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.

See also:

R1b-V88 migration through Southern Italy into Green Sahara corridor, and the Afroasiatic connection

palaeolithic

Open access article The peopling of the last Green Sahara revealed by high-coverage resequencing of trans-Saharan patrilineages, by D’Atanasio, Trombetta, Bonito, et al., Genome Biology (2018) 19:20.

Abstract:

Background
Little is known about the peopling of the Sahara during the Holocene climatic optimum, when the desert was replaced by a fertile environment.

Results
In order to investigate the role of the last Green Sahara in the peopling of Africa, we deep-sequence the whole non-repetitive portion of the Y chromosome in 104 males selected as representative of haplogroups which are currently found to the north and to the south of the Sahara. We identify 5,966 mutations, from which we extract 142 informative markers then genotyped in about 8,000 subjects from 145 African, Eurasian and African American populations. We find that the coalescence age of the trans-Saharan haplogroups dates back to the last Green Sahara, while most northern African or sub-Saharan clades expanded locally in the subsequent arid phase.

Conclusions
Our findings suggest that the Green Sahara promoted human movements and demographic expansions, possibly linked to the adoption of pastoralism. Comparing our results with previously reported genome-wide data, we also find evidence for a sex-biased sub-Saharan contribution to northern Africans, suggesting that historical events such as the trans-Saharan slave trade mainly contributed to the mtDNA and autosomal gene pool, whereas the northern African paternal gene pool was mainly shaped by more ancient events.

y-dna-r1b-v88-e-m78
Maximum parsimony Y chromosome tree and dating of the four trans-Saharan haplogroups. a Phylogenetic relations among the 150 samples analysed here. Each haplogroup is labelled in a different colour. The four Y sequences from ancient samples are marked by the dagger symbol. b Phylogenetic tree of the four trans-Saharan haplogroups, aligned to the timeline (at the bottom). At the tip of each lineage, the ethno-geographic affiliation of the corresponding sample is represented by a circle, coloured according to the legend (bottom left). The last Green Sahara period is highlighted by a green belt in the background

Also, interesting excerpts:

The fertile environment established in the Green Sahara probably promoted demographic expansions and rapid dispersals of the human groups, as suggested by the great homogeneity in the material culture of the early Holocene Saharan populations [62]. Our data for all the four trans-Saharan haplogroups are consistent with this scenario, since we found several multifurcated topologies, which can be considered as phylogenetic footprints of demographic expansions. The multifurcated structure of the E-M2 is suggestive of a first demographic expansion, which occurred about 10.5 kya, at the beginning of the last Green Sahara (Fig. 2; Additional file 2: Figure S4). After this initial expansion, we found that most of the trans-Saharan lineages within A3-M13, E-M2 and R-V88 radiated in a narrow time interval at 8–7 kya, suggestive of population expansions that may have occurred in the same time (Fig. 2; Additional file 2: Figures S3, S4 and S6). Interestingly, during roughly the same period, the Saharan populations adopted pastoralism, probably as an adaptive strategy against a short arid period [1, 62, 63]. So, the exploitation of pastoralism resources and the reestablishment of wetter conditions could have triggered the simultaneous population expansions observed here. R-V88 also shows signals of a further and more recent (~ 5.5 kya) Saharan demographic expansion which involved the R-V1589 internal clade. We observed similar demographic patterns in all the other haplogroups in about the same period and in different geographic areas (A3-M13/V3, E-M2/V3862 and E-M78/V32 in the Horn of Africa, E-M2/M191 in the central Sahel/central Africa), in line with the hypothesis that the start of the desertification may have caused massive economic, demographic and social changes [1].

Finally, the onset of the arid conditions at the end of the last African humid period was more abrupt in the eastern Sahara compared to the central Sahara, where an extensive hydrogeological network buffered the climatic changes, which were not complete before ~ 4 kya [6, 62, 64]. Consistent with these local climatic differences, we observed slight differences among the four trans-Saharan haplogroups. Indeed, we found that the contact between northern and sub-Saharan Africa went on until ~ 4.5 kya in the central Sahara, where we mainly found the internal lineages of E-M2 and R-V88 (Additional file 2: Figures S4 and S6). In the eastern Sahara, we found a sharper and more ancient (> 5 kya) differentiation between the people from northern Africa (and, more generally, from the Mediterranean area) and the groups from the eastern sub-Saharan regions (mainly from the Horn of Africa), as testified by the distribution and the coalescence ages of the A3-M13 and E-M78 lineages (Additional file 2: Figures S3 and S5).

green-sahara-r1b-v88-em-78
Time estimates and frequency maps of the four trans-Saharan haplogroups and major sub-clades. a Time estimates of the four trans-Saharan clades and their main internal lineages. To the left of the timeline, the time windows of the main climatic/historical African events are reported in different colours (legend in the upper left). b Frequency maps of the main trans-Saharan clades and sub-clades. For each map, the relative frequencies (percentages) are reported to the right

R-V88 has been observed at high frequencies in the central Sahel (northern Cameroon, northern Nigeria, Chad and Niger) and it has also been reported at low frequencies in northwestern Africa [37]. Outside the African continent, two rare R-V88 sub-lineages (R-M18 and R-V35) have been observed in Near East and southern Europe (particularly in Sardinia)[30, 37, 38, 39]. Because of its ethno-geographic distribution in the central Sahel, R-V88 has been linked to the spread of the Chadic branch of the Afroasiatic linguistic family [37, 40].

