December 5, 2012

Mitochondrial haplogroups M1 and U6

A new study has been published on the two most relevant African matrilineages of Eurasian origin: M1 and U6. There are others but these two are the ones, together with X1 maybe, who have a more typically African distribution with limited presence in Eurasia.

Erwan Pennarun et al., Divorcing the Late Upper Palaeolithic demographic histories of mtDNA haplogroups M1 and U6 in Africa. BMC Evolutionary Biology, 2012. Open accessLINK [doi:10.1186/1471-2148-12-234]

I must say that I am rather unpersuaded of what the authors have to say, very especially in regards to U6 (but not either on M1) but it is still a study with interesting data for the record.

Very briefly the authors use molecular-clock-o-logy (academic pseudoscience when used as alleged evidence of anything, being as it is mostly speculation with a mathematical pretext), along with very debatable archaeological interpretations, to propose that the expansion of U6 and M1 originate in the mid to late Upper Paleolithic and not, as Olivieri proposed in 2006 (and I rather support) from around the time of the (re-)colonization of West Eurasia c. 50-30 Ka ago.

Whatever the case they leave us with some data on these two lineages and some of their main subclades (not all). These can be found in table 1 and the supplementary materials. 

They also provided us with these frequency maps (enriched with approx. linguistic boundaries):

Absolute frequency of (A) M1 and (B) U6 (and approx. linguistic family boundaries)

Dabban industries

One of the contention points is the Dabban industries of Cyrenaica, which have been argued to be Aurignacoid and therefore correspond to the Homo sapiens (back-)migration into West Asia, Europe and probably also North Africa. The authors cite some references to question that the Dabban are derived from Palestinian technologies but I can only imagine (without getting into the matter in depth as of now) that the issue is under debate (as happens with other technologies from Europe, etc.)

A bigger problem is that no Dabban nor related industries are known to have existed in NW Africa, which is the region where U6 has its greatest basal diversity (clearly) and one of a few regions with greatest M1 basal diversity (the other two being Egypt and Arabia). 


U6 from NW Africa or what?

This is another element of the paper that irks me: that the authors happily reject the weight of basal diversity, which is clearly in NW Africa (Morocco notably) and around it (Canary Islands, Iberia). Yet the authors contend:

Whilst several U6 sub-clades seem to be confined to Northwest Africa, this pattern may be the result of drift and founder effects over many thousands of years and does not necessarily suggest that Northwest Africa was the geographic source of U6 dispersals in Africa.

The reasoning is simply not sound, contradicting the logic of greatest parsimony. What they say might hypothetically have happened but has a low chance, notably in absence of any other evidence.

Also the authors argue, based mostly on their own age estimates (which, I insist, are speculations, educated guesses, and cannot ever be used as evidence) that U6 may have arrived with Oranian (aka Iberomaurusian), of quite likely Gravetto-Solutrean South Iberian relatedness. This Western European connection is not only supported by some quite reasonable typological likenesses and mutual influences (back-tipping weapon points in Iberia may have an Aterian origin for example) and chronology of the sites (generally older towards the West) but also by anthropometric considerations (many of the best known Crô-Magnon type specimens, related to Gravettian industry in Europe, are North African Oranian) but also the fact that North Africa has loads of mtDNA of likely West European origin¹ (H1, H3, H4, H7 and V, amounting together to some 30% of the lineages of the region) and that Oranian ancient mtDNA was found to be consistent with this kind of mtDNA pool by Kefi in 2005² (however, as only HVS-I was tested for, the certainty is not 100% - but not your usual African L(xM,N) in any case).

So even if U6 would be of Oranian origin, it would still most likely have coalesced in Morocco (or somewhere nearby), something the authors seem reluctant to admit without offering any clear alternative.


And M1?

M1 has three regions of high basal diversity (only two subclades exist however M1a and M1b, making this kind of evaluation a bit less certain); they are: Arabia, Egypt and NW Africa.

The closest relative of this haplogroup is M20'51, which is found in SE Asia (Vietnam, South China, Indonesia, etc.) as well as Nepal. I do use this kind of "sister" references also to estimate the most likely origin jointly with basal diversity, and, in this case it suggests that M1 is from Arabia and that North African (and also East African and Highland West Asian) M1 is derived from this origin.

The authors don't seem to take a clear stand in this case either and they even do not mention the East Asian relative, insisting on analyzing only their molecular-clock-o-logic speculations, what are at best only marginally relevant.

However the concede that M1 and U6 were in Africa long before the Afroasiatic expansion.


In brief

In the end a somewhat messy study with too much emphasis in the wrong stuff but still good for the data. Check, please table 1 and, if you feel like researching the matter in greater depth the supplemental materials, which are no doubt informative in their own right.


