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…
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.

Redating gives older age to Chinese Homo erectus

Paleoanthropology news so big, it makes the cover of the current Nature (left), albeit with a lame headline that says, “Peking Man was Cool.” I know, lame, right? Like the editor was trying to be ‘hip’ and reach a new generation of nerds. Although ‘cool’ has been used since like the 1950s. And I peruse Nature, so it’s not nerdy.

Anyway, yes, researchers working at the Zhoukoudian (aka Chokoutien, etc.) cave site in China have redated the Homo erectus-bearing layers. As is the problem with the South African cave sites from which we get some great Australopithecus fossils (and “Paranthropus,” if you’re so inclined), cave sites are difficult to date radiometrically, with absolute dates based on the decay of radioactive materials. Before now, Zhoukoudian had been dated to about 500-250 ka (thousand years ago). Such a date is roughly contemporaneous with some “archaic Homo sapiens” (whatever that really means) fossils, which are generally thought to be intermediate in morphology between Homo erectus and modern people such as you or I. Could this indicate the coexistence of Homo erectus and more ‘modern’ people?

Well, the site has now been dated to about 770 ka, plus or minus 80ka (Shen et al. 2009). This means that the hominin occupations at the site start at nearly 800 ka, and end around 400 ka. Does this older date mean that now the overall picture of human evolution is clear? Of course not. But, it does fit the same kind of pattern we see elsewhere. The new dates show that, like in other areas, older more classically erectus-like hominins predate more intermediate “archaic” forms of humans. Moreover, it’s now certain that these fossils don’t (at least not the older ones) overlap in time with younger, “archaic” humans, such as that from Jinniushan.

So I guess that’s the main point, that the earlier dates of Zhoukoudian emphasize the fact that evolutionary trends in Asian middle Pleistocene hominins are the same as those throughout the world. If you really think that all these ancient regions represent separate species or distinct lineages, I suppose you could argue that this pattern of similarity means parallel evolution. A much more sensible interpretation, in my opinion, is that these populations were connected, even if only sporadically, by gene flow.

Busidima female pelvis

Yay! A female, mostly complete, Homo erectus pelvis has been found (Simpson et al 2008)! I say “yay” because female pelves are the best way to learn about the effects of birthing on pelvic morphology, despite the numerous studies that claim to glean this knowledge from male pelves (I’m basing this mainly on Neandertal examples as I am admittedly not as well versed with H erectus papers).

Quick summary of the facts: Found in Gona, Ethiopia, BSN49/P27a-d (a.k.a. the Busidima pelvis) consists of both os coxae (hipbones) and the upper part of the sacrum of what has been identified as an adult female Homo erectus dated to 0.9 to 1.4 Ma. The hipbones are mostly complete, except for portions of the ilium missing on both sides and portions of the ischial tuberosities. Overall, the pelvis has been reconstructed so that measurements can be taken (see image below, taken from the Simpson et al 2008 article).

Why I’m excited: Since this fossil is female and has the first complete early Pleistocene pubis, and thus the first complete pelvic inlet, it means legitimate inferences about birthing can be made. The initial paper addresses this by exploring neonate (i.e. infant at birth) head sizes compared to inlet dimensions. The authors found that Busidima would have been capable of birthing infants 30% larger than predicted based on the Homo erectus pelvis KNM-WT 15000 (male subadult). Already this find is showing why making assumptions about birth based on male specimens is flawed! Yay!!

Why I’m concerned: The authors draw conclusions about the width of the trunk based on the bi-iliac breadth. As the picture shows, the ilia are largely reconstructed, making any measurement between the widest points on the iliac crest dubious. I’m not saying it’s a bad reconstruction; in fact it looks reasonable to me. I just believe caution must be used when drawing conclusions based on reconstructed measurements. Especially since I cannot find details on how the ilia were reconstructed (if I’ve missed something in my reading of the online supporting material, please, someone correct me). Given that this is the case, it seems extreme to use incomplete ilia as evidence against an endurance running hypothesis and as evidence for what type of environment this specimen lived in.

What others are saying: Hawks goes into more anatomical detail in his blog post about the article. He discusses questions raised by the study and points out that you really have to read the supporting stuff to get the full picture as the Science article is “superficial”. Beast Ape’s blog post discusses the bi-iliac interpretation and what it means. For a multilingual look, Mundo Neandertal also discusses the article, and based on my imperfect Spanish capabilities, I think they discuss the infant head size findings. Finally, New Scientist describes the findings in a newsy way, detailing the bigger birth canal (compared to WT 15000) and the squat proportions of Busidima. Enjoy!

Simpson et al. 2008 A Female Homo erectus Pelvis from Gona, Ethiopia. Science 322: 1089-1092.