Denisova the Menace

Johannes Krause and colleagues reported yesterday in Nature‘s advance online publication, on a new hominin mitochondrial DNA (mtDNA) genome. The genetic material is derived from a finger bone which lacks diagnostic morphology, from a southern Siberian site called Denisova dating to between 30 – 50 thousand years ago. Of note, the authors describe that the mtDNA is about twice as different from humans as any neandertal mtDNA is from modern humans. If the human-neandertal mtDNA divergence is accurately estimated at around 450 thousand years ago, that means this mystery specimen’s mtDNA lineage diverged from the human-neandertal line around 1 million years ago.

This is really interesting, because also around 40 thousand years ago, but from a site some 100 km to the west of Denisova, bones that were morphologically non-diagnostic yielded mtDNA basically identical to Neandertals.
Does this speak to the presence of at least 3 human species running around the Old World around 40 thousand years ago? Not necessarily. Most claims of a speciose recent human fossil record are based on cranial morphology. For example, modern human skulls are fairly different from “classic” neandertal skulls of western Europe (which is why the Skhul and Qafzeh hominins which display characteristics of both groups are so interesting). However, the mtDNA we have of most of these specimens comes from non-diagnostic specimens. The first Neandertal mtDNA studied came from a piece of tibia (shin bone); this bone is basically non-diagnostic morphologically between recent hominins, and the site it came from (Vindija, Croatia) has both human and Neandertal remains. The Denisova finger, similarly, is non-diagnostic in morphology so far as I can tell, and the archaeological layer contains both Middle and Upper Paleolithic cultural materials: we have no idea what these mtDNA bearers looked like.
I think people thinking “new species at Denisova” (NB: Krause and colleagues never make this claim!) would be shocked if it turns out that the Denisova remains, or those from which the Vindija specimens came, were morphologically modern humans, but this is entirely possible.
Humans today are not so diverse genetically as superficial appearances may suggest to many people. I wouldn’t be surprised if humans simply displayed more genetic diversity in the past. It is certainly interesting just how different the Denisova genome is from both humans and Neandertals. What exactly this difference means is just not clear. It is further interesting to note that the coding regions of the Denisova mtDNA show signs of strong purifying selection. Assumptions of neutrality are so important for genetic studies that I think people often forget that mtDNA actually serves functions necessary to survival, and is not actually neutral. Maybe this ancient mtDNA lineage lasted so long because the mitochondria provided some selective advantage, hence the purifying selection? Who knows?!
The authors make a funny deduction that I can’t quite follow, that because the Denisova specimen’s mtDNA diverged from humans-neandertals some 1 million years ago, “it was distinct from the initial radiation of H[omo] erectus that first left Africa 1.9 million years ago, and perhaps also from the taxon H. heidelbergensis,” which is the name given to mainly European but also African fossils between 1 and 0.5 million years ago. I just don’t follow this. We don’t know what mtDNA diversity was like at any of these times, so there is no reason to think that this specimen’s ancestors were from some undocumented dispersal from Africa. The implicit assumption is that mtDNA lineages arise sporadically and discretely from Africa and then spread to different parts of the world, repeatedly over the course of human evolution. If there’s gene flow all around from the get-go, then the Denisova specimen simply represents an especially ancient mtDNA lineage – not necessarily an ancient population (recall that mtDNA is only inherited from mothers).
Oh well, should be interesting to see the nuclear DNA from this specimen, surely to be described in the near future…
Reference
Krause J, Fu Q, Good JM, Viola B, Shunkov MV, Derevianko AP, Paabo S. 2010. The complete mitochondrial DNA genome of an unknown hominin from southern Siberia. Nature, in press.
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