I suppose that this "Letter to the Editor" has to do with the content of the BBC piece I referred to in my earlier note ("Vive la BBC". I would love to get to the source of it all.
Thanks in advance,
Jacques Cinq-MarsI've read the paper. The basic story is that they discovered a heretofore unidentified mutational variant that is very common (nearly ubiquitous) in the Y chromosomes of New World populations, and is rare, but present, in several Asian populations. By examining the microsatellite loci of Y chromosomes bearing the mutation in Asia, they attempt to date the origin of the mutation in Asia, thus placing a limit on when the mutation could have reached the Americas. Their preferred time limit is 15,000 years ago.
Now a critical perspective: the estimate is basically garbage, because it is highly model-bound. None of the assumptions are particularly likely to be true. Most critical is not the mutation rate or the generation time, both of which could be wrong, but the population history of Asia, which they assume to be uniform and panmictic. And they have no error bars on their estimate--not worse than having no reason to believe their assumptions, but highly problematic for excluding an early entry to the Americas.
--John
First, my sincere apologies for this excessive delay in responding to your post and thanking you for the information. I particularly like your characterization of the "estimate(s)" these researchers have come up with. If only because I suspected this all along!
As I have said before, people dealing with chronometry in general, know or should know that all dates are not born equal and that "correlation games" have to be approached with great caution. This is even true, today, in the case of radiocarbon dating which, after all, has gone through half a century of experimentation, testing and transformations.
What brings this up again is the following piece from The Scientist (Website:
http://www.the-scientist.com/).
Marking the First Americans' Arrival
Maria W. Anderson
The Scientist 17 (10 ), Oct. 20, 2003.
Y-chromosome genetic markers show that people first arrived on the North American continent about 14,000 years ago, according to two papers in the American Journal of Human Genetics.1,2 This is more recent than previously thought; mitochondrial DNA (mtDNA) studies had suggested an entry date of 30,000 years ago.
Researchers led by Mark Seielstad, Harvard School of Public Health, identified a single nucleotide polymorphism (SNP) called M242 on the Y chromosome that appeared before, but close to, human entry into the New World. Chronologically, M242 falls between two previously identified Y-chromosome mutations: The M45/M75 changes developed in Asia before M242, while the M3 mutation arose later in Native American populations.1 The team used two different methods to date the first appearance of M242 in the Asian population.
Andres Ruiz-Linares, University College London, and his team used M242 to demonstrate that two migratory waves came from Siberia into the Americas.2 They also showed that the polymorphism is present in all indigenous American populations. Ruiz-Linares agrees with Seielstad's proposed entry date. "[It] fits very well with the standard archaeological data," says Ruiz-Linares. "We used an entirely independent approach [from the archaeologists], and the dates we came up with were the same."
References:
1. M. Seielstad et al., "A novel Y-chromosome variant puts an upper limit on the timing of first entry into the Americas," Am J Human Genet, 73:700-05, September 2003.
2. M. Bortolini et al., "Y-chromosome evidence for differing ancient demographic histories in the Americas," Am J Human Genet, 73:524-39, September 2003.
What I am particularly concerned about is the last sentence (quoting Ruiz-Linares) which flies in the face of (completely ignores) a broad range of interdisciplinary evidence (as I have noted before in an earlier post) that, on the contrary, indicates or, at the very least, suggests that, in all likelihood, human dispersal in the New World predates the LGM, and this, by quite a bit.
In my view, it is very clear demonstration that "dates are not born equal" and that (some) "molecular chronometry" and "chronometrists" definitely leave something to be desired. So do many North American prehistorians and archaeologists who rely on such "estimates" and, in so doing , keep adding to the circularity (confusion) of the short chronology arguments regarding the dating of the earliest human dispersals in the New World.
Jacques Cinq-Mars
