Not everything Cushitic south of the Horn is South Cushitic

Not all of what seems to be Cushitic genetic influences South of the Horn of Africa should be perceived as South Cushitic admixture which is something I've hoped to mention for a while now for various reasons.

Map of languages spoken in Kenya

Some are overt & obvious and likely known to anyone at all familiar with a country like Kenya which is that there's a strong "East Cushitic" presence (Lowland East to be exact) via the presence of Oromos, Rendilles & Somalis all over Northern Kenya with Somalis even being the predominant ethnic group in Northeastern Kenya for one.

The Azanians: A Cushitic speaking people

Other examples go more deep than that though with anthropologists like Daniel Stiles explicitly stating that the Azanians of antiquity seemed to be culturally more East Cushitic (as in more similar to Lowland East, Sidamic and Dullay speakers). [1]

Mainland Southeast Africa

Azanians were essentially a rather advanced culture in antiquity (going back to the 1st Century CE or earlier) which traded with outside groups like Greeks, Romans & South Arabians and are thought to have been found perhaps as far south as the Rufiji river in Tanzania. [2] It says something that upon examining sites associated with these "Azanians" in Northern Kenya; Daniel Stiles figured they looked more like an East Cushitic culture than a South Cushitic one.

"The solution that Mous proposes is that the Mbugu people originally spoke a form of (Old Kenyan) Cushitic but, probably some time after arriving in the Pare mountains (49), there was a rapid shift to a completely Bantu (Pare) grammar (83)
The IMb lexicon has various sources, including Nilotic (Maasai), Southern Cushitic (Gorwaa and related languages), Eastern Cushitic (represented nowadays by Boraana Oromo and Dahalo, but possibly closer to Yaaku (Heine 1975)), and Bantu (notably Shambaa and phonologically adapted NMb).  

Mous argues convincingly that the Eastern Cushitic contributions are from an Old Kenyan Cushitic source and constitute the oldest lexical elements in IMb. This hypothesis is consistent with the most likely migratory route of the Mbugu and, if we assume that the Mbugu have always had a cattle culture, it is supported by the fact that much of the most detailed cattle terminology in IMb is from this source (43). 

Mous addresses the possibility that some of the Eastern Cushitic words in IMb could have come from Taita (Sagala, Davida) which although Bantu has undergone significant Cushitic influence (see Ehret & Nurse 1981). Of the IMb words shared with Taita that Mous lists on p.36 none that I am aware of have cognates in Mijikenda, despite the fact that there has been a significant Mijikenda influence on Taita (Sagala). This suggests either that “the Taita words in Inner Mbugu are remnants of an Old Kenyan Cushitic presence” (36) or that they are later borrowings from Taita which were either taken before the Mijikenda influx or were selectively borrowed in order to ‘screen out’ obviously Bantu words. The first scenario seems the more likely."

Above are excerpts from a paper on the Mbugu language a mixed Bantu language from Tanzania with strong Cushitic elements some of which are not entirely South Cushitic but instead East/Eastern Cushitic in nature from a rather old source. [3]  And this linguist isn't alone in finding this influence since Roland Kießling found substantial East Cushitic influences in Mbugu as well. [4]

There's also the presence of lineages like E-V32 and E-V22 among groups like the Turkana in Kenya and even some Maasais like in a recent paper.

E-V32 is very common in East Cushites like Somalis & Oromos [3] and even "North Cushites" like Bejas [5] while E-V22 via this new paper (Trombetta et al. 2015) seems to possibly extremely common among Sahos who are Lowland East Cushitic speakers like Somalis & Oromos. [6] 

However E-V32 and E-V22 while common in various Nilo-Saharan speakers although those speakers of Nilo-Saharan are mostly from Western Sudan [8] as a friend notes below:

"Turkana are Northwest Kenyans and Maasai migrated from there recently, so it seems plausible that they have some East Cushitic admixture, which could explain the V32. Or it could be from their Nilo-Saharan ancestry, although V32 is mostly found in Western Sudan."

So these lineages could seriously be owed to East Cushitic input or perhaps not and I'm just speculating too much here. But it is a fact that there has been intermingling between Nilo-Saharans and East Cushites; a fact Rendilles are a living testament to with the substantial Nilotic influences in their culture that nearly everyone that's every studied them in one way or the other has noted.

