On Eurasian mtDNA

A quite lengthy discussion with (primarily) Terry Toothill at Dienekes' Anthropology Blog on Eurasian genetics and particularly mtDNA and the possible routes of migration and coalescence areas has caused me to review and re-consider Eurasian mtDNA genetics, often poorly understood because of higher complexity, less clear-cut nomenclature and somewhat less interest in general as of late.

After careful consideration of the genealogical tree (see Ian Logan's mtDNA site for reference), I came with the following simplified map:


Legend:

• Red large dots represent top level subclades of M, small red dots mean significative presence of an M subclade likely to have originated in some other region.
• Blue large dots represent top level subclades of N(xR), small blue dots mean significative presence of an N subclade likely to have originated in some other region (notably Australian N* related to West Eurasian W), blue squares represent N* (normally a single top-level N subclade but hard to confirm). An exception is East Asian N9-Y, actually related to R at its genesis but considered a separate haplogroup everywhere, which has been here considered like any other N(xR) subclade for simplicity.
  
• Green large dots represent top level subclades of R, small green dots mean significative presence of an R subclade likely to have originated in some other region. R is a derivate subclade of N, linked to N9-Y at its origin but because of its widespread distribution it is always considered separately.
• Grayed out areas are not really considered for various reasons.
  
• Regions are marked by gray borders and 3-letter abbreviations in black: WEA (West Eurasia), SOA (South Asia), AND (Andaman Islands), SEA (SE Asia), AUS (Australian Aborigines), MEL (Melanesia), EAS (East Asia), CAS (Central Asia), KET (Ket people of the Yenisei basin), NAM (North American Natives), SAM (South American Natives).
  

Discussion:

It seems self-evident from the top-level diversity (and central geographical position) that South Asia was with all likehood the place of coalescence of haplogroups M and R. The case is less clear for haplogroup N and the genesis of R itself is directly associated with a major East Asian clade (N9-Y).

Pre-R (i.e. the N-derived ancestor of R and N9-Y) could have been South Asian or not (in which case we would have to look to East Asia). But considering the context, I'm more inclined to think that pre-R lived in South Asia and that R is its direct derivate in that region, with N9-Y representing maybe an early migration into East Asia.

The context is not just geography and common sense but also the problem of N as such. If we are to follow the clue of highest top level diversity, West Eurasia stands (by narrow margin admittedly) as the most likely candidate for the origing of N. It is not a too solid conclusion but it does make some good sense anyhow.

Specially if, as I do, you suspect, based on archaeology, that the presence of H. sapiens in Asia may be as old as c. 100,000 BP, long before the Toba supervolcano event and the expansion of H. neanderthalensis into West and Central Asia.

I do understand that the main expansion of H. sapiens in Eurasia happened after these two events: towards the east after Toba (i.e. after c. 74,000 BP) and towards the west after a phase of losing ground to Neanderthals (i.e. after c. 60,000 BP).

But some H. sapiens may well have survived in West Asia after the Neanderthal expansion and, in my opinion they may well have carried haplogroup N, either still undifferentiated or in the process of evlution towards the modern West Eurasian N subclades (all them quite rare): N1, X and W. These N carriers would have also migrated eastward after Toba through South Asia, mixing to an extent with the carriers of M, who were already in the subcontinent. Together (more or less, the exact process is not known) they must have migrated towards East Asia and Sahul, roughly at the same time when N was evolving into "pre-R" (R plus N9-Y) and into "pre-W" (W plus directly related Australian N*), as well as when pre-R was evolving into R (soon after).

This expansive epysode must have happened soon after Toba, I think, because only something of the dimensions of the Toba catastrophe would have left so much empty land for such a massive migration, evident in the huge ammount of new clades at this genealogical "point".

After R had already coalesced (necesarily in South Asia), there was surely a "back-migration" towards West Asia that brought R (already evolving into U and maybe also into R0, aka pre-HV, and pre-JT). This wave eventually colonized Europe, as it's well known, and we do have a clear date for that: c. 45-40,000 years ago - and at least U subclades participated in that migration. So the whole process of divergence of N and then of "pre-R" and of R itself happened probably between 74,000 and 40,000 years ago. This was surely also the moment of diversification of M (I'd say that rather to the beginning, soon after Toba).

And this timeframe (c. 60-40,000 BP) is when we do have the best available archaeological evidence for expansion of H. sapiens east of South Asia and west of West Asia. Though archaeological evidence in some cases may be of later date than the actual events, specially in those areas not throughtly researched.

West and South Asia though, as well as North Africa, do have some evidence pointing to older presence of H. sapiens. In the case of West Asia and North Africa this includes the oldest skulls north of the Sahara and are dated to c. 90,000 BP (North Africa, modern solid datation) and c. 130,000 BP (West Asia, old contoversial datation).

In the case of South Asia, it has the oldest evidence worldwide of stone blade technology (dated c. 103,000 BP) as well as strong indications of survival and continuity through the Toba epysode. There are no human remains though until much later, possibly because the tropical climate (wet and hot) makes preservation less likely even for bones.

Below there is a simplified map for a plausible scenario:


Notes:

1. I have depicted migration into Sahul (Melanesia and Australia) as two separate events. This matter is not fully agreed upon and, admittedly, I depicted it that way on a mere whim.
2. There is an interesting problem regarding SE Asia: it must have been necesarily a passage for the migrations into East Asia and Sahul, yet it appears to keep a relatively low index of high-level diversity (excepted the Andaman islands, where not just two unique subclades of M but also a distinct fossil clade of Y-DNA D appear to suggest that the regional diversity was much higher in the past). In comparison East Asia appears as much more diverse. I guess that Terry might want to make a case out of this matter but, sadly for his hypothesis, Central Asia does not appear to show any autonomous top-level diversity at all. Only the rare Ket N* might suggest that some N-derived clades (but not anything else) might have been in Northern Asia (rather than Central Asia) since old but how N arrived there and exactly when is a total mystery.
   

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