(…) the R-V88 lineages date back to 7.85 kya and its main internal branch (branch 233) forms a “star-like” topology (“Star-like” index = 0.55), suggestive of a demographic expansion. More specifically, 18 out of the 21 sequenced chromosomes belong to branch 233, which includes eight sister clades, five of which are represented by a single subject. The coalescence age of this sub-branch dates back to 5.73 kya, during the last Green Sahara period. Interestingly, the subjects included in the “star-like” structure come from northern Africa or central Sahel, tracing a trans-Saharan axis. It is worth noting that even the three lineages outside the main multifurcation (branches 230, 231 and 232) are sister lineages without any nested sub-structure. The peculiar topology of the R-V88 sequenced samples suggests that the diffusion of this haplogroup was quite rapid and possibly triggered by the Saharan favourable climate (Fig. 2b).

One of the theories I proposed in the Indo-European demic diffusion model since the first edition – based mainly on phylogeography – is that R1b-V88 lineages had probably crossed the Mediterranean through southern Italy into a Green Sahara region, and distributed from there throuh important green corridors, humid areas between megalakes. Even though this new study – like the rest of them – is based solely on modern samples, and as such is quite prone to error in assessing ancient distributions – as we have seen in Europe -, it seems that a southern Italian route (probably through Sicily) for R1b-V88 and a late expansion through Green Sahara is more and more likely.

If we accept that the migration of R1b-V88 lineages is the last great expansion through a Green Sahara, then this expansion is a potential candidate for the initial Afroasiatic expansion – whereas older haplogroup expansions would represent languages different than Afroasiatic, and more recent haplogroup expansions would represent subsequent expansions of Afroasiatic dialects, like Semitic, Hamitic, Cushitic, or Chadic – as I explained in an older post.

In absolutely shameless speculative terms, then – as is today common in Genetic studies, by the way, so let’s all have some fun here – instead of some sort of R1b/Eurasiatic continuity in Europe, as some autochthonous continuists would like, this could mean that there would be an old Afroasiatic – R1b connection. That would imply:

NOTE. Regarding the contribution of CHG ancestry in the Pontic-Caspian steppe cultures, it is usually explained as caused by exogamy, or by absorption of a previous population (as in the Indo-Iranian case), although a contribution of communities of mainly J subclades to the formation of Neolithic steppe cultures cannot be ruled out. As for some autochthonous continuists’ belief in some sort of mythical mixed steppe people with mixed haplogroups and mixed language, well…

nostratic-tree
Simple Nostratic tree by Bomhard (2008)

The Pre-Indo-European linguistic situation, before the formation of Neolithic steppe cultures, seems like pure speculation, because a) language macro-families (with the exception of Afroasiatic) are highly speculative, b) sound anthropological models are lacking for them, and c) migrations inferred from haplogroup distributions of modern populations are often incorrect:

  • Haplogroup R could then be argued to be the source of Nostratic, and earlier subclades the source of Starostin’s Borean, given the distribution of its subclades in Asia and the timing of their migrations.
  • But of course one could also argue that, given the comparatively late population expansions that Genomics is showing, supporting Western European linguistic schools – where Russian Nostraticists tend to date languages further back in timeR1b (and not R) expansion could be the marker of Nostratic languages, due to its most likely southern path (and their old subclades found in Iran and the Caucasus), which would be more in line with the wet dreams of Europeans proposing R1b autochthonous continuity theories. I like this option far less because of that, but it cannot be ruled out.

If you have read this blog before, you know I profoundly dislike lexicostatistical and glottochronological methods, and I don’t like mass comparisons either. Whereas these methods pretend to apply mathematics to big (raw) data where there is almost no knowledge of what one is doing, comparative grammar applies complex reasoning where there is a lot of partially processed data.

But, it is always fun to ask “what if they were right?” and follow from there…

See also:

Potential Afroasiatic Urheimat near Lake Megachad

palaeolithic-migrations

The publication of new ancient DNA samples from Africa is near, according to people at the SMBE meeting. As reported by Anthropology.net, a group by Pontus Skoglund has analysed new samples (complementing the study made by Carina Schlebusch), so we will have ancient samples of Africans from 300 to 6,000 years ago. They have been compared to the data of modern African populations, and among their likely conclusions (to be published):

  • Several thousand years ago, likely Tanzanian herders migrated far and wide, reaching Southern Africa centuries before the first farmers.
  • West Africans were likely early contributors to the gene pool of sub-Saharan Africans.
  • One ancient African herder showed influence from even farther abroad, with 38% of their DNA coming from outside Africa. 9-22% of the DNA of modern farmers, including the southern Khoe-San, comes from East Africans and Eurasian herders
  • Modern farmers, the ones as old as 500 years old, did have Bantu DNA in their genomes, but the ancient hunter-gatherers predated the spread of the Bantu.

