Sahelanthropus_tchadensis, Sahelanthropus_tchadensis Information, Sahelanthropus


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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
Subtribe:	Hominina
Genus:	Sahelanthropus Brunet et al, 2002
Species:	S. tchadensis

Binomial name

†Sahelanthropus tchadensis
Brunet et al, 2002

Sahelanthropus tchadensis is a fossil ape, thought to have lived approximately 7 million years ago. It is sometimes claimed as the oldest known ancestor of Homo (humans) post-dating the most recent common ancestor of humans and chimpanzees. It was a species of Miocene ape, related to humans and other living African apes.

Fossils

Existing fossils, a relatively small cranium, five pieces of jaw, and some teeth, make up a head that has a mixture of derived and primitive features. The braincase, being only 340 cm³ to 360 cm³ in volume is similar to that of extant chimpanzees and is notably less than the approximate human volume of 1350 cm³. The teeth, brow ridges, and facial structure differ markedly from those found in Homo sapiens. Due to the distortion that the cranium has suffered, a 3D computer reconstruction has not been produced. Since no postcranial remains (bones below the skull) have been discovered, it is as of yet unknown whether Sahelanthropus tchadensis was indeed bipedal, although claims for an anteriorly placed foramen magnum suggests that this may have been the case, although some paleontologists have disputed this interpretation of the basicranium. Its canine wear is similar to other Miocene apes.

The fossils were discovered in the desert of Chad by a team of four; three Chadians, Mahamat Adoum and Ahounta Djimdoumalbaye (who found the skull on July 19, 2001), Fanone Gongdibe, and Alain Beauvilain, the French team leader. All fossils of Sahelanthropus were found between July 2001 to March 2002. 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.

Perspective

The fossil skull TM 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) but there is now general acceptance among paleontologists, and even among molecularists, that such a late divergence is no longer tenable. 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 a direct human ancestor, then its facial features bring the status of Australopithecus into doubt because its thickened brow ridges were reported to be similar to those of some later fossil hominids (notably Homo erectus), whereas this morphology differs from that observed in all australopithecines, most fossil hominids and extant humans.

Another possibility is that Toumaï is related to both humans and chimpanzees, but is the ancestor of neither. Brigitte Senut and Martin Pickford, the discoverers of Orrorin tugenensis, suggested that the features of S. tchadensis are consistent with a female proto-gorilla. Even if this claim is upheld, then the find would lose none of its significance, for at present precious few chimpanzee or gorilla ancestors have been found anywhere in Africa. Thus if S. tchadensis is an ancestral relative of the chimpanzees (or gorillas) then 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 by some paleontologists.

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