Narrowing down Flores microcephaly22 Jun 2006
Well, I'm still writing from Zagreb, so I don't have a lot of time for review. But I do want to point out the new paper by Gary Richards in Journal of Evolutionary Biology (DOI link).
It is clear from the paper that Richards really knows his microcephaly, from a clinical medical perspective. This is the kind of knowledge that the skeletal biologists should have brought in from the beginning, as I've argued for since early 2005:
Has anyone involved any experts in human brain pathologies in studying the skull yet? Is there even a single expert on microcephaly who has said anything about the skull? I know enough not to pretend that I can say this is or isn't pathological. I suspect that any qualified clinician would say the same; there is just no telling how microcephaly would manifest in a Stone Age specimen.
At this point, after this new paper, I think I can restate that question a different way: Has anybody with clinical knowledge of microcephaly now argued in support of the idea that this is a new species? Because we have now had three published arguments about the specimen (from Weber and colleagues, Martin and colleagues, and now Richards) that support the hypothesis of microcephaly, based on comparisons with endocasts, critiques of the original descriptions, and now physiological and anatomical comparisons with human pygmy populations.
There is an enormous amount of detail in this new paper about the genetic mechanisms behind small body size in pygmies, the genetics of primary microcephaly, the clinical and developmental manifestations of different types of microcephaly, and the consequences of all these for interpreting the Liang Bua remains. Richards has studied microcephaly and dwarfism in archaeological remains before, and clearly knows this literature. He is critical in several places of the original descriptions of the fossils, for their failure to cite pertinent recent clinical literature and their misuse of terms; either by failing to note changes in definitions in modern practice, or by failing to note variability in the diagnoses and manifestations of different conditions.
However, Richard's conclusion is not that the LB1 specimen is pathological. He is persuaded by the evidence of additional individuals from the site, and concludes that the sample represents a population with a distinctive suite of genetic characteristics due to insular isolation and dwarfism. I can't explain it better than citing his conclusion:
I agree that these remains do not represent diseased, pathological, or aberrant individuals. I consider these individuals to manifest physiological differences from other modern humans in similar ways to modern pygmies and additional modern humans possessing MCPH or GHRH-R mutations. When developmental differences result in divergent morphologies that then define a population, they can provide the basis for speciation, but by themselves they do not confer specific status. The fact that the Liang Bua remains present with a currently distinct suite of characters relative to modern humans does not necessarily mean that they represent a separate species. We currently lack the kind of detailed morphological assessments of both pygmy populations and individuals possessing the kinds of anomalous conditions suggested to account for the observed morphology. Prior to suggesting that the observed morphological suite of the LB1-LB9 individuals is indicative of speciation, it will be necessary to collect a substantial amount of new data.
When one applies the concept of maximum parsimony to the totality of evidence available on the Flores remains, one finds significant support for the remains being a variant of H. sapiens and little support for a species-level distinction. Given this position, I suggest that the LB1-LB9 individuals represent the remains of a H. sapiens group which became dwarfed in an island environment via changes in the GH-IGF-I axis. Acquisition of a dwarfing condition may either have occurred prior to or after the group arrived on the island. If it can be demonstrated that the totality of the recovered remains sample the same population, it appears that a mutation in the MCPH gene family or a secondary modification of the GH-IGF-I axis arose in teh later part of their occupation of the island and was transmitted within a local group. Whereas I consider the 'primitive' features identified in the LB1-LB9 individuals to be consistent with the scenario presented above, only a detailed analysis will be able to clarify the value of these features for phylogeny reconstruction (Richards 2006:19).
So the hypothesis is that this actually was a distinctive local variant population, but not a species that had diverged from some earlier hominid, like H. erectus. That is a fair enough hypothesis, but it doesn't quite make sense to me as following from the evidence presented. The only specimen for which there is any evidence about the brain at all is LB1. Now, Richards presents several examples of microcephaly variants that are very common in consanguineous marriages or small villages. So there is the possibility of such a condition becoming locally common even in modern humans. But in this case, we really have no reason to think that the condition was locally common. There is only one skull!
