Saturday, July 08, 2006
More on the "Lost Common Ancestor"
Happened upon John Hawks website cum blog, which has a recent post regarding Sahelanthropus. Hawks has had a long standing collaboration with Milford Wolpoff, and he, Wolpoff, Brigitte Senut, Martin Pickford and James Ahern have published a critique of Toumai as an ancestral hominid. Interestingly enough, Senut and Pickford have made claims about the hominid status of Orrorin tugenensis, a 6 million year old hominoid from the Turgen Hills of Kenya, similar to those made by Brunet et al. about Sahelanthropus. Orrorin, discovered in 2000 and nicknamed "millenium man," consists of a proximal femur and other fragmentary skeletal and dentognathic remains, that have been described as "the first human" by Pickford and Senut. As the search for the "first human" is a very competitive business, it is not surprising that Pickford and Senut concur with Wolpoff et al's analysis of Sahelanthropus as an ape, although they differ in their interpretation as to its overall affinities. Pickford and Senut's insistance that Orrorin was a bipedal hominid with a modern human gait, has also been challenged, so there is a certain irony in their collaboration with Wolpoff et al. in deconstructing Sahelanthropus.
Although there is a general willingness to accept Orrorin as a biped and hence an early hominid, the nature of its bipedalism has been called into question, with White et al. suggesting that the anatomy of the femur was more primitive than seen in australopithecines. Of course bipedalism in late Miocene hominoids has been argued for some unequivocally non-hominid apes such as the 8 million year old remains of Oreopoithecus from Tuscan lignites in Italy, so bipedalism in and of itself should not necessarily serve as the sole criterion for being human. As I argued in my last post, faculative bipedalism may have been part of the morphotype of the African ape/human LCA and not indicative of true homind status. It is interesting that no one seems to be arguing for this interpretation. It's either argued that a six million year old divergence date for the human/chimp split is too recent or that the fossils in question do not possess the features they are said to possess and are not really hominids after all. This is exemplified by a quote of Vince Sarich's from an article in Science by Ann Gibbons:
The first molecular study back in 1967 dated the split between
humans and apes to 5 million years ago, and Vince Sarich of the University
of California, Berkeley, co-author of the study, still stands by
that date. “I still bet that either the morphology or dates or both will
be found wanting for these ‘6-million-year-old hominids,’ ” he says.
The idea that the LCA of African apes and humans may have had preadatations for bipedalism that are expressed morphologically as early signs of a bipedal adaptation, does not seem to have been entertained by anyone besides yours truely. I agree with White et als. contention that Sahelanthropus, Orrorin and Ardipithecus represent the same taxon, but differ as to their interpretation that it is a hominid. As I said in my last post all three should be recognized as representative of the "lost" common ancestor of chimps and humans, a view consistent with their chronometric age and the consensus 5-6 million year old divergence date for the LCA established by genomic analysis. Again let me reiterate, because it seems to be a point no one is willing to concede, based on current evidence, the most parsimonious position regarding the phylogenetics of Sahelanthropus, Orrorin, and early forms of Ardipithecus is that they are indistinguishable from the human/chimp LCA and that this LCA possessed more hominid-like features vis a vis its locomotor and dietary adaptations than anyone has been willing to acknowledge.