Tuesday, 4 December 2012

Disentangling the histories of mtDNA haplogroups M1 and U6

mtDNA haplogroups M1 and U6 are often mentioned in terms of Eurasian back-migration in Africa. The former is the only clade of the Asian haplogroup M which occurs in Africa at all; the latter is the only clade of the West Eurasian haplogroup U that does the same. These haplogroups also tend to co-exist in North and East Africa, although they are largely absent in sub-Saharan Africa. Different ideas have been offered for their occurrence, including a "Paleolithic" spread or a more recent one associated with the spread of Afroasiatic languages.

The new paper offers useful new data on this debate. The most important conclusion is that despite their oft-mentioned association, these two haplogroups appear to have distinct histories. One argument for this is their separate geographic distribution:

M1 (on panel A) is much more common in Northeast Africa and the Near East (including the Caucasus), whereas U6 (panel B) is more confined in Africa, and has its stronger peak in NW Africa, being rare in NE Africa.

An interesting aside, is that all the mysterious M1 from the Caucasus belongs to subclade M1a, while the smaller M1b clade tends to co-occur with M1a in other parts of Africa and the Near East. This indicates a founder effect for the origin of Caucasian M1a, but leaves open the issue of the immediate origins of M1. Hopefully it will become possible to place this haplogroup within the broader M phylogeny in the future.

The Bayesian skyline plots also contrast M1 and U6 in terms of their demographic histories:

The authors argue that these histories are inconsistent with either a very early dispersal history with the Dabban industry, as well as a more recent spread with Afroasiatic. From the paper:
The transition from the Middle Palaeolithic to Upper Palaeolithic in North Africa is characterised by the appearance of the “Dabban”, an industry that is restricted to Cyrenaica in northeast Libya and represented at the caves of Hagfet ed Dabba and Haua Fteah [19]. Whilst a techno-typological shift occurred within the Dabban ~33 KYA [19], starker changes in the archaeological record occurred throughout North Africa and Southwest Asia ~23-20 KYA, represented by the widespread appearance of backed bladelet technologies. The appearance of these backed bladelet industries more or less coincides with the timing of the Last Glacial Maximum (LGM) (~23-18 KYA), including: ~21 KYA in Upper Egypt [20]; ~20 KYA at Haua Fteah with the Oranian [21]; the Iberomaurusian expansion in the Jebel Gharbi ~20 KYA [22]; and the first Iberomaurusian at Tamar Hat in Algeria ~20 KYA [23]. The earliest Iberomaurusian sites in Morocco appear to be only slightly younger ~18 KYA [24].
A disassociation of these haplogroups from the UP in North Africa might be consistent with my idea that the UP was in part a cultural revolution that spread not only with people, but often with ideas across a species that already had the "biological machinery" for behavioral modernity and was already established in both Africa and the Near East.

As for the connection to Afroasiatic, the authors detect a linguistic correlation with M1a, which, however, appears too old to have been involved directly in the spread of this language family:
Concerning haplogroup M1 individually, a significant correlation with languages was observed. Furthermore, within M1, it appears that the correlation is mostly due to M1a. However, given the small sample size of M1b, any potential signal correlating with language might not be detectable. Interestingly, M1a has a likely East African origin, but its coalescent age of ~21 KYA still largely predates that of the proto-AA. Maybe a sub-clade of M1a would still give a similar correlation, but there are not sufficient samples to allow splitting M1a into its various sub-clades, and to test for a correlation. Although we found a correlation, limited sample sizes do not allow drawing unambiguous connection between genes and languages. Furthermore, it is also possible that this putative sub-clade of M1 does not testify for the expansion of AA speaking people, but was already present among the people who inhabited the area before the spread of the AA languages.
Personally, I am in favor of an East African origin of Afroasiatic, as this makes sense of various lines of evidence, one of which is the African shift of the "Southwest_Asian" component that is modal in Semitic populations. I envision that M1 was geographically circumscribed in a NE African population after its much earlier arrival from Asia and piggy-backed onto the expansion of Afroasiatic speakers, thus explaining the observed correlation. A good analogy would be with the expansion of, say, haplogroup H in the Americas which piggybacked on the European colonization, even though the coalescence age of H predates the arrival of Europeans in the New World by many millennia.

BMC Evolutionary Biology 2012, 12:234 doi:10.1186/1471-2148-12-234

Divorcing the Late Upper Palaeolithic demographic histories of mtDNA haplogroups M1 and U6 in Africa

Erwan Pennarun et al.

Abstract (provisional)
A Southwest Asian origin and dispersal to North Africa in the Early Upper Palaeolithic era has been inferred in previous studies for mtDNA haplogroups M1 and U6. Both haplogroups have been proposed to show similar geographic patterns and shared demographic histories.

We report here 24 M1 and 33 U6 new complete mtDNA sequences that allow us to refine the existing phylogeny of these haplogroups. The resulting phylogenetic information was used to genotype a further 131 M1 and 91 U6 samples to determine the geographic spread of their sub-clades. No southwest Asian specific clades for M1 or U6 were discovered. U6 and M1 frequencies in North Africa, the Middle East and Europe do not follow similar patterns, and their sub-clade divisions do not appear to be compatible with their shared history reaching back to the Early Upper Palaeolithic. The Bayesian Skyline Plots testify to non-overlapping phases of expansion, and the haplogroups' phylogenies suggest that there are U6 sub-clades that expanded earlier than those in M1. Some M1 and U6 sub-clades could be linked with certain events. For example, U6a1 and M1b, with their coalescent ages of ~20,000-22,000 years ago and earliest inferred expansion in northwest Africa, could coincide with the flourishing of the Iberomaurusian industry, whilst U6b and M1b1 appeared at the time of the Capsian culture.

Our high-resolution phylogenetic dissection of both haplogroups and coalescent time assessments suggest that the extant main branching pattern of both haplogroups arose and diversified in the mid-later Upper Palaeolithic, with some sub-clades concomitantly with the expansion of the Iberomaurusian industry. Carriers of these maternal lineages have been later absorbed into and diversified further during the spread of Afro-Asiatic languages in North and East Africa.