Sahelanthropus tchadensis
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Sahelanthropus tchadensis is an extinct 7 million year old species of hominid. The species could be an ancestor of humans and chimpanzees, which diverged about five million years ago.
Fossils
The species is known from an almost complete fossilised skull (cranium and fragments of jaw and teeth), named 'Toumaï' by its discoverers. The fossil was discovered by Ahounta Djimdoumalbaye in Toros-Menalla, Chad, in July 2002. The discovery of a fossil hominid in the Chad desert was unexpected, as previous fossil hominid discoveries were mainly made around the Rift Valley and South Africa, and it was thought that in the Miocene, the period of early human evolution, the areas to the east of the Rift Valley were populated by ancestors of the modern apes.
Anatomy and lifestyle
In the Miocene what is now the Chad desert was a wet forrested area, and may have been within the range of hominid ancestors. Sahelanthropus had relatively small canines and thicker layers of tooth enamel than most apes, suggesting a mainly herbivorous diet. The areas of the skull to which the neck muscles attached are shaped such that it is possible that Sahelanthropus was bipedal, or walked partially upright. However, the foramen magnum is not situated directly under the skull, so the head must have been held forward, rather than balanced on an erect body as in modern humans. The lack of post-cranial fossils makes it impossible to know whether Sahelanthropus was bipedal. It is already known that hominids adopted bipedal locomotion before the brain began to expand, so proof of bipedality would back up the claim that Sahelanthropus is an ancestor of modern hominids.
The cranium showed that the Sahelanthropus brain was closer in size to those of modern chimpanzees than that of modern Humans, at 320 to 380 cubic centemetres. The face is relatively flat, but with pronounced brow ridges, the opposite of the later hominid ancestors, the Australopithecines, who had prognathic faces and no brow ridges, while modern humans have flat faces and no brow ridges. The back of the skull is very ape-like.
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