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¹ See these papers:
  • H. Enafaa, V. M. Cabrera et al., Mitochondrial DNA haplogroup H structure in North Africa. BMC Genetics 2009. Open Access. [LINK]
  • L. Cherni et al., Post-last glacial maximum expansion from Iberia to North Africa revealed by fine characterization of mtDNA H haplogroup in Tunisia. American Journal of Physical Anthropology. Pay per view. [LINK]
  • Claudio Ottoni et al., Mitochondrial Haplogroup H1 in North Africa: An Early Holocene Arrival from Iberia. PLoS ONE 2010. Open Access. [LINK]
² R. Kéfi et al., Diversité mitochondriale de la population de Taforalt (12.000 ans bp - maroc): une approche génétique a l’étude du peuplement de l’afrique du nord. Anthropologie 2005. [PPT presentation direct download - Institut Pasteur]

5 comments:

  1. Sticking to the basics, one of the clear indications of the study is that U6 and M1, while often lumped together as a single old backmigration to African from West Eurasia, actually derived from completely separate historical migrations/expansions.

    Regardless of the time that these haplogroups appeared, U6 is almost completely within the area that was linguistically Berber immediately prior to the arrival of Arabic in the 7th century CE.

    Also, while it isn't necessarily the only source, the low frequency distribution of U6 in Iberia could quite plausibly have at least a partial source in the Moorish presence in Iberia from the 8th to the 15th centuries.

    I agree with you that U6 very likely predates AA expansion, however, so Berber was likely a result off language shift from a non-AA language among members of a pre-existing U6 rich NW African population. Indeed, it is quite remarkable how little NW African genetics had an eastbound influence in North Africa.

    Meanwhile, M1 with a couple of minor exceptions (Tanzania and South Sudan adjacent to AA areas, and the Pontic-Caspian steppe which looks like a long distance migration with a founder effect that it would be quite interesting to figure out historically), is within most of the range of the linguistically Afro-Asiatic area. Even if M1 was in Africa long before AA expansion, the argument that M1 expanded as part of AA expansion seems very plausible.

    Likewise, even if mtDNA molecular clocks aren't particularly well calibrated, the suggestion that U6 looks older than M1 is quite plausible. The case that U6 was common in NW Africa before M1 arrived there seems like a reasonable inference to draw from the data.

    As far as locating M1's source, I am also inclined to think that the overall fact that all Eurasian mtDNA is rooted in M or N, that M1 is the only branch of M that is predominantly African, that M1 has a relatively basal position in the M clade, and that M1 is most common at plausible points of Out of Africa migrations, all point of origins for M1 somewhere in the general vicinity of the Red Sea, regardless of which side it originated on. Geographically, M1 is right about where we would expect M to have branched off from L3, it is the proverbial apple that did not fall far from the tree.

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    1. I agree that M1 and U6 appear relatively unrelated to each other. I only partly agree with your statement of U6 being "Berber": yes about the greatest concentrations... but what about Ethiopia and in general the Nile, West Asia and notably Iberia?

      Iberian U6, as other North African influences E-M81, L1b... appear to have no relation whatsoever with the Muslim period (which only saw limited Berber colonization and effectively zero Arab one). The most clear evidence is that the lineages are not found in Southern but Western Iberia, including areas that were never or almost never under Muslim domination. The correlation is effectively impossible and we should look at Neolithic or Upper Paleolithic founder effects for an explanation.

      "... so Berber was likely a result off language shift from a non-AA language among members of a pre-existing U6 rich NW African population"...

      I think that the expansion of Afroasiatic (eventually Berber as we know it) in NW Africa is linked to extensive colonization, notably by male lineages E-M78 and J1 and also, to lesser extent maybe, matrilineages to be determined (maybe even some branches of U6a, M1a...)

      IMO this happened with Capsian culture, which is not (as the authors seem to claim) a local evolution from Oranian but appears to stem from Nubian/Upper Egyptian sources. Also their distribution is rather different, with Oranian being concentrated near the coasts and Capsian towards the interior (at least initially). Capsian did evolve into a local Neolithic however.

      The correlation of M1 with Afroasiatic is very much weak. The exceptions of East Africa are most notable and also if my basal diversity logic makes any sense, the Blue Nile are, which is probably at the origin of AA, is NOT the origin of M1. However some subclades of M1a, etc. may well have been scattered and stirred around with Afroasiatic expansion. But not the whole haplogroup.

      ... "the suggestion that U6 looks older than M1 is quite plausible"...

      I would tentatively agree with that but without much certainty: their respective (coding region) mutation counts or stems to the common ancestor L3 are longer for M1 than for U6.

      As for the origins of M1, I have already said in text: it looks Arabian but I could also accept Egypt or NW Africa if some other evidence would support such inferences.

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    2. PS- Actually I still think that Arabia is the best candidate for M1 origins because it's also the most logical source for the East African expansion. Arabia has the basal diversity, it is near the center of gravity of the spread of M1 (the Red Sea zone as you say, rather to the South) and it is also closest to the geography of its closest relatives from Asia, so Arabia with 99% likelihood.

