Sahelanthropus_tchadensis

Sahelanthropus tchadensis

Sahelanthropus tchadensis is a fossil ape that lived approximately 7-6 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 the living African apes.

Fossils

Existing fossils – a relatively small cranium nicknamed Toumaï ("hope of life" in the local Goran language of Chad), 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 Djurab desert of Chad by a team of four; three Chadians, Adoum Mahamat and Djimdoumalbaye Ahounta (who found the skull on July 19, 2001) and Gongdibé Fanone, and the French team leader Alain Beauvilain. 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. 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 the Sahelanthropus' discoverers as early as 1993.

Relationship to modern humans and great apes

Sahelanthropus may represent 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 molecular biologists that such a late divergence is no longer tenable. 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 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 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.

Unfortunately, the exact age of the fossil is somewhat hard to determine. While molecular clocks are increasingly found to be far more unreliable than initially believed, sediment isotope analysis which yielded an age of about 7 million years is generally considered quite reliable. In this case however, the fossils were found partially exposed in loose sand; co-discoverer Beauvilain cautions that such sediment can be easily moved by the wind, unlike packed earth. The sediment surrounding the fossils might thus not be the material the bones were originally deposited in, making it necessary to corroborate the fossil's age by some other means. 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 extinct already by that time.

Footnotes

References

(2008): The contexts of discovery of Australopithecus bahrelghazali (Abel) and of Sahelanthropus tchadensis (Toumaï): unearthed, embedded in sandstone, or surface collected? South African Journal of Science 104(5/6): 165-168
(2002): A new hominid from the Upper Miocene of Chad, Central Africa. Nature 418(6894): 145–151. PDF fulltext Erratum: Nature 418(6899): 801. PDF fulltext
(2005): New material of the earliest hominid from the Upper Miocene of Chad. Nature 434(6894): 752-755. PDF fulltext
(2005): Morphological affinities of the Sahelanthropus tchadensis (Late Miocene hominid from Chad) cranium. PNAS 102(52): 18836–18841. PDF fulltext Supporting Tables
(2006): An Ape or the Ape: Is the Toumaï Cranium TM 266 a Hominid? PaleoAnthropology 2006: 36–50. PDF fulltext