Sinanthropus

Gary Richards, an old friend and colleague from my days at the Laboratory for Human Evolutionary Studies (now the Human Evolution Research Center) at the University of California, Berkeley has a recent article in the Journal of Evolutionary Biology (DOI link) challenging the validity of Homo floresiensis. He argues that changes in developmental pathways of an isolated population of modern humans could have produced the idiosyncratic features seen in the Flores remains. Both John Hawks and Carl Zimmer offer useful reviews of Gary’s contribution, which can be found here and here.

I’ve always been skeptical of the view that the Flores hominid was an insular dwarf derived from an antecedent population of Indonesian H. erectus. The association of “H. floresiensis” with a modern human tool-kit and the presence of short-statured peoples throughout the region led me to suspect that if insular dwarfing was the cause for the features seen in the Flores remains, the process was acting on a population of modern humans rather than a relic population of H. erectus.

Gary’s article clearly shows that populational dwarfism among extant humans varies as to etiology and that the stature of LB1 does not fall too far below the lower limit of the range of variation seen in modern humans. Many of the seemingly “primitive traits” seen in the Flores remains may be due to allometric scaling effects or the result of alterations in developmental pathways brought on by changes in the expression of human growth hormones and associated factors. In addition, mutational effects leading to extreme size reduction, combined with inbreeding may have led to an increase in the incidence of microcephaly within individual demes of Flores hominids. This would help account for the unlikely recovery of an individual with such an extremely small cranial capacity as the LB1 specimen.

The article argues that more wide-ranging comparisons must be made between the Flores sample and localized human populations to better understand how “Homo floresiensis” fits into the panoply of human evolution and endemic adaptation. Gary concludes that:

When one applies the concept of maximum parsimony (Sober, 1988) 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 (Human Growth Hormone-Insulin-like Growth Factor 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 the 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.

This seems to me to be an extremely well balanced analysis and one that addresses the major contradictions generated by the Flores remains.

Folkloric accounts by local residents of Floresian “hobbits” suggests that there was ongoing contact between the dwarfed population and settled villagers, leading to interbreeding and resultant genetic exchange between the two populations, again mitigating against “Homo floresiensis” being a distinct human species. No matter how one cuts it, however, the recovery of the Liang Bua remains expands the range of modern human variation to the brink of an incipient speciation event and helps document an extremely interesting episode in recent human evolution.

# posted by DAE @ 3:40 PM