Sahelanthropus Tchadensis: Milford Wolpoff and Michel Brunet

Milford Wolpoff and Michel Brunet were both published in Nature in regard to the evolutionary significance of Sahelanthropus tchadensis. Despite the species' small canines, reduced prognathism, and supraorbital torus, it remains clear to Wolpoff that Sahelanthropus was not bipedal and was therefore an ape. The following four areas stress Wolpoff's belief that Sahelanthropus has very little evolutionary significance to the hominid clade:

1) Wolpoff responds to the fact that the canines of Sahelanthropus are small. According to Brunet, canines of this size, shape, and configuration should include Sahelanthropus in the hominid clade. However, as Wolpoff clearly notes in Nature, the "canine breadth is similar to the chimpanzee mean, being within the range of both chimpanzee and gorilla females and of chimpanzee males" (p. 581). 2) Brunet believes the supraorbital torus (and other mixed cranial features, including reduced prognathism) can somehow be attributed to genus Homo. A supraorbital torus is a ridge above the eyes that can be seen in extant apes, early humans, and some modern, human groups, as well. Wolpoff, on the other hand, feels the pronounced supraorbital torus of Sahelanthropus is simply an evolutionary "response to strain" and, again, does not supply sufficient evidence to include Sahelanthropus in the hominid clade (p. 581). 3) Brunet claims that the position of the foramen magnum (one clear indicator of habitual bipedalism) suggests that Sahelanthropus was an obligate biped. Nevertheless, the same positioning of the foramen magnum has been observed in modern chimpanzees, and, as one can easily see, chimps are hardly obligate bipeds. It requires some effort and strain for chimpanzees to walk bipedaly. 4) Finally, Wolpoff notes that there are many characteristics of the Sahelanthropus find that more closely resemble chimpanzees and gorillas. One of these is the angle of the nuchal plane, suggesting a typical quadruped.

In sum, Wolpoff's conclusion in regard to the inclusion of Sahelanthropus tchadensis in the hominid clade is:

We believe that Sahelanthropus was an ape living in an environment that was later inhabited by australopithecines and, like them, it adapted with a powerful masticatory complex. A penecontemporaray primate with a perfect and well-developed adaptation to obligate bipedalism is more likely to have been an early hominid. (p. 582).

This statement does put a great deal of emphasis on bipedalism, but this emphasis is warranted when one considers that habitual bipedalism is an important factor in differentiating our species from other primates at the family level.

Michel Brunet does offer a bit of argument toward Wolpoff in the following:

Those who ignore derived characteristics and concentrate on primitive ones will reach the conclusion that early hominids [...] are related to modern apes. For Wolpoff to revert to the use of primitive characters in an attempt to undermine a clear statement of affinity of Toumai is curious. (p. 582)

Certainly, Wolpoff's argument would have been strengthened with a deeper focus on derived, as opposed to primitive, characteristics. They should be addressed. Nonetheless, despite this assertion made by Brunet, it does not change the fact that it is highly unlikely Sahelanthropus could have been bipedal - again, a clear indication of affiliation in the hominid clade. As a final note, in human evolutionary history, it is thought that "bipedalism was strongly and quickly selected for, and all other traits that came to characterize our tribe of primates came latter" (Park, 2008). This statement, in itself, implies that our oldest ancestor must surely have been bipedal, and, again, Sahelanthropus displays very little evidence of this. Perhaps better candidates might include Ardipithecus ramidus or Kenyanthropus platyops, although both are doubtful.

References:

Park, M. (2008). Biological Anthropology. New York: McGraw-Hill.

Wolpoff, M. & Brunet, M. (2002). Sahelanthropus or Sahelpithecus? Nature, 419, 581-582.