A challenge to Sahelanthropus

And it comes from me! My paper with Milford Wolpoff, Brigitte Senut, Martin Pickford, and Jim Ahern is now available online from PaleoAnthropology! The PDF is freely accessible -- a big advantage with this journal. So go download!

Here's the abstract:

The Toumaï cranium TM 266 is the first known from any Late Miocene African hominoid clade, and is one of the best preserved crania of any Miocene hominoid. Since its publication there has been debate in the scientific literature and discussion in the popular press over the assertion that this cranium is significant because it is the earliest known hominid. The basis of the hominid assessment rests on two interpretations of the anatomy: a hominid-like, small, flat-wearing canine; and, cranial features reflecting an upright stance and bipedal locomotion. In fact, it is widely reported that the specimen is an upright hominid biped (Haile-Selassie et al., 2004; Kimbel, 2004; Lieberman, 2002), although this is yet to be verified by independent observations and study. The history of paleoanthropology may be relevant to this assessment, because there have been similar claims for other extinct primate species. Here, we evaluate the hypothesis that Sahelanthropus (the genus TM 266 is attributed to) is a hominid by examining features of the canine and of the cranial base that are said to reflect canine reduction and change of function, and upright posture and bipedal locomotion. These are hominid autapomorphies and their presence or absence in late Miocene hominoids has fundamental importance for identifying the hominid clade (Wolpoff et al. 2006:36).

There are two important differences between our analysis and earlier analyses of the skull. First, we provide more comparative data from Miocene primates. Some of the apparent similarities with hominids -- especially considering the morphology, size, and wear on the canines -- are clearly present in other Miocene ape lineages. This is of course the primary difficulty in defining hominids on dental remains alone, since several lineages of Miocene apes appear to have been convergent on some hominid dental features. These similarities don't preclude Sahelanthropus as a hominid, but they remove a major support for that hypothesis.

And second, we provide a biomechanical assessment of the reconstructed skull and its relevance for bipedality and posture. Rather than simply looking for similarities with hominids or chimpanzees, we actually developed a model for how the skull and neck musculature must have functioned. One angle in reconstruction makes the skull look like modern humans -- the foramen magnum - orbital plane angle. But regardless of this angle, the skull actually cannot have functioned in a vertical posture because of the length of the nuchal plane and vertical height of inion. Also, this angle in Toumaï doesn't look anything like early hominids -- australopithecine skulls have FMOP angles similar to chimpanzees!

I think this line captures the point:

The point is not that the TM 266 cranial rear and posterior portion of the cranial base was unlike hominids because the region looks like apes, but that TM 266 had a posture that is not upright because the region reflects nuchal functions similar to those of apes (Wolpoff et al. 2006:46).

There's lots of other interesting stuff in the paper also -- including a section about the hominid-chimpanzee divergence date that I never thought we would say, but is looking very prescient considering recent data. It was a real pleasure to explore several different topics and bring them together, and we'll have much more to do!

References:

Wolpoff MH, Hawks J, Senut B, Pickford M, Ahern J. 2006. An ape or the ape: is the Toumaï TM 266 cranium a hominid? PaleoAnthropology 2006:36-50.