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Wednesday, March 5, 2008

Cretins and Omomyids

What's new in the world of Paleoanthropology?

The first bit of news is older than dirt: Christopher Beard reports in the 11 March issue of PNAS on new evidence of the earliest primates in North America around the time of the Paleocene, some 60-55 Ma. Fossil teeth discovered in Mississippi have been allocated to a new species, Teilhardina magnoliana, arguably the most primitive Teilhardina in North America or Europe. Beard argues this suggests that, contrary to current understanding of euprimate dispersal, early omomyids colonized North America around the Paleocene-Eocene Thermal Maximum via a Bering Strait land bridge. Possibly, these early critters thence colonized Europe. Pretty neat stuff: early primate forebears disperse from Asia to N. America, and some 55 Ma later modern humans follow their tiny footsteps. I don't know very much about primate origins (my interests like some 50 Ma after this event), but it's always nice to have a reason to say "omomyid" or "omomyiform."

Next, one of the most sensational paleoanthropological news bits, the famed hobbit, is back in the spotlight, in a paper in the Proceedings of the Royal Academy B. Recall that when the Flores hominin material was first reported about five years ago, it was argued to be a new species of an insular-dwarfed erectus-like hominin: Homo floresiensis. Others later argued that the Liang Bua material was pathological (i.e. microcephalic dwarf) H. sapiens. Most recently, Hershkovitz et al. made a pretty convincing case that the LB 1 was a modern human with Laron Syndrome, a pituitary disease. Their case is pretty strong, but the most troubling evidence, in my mind, against a pathological human is the fact that the diminutive Liang Bua specimens are very small and span a period of thousands of years. I look forward to Falk et al.'s presentation at the AAPAs in April ("LB1 did not have Laron Syndrome").

But the new Obendorf et al. paper gives the hobbit a new diagnosis: "myxoedematous cretinism." Thus, the diminutive stature and small brain of LB1, Obendorf and team hypothesize, is due to a non-functioning thyroid. Support for their hypothesis comes from, among other things, the enlarged pituitary fossa of the LB1 sphenoid and the morphology of the LB1 trapezoid. I must admit now that I have not yet read the entire paper (I have to teach in 10 minutes) but the paper looks interesting, and I am interested in how they propose to explain how only the "cretins" were preserved in Liang Bua cave, while non-pathological humans were not. The paper also reminds me of Dobson's (1998) paper that suggested that the thyroid and iodine deficiency were responsible for the anatomical differences between humans (as we generally know them today) and neandertals. I guess the evolutionary significance of the thyroid is really beginning to be appreciated by anthropologists...

Given this new diagnosis for LB1 and others, what might be interesting is a Bayesian approach to testing these hypotheses. Which hypothesis (diagnosis) is most likely given the data--new species, microcephalic or microcephalic dwarf H. sapiens, human with Laron Syndrome, or human cretin? Or, under which hypothesis(-es) are the observed data the most likely? The new diagnosis for the Liang Bua material is interesting and certainly provides good research questions. But, as is usually the case with fossil hominins, what would be really nice are more specimens against which to test our hypotheses.

References
Beard C. 2008. The oldest North American primate and mammalian biogeography during the Paleocene-Eocene Thermal Maximum. Proc Nat Acad Sci 105: 3815-3818.

Obendorf PJ, Oxnard CE and Kefford BJ, in press. Are the small human-like found on Flores human endemic cretins? Proc R Soc B xx: 1-10.

1 comment:

zacharoo said...

I know it's super cool to comment on your own post, but I'm gonna do it anyway: for a great treatment of the Cretinism hypothesis, see John Hawks's blog.
http://johnhawks.net/weblog/