Thursday, March 25, 2010

New human species?

Cool news of the week: Ancient DNA from a bone shard implies a human species coexisted with modern humans and Neandertals in ice age Siberia. Krause et al. report their results in an online paper in Nature. Based on the complete mitochondrial DNA sequence from a finger found in the Denisova Cave in the Altai Mountains, they estimate that the ancestor of this sequence and modern humans had a conventional date of a million years before present. For comparison, the ancestor of Neandertals and modern humans is about half that. In terms of nucleotide differences, the modern human and Neandertal mitochondrial genomes differ by about 202 nucleotide positions, but the Siberian mtDNA differed from modern humans at 385 nucleotide positions. (That's based on comparisons with 6 Neandertal and 52 modern human mtDNA genomes.)

Is this a new species? Krause et al. write,
...nuclear DNA sequences are needed to clarify definitively the relationship of the Denisova individual to present-day humans and Neanderthals.... An unambiguous association of the Denisova mtDNA with morphologically defined hominin taxa awaits determination of mtDNA sequences from more complete skeletal remains.
Fair enough. They're being cautious. In the accompanying news article, Eske Willerslev from the University of Copenhagen
emphasizes that, on its own, the mtDNA evidence does not verify that the Siberian find represents a new species because mtDNA is inherited only from the mother. It is possible that some modern humans or Neanderthals living in Siberia 40,000 years ago had unusual mtDNA, which may have come from earlier interbreeding among H. erectus, Neanderthals, archaic modern humans or another, unknown species of Homo. Only probes of the nuclear DNA will properly define the position of the Siberian relative in the human family tree.
Since the specimen this DNA came from had a conventional date of 30-50 thousand years ago, this "unusual mtDNA" theory would mean that this mtDNA type survived for nearly a million years but showed up in no other individuals yet sequenced (and we've sequenced lots of modern individuals and no small sample of fossil individuals). Does that seem really improbable to anyone else? I'd say that the likelihood of this being a new species is pretty high.

Too bad we don't have any fossils to tell us what they looked like. Still, the mtDNA results are pretty amazing, and Nature reports that the nuclear genome is now being sequenced by the same team.

I suppose I should make some creationist application. Since this bone fragment is associated with toolmaking, I'd say this stretches the genetic range of "human" quite a bit. It also further reinforces my point that mitochondrial Eve is NOT biblical Eve. Mitochondrial Eve is only the most recent ancestral sequence of modern human mtDNA, but modern humans aren't the only humans that ever lived.

Makes me wish we could get some erectus or ergaster DNA...

Krause et al. 2010. The complete mitochondrial DNA genome of an unknown hominin from southern Siberia. Nature doi:10.1038/nature08976.

Dalton. 2010. Fossil finger points to new human species. Nature 464:472-473.

Wood. 2008. Four Women, a Boat, and Lots of Kids: A Reevaluation of Mitochondrial Eve. Answers 3(2):70-74.