I wouldn't at all put it beyond these Turkanas and certain Maasais to honestly have some East Cushitic admixture.

One thing I and some others have noticed that could be an autosomal distinction between South and East Cushites could be what turns up at the higher Ks of various ADMIXTURE analyses whether its in studies or those made by independent more genome bloggers running ancestry projects.

ADMIXTURE analysis from Hodgson et al. 2014 [7]

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And that's that like at K=5 (look at the numbers to the left on the ADMIXTURE analysis from Hodgson et al.) where Somalis and other Horn populations like Oromos and Habeshas alongside Wolaytas show more "Northern West Asian" like influences by showing "European" at this K alongside "Arabian".

And in various ADMIXTURE runs like HarappaWorld, Eurogenes K13 and Dodecad Globe13 where Somalis will consistently show "Mediterranean" to sometimes also "Caucasian" like influences while Maasais and some other seemingly mostly South Cushitic admixed will be utterly bereft of such influences in the same analyses (granted, the Maasai show some "East Med" in K13 but no "West Med").

Myself and others think this may be indicative of West Eurasian ancestry carrying gene flow into populations like Agaws & Somalis that perhaps never managed to hit South Cushites.

But the thing is while I haven't managed to confirm this as most of these runs tend to add all of Pagani et al.'s Oromos together for these kinds of analyses which skews things given their heterogeneity; I've been told by some reliable individuals that Borana Oromos may also lack such "Mediterranean" to "Caucasian" like influences.

You can also easily see that Ari Culivators who are Horn residing Omotic speakers with non-negligible Cushitic admixture like the Maasai don't show such influences and while I can't of course confirm this; I think it's more plausible that the admixture in them might be East Cushitic rather than Southern but we'll need more data and perhaps ancient genomes to really be sure.

So the point is; this may not entirely be indicative of a true "East-South genetic divide" among Cushitic speakers but it's the closest thing we've got on an autosomal level that at least I and those I tend to discuss East African population genetics with have noticed. It could turn out to not be much or anything...

But this could still prove relevant in that if a Southeast African population via various ADMIXTURE analyses demonstrates more "Mediterranean" to "Caucasian" like influences this could be indicative of more recent gene flow from East Cushites like how Datogs/Datoogas show tiny hints of the pink "Early European Farmer" component (essentially an equivalent "Mediterranean" in this run) at K=20 in Lazaridis et al. 2013's ADMIXTURE analysis [8]:

Some of the Somalis used in this study in some cases may not be entirely "representative" and the "Afars" aren't actually Afars

But this is merely educated speculation on my part. We clearly need more study into this but nevertheless; the point behind this entire post is quite simple--- not everything Cushitic in Southeast Africa or South of the Horn of Africa, Sudan and South Sudan or Northern Kenya should be assumed to be entirely South Cushitic. There are noticeable influences from East Cushites via archaeology, linguistics and at the very least; genetic hints.

Reference List:

1. The Azanian Civilization Revisited, by Daniel Stiles

2. The Lost Kingdoms of Africa, by Gus Casely-Hayford

3. Review of The making of a mixed language: The case of Ma'a/Mbugu, by Maarten Mous, reviewed by Steve Nicole

4. South Cushitic links to East Cushitic, by Roland Kießling

5. Phylogeographic Analysis of Haplogroup E3b (E-M215) Y Chromosomes Reveals Multiple Migratory Events Within and Out Of Africa, Cruciani et al. 

6. Y-Chromosome Variation Among Sudanese: Restricted Gene Flow, Concordance With Language, Geography, and History, Hassan et al.

7. Early Back-to-Africa Migration into the Horn of Africa, Hodgson et al.

8. Ancient human genomes suggest three ancestral populations for present-day Europeans, Lazaridis et al.

Notes:

1. In Cruciani et al. E-V32 is listed E-M78γ ("The Gamma cluster", basically).

2. Although it's been brought to my attention via the massive Hirbo thesis that Iraqw do show E-V22 so it's rather E-V32 showing up in some of these Southeast African Cushitic admixed groups that perhaps has more of a dubious origin.