Friday, December 24, 2010

Denisova again

Remember that Denisova mtDNA sequence from Siberia? The one published earlier this year that was more different from modern humans than Neandertals? In this week's Nature, the same research team has published the full nuclear genome of Denisova. It answers some of my questions and raises new ones. Some thoughts:

1. The genome is more similar to the Neandertals than to modern humans, which is different from the mitochondrial genome analysis, which placed Neandertals and humans together as sisters, with Denisova as the outgroup. The authors (Reich et al.) hypothesize that this represents incomplete lineage sorting, which is a phenomenon that occurs in the early stages of speciation when different genes have different phylogenetic histories.

2. They sequenced a second Denisovan mtDNA genome from a tooth found at the same site. This mtDNA genome differed from the first Denisovan mtDNA genome by only two nucleotide positions. This second mtDNA genome gives independent confirmation of the biological reality of the Denisovan mtDNA type.

3. The Denisovan tooth is morphologically unlike either Neandertals or modern humans. It's actually in the size range of australopith teeth. There are some Asian hominid skulls that resemble neither Neandertals nor sapiens that are the accompanying news item suggests could be a candidate for a Denisovan.

4. Most surprising (and informative) of all, using the same analysis used to show Neandertal/Eurasian interbreeding, Reich et al. found evidence of interbreeding between Denisovans and Homo sapiens from Melanesia (esp. Papua New Guinea). Following Frank Marsh's idea that organisms can only breed with members of the same kind, I would interpret this interbreeding as evidence that Denisovans were human (i.e. descendants of Adam and Eve). The sister-taxon relationship with the Neandertals would also support the humanity of Denisova. This interbreeding evidence is really helpful to me, since I've been trying to think of ways (other than handwaving) to argue that Denisova is human.

5. Reich et al. conclude with this passage:
The Denisova individual belongs to a hominin group that shares a common ancestor with Neanderthals but has a distinct population history. We define this group based on genomic evidence and call it Denisovans, but refrain from any formal Linnaean taxonomic designations that would indicate species or subspecies status for either Neanderthals or Denisovans.
You might recall when the Neandertal genome came out earlier this year, both Hugh Ross and Fuz Rana claimed that the paper describing the genome showed that it was a different species from modern humans. That was a false claim at the time, but this time, Reich et al. have made it crystal clear that they are not making any judgments on the specific or subspecific status of Denisovans. I wonder how Ross and Rana will use the Denisovan genome report to claim that it's a different species from humans? Will they acknowledge the error in their Neandertal genome commentary?

And speaking of Reasons to Believe and the Neandertal genome, you might also recall RTB's conclusion that humans engaged in bestiality with Neandertals and produced half human, half animal offspring. (RTB only accepts modern Homo sapiens as descendants of Adam and Eve.) Ross and Rana insisted that this ongoing bestial interbreeding was no big deal, despite a previous claim (discussed here) that interbreeding between Neandertals and modern humans would indicate that they were the same species. Well now we have another "archaic hominin" lineage that interbred with the ancestors of modern Melanesians. Will RTB spin this as yet more bestiality? People back then were having sex and children with more than one animal species? Given their investment in Homo sapiens as the only human species, I think it's inevitable that they'll simply acknowledge more bestiality. In fact, I'm going to predict that they'll spin this as "confirmation" of their model that Neandertals and humans commited ongoing bestiality. I'll have more words for RTB next week, but for now, I think the young-age creation model that embraces a plurality of human species (or "variations" if you must) is preferable to the RTB model of people having half-breed children with animals.

Reich et al. 2010. Genetic history of an archaic hominin group from Denisova Cave in Siberia. Nature 468:1053-1060.

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