Friday, 22 March 2013

Revised timescale of human mtDNA evolution (Fu et al. 2013)

An important new paper has just appeared in Current Biology. It is very exciting for a couple of reasons:
  • The paper uses the idea of branch shortening to infer dates for the mtDNA phylogeny. Briefly, if one counts differences between a present-day sample P and an ancient sample A, one can get a sense of when their most recent common ancestor X, lived. But PX and PA are not equal: PA is shorter, because A has "missed" a few thousand years of evolution (depending on its age). If we know the age of A (and this can be reliably known by direct dating for many samples), then we can infer the time of X.
Looking at the ages of many haplogroups, I don't see any that immediately strike me as inconsistent with recent published age estimates based on modern mtDNA alone, such as those from the Copernican reassessment paper. In any case, this is an exciting new application of an idea that will yield good dates for haplogroup ages in the future. For example, it could be used to date Y-chromosome lineages as well, when high-quality sequences of ancient human Y-chromosomes become available.
  • The paper also presents a number of new and exciting ancient DNA samples:

Please note that the "Cro Magnon" sample is actually revealed to be non-ancient in the study; that is another utility of the "branch shortening" idea, since it can demonstrate that purported very old samples are in fact fairly recent.
Note the important new samples from Dolni Vestonice, Oberkassel, Continenza, all of which are Paleolithic Europeans and all of which belong, without exception to subgroups of haplogroup U. It appears that Europe was indeed dominated by this haplogroup down to Mesolithic times, with a sharp discontinuity with early Neolithic Europeans.
The only outlier in the European context is Paglicci Str. 4b which is assigned to haplogroup H1. However:
Using this criterion, we excluded Paglicci Str. 4b from further analysis as the rate of C to T misincorporation at the 50 end was only 8.8%, thus making an ancient origin for the DNA in this sample uncertain [14].
Personally, I doubt there was any mtDNA haplogroup H in pre-Neolithic Europe, as the first author of this paper has also argued for in a previous one.
There is also a new sample from Boshan; it is probably too early to detect a pattern, but it is nonetheless noteworthy that it also belonged to mtDNA haplogroup B like the much earlier sample from Tianyuan.
Finally, the paper also discusses the issue of the mutation rate, noting that the divergence between Eurasian (M+N) and African (L3) mtDNA is much later than that inferred for autosomal DNA using new and "slow" de novo autosomal mutation rates. In my opinion there are two possible interpretations for this: one that there's something wrong with the slow rates, while another is that the earlier divergence using autosomal DNA may be a consequence of inflation due to admixture events in Africa.
Current Biology doi:10.1016/j.cub.2013.02.044
A Revised Timescale for Human Evolution Based on Ancient Mitochondrial Genomes
Qiaomei Fu et al.
Recent analyses of de novo DNA mutations in modern humans have suggested a nuclear substitution rate that is approximately half that of previous estimates based on fossil calibration. This result has led to suggestions that major events in human evolution occurred far earlier than previously thought.
Here, we use mitochondrial genome sequences from ten securely dated ancient modern humans spanning 40,000 years as calibration points for the mitochondrial clock, thus yielding a direct estimate of the mitochondrial substitution rate. Our clock yields mitochondrial divergence times that are in agreement with earlier estimates based on calibration points derived from either fossils or archaeological material. In particular, our results imply a separation of non-Africans from the most closely related sub-Saharan African mitochondrial DNAs (haplogroup L3) that occurred less than 62–95 kya.
Though single loci like mitochondrial DNA (mtDNA) can only provide biased estimates of population divergence times, they can provide valid upper bounds. Our results exclude most of the older dates for African and non-African population divergences recently suggested by de novo mutation rate estimates in the nuclear genome.

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