Monday, 11 August 2008

Neanderthal genome

The announcement of the sequencing of the Neanderthal mitochondion genome is a major technological and cultural advance. The feeling in the scientific press is that the sequence published is reliable because it has been replicated over thirty times. The excitement about the results has to be tempered with slight caution, since only one bone has been studied, a bone that represents a late Neanderthal human.

That said, it seems as if the genome is distinctive and is not represented in any of the known modern human genomes. The mitochondrial DNA of modern humans is sequenced quite extensively and we have a good feel for the kind of variation found in modern humans.

The tentative conclusions drawn by the researchers are that is no evidence of interbreeding between Neanderthals and early modern humans. This is quite reasonable. Had the Neanderthal sequences been identical to the typical modern sequences, then we would have concluded that the interbreeding must have been commonplace because we found evidence of it at our first attempt at looking. This is not to say that no interbreeding occurred; it is unlikely that we can find evidence of rare events on the first attempt.

The second conclusion is that the effective population size was small; this implies high levels of inbreeding, high levels of genetic similarities within populations, but not between populations. This is not really detectable with mitochondrial DNA, which is transmitted down the female line only. Strange things happen to pools of genes in small populations.

There are other studies that do suggest some levels of interbreeding between early modern humans and Neanderthals, but they were rare events because the populations were small and probably widely dispersed through the terrain.

The mitochodrial DNA studies are not thst sensitive to picking up interbreeding. Of the four possible crosses between Neanderthals and early modern humans, only 1 in 4 will allow Neanderthal mtDNA to persist beyond the first generation cross, assuming that the Neanderthal females are rare in the population.

One fascinating twist to this wonderful story is the new question of just how did so much biochemical evolution take place in the early modern humans in such a short time following the seapration from the common ancestor. The next year promises to be fascinating for Neanderthal watchers!

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