Razib Khan, asked about the Afroasiatic homeland by David Reich, has taken this opportunity to publish his own hypothesis on the expansion of Afroasiatic, given the known Admixture analyses, using Y-DNA phylogeography, and with reasonable assumptions. He concludes that Afroasiatic expansion might also be associated with the western expansion of E1b1b subclades from a Levantine (“Natufian”) homeland.

I think it is necessary to remind everyone of the many problems unsolved by Indo-European studies – a much older discipline (and with more research published) than Afroasiatic studies. It is already quite revealing that we can’t still trace back Proto-Semitic to its homeland, and that Proto-Semitic is probably as old as Late Proto-Indo-European. We are talking, then, about an ancient proto-language – Afroasiatic – possibly older than Middle Indo-European (or Indo-Hittite), and whose dialects are still not well studied – but for the Semitic and Egyptian branches. Linguistic guesstimates or phylogenetic speculation date the proto-language (and thus the homeland) within a wide range, from 15,000 to 6,000 years ago.

There is an obvious trend (probably driven by Semitic and Egyptian researchers) to place the Afroasiatic Homeland near one of the many proposed Semitic homelands, i.e. in East Africa. This is similar to the trend seen in the first half of the 20th century in Indo-European studies, with most proposals locating the Proto-Indo-European homeland in Europe. European languages were the best known, and only the perceived antiquity of Vedic Sanskrit made some propose South Asian origins for the proto-language. However, it was only careful interpretation of linguistic finds, combined with archaeological data, what eventually yielded the Kurgan hypothesis, which has been since refined.

afroasiatic-homeland
A model for the homeland and expansion of Afroasiatic, from Wikipedia

Razib Khan’s proposal makes sense in that it fits what others have proposed before, i.e. an east African or Middle Eastern Afroasiatic homeland, and that it links it with the expansion of farming. However, we have to keep in mind that until 5,000 years ago the Sahara was not the desert we know: it had certain important green corridors, humid areas between megalakes. The Sahara might not have been exactly green 10,000 to 5,000 years ago (roughly the time when Afroasiatic must have been spoken), but it had certain regions that allowed for an east-west migration. However, it also allowed for a west-east migration, and – perhaps more importantly – for a sizeable population expansion in central Saharan territory. To forget that is to allow for potentially wrong assumptions to be made.

What we expect from the next papers on ancient African DNA samples are the result of certain (more recent) population – and thus potentially ethnolinguistic – movements, but they probably won’t solve the question of the Afroasiatic homeland, which has an older time span than the samples studied. There is a wide void in African prehistory – compared with Near Eastern history – and this research will be closing that gap, just like European samples are helping close the gap in the prehistory of western, northern, and eastern Europe, compared to the history of the eastern Mediterranean regions.

palaeolithic-europe-africa
Diachronic map of Paleolithic migrations of R1b lineages in Europe and Africa

I already wrote, regarding the potential ethnolinguistic link between Indo-European and Afroasiatic, that a close look at the migration of R1b-V88 lineages from Europe (through southern Italy?) into the Sahara – through the Fezzan-Chad-Chotts, and Chad-Chotts-Ahnet-Moyer megalake green corridors – could have been the key to the successful expansion of Afrasians.

Interesting aspects to take into account are the distribution of R1b-V88 lineages, compared to the location of Chadic languages (probably the most divergent and least known of the group) and to the potential North Afroasiatic (composed by Egyptian, Berber, and Semitic) and South Afroasiatic group (made of Cushitic and Omotic). Chadic has been argued to be connected variously to North Afroasiatic, or to the Berber branch, but the Northern group has also been argued to be connected with Cushitic, with Omotic as an independent branch. Also interesting would then be the potential connection between Indo-European (or Indo-Uralic) and Afroasiatic.

r1b-map
Modern distribution of haplogroup R1b, from Wikipedia

We could speculatively place the potential primary Afroasiatic homeland in the south-central Sahara, near the Megachad lake (i.e. near the peak of R1b-V88 lineages), with a secondary homeland in eastern Africa (as in the map above) – and maybe a tertiary homeland (of North Afroasiatic) in the Middle East, associated with the expansion of “Natufians” and E1b1b subclades. The identification of the spread of Afroasiatic languages with the expansion of R1b-V88 lineages needs an anthropological context (linguistic and archaeological) that is obviously lacking today.

It is important to keep all possibilities in sight when reviewing genetic analyses.

Related:

EDIT (16/7/2017): Added link to Neby’s post on a potential Semitic homeland, and Nature article on Schlebusch and Skoglund research.