The paper by Justin C. Tackney et al., (2015) Two contemporaneous mitogenomes from terminal Pleistocene burials in eastern Beringia, PNAS doi: 10.1073/pnas.1511903112, published today has found that the remains of a stillborn baby and a very young infant that were buried at the same time in a grave in Upward Sun River, Alaska 11,500 years ago, belong to Mitochondrial DNA types C1b and B2, and these "allowing us to refine younger coalescence age estimates for these two clades. C1b and B2 are rare to absent in modern populations of northern North America. Documentation of these lineages at this location in the Late Pleistocene provides evidence for the extent of mitochondrial diversity in early Beringian populations, which supports the expectations of the Beringian Standstill Model.".
Though it is behind a paywall, the information published online indicates the ages of "about 12,800 years ago for C1b and 12,000 years ago for B2" and that one of the paper's co-authors "O'Rourke suspects the real times were even earlier, but that nonetheless both 11,500-year-old infants were at or near the root of their respective genealogical trees".
Additionally, O'Rourke said "'You don't see any of these lineages that are distinctly Native American in Asia, even Siberia, so there had to be a period of isolation for these distinctive Native American lineages to have evolved away from their Asian ancestors. We believe that was in Beringia,'".
This value of 12 to 12.8 kya is interesting. Why not 20 or 25 kya? It is convenient that the these 12,000 years situate the origin of these haplotypes at the time of a purported Beringian standstill. Lets see how they were calculated.
The Supplemental information, on page 4 tells us that "... Mutational distances were converted into years using a corrected molecular clock proposed by ref. 27 or a whole-genome substitution rate of 2.67 × 10−8 sub per site per year".
And this is a critical point, because based on this mutation rate is that the authors have estimated the age of these mtDNA haplotypes.
However this mutation rate is questinable. For instance a value of 0.43 × 10−9 per site per year was reported for the 45,000 year old shin bone from Ust’-Ishim.
The difference between 0.43 x 10-9 and 2.67 x 10-8 (which I will express as 26.7 x 10-9) is 62 times!
I eliminated the previous paragraphs because I was using autosomal and mtDNA mutation rates and they are not comparable. (see the comment below dated 27/Oct.) However, the following is valid. Because as I mention in Comments below, the mtDNA mutation rates estimated by different authors vary and considerably, from 1.3 to 3.2 x 10-8. Even the range for a 95% Confidence Interval is too wide for my liking, so...
Fidgeting with mutation rates one can get a wide range of coalescence dates...and that means you can get mistaken ones too.
I posted in June 2014 about mtDNA C1 haplogroup and in that post, I cited a paper that gave the following age to C1b: 17.9 +⁄- 2.3kya. Which is at odds with the figure calculated by Tackney's team.
In that post, besides questioning the molecular clock (I always seem to be irked by this subject), I wrote: "Interesingly, C1 has a high values for nucleotide diversity indices, and show a South to North cline (with most variations in South America), indicating that it has deeper roots in the southern part of the New World or that bottlenecks reduced its diversity in the North. In my opinion in points at an older date of entry into America than those mentioned above.", which again seems to be at odds with such early (12 kya) dates and a late Beringian standstill.
Copyright © 2015 by Austin Whittall
Let's wait and see if they can get a Y chromosome haplogroup from the remains (but it seems both were girls).
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