Sahelanthropus

Sahelanthropus tchadensis "Toumaï" Temporal range: Late Miocene, 7Ma PreЄ Є O S D C P T J K Pg N ↓
Cast of a Sahelanthropus tchadensis skull (Toumaï)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Primates
Family:	Hominidae
Subfamily:	Homininae
Tribe:	Hominini?
Genus:	†Sahelanthropus Klages et al., 2008[1]
Species:	†S. tchadensis
Binomial name
Sahelanthropus tchadensis Klages et al., 2008[1]

Sahelanthropus tchadensis is an extinct hominine species that is dated to about 7 million years ago, possibly very close to the time of the chimpanzee/human divergence, and so it is unclear whether it can be regarded as a member of the Hominini tribe.^[1] Few, if any, specimens are known, other than the partial skull nicknamed Toumaï ("hope of life").

Fossils

Location of discovery.

Detail of map.

Existing fossils include a relatively small cranium known as Toumaï ("hope of life" in the local Dazaga language of Chad in central Africa), five pieces of jaw, and some teeth, making up a head that has a mixture of derived and primitive features. The braincase, being only 320 cm³ to 380 cm³ in volume, is similar to that of extant chimpanzees and is notably less than the approximate human volume of 1350 cm³.^{[citation needed]}

The teeth, brow ridges, and facial structure differ markedly from those found in Homo sapiens. Cranial features show a flatter face, u-shaped dental arcade, small canines, an anterior foramen magnum, and heavy brow ridges. No postcranial remains have been recovered. The fossil suffered a large amount of distortion during the time of fossilisation and discovery.^{[citation needed]}

Because no postcranial remains (i.e., bones below the skull) have been discovered, it is not known definitively whether Sahelanthropus tchadensis was indeed bipedal or two-footed, although claims for an anteriorly placed foramen magnum suggests that this may have been the case. Some paleontologists have disputed this interpretation of the basicranium. Its canine wear is similar to other Miocene apes.^[2] Moreover, according to recent information, the femur of a hominid might have been discovered alongside the cranium but never published.^[3]

Discovery

The fossils were discovered in the Djurab Desert of Chad by a team of four led by Michel Brunet; three Chadians, Adoum Mahamat, Djimdoumalbaye Ahounta and Gongdibé Fanoné, and Frenchman, Alain Beauvilain et al.^[4][5] All known material of Sahelanthropus were found between July 2001 to March 2002 at three sites (TM 247, TM 266 (which yielded most of the material), and TM 292). The discoverers claimed that S. tchadensis is the oldest known human ancestor after the split of the human line from that of chimpanzees.^[6]

The bones were found far from most previous hominin fossil finds, which are from Eastern and Southern Africa. However, an Australopithecus bahrelghazali mandible was found in Chad by Beauvilain A., Brunet M. and Moutaye A.H.E. as early as 1995.^[6] With the sexual dimorphism known to have existed in early hominids, the difference between Ardipithecus and Sahelanthropus may not be large enough to warrant a separate species for the latter.^[7]

Relationship to humans and chimpanzees

Sahelanthropus may represent a common ancestor of humans and chimpanzees; no consensus has been reached yet by the scientific community. The original placement of this species as a human ancestor but not a chimpanzee ancestor would complicate the picture of human phylogeny. In particular, if Toumaï is a direct human ancestor, then its facial features bring into doubt the status of Australopithecus 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 it represents the first known member of their lineage. 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.^[8][9]

A further possibility, highlighted by research published in 2012, is that the human/chimpanzee split is earlier than previously thought, with a possible range of 7 to 13 million years ago (with the more recent end of this range being favoured by most researchers), based on slower than previously thought changes between generations in human DNA. Indeed, some researchers (such as Tim D. White, University of California) consider suggestions that Sahelanthropus is too early to be a human ancestor to have evaporated.^[10]

Sediment isotope analysis of cosmogenic atoms in the fossil yielded an age of about 7 million years.^[11] In this case, however, the fossils were found exposed in loose sand; co-discoverer Beauvilain cautions that such sediment can be easily moved by the wind, unlike packed earth.^[12]

In fact, Toumaï was probably reburied in the recent past. Taphonomic analysis reveals the likelihood of one, perhaps two, burial(s) which seemingly occurred after the introduction of Islam in the region. Two other hominid fossils (a left femur and a mandible) were in the same “grave” along with various mammal remains. The sediment surrounding the fossils might thus not be the material in which the bones were originally deposited, making it necessary to corroborate the fossil's age by some other means.^[13] The fauna found at the site – namely the anthracotheriid Libycosaurus petrochii and the suid Nyanzachoerus syrticus – suggests an age of more than 6 million years, as these species were probably already extinct by that time.^[14]

See also

• Human evolution
• Humanzee
• List of human evolution fossils (with images)

Notes

References

• Beauvilain Alain, 2003. Toumaï, l’aventure humaine. La Table Ronde, Paris, 239 p. ISBN 978-2-7103-2592-5.
• Brunet Michel, 2006. D’Abel à Toumaï. Nomade, chercheur d’os. Odile Jacob, Paris, 254 p. ISBN 978-2-7381-1738-0.
• Gibbons Ann, 2006. The first human.. Doubleday, New York, 306 p. ISBN 978-0-385-51226-8.

External links

Wikimedia Commons has media related to Sahelanthropus.

Wikispecies has information related to: Sahelanthropus

• Sahelanthropus.com
• Fossil Hominids: Toumai
• National Geographic: Skull Fossil Opens Window Into Early Period of Human Origins
• New Findings Bolster Case for Ancient Human Ancestor
• Sahelanthropus news reporting by John Hawks
• Sahelanthropus tchadensis - The Smithsonian Institution's Human Origins Program
• S. tchadensis reconstruction

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