Sahelanthropus tchadensis

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Sahelanthropus tchadensis
Fossil range: Miocene

Scientific classification

Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Primates
Family:	Hominidae
Subfamily:	Homininae
Tribe:	Hominini
Genus:	Sahelanthropus Brunet et al, 2002
Species:	S. tchadensis

Binomial name

†Sahelanthropus tchadensis
Brunet et al, 2002

Sahelanthropus tchadensis is a fossil ape originally classified as the oldest possible member of the human family tree, but more recently as a Miocene ape related to humans and other living African apes, that is thought to have lived approximately 7 million years ago. The fossils found indicate a relatively small cranium, five pieces of jaw, and some teeth, making up a head that has a mixture of derived and primitive features. These were discovered in the desert of Chad by a team of four people. The team, made up of three Chadians, Mahamat Adoum and Ahounta Djimdoumalbaye (who found the skull on July 19, 2001), Fanone Gongdibe, and the French Alain Beauvilain, leader of this team, found all the fossils of Sahelanthropus from July 2001 to March 2002. The braincase, being only 340 cc to 360 cc in volume is notably less than the approximate human volume of 1350 cc. The teeth, brow ridges, and facial structure in many ways resemble those found on Homo sapiens. Due to the distorted matrix of the cranium, a 3D computer reconstruction has been produced. Since no bones below the skull itself have been discovered, it is unknown to this day whether or not Sahelanthropus tchadensis was indeed bipedal, though an anteriorly placed foramen magnum suggests that this may have been the case. Its canine wears similarly to other Miocene apes.

The discoverers claimed that S. tchadensis is the oldest known human ancestor after the split of the human line from that of chimpanzees. The bones were found in Chad, far from most previous hominin fossil finds, i.e. Eastern and Southern Africa. However, an australopithecine mandible was also found in Chad by Sahelanthropus' discoverers in 1993 belonging to Australopithecus bahrelghazali.

[edit] Perspective

The fossil skull TH 266, nicknamed "Toumaï" ("hope of life" in the local Goran language of Chad), may be a common ancestor of humans and chimpanzees; most molecular clocks suggest humans and chimps diverged 1–2 million years after S. tchadensis (5 mya). The original placement of this species as a human ancestor but not a chimpanzee ancestor complicated the picture of the human family tree. In particular, if Toumaï is only a direct human ancestor, its facial features bring the status of Australopithecus into doubt because the thickened brow ridges are similar to later hominids, but not earlier ones. Another possibility is that Toumaï is anatomically related to both humans and chimpanzees, but the ancestor of neither. Brigitte Senut, the discoverer of Orrorin tugenensis, claims that the features of S. tchadensis are consistent with a female proto-gorilla.

If Senut's claims are true the find would lose none of its significance, for at present no chimpanzee or gorilla ancestors have been found anywhere in Africa, thus if S. tchadensis is an ancestral relation of the chimpanzees the first light would be shed on their family trees. Furthermore, S. tchadensis does indicate that the last common ancestor of humans and chimpanzees is unlikely to resemble chimpanzees very much, as had been previously supposed.

[edit] References

Michel Brunet, Franck Guy, David Pilbeam, Hassane Taisso Mackaye, Andossa Likius, Djimdoumalbaye Ahounta, et al. "A new hominid from the upper Miocene of Chad, central Africa". Nature, 418:145-51, 2002.
Nature, "The look of Toumaï", 7 April 2005, pp. 752-755, ISSN 0028-0836
Wolpoff, M.H., J. Hawks, B. Senut, M. Pickford, and J. Ahern: "An Ape or The Ape: Is The Toumaï Cranium TM 266 a Hominid?" PaleoAnthropology 2006:36-50. University of Wyoming news release and abstract