But I think the paper is a very important source of information, not only about the clinical manifestations of microcephaly and genetic mechanisms of small stature in human populations, but also because it draws together more considerations of how LB1 and the other specimens compare to different populations of modern humans, and not "modern humans" as a whole. Quite clearly, when you consider a small subset of modern humans as a comparative sample, some individuals who were not part of this small sample are going to look different or even strange. But you have to consider the full extent of variation in humans -- especially local humans -- when considering whether an unusual feature is part of the range of modern human morphology:
Brown and Morwood (2004) have recently argued that the LB1 mandible possesses a combination of nine features that eliminated any possibility it could derive from a modern human. At first glance this feature set would seem to represent '...component parts of a total morphological pattern of associated parts...' (LeGros Clark, 1964, p. 16-17) that would characterize a species-level difference. However, seven of the nine features are normal variants in modern humans, generally, and Indonesian, Malaysian and African groups, specificially [citations elided]. The two remaining features, absence of a chin and presence of well-developed superior and inferior transverse tori, not normal H. sapiens variants, are of interest. A significantly reduced chin (vertically aligned corpus and alveolar units) and developed transverse tori are, however, features found in some African and Indonesian pygmies (Jacob, 1967; Marquer, 1972) and Australomelanesians (Keiter, 1933; Birdsell, 1993) respectively (Richards 2006:2).
By compiling a list of 90 skeletal features from discussions in Brown et al. (2004) and Morwood et al. (2005), this author found that 77.8 % (70/90) either occur in H. sapiens generally or are known variants with more variable frequencies of occurrence. Also, many of these features are reminiscent of those found in various modern and prehistoric Australo-Melanesian populations, as also observed by Henneberg and Thorne (2004) .... Of the 20 features that are not normally found in H. sapiens, the majority are related to brain size reduction and modifications of ontogenetic pathways, as discussed below (Richards 2006:3, emphasis in original).
Of course, the hypothesis that these remains represent a species descended from some earlier hominid has to explain all these features shared with modern humans as parallelisms. And the list of these parallelisms is long, including physiological and anatomical changes related to small body size (shared with human pygmies), features of the dentition and skull not shared wtih H. erectus or earlier hominids, and the "behavioral" evidences represented by the stone tools. In contrast, most (if not all) of the "primitive" characters can be explained with reference to either dwarfism or microcephaly.
This is what many anthropologists have been saying all along. I hope that we will be seeing an alternative review of the skeletal evidence before too long -- from my own impressions I would say that some of the evidence supporting the series of remains as representative of non-modern humans is not very substantial.
What to think? The new paper doesn't give any new information about the specimens, and the suspicions raised about the nature of the sample are not new. But there is much new context here about the nature of microcephaly, its variability both clinically and genetically, and the complexity of examining it in the contexts of traditional socieities and archaeological specimens, and the problems of examining it in combination with possible dwarfism. The paper concludes that there is every likelihood that the pathways causing primary microcephaly in living people are the same ones responsible for the LB1 specimen.
Now, we will have to wait for better descriptions of the other specimens and how they relate to modern human variability, as well as a final determination on the evidence for pathology on the LB1 endocast itself.
UPDATE (6/22/2006): As I was posting this and connected to the Internet to do it, I found that Carl Zimmer also has a description of the paper, along with comment by Peter Brown.
Again, with the mysterious thesis from Leiden University. Can somebody send me a copy of that thesis? Because the other direct quotes I've read from the thesis seem to contradict stuff that has appeared in other papers by the same group! Please, somebody from Leiden, help!
Richards GD. 2006. Genetic, physiologic and ecogeographic factors contributing to variation in Homo sapiens: Homo floresiensis reconsidered. J Evol Biol (online before press). DOI link