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Friday, March 26, 2010

Molecular clock and the Denisova hominin

Yesterday I echoed the sequencing of ancient mtDNA from a finger tip at Denisova cave in Altai, dated to c. 40,000 years ago, when the area was also populated by H. sapiens and H. neanderthalensis.

I thought that would be about enough but as I read more and more discussions there seems to be some major confusion sparked by the use of molecular clock estimates, which the paper estimates in only 300-700 Ka for Neanderthal most recent common ancestor, hence producing a 1300-900 Ka estimate for the MRCA of us and the Denisova hominin, aka "X Woman". This, some say, calls for a major revision of the history of hominin migrations out of Africa because we know of no such migration.


The problem is caused by the use of a quick-ticking version of the molecular clock, because there's absolutely no archaeological reason to claim that Neanderthal and Sapiens MRCA is dated to only 700 Ka much less 300.

In fact the last migration out of Africa before H. sapiens we know of from the archaeological record is the Acheulean migration which is dated to c. 900,000 years ago. In my not so humble opinion, this is the real chronology of the divergence of Neanderthal and H. sapiens, a divergence that began most likely at the stage of H. ergaster.

Hence the Neanderthal-Sapiens MRCA must be an H. ergaster woman and should be dated to c. 900,000 or 1 million years ago.

That fixes all. Because, when we use this realistic reference, the MRCA of Denisova and the other hominins (Neanderthals and us) would have an age of 1.8 or 2 million years ago, which is precisely the time of the earliest H. erectus migration out of Africa.

So the Denisova hominin is, with all likelihood, an H. erectus or a descendant from this lineage (maybe by admixture with Neanderthals?) and everything fits nicely.

Everything but the short count molecular clock, which is obviously the one in need of a good fix, because it's demonstrating once and again that it only causes confusion and it always fails to fit well with the archaeological record.

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