The Orrorin identity

There's nothing especially surprising about the functional interpretations in Richmond and Jungers' paper about the Orrorin BAR 1002'00 femur. They conclude it was an australopithecine-like biped, because it shared several features with australopithecine femora: in particular, it has a long, narrow, anteroposteriorly flattened neck and a broad thick proximal shaft.

In this, they mirror the conclusions of the original description of the Lukeino fossils by Senut et al. (2001). Richmond and Jungers also reiterate the evidence for arboreality in the Lukeino fossils, including the well-developed musculature of the distal humerus and the chimpanzee-like curved finger bone. I wonder why their analysis could not have made something more out of the other two femoral fragments, one of which is fairly large (but lacking the head). Still, the paper reiterates the quite good evidence for bipedality in the most complete femoral specimen.

I wonder sometimes how closely people actually read the papers they comment on. The associated coverage, including Ann Gibbons' article, has made a lot out of a small point in the paper, but I think that the commenters have it wrong.

Here's the story: When the Orrorin materials were first published, Brigitte Senut and Martin Pickford put forward the argument that these may be more closely related to Homo than to known australopithecines. They based their argument mainly on Orrorin's relatively thick-enameled molars, which they viewed as different from the thin-enameled molars of Ardipithecus, but lacking the enlarged dentition of Australopithecus. So, they suggested that Orrorin might be a plesiomorphic ancestor of Homo, and that Ardipithecus and Australopithecus represent divergent lineages derived in their dental anatomy.

I don't find that suggestion very compelling, because it seems to put too much faith in the absence of evolutionary reversals. There's no reason why a large-molared australopithecine should not have given rise to small-molared Homo, particularly since smaller-toothed Homo habilis is apparently derived from earlier, larger-toothed "Homo" specimens like A. L. 666-1 and Omo 75-14. And Haile-Selassie, Suwa and White (2004) claimed that the Orrorin, Sahelanthropus, and Ardipithecus dentitions were so similar that they might represent one taxon. So the dental contrasts among these early hominids are probably not great enough to justify the idea that Orrorin is an exclusive Homo ancestor.

The femur also formed a part of this phylogenetic story, with Senut and Pickford having noted the lack of extreme australopithecine-like features in the femur. The Orrorin femur has a less exaggerated neck length than many australopithecine specimens, it is larger than many, and appears to have a higher neck-shaft angle. To the extent those features differ from later Australopithecus, they resemble the human anatomy.

Richmond and Jungers address this argument very briefly in their last paragraph, by noting that the functional elements of the Orrorin femoral anatomy are entirely consistent with the australopithecine pattern of bipedality:

The similarity between O. tugenensis and australopith femora weakens support for scenarios in which O. tugenesis is ancestral to Homo to the exclusion of A. afarensis (4). Instead, the overall primitive hominin morphology of the O. tugenensis femur, along with primitive dental anatomy, is consistent with the more parsimonious hypothesis that it is a basal member of the hominin clade.

I think that's fair, as far as it goes. The overall morphological pattern of this femur, with its long neck and broad shaft, is much like known australopithecine femora. But to go a bit further, their metric comparisons show BAR 1002'00 to be the most Homo-like of the early hominid femora they examined, and their phenetic cluster puts it basal to the other australopithecines. That's pretty much exactly what Senut et al. have consistently said. So I have a hard time understanding how those observations refute the idea that Orrorin has a more Homo-like femur than later australopithecines!

Again, I don't put much stock in the phylogenetic argument for an Orrorin-Homo link. I don't see any difficulty deriving Homo from Australopithecus, especially given the likely effects of body size evolution on the locomotor pattern. And at least one or two early Homo femoral specimens, like KNM-ER 1481, share most of the Australopithecus-like pattern of proximal femur anatomy. But this paper surely doesn't add anything new to the critique of Senut and Pickford's preferred phylogenetic hypothesis. The details simply don't detract from their story.

References:

Richmond BG, Jungers WL. 2008. Orrorin tugenensis femoral morphology and the evolution of hominin bipedalism. Science 319:1662-1665. doi:10.1126/science.1154197

Gibbons A. 2008. Millennium ancestor gets its walking papers. Science 319:1599-1601. doi:10.1126/science.319.5870.1599

Haile-Selassie Y, Suwa G, White T. 2004. Late Miocene teeth from Middle Awash, Ethiopia, and early hominid dental evolution. Science 303:1503-1505.

Senut B, Pickford M, Gommery D, Mein P, Cheboi K, Coppens Y. 2001. First hominid from the Miocene (Lukeino Formation, Kenya). C R Acad Sci Paris, Sciences de la Terre et des planètes 332:137-144.