      But for U6, I say Morocco (or Algeria?) and it's hard to argue for any other origin. But no direct relation with Berbers or Muslims by origin. Only the apparent backflow of U6a from the Nile (following Maca-Meyer here) could be attributed to the Capsian/AA expansion. Possibly M1b, M1a arrived to NW Africa also then but more detailed analysis would be needed.

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  2. U6 may have become U6 in Morocco or Algeria, but where did the parent from which it mutated to become the local U6 variant come from?

    U is not rooted in Africa. U is rooted in Eurasia. Therefore, U6 must have a Eurasian parent somewhere down the line. By what route did that parent end up in NW Africa? Did it come from Iberia across the Straight of Gibralter? Did it cross the Straight of Sicily? Did it arrive from SW Asia via Egypt and North Africa?

    Surely, given its distribution, a source that arrived in NW Africa via Iberia across the Straight of Gibralter is the most plausible scenario.

    Whether the mutation that defines U6 occured shortly after U arrived in NW Africa, or occured first in SW Europe and then migrated with the mutation defining U6 in place, is really immaterial to the larger prehistoric narrative that is the prize we are chasing after. This is getting overtechnical about the issue. Assuming that there has been some level of back and forth gene exchange between Iberia and NW Africa for thousands of years including the time at which the matriline ancestors of today's U6 women arrived in NW Africa arrived, it may be beyond the capacity of modern researchers to ever find an answer to this question for sure, even in theory.

    What we really want to know where the maternal ancestors of NW Africans came from, by what route, and more or less when. Ancestry informative mtDNA is merely a means to that end. And, the phylogeny of U6 and its geographic distribution clearly point to an ancestral source somewhere in Iberia at some point after the defining mutation for U and before most of the expansion of U6 that created the current mtDNA population genetic mix in NW Africa.

    The evidence strongly suggests that some woman or women who had either U6 mtDNA or a clade of mtDNA U ancestral to U6 must have crossed the Strait of Gibralter from Iberia to NW Africa at some point, and must have then been fruitful and multiplied. The evidence also, although less strongly, suggests that these women arrived in NW Africa sometime after modern humans arrived in Iberia (i.e. less than 43,000 years ago, give or take a few thousand years), but probably prior to the Neolithic revolution (i.e. more than 8,000 years ago, give or take a few hundred years). You and the authors, for somewhat different reasons and with less unequivocal evidentiary support, both see about 20,000 years ago as a particularly likely time given the archeological cultures on both sides of the Strait at the time and the rough age range suggested by the number of mutations defining this particular mtDNA haplogroup.

    In contrast, the evidence does not favor an interpretation in which the women who are matriline ancestral to today's U6 bearers arrived in NW Africa from an immediately source in either Sicily across the Strait of Sicily, or via Egypt and North Africa (not passing through Europe) from SW Asia. And, the evidence does not favor either an arrival of U6 in NW Asia with the Neolithic revolution or later, or at a time before modern humans first arrived in Iberia ca. 43,000 years ago. Moreover, of the archaelogical cultures present in NW Africa in that time frame, none seem more likely than the Oranian to have been the one in place in NW Africa at the time, given what we currently know, although there are alternative possibilities.

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    1. "... but where did the parent from which it mutated to become the local U6 variant come from?"

      The answer, considering the overall spread of U is rather evident: West Asia. However what is not fully clear is if it did via Egypt or via Europe. It does not seem to matter too much because it was a "parachutist" founder effect in any case and the track of the migration has been erased since then, if it ever existed at all.

      "Did it cross the Straight of Sicily?"

      That does not look plausible on light of the archaeological and genetic evidence. The other two options (Egypt or Iberia) remain open however.

      "Whether the mutation that defines U6 occured shortly after U arrived in NW Africa, or occured first in SW Europe and then migrated with the mutation defining U6 in place, is really immaterial to the larger prehistoric narrative that is the prize we are chasing after".

      Yes and no, because each scenario implies a somewhat different prehistoric narrative for NW Africa (and by extension Iberia and other regions). It would be nice to know but maybe we will never know: the process may have been too fast and subtle to leave any mark in a such a slow-ticking "clock" as is mtDNA.

      "The evidence strongly suggests that some woman or women who had either U6 mtDNA or a clade of mtDNA U ancestral to U6 must have crossed the Strait of Gibralter from Iberia to NW Africa at some point"...

      Not necessarily: they could well have arrived from West Asia via Egypt. I don't see why you claim that "the evidence does not favor an interpretation in which the women who are matriline ancestral to today's U6 bearers arrived in NW Africa (...) via Egypt and North Africa (not passing through Europe) from SW Asia".

      All we have is a dotted line ending in arrow from West Asia (origin of U overall) to NW Africa (origin of U6). That's the genetic evidence. Whether this happened with Dabban or a similar process still to be detected (Dabban itself is a recent discovery) or with the Iberomaurusian genesis (which may explain H and even V but does not seem consistent with U6 in the Iberia → Africa direction because there are no known Iberian precursors) is an open question.

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