Sahelanthropus

Sahelanthropus
"Toumaï"
Temporal range: Late Miocene, 7Ma
↓
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
Brunet et al., 2002[1]
Species: †S. tchadensis
Binomial name
Sahelanthropus tchadensis
Brunet et al., 2002[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 specimens are known, other than the partial skull nicknamed Toumaï ("hope of life").

Fossils

Location of discovery.
Detail of map.
Reconstruction of S. tchadensis by sculptor Élisabeth Daynès

Existing fossils include a relatively small cranium named Toumaï ("hope of life" in the local Daza 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.

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 only known skull suffered a large amount of distortion during the time of fossilisation and discovery, as the cranium is dorsoventrally flattened, and the right side is depressed.[1]

Because no postcranial remains (i.e., bones below the skull) have been discovered, it is not known definitively whether Sahelanthropus tchadensis was indeed bipedal, although claims for an anteriorly placed foramen magnum suggests that this may have been the case. Upon examination of the foramen magnum in the primary study, the lead author speculated that the utilisation of a bipedal gait "would not be unreasonable" based on basicranial morphology similar to more recent hominids.[2] Some palaeontologists [3] have disputed this interpretation, stating that the basicranium, as well as dentition and facial features, do not represent adaptations either unique to the hominid clade or indicative of bipedalism. Its canine wear is similar to other Miocene apes.[4] Moreover, according to recent information, the femur of a hominid might have been discovered alongside the cranium but never published.[5]

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 a Frenchman, Alain Beauvilain et al.[6][7] All known material of Sahelanthropus were found between July 2001 and 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.[8]

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.[8] 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.[9]

Relationship to humans and chimpanzees

Sahelanthropus may represent a common ancestor of humans and chimpanzees, though 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 indeed 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 brow ridge 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.[10][11]

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.[12]

Sediment isotope analysis of cosmogenic atoms in the fossil yielded an age of about 7 million years.[13] 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, making the the date of 7 million years less certain.[14]

In fact, Toumaï may have been reburied in the recent past. Taphonomic analysis reveals the likelihood of one, perhaps two, burial(s). 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.[15] 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.[16]

See also

Notes

  1. ↑ 1.0 1.1 1.2 1.3 Brunet et al. (2002) "A new hominid from the Upper Miocene of Chad, Central Africa," Nature: Vol. 418: Pages 145-151. http://dx.doi.org/10.1038/nature00879
  2. ↑ Brunet, M., Guy, F., Pilbeam, D., Mackaye, H.T., Likius, A., Ahounta, D., Beauvilain, A., Blondel, C., Bocherens, H., Boisserie, J.R., de Bonis, L., Coppens, Y., Dejax, J., Denys, C., Duringer, P., Eisenmann, V., Gongdibé, F., Fronty, P., Geraads, D., Lehmann, T., Lihoreau, F., Louchart, A., Mahamat, A., Merceron, G., Mouchelin, G., Otero, O., Pelaez Campomanes, P., Ponce de León, M., Rage, J.-C., 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, 145e151.
  3. ↑ Milford H. Wolpoff1, Brigitte Senut2, Martin Pickford3 & John Hawks. 2002. Palaeoanthropology (communication arising): Sahelanthropus or 'Sahelpithecus'? Nature, 419, 581-582. doi:10.1038/419581a
  4. ↑ Brunet, Guy, Pilbeam, Mackaye, Likius, Djimdoumalbaye, Beauvilain, Blondel, Bocherens, Boisserie, De Bonis, Coppens, Dejax, Denys, Duringer, Eisenmann, Gongdibé, Fronty, Geraads, Lehmann, Lihoreau, Louchart, Adoum, Merceron, Mouchelin, Otero, Pelaez Campomanes, Ponce De Leon, Rage, Sapanet, Schuster, Sudre, Tassy, Valentin, Vignaud, Viriot, Zazzo & Zollikofer, 2002. "A new hominid from the Upper Miocene of Chad, Central Africa", Nature, 418 (6894): 145–151.
  5. ↑ Hawks 2009 "Sahelanthropus: The femur of Toumaï?"
  6. ↑ Tchad Actuel Toumaï : "Histoire des Sciences et Histoire d’Hommes"
  7. ↑ Web site of Alain Beauvilain
  8. ↑ 8.0 8.1 Brunet M., Beauvilain A., Coppens Y., Heintz E., Moutaye A. H. E. et Pilbeam D., 1995. "The first australopithecine 2,500 kilometres west of the Rift Valley (Chad)" Nature, 378 (6554): 273–275. résumé]
  9. ↑ Haile-Selassie, Yohannes; Suwa, Gen; White, Tim D. (2004). "Late Miocene Teeth from Middle Awash, Ethiopia, and Early Hominid Dental Evolution". Science 303 (5663): 1503–1505. doi:10.1126/science.1092978. PMID 15001775.
  10. ↑ Guy F., Lieberman D. E., Pilbeam D., Ponce de Leon M. S., Likius A., Mackaye H. T., Vignaud P., Zollikofer C. P. E. et Brunet M., (27 December 2005). "Morphological affinities of the Sahelanthropus tchadensis (Late Miocene hominid from Chad) cranium" PDF fulltext Supporting Tables, PNAS, 102 (52) : 18836–18841.
  11. ↑ Wolpoff M. H., Hawks J., Senut B., Pickford M. et Ahern J., 2006. "An Ape or the Ape : Is the Toumaï Cranium TM 266 a Hominid?", PaleoAnthropology, 2006: 36–50.
  12. ↑ Catherine Brahic (24 Nov 2012). "Our True Dawn". New Scientist (Reed Business Information) (2892): 34–7. ISSN 0262-4079., citing research by Augustine Kong (Decode Genetics, Reykjavik), David Reich (Harvard) and others
  13. ↑ Lebatard A.-E., Bourles D. L., Duringer P., Jolivet M., Braucher R., Carcaillet J., Schuster M., Arnaud N., Monie P., Lihoreau F., Likius A., Mackaye H. T., Vignaud P. et Brunet M., 2008. "Cosmogenic nuclide dating of Sahelanthropus tchadensis and Australopithecus bahrelghazali: Mio-Pliocene hominids from Chad" PNAS 105(9): 3226–3231 PDF fulltext
  14. ↑ Beauvilain 2008 "The contexts of discovery of Australopithecus bahrelghazali and of Sahelanthropus tchadensis (Toumaï) : unearthed, embedded in sandstone or surface collected?" South African Journal of Science, 104 (3): 165–168.
  15. ↑ Beauvilain and Watté, 2009. "Toumaï (Sahelanthropus tchadensis) a-t-il été inhumé ?" Bulletin de la Société géologique de Normandie et des Amis du Museum du Havre, 96 (1): 19–26.
  16. ↑ Brunet M., Guy F., Pilbeam D., Lieberman D. E., Likius A., Mackaye H. T., Ponce de Leon M. S., Zollikofer C. P. E. et Vignaud P., 2005. "New material of the earliest hominid from the Upper Miocene of Chad" Nature, 434 (7034): 752–755

References

External links

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