dominated by E-M2 lineages carried by Bantu-speaking farmers as they expanded from West Africa starting < 5 kya, reaching South Africa within recent centuries [4]. The E-M2-Bantu lineages lie phylogenetically within the E-M2-Green Sahara lineage and show at least three explosive lineage expansions beginning 4.9–5.3 kya [5] (Fig. 1a). These events of E-M2-Bantu expansion are slightly later than the R-V88 expansion, and highlight the range of male demographic changes in the mid-Holocene. North of the Sahara, in addition to the four trans-Saharan haplogroups, haplogroup E-M81 (which diverged from E-M78 ~ 13 kya) became very common in present-day populations as a result of another massive expansion ~ 2 kya [6] (Fig. 1a).

african-sahara-y-dna
Simplified Y-chromosomal phylogeny and inferred past or observed present-day distribution of relevant Y-chromosomal lineages. a Calibrated phylogenetic tree of Y-chromosomal lineages discussed in the text. Green shading represents the period when the present-day Sahara Desert was green and fertile. Lineages represented by filled pentagons have undergone very rapid expansions. b [featured image] The Green Sahara period 5–12 kya. Green shading indicates that the present-day Sahara Desert was green and fertile. The colors within the large oval represent the four Y-chromosomal haplogroups deduced to be present in the region at this time; specific locations are not implied. The arrows indicate the inferred origins of these haplogroups to the north or south, but specific origins and routes are not implied. c The present-day distributions of the four Green Saharan Y-chromosomal haplogroups. Yellow shading indicates the Sahara Desert. Each circle represents a sampled population, with the presence or absence of the four Green Saharan haplogroups shown by the colored sectors; other haplogroups may also be present in these populations, but are not shown. The small arrows indicate the inferred northwards and southwards movements of these haplogroups when the Sahara became uninhabitable.

Although Y chromosomes exist within populations and so share and reflect the general history of those populations, they can sometimes show some departures from other parts of the genome that result from differences in male and female behaviors. D’Atanasio et al. [1] highlight one such contrast in their study. Present-day North African populations show substantial sub-Saharan autosomal and mtDNA genetic components ascribed to the Roman and Arab slave trades 1–2 kya [7], but carry few sub-Saharan Y lineages from this source, probably reflecting the smaller numbers of male slaves and their reduced reproductive opportunities when compared to those of female slaves. The sub-Saharan Y chromosomes in these North African populations thus originate predominantly from the earlier Green Sahara period.

In this part of Africa, the indigenous languages that are spoken belong to three of the four African linguistic families (Afro-Asiatic, Nilo-Saharan and Niger-Congo). Interestingly, these languages show non-random associations with Y lineages. For example, Chadic languages within the Afro-Asiatic family are associated with haplogroup R-V88, whereas Nilo-Saharan languages are associated with specific sublineages within A3-M13 and E-M78, further illustrating the complex human history of the region.

The main question after D’Atanasio et al. (2018) is thus:

(…) what are the reasons for the very rapid R-V88 expansion 5–6 kya [1] and E-M81 expansion ~ 2 kya [6], and how do these expansions fit within general worldwide patterns of male-specific expansions, which in other cases have been linked to cultural and technological changes [5]?

I think that the only known haplogroup expansion that might fit today the spread and dialectalization of Afroasiatic, a proto-language probably contemporaneous or slighly older than Middle Proto-Indo-European, is that of R1b-V88 lineages. However, without ancient DNA samples to corroborate this, we cannot be sure.

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