Reply to Lancaster

In January 2010, we published in this journal a report¹ on the frequency distribution of the Y chromosome haplogroup R1b1a (R-V88) in Africa, where it can be found at frequencies as high as about 90%. This haplogroup (or its ancestor) most likely traces its origins back to Eurasia, but is presently found very rarely outside Africa.

In our original publication,¹ we described two important patterns in the genotyping data. The first observation was that the highest frequencies of the R1b1a haplogroup were found among Afro-Asiatic-speaking populations from the Central Sahel, with Chadic mostly contributing to this pattern. We have now extended our analysis to a further 258 unrelated male subjects from northern Cameroon (Table 1). As can be seen from Table 1, the extended data fully confirm the pattern originally observed.

Table 1 Frequencies (%) of the Y chromosome R1b1a and R1b1a4 haplogroups in Central Africa

The second observation was regarding a genetic contiguity between the Chadic-speaking peoples from the Central Sahel and several other Afro-Asiatic-speaking groups from North Africa, including Ouarzazate Berbers from Morocco, Mozabite Berbers from Algeria, Siwa Berbers and several Semitic groups from Egypt, and, possibly, different groups from Algeria,² Tunisia³ and Egypt,^{3, 4} with R1b1a frequencies ranging from 1 to 3% in Algeria to about 4% in Tunisia, to 26.9% in the Siwa. We interpreted these data by suggesting that they are more compatible with Ehret's hypothesis, which proposes that Chadic peoples arrived from the North through the Sahara (the ‘trans-Saharan’ hypothesis),⁵ rather than with Blench's theory, which states that Chadic-speaking pastoralists reached the Chad Basin through the Sahel from an eastern Sudanic Cushitic-Chadic motherland (the ‘inter-Saharan’ hypothesis).⁶

Considering the mitochondrial DNA, the populations from the Chad Basin also show some genetic peculiarities when compared with other populations living south of the Sahara. Mitochondrial DNA haplogroups L3f3⁷ and L3e5,⁸ which are uncommon in the sub-Saharan area, were found to be relatively frequent in the Chad Basin region, with estimated coalescence ages similar to those we obtained for the Y chromosome R1b1a haplogroup. The lineage L3f3 can be traced back over the millennia to L3f,⁹ and this led the researchers who analyzed L3f3 from the Chad Basin⁷ to propose ancient links between this haplogroup and Chadic-speaking peoples coming from East or North East Africa. However, the presence in North Africa of the supposedly autochthonous Chad Basin haplogroup L3e5^{8, 10} would seem to suggest another possible scenario, which is more compatible with the ‘trans-Saharan’ migration route.

In his Letter, Lancaster¹¹ revisits our original data and provides valuable comments on our paper. Following his own previous review,¹² he argues that our interpretation may have been affected by poor population coverage in relevant regions from East and North East Africa. We agree that data from those areas that are particularly important in order to discriminate between the two theories (eg, Eastern Egypt and Sudan) would be very important. As far as we are aware, there are no data for Eastern Egypt. The investigation by Hassan et al¹³ in Sudan fills in the map of North East Africa, at least in part, by providing Y-chromosome haplogroup data on additional relevant population samples. However, the power of these data is limited by the low level of resolution, as no R1b1 internal markers were analyzed. Furthermore, even if one assumes that all the R1b1 Y chromosomes found in Sudan harbor the V88 mutation, there are little data to support the hypothesis that these chromosomes are the product of an ancient migration from the East. The highest frequencies of R1b1 chromosomes from Sudan were observed in the Hausa, a Chadic-speaking population that has migrated from the West, and in the Copts, a population group that is known to be largely the result of recent migrations from Egypt over the past two centuries.¹³ By contrast, only two R1b1 chromosomes were found among the Beja in Sudan,¹³ confirming our previous results that Chadic-speaking populations are distinguished from Cushites, at least at the Y chromosome variation level.¹

In summary, currently available genetic evidence seems to favor our previous hypothesis¹ that the Y chromosome haplogroup R1b1a is a paternal genetic record of the proposed ‘trans-Saharan’ migration.⁵ It will be interesting to see how the proposed pattern develops as more detailed information about the phylogeographic structure of this haplogroup and a more refined method to estimate coalescence times become available.