08 November 2006
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The scientists said they have developed the most robust genetic evidence to date that suggests humans and Neanderthals interbred when they existed together thousands of years ago. The interbreeding hypothesis contrasts with at least one prominent theory that posits that no interbreeding occurred when the two species encountered one another.
Lahn collaborated on the studies with Patrick D. Evans, Nitzan Mekel-Bobrov, Eric J. Vallender and Richard R. Hudson, all of the University of Chicago.
In their studies, Lahn and his colleagues performed a detailed statistical analysis of the DNA sequence structure of the gene microcephalin, which is known to play a role in regulating brain size in humans. Mutations in the human gene cause development of a much smaller brain, a condition called microcephaly.
Earlier studies by Lahn's group yielded evidence that the microcephalin gene has two distinct classes of alleles. One class, called the D alleles, is comprised of a group of alleles with rather similar DNA sequences. The other class is called the non-D alleles. Lahn and colleagues previously showed that all modern copies of the D alleles arose from a single progenitor copy about 37,000 years ago, which then increased in frequency rapidly and are now present in about 70 percent of the world's population. This rapid rise in frequency indicates that the D alleles underwent positive selection in the recent history of humans. This means that these alleles conferred a fitness advantage on those who possessed one of them such that these people had slightly higher reproductive success than people who didn't possess the alleles, said Lahn.
Specifically in this case GNXP has two posts with links to two others by John Hawks which talk about a recent paper on the microcephalin gene (available free here). It is interesting to see how this genetic evidence of introgression links up to the fossil eveidence as noted in those Romanian fossils that turn out to be about 30,000 years old and that show a number of different crossover bone types. As Hawks explains this variety is key:Why do I think these critiques have little force? Because at this point, we have enough early Upper Paleolithic specimens with such features to notice something very important about them: different specimens have different Neandertal features.
It's like a shotgun approach to Neandertal intermixture. These are not one or two things appearing in parallel, and they aren't chance resemblances in this small early Upper Paleolithic sample, when they almost all decline systematically in later samples.
So when we see each new specimen, like Muierii 1, carrying not only Neandertal features, but its own distinctive set of Neandertal features, that emphasizes the early role of genome-wide intermixture.
It's nice how that ties right in to the genetic introgression discoveries.