[Taxacom] chimp-human interbreeding & fuzzy origins?

John Grehan jgrehan at sciencebuff.org
Tue May 23 08:14:31 CDT 2006

Barry Roth wrote:

>   (2)  if the Toumai skull, which is said to combine features of both
> humans and chimpanzees, precedes the definitive human-chimp split

Patterson et al claim (p. 4) that "The Toumai fossil (Sahelanthropus
tchadensis), with its bipedalism and hominin dental features, is usually
interpreted as being on the hominin line...". 

Both features are problematic to say the least. The bipedalism is
inferred only through modeling and extrapolation that ignores similar
positional characteristics of the foramen magnum in non-bipedal
primates. As for "hominin" dentition one might wonder what that would
be. Two papers are cited. 

The first is Brunet et al (2002). This paper is rather tricky when it
comes to explicit hominin dental characters. On page 146 they
distinguish Sahleanthropus as a hominid from large [note the caveat]
living and known fossil hominoid genera in various respects, but do not
specify in what way such differences constitute hominid synapomorphies.
For example (p. 146), they note that the cheek-teeth enamel is thinner
that Homo, but thicker than chimp. For orangutans they just say to see
further into the article. 

On. P. 150 they say that a lower c-P3 diastema is absent and therefore
Sahelanthropus "probably represents an early stage in the evolution of
the non-honing C-P3 complex characteristic of later hominids". The only
trouble is that Australopithecus afarensis does have a diastema, So if
Sahelanthropus is diagnostically hominid through absence of a diastema,
then afarensis is diagnostically not a hominid at all.

Finally on p. 151 they refer to derived hominid dental features as (1)
non-honing cupper canine-third upper premolar complex (problematic as
above), and intermediate postcanine enamel thickness. It is this enamel
thickness that is problematic. Humans and orangutans have thicker molar
enamel overall than chimps or gorillas. Sahelanthropus is supposed to
have enamel "intermediate" enamel thickness. Intermediate between what?
I have to infer that it is between chimps and Australopithecus because
that seems to be the only comparison given (bottom right column p. 150.
The implication seems to be that anything "intermediately" thicker than
chimps but thinner than australopiths is diagnostically hominid. That
would make the orangutan hominid because they also have thick molar
enamel. One of the authors (Pilbeam) in other publications claims that
the ancestral largebodied hominoid had thick enamel so it got thinner in
chimps. If that is the case an "intermediate" thickness must also
represent a derived state since the thick enamel of hominids
(Australopithecus, Homo) is primitive.

So if the above dental features are anything to go by there is nothing
to suggest Sahelanthropus is hominid. The authors also claim that
Sahelanthropus is similar to later hominids in having a large continuous
supraorbital torus. Now this one I will dare to call "garbage" as an
accurate description because no australopiths have the continuous
African ape torus (and also lack the continuous sulcus), and in Homo it
(or at least a similar structure) is present only for some fossils
currently assigned to Homo erectus (and the torus of Sahelanthropus
looks like no other hominoid at all - very autapomorphic). And in
afarensis the supra-orbital tori and more robust than in orangutans, but
otherwise they follow the same configuration as mounded structures over
each eye orbit.

The other paper is Vignaud et al (2002) and in a quick read I did not
see anything substantiating hominid dental characters.

I do wonder about the quality of this kind of hominid study. Would I be
justified to say that a lot of it is "garbage"? Perhaps according to the
third author of the Sahelanthropus paper - David Pilbeam - who in other
publications has said that morphology is completely untrustworthy - and
yet the authors ask us to trust their morphological work as scientific
in accepting their hominid claims.

If anyone can refute my above contentions I would be very interested (I
may not be happy to be wrong, but very interested nonetheless).


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