A challenge to Sahelanthropus

[Daryl Habel]

Wolpoff, M.H., Hawks, J., Senut, B., Pickford, M. & Ahern, J. (2006). An Ape or the Ape: Is the Toumai Cranium TM 266 a Hominid?  PaleoAnthropology 2006: 36-50.

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 hominid1. 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.

The paper concludes:

"TM 266 is a young specimen of indeterminate sex.  Its canines are large for a Miocene ape female and small for a Miocene ape male, in absolute and relative dimensions.  Canine wear is apical and transversely distal, a condition found in many other Miocene ape specimens, but it is possible that the TM 266 canine is more heavily worn than most other Miocene canines of similar dental age.  Unlike Australopithecines, the evidence indicates that TM 266 did not habitually hold its head in an upright position over the spine and thus lacked this significant obligate bipedal adaptation.  By itself this contrasts with all known hominids, and even in the absence of postcranial remains this anatomy is sufficient to exclude Sahelanthropus from the hominid clade as we understand it, an exclusion compatible with genetic estimates of the chimpanzee/hominid divergence.  It is this exclusion, and not any combination of synapomorphies, which demonstrates that TM 266 was an ape" (Wolpoff et al. 2006:48)

This question was raised previously, a few years ago:

Wolpoff, M.H., B. Senut, M. Pickford & J. Hawks (2002). Sahelanthropus or 'Sahelpithecus'? Nature 419: 581-582.

But the paper in Nature is only two short pages, while this new paper (with references) is a much more comprehensive 15 pages.  The paper (Wolpoff et al. 2006) is available free (as are all papers from PaleoAnthropology)  in pdf .  Instructions on how to reach the free pdf download as well as  further comment from a co-author can be found on John Hawks' weblog, at: CLICK HERE.

Dar

[rmacfarl]

Dar,

John Hawks has posted comments about this paper on his weblog:

http://johnhawks.net/weblog/fossils/sahelanthropus/sahel_wolpoff_paper_2006.html

In it he makes the following comment:

"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! "

So do I read this correctly? The feature of Sahelanthropus that has led to it being considered a probable early hominid, the position of its foramen magnum, is misleading because analysis of the "FMOP" angle shows Sahelanthropus to look *more* human than the much later, unquestionably hominid australopiths. But in fact Wolpoff et al's analysis shows that it could not have held its head upright like a biped, for other reasons...

Ross Macfarlane

[Daryl Habel]

Dar,

John Hawks has posted comments about this paper on his weblog:

http://johnhawks.net/weblog/fossils/sahelanthropus/sahel_wolpoff_paper_2006.html

In it he makes the following comment:

"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! "

So do I read this correctly? The feature of Sahelanthropus that has led to it being considered a probable early hominid, the position of its foramen magnum, is misleading because analysis of the "FMOP" angle shows Sahelanthropus to look *more* human than the much later, unquestionably hominid australopiths. But in fact Wolpoff et al's analysis shows that it could not have held its head upright like a biped, for other reasons...

Ross Macfarlane

Ross,

I can't speak for John Hawks, and since he is an active member of the Palanth forum I had hoped he would answer this himself sometime today .  Hopes dashed, I'll give it my best shot.

The foramen magnum-orbital plane (FMOP) angle is described on page 44 of the PaleoAnthropology paper.  According to Zollikofer and colleagues (who claim bipedal locomotion for Sahelanthropus) FMOP angle can be used to assess posture.  But  see Wolpoff et al. 2006: Fig. 11; the FMOP of four australopithecines (upright bipeds) falls within the range of a series of 63 chimpanzees (quadrupeds).  Consequently, Wolpoff et al. "deduce from this comparison that the foramen magnum-orbital plane  angle does not distinguish the posture and locomotion of australopithecines and chimpanzees, and therefore it cannot be expected to address the posture and locomotion of a species said to be close to the last common ancestor of australopithecines and chimpanzees" (Wolpoff et al. 2006:46).

I can't find where (you say) that Wolpoff et al. (or Hawks) says that about theTM 266 FMOP angle looking *more* human, but whatever I may have missed, it is obvious that Wolpoff et al. are arguing that the FMOP angle does not reliably distinguish bipedalism from quadrupedalism.  Therefore it can't be used as an indicator for bipedalism.

Yes, Wolpoff et al also argue with other reasons that Sahelanthropus was not bipedal: "The TM 266 cranial rear and posterior portion of the cranial base, including the size, shape and orientation, of the nuchal plane, are compatable with a chimpanzee mode of locomotion......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).

Keeping in mind physical anthropology isn't my strong suite, hope this helps.
Dar