Friday, November 26, 2010

What happened to Sahelanthropus tchadensis?

This Wednesday I had to do a Presentation on the description and classification of Sahelanthropus tchadensis for my Seminar on Human Evolution. I loved working on this topic, because two years ago, while I already attended a similar course, I watched a pretty impressive presentation about the same topic. This Presentation was one of those “enlightening” moments I had in this time and I always wanted to dig deeper into this whole story.
Anyways, I think I did pretty well, although I again realised that many other students do not share my enthusiasm for cladistics and the search for “good” characters. This or maybe it was just boring.

To come to my original reason for this post: While I prepared my talk, reading all those papers about the original discovery of Sahelanthropus and the upcoming critique of its classification, I could not help but ask myself: “What happened to this discussion?”
To give short recap on this story: Michel Brunet and his colleagues base their interpretation of Sahelanthropus tchadensis being a hominid on the following traits:

-A large supraorbital Torus, which Brunet et al. (2002) associated with a male individual (this sex estimation is pretty important for the next trait).

Brunet et al. (2002)

-A small canine, with  a  distal and apical wear facet and no C/P3 honing facet. (There were more characters described for the Canines, but I will skip them here)

-An enamel thickness between the chimpanzee and the Australopithecus condition.

-A flat, horizontally orientated Planum nuchale

-A more anterior positioned Foramen magnum

Zollikofer et al. (2005)

-A more perpendicular angle between the Foramen magnum and the Orbital Plane

The last three characters were associated with the probability of Sahelanthropus being a Biped.

Now, two years ago when I first heard about above listed traits I nearly went mad on their assumption that Sahelanthropus was a male. I just couldn’t, and still can’t understand how you’re able to estimate the sex of a single individual without knowing how those characters you use for your Sex estimation vary within this species.
This estimation is crucial because if we look at the canines, they are only small if Sahelanthropus was a male individual. If it was female, the size of the canine falls within the variation of other female Miocene apes and becomes much less diagnostic.
Speaking of canine morphology, there is a huge probability, that the morphology of the canine and its wear pattern is not an apomorphic character for hominids, but in fact a plesiomorphic character of at least all African Apes, which renders it almost useless for classification. Wolpoff and Colleagues (2006) stated that the wear pattern of the Sahelanthropus canines is probably the result of powerful masitcation, because similar patterns could be found in other Miocene Ape Taxa, such as Ouranopithecus and Gigantopithecus.

Canine of Gigantopithecus showing slight distal and apical wear. According to Wolpoff et al. (2006) this tooth shows an earlier stage of the wear pattern of Sahelanthropus tchadensis. (Wolpoff et al., 2006).

They continue in their critique, showing that none of the traits, Brunet et al. (2002) and Zollikofer et al. (2005) showed, could be interpreted as clear signs for bipedalism. This doesn’t mean of course, that those traits couldn’t be seen as apomorphic for hominids*, but at least you’re not able to state that it’s more possible that Sahelanthropus was a biped, than that it was not (as ist was by Zollikofer et al., 2005).

From my point of view, Wolpoff et al. (2006) raised some serious doubts on the classification of Sahelanthropus being a hominid and I was pretty sure that there should be some kind of answer on these issues by now. Well, apparently I was wrong about that,The only thing I found so far was this little passage:

“Scientifically it is impossible to understand why some authors ignore these derived characters and concentrate on primitive ones to reach the conclusion that S. tchadensis is related to modern apes and even more precisely to a palaeogorilla (Wolpoff et al. 2002, 2006; Pickford 2005). This attempt to undermine the clear affinity of the Chadian hominid is curious mainly when it is coming from, among others, two who have not yet had the opportunity to check Toumaı¨ casts in their laboratory. Is it what they believe, or is it only because they want to keep Orrorin as the earliest hominid?“ (Brunet, 2010, S.3318)

I found this to be pretty disappointing. Mostly because, a little bit before that passage, Brunet listed exactly the same “derived” characters, Wolpoff et al. criticized in their 2006 paper.

And instead of countering the critique on a scientific base, Michel Brunet points to the involvement of two of his “critics” (Martin Pickford and Brigitte Senut) into the discovery and description of Orrorin tugenensis. Well you can do exactly the same kind of argument with Michel Brunet, pointing out his extensive work in Tchad and the fact that it surely is pretty lucrative to announce an “earliest known hominid”.
Surely, there might be some kind personal interest behind many critical papers, but you still have to confront those critics in a scientific way. And the only way to get some kind of scientific progress is to falsify hypothesises of other people. In fact, the only thing we can be sure of in science is that if something is false, it is false (to put it simple). So the best way, to show that you’re right, is to show to other people that they are wrong.  But if we look at this case here, I can’t see that this process is happening right now.

Judging from what I know so far about this story, I would not classify Sahelanthropus tchadensis as a hominid. This doesn’t mean on the other hand that this conclusion diminishes the scientific value of Sahelanthropus.
What I exactly mean by this sentence I will explain in my next post, which I hopefully manage to put in a few days.

* I’m a little bit confused about that right now, since I got this thought while writing this post.


Brunet, M., Guy, F., Pilbeam, D., Lieberman, D., Likius, A., Mackaye, H., Ponce de León, M., Zollikofer, C., & Vignaud, P. (2005). New material of the earliest hominid from the Upper Miocene of Chad Nature, 434 (7034), 752-755 DOI: 10.1038/nature03392
Brunet, M., Guy, F., Pilbeam, D., Mackaye, H., Likius, A., Ahounta, D., Beauvilain, A., Blondel, C., Bocherens, H., Boisserie, J., De Bonis, L., Coppens, Y., Dejax, J., Denys, C., Duringer, P., Eisenmann, V., Fanone, G., Fronty, P., Geraads, D., Lehmann, T., Lihoreau, F., Louchart, A., Mahamat, A., Merceron, G., Mouchelin, G., Otero, O., Campomanes, P., De Leon, M., Rage, J., Sapanet, M., Schuster, M., Sudre, J., Tassy, P., Valentin, X., Vignaud, P., Viriot, L., Zazzo, A., & Zollikofer, C. (2002). A new hominid from the Upper Miocene of Chad, Central Africa Nature, 418 (6894), 145-151 DOI: 10.1038/nature00879
Brunet, M. (2010). Two new Mio-Pliocene Chadian hominids enlighten Charles Darwin's 1871 prediction Philosophical Transactions of the Royal Society B: Biological Sciences, 365 (1556), 3315-3321 DOI: 10.1098/rstb.2010.0069
Wolpoff, M. H., Hawks, J., Senut, B., Pickford, M., & Ahern, J. (2006). An Ape or the Ape: Is the Toumaï Cranium TM 266 a Hominid? Paleoanthropology, 36-50
Zollikofer, C., Ponce de León, M., Lieberman, D., Guy, F., Pilbeam, D., Likius, A., Mackaye, H., Vignaud, P., & Brunet, M. (2005). Virtual cranial reconstruction of Sahelanthropus tchadensis Nature, 434 (7034), 755-759 DOI: 10.1038/nature03397