The teeth that didn't bark05 Dec 2009
Earlier in the week, I wrote about the new interpretation of fossil teeth from Woranso-Mille, Ethiopia (“Woranso-Mille: A ladder not a bush”). There was one aspect of the paper that I wanted to comment at some greater length: Why didn’t the paper include comparisons with the Lomekwi sample of teeth and mandibles?
The Lomekwi sample includes more teeth than the sample reported from Woranso-Mille. At around 3.5 million years old, Lomekwi is a more relevant comparison than Hadar or Kanapoi, closer in age to Woranso-Mille than any sample other than Laetoli. So why didn’t they include the comparison?
The obvious answer is that these teeth belong to Kenyanthropus platyops, a different species off the line leading from A. anamensis to A. afarensis. So they’re not relevant to testing hypotheses about evolution within that lineage.
But…I don’t see why that answer is obvious if Kenyanthropus is a fictitious species, based on a distorted skull (e.g., White 2003).
Suppose that the teeth don’t represent Kenyanthropus. Then they ought to belong to the one cosmopolitan species that exists both north and south of Lomekwi, both earlier and later in time. They ought to be A. afarensis. Which makes them directly relevant to the Woranso-Mille hominins. At the very least, they add to the variation of the A. afarensis sample, helping to inform about the temporal trend in that lineage.
Besides that, even if we thought that Kenyanthropus was a real taxon, most of the Lomekwi teeth might still be A. afarensis. Only two specimens were assigned to K. platyops by Leakey and colleagues (2001): the KNM-WT 40000 skull and the unassociated maxilla fragment KNM-WT 38350. Both those have relatively small molars compared to the known A. afarensis sample, but in both cases the teeth are broken and dimensions are estimated. The rest of the dental sample is unassociated. The only thing keeping them from being A. afarensis is what we’re willing to assume about the number of species at Lomekwi.
So, it seems like a comparison that ought to be done. The information content is not going to be really high – we’re only talking about gross dimensions of the crowns, and the only ones that look informative at all are upper molars. But Leakey and colleagues argued that the KNM-WT 40000 and KNM-WT 38500 molars are at the very lower end of the range of A. afarensis. Guess what? The Woranso-Mille sample extends this range downward, right around the Lomekwi range.
UPDATE (2009-12-08): I should add that Brown and colleagues (2001) reported additional teeth from Lomekwi and other localities in the Turkana Basin, which they referred to A. afarensis on the basis of their temporal position. They’re not in the Woranso-Mille comparisons either.
Haile-Selassie Y, Saylor BZ, Deino A, Alene M, Latimer BM. 2010. New hominid fossils from Woranso-Mille (Central Afar, Ethiopia) and taxonomy of early Australopithecus. Am J Phys Anthropol (in press) doi:10.1002/ajpa.21159
Kimbel WH, Lockwood CA, Ward CV, Leakey MG, Rak Y, Johanson DC. 2006. Was Australopithecus anamensis ancestral to A. afarensis? A case of anagenesis in the hominin fossil record. J Hum Evol 51:134-152. doi:10.1016/j.jhevol.2006.02.003
Leakey MG. Spoor F, Brown FH, Gathogo PN, Kiarie C, Leakey LN, McDougall I. 2001. New hominin genus from eastern Africa shows diverse middle Pliocene lineages. Nature 410:433-440.
White T. 2003. Early hominids -- diversity or distortion? Science 299:1994-1996.