Nilotic father tongues

Back in the late 1990s as human genetic data started piling up, it became increasingly clear that there were a lot of language families where most speakers shared relatively recent common male-line ancestry, visible by looking at Y-haplogroups. George van Driem memorably turned this observation into the Father Tongue Hypothesis: that language expansions are typically male-led, with children often raised to speak their father's language rather than their mother's. Berber is one of the many families where this holds true; Afroasiatic, on the other hand, shows several quite different dominant Y-haplogroups depending on the subgroup, indicating a more complex story at an earlier stage. What about Nilotic?

Nilotic, the most geographically widespread family within the rather questionable "Nilo-Saharan" phylum, divides into three primary subgroups:

• West Nilotic was originally concentrated around the White Nile, in modern South Sudan, including such languages as Dinka, Nuer, and Shilluk. Medieval-era expansions brought Luo speakers as far south as Kenya.
• East Nilotic languages are spread from southern South Sudan down to Tanzania, including such languages as Bari, Turkana, and Maasai.
• South Nilotic languages are concentrated in mountainous areas of Kenya and Tanzania, including languages like Nandi and Kipsigis.

It turns out that each of these subfamilies has a reasonable correlation with a Y-haplogroup. West Nilotic shows high rates of A1b1b2b-M13 (62% Dinka, 53% Shilluk, 50% Kenya Luo, 38% Nuer, 22% Alur). Its northern members also have a high frequency of B (54% Nuer, 27% Shilluk, 23% Dinka), which is nearly absent from the more southerly ones (6% Kenya Luo, 0% Alur). A1b1b2b-M13 is also frequent, to a lesser extent, in East Nilotic (33% Karimojong, 28% Maasai and Turkana, 17% Samburu - but 0% Camus), though significant rates of B are recorded only for Karimojong (33%). In South Nilotic, on the other hand, A1b1b2b-M13 is much less frequent (13% Pokot, 10% Marakwet, 8% Ogiek, and so on down to 2% Datog and 0% Sabaot), with B even rarer (11% Pokot), and the plurality of lineages usually belong to E1b1b1-M35 - a Y-haplogroup otherwise notably associated with Cushitic and Nubian speakers (50% Ogiek, 46% Datog, 45% Marakwet, 38% Sengwer...) - or to E2. E1b1b1-M35 is not unknown further north, but is far rarer (20% Shilluk, 15% Dinka, 8% Nuer).

None of this looks much like the result of a single male-led expansion. An obvious interpretation would be that South Nilotic primarily reflects communal language shift, probably from Cushitic judging by the well-studied stratum of Cushitic vocabulary in these languages. One might reasonably postulate a classical male-led expansion to explain the spread of West Nilotic within South Sudan; but, if so, one is led to the conclusion (already plausible on linguistic and historical grounds) that the Luo expansion southwards involved considerable assimilation of local men, notably Bantu-speaking (the Bantu-associated E1b1a1-M2 accounted for 33% of Kenya Luo sampled). Such assimilation also appears probable in East Nilotic, for which I unfortunately lack data from South Sudan.

In a broader perspective, A1b1b2b-M13 is frequent in several far-flung "Nilo-Saharan" groups along the southeastern fringes of the Sahara whose languages are only very distantly related, if at all, to Nilotic: Fur (31%), various Sudanese Maban (26%), and even Cameroon Kanuri (27%). It does not, however, seem to be frequent among Nubian speakers, much closer at hand.

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I won't attempt to exhaustively reference this post, which is basically open notes on work in progress, but key sources include Wood et al. 2005, Tishkoff et al. 2007, Hassan et al. 2008, Gomes et al. 2010, and Hirbo 2011. Note that I've combined different samples for Nuer and Dinka.