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Sahelanthropus tchadensis

Sahelanthropus tchadensis is an early fossil hominid. The fossils found, a cranium, two pieces of jaw, and some teeth, make up a head that has a mixture of human and chimpanzee features. The braincase suggests a chimpanzee, but the teeth are closer to those of humans, and the face includes brow ridges, a human feature not found on any living great ape. The point at the back of the skull where the neck muscles attach suggests that this species walked upright.

The discoverers claim that S. tchadensis is the oldest known hominid. The bones were found in Chad, nowhere near the previous known hominid fossils, which come from eastern and southern Africa. The location in which they were found is, however, a very difficult one in which to work, and had not been explored in any detail.

The fossil skull, nicknamed "Toumai", may be a common ancestor of humans and chimpanzees. If so, the molecular clock that has been calculated for the two species is incorrect: Toumai is between six and seven million years old, and the molecular clock puts the divergence between the two lines at approximately five million years ago.

This find complicates the picture of the human family tree. In particular, if Toumai is a direct human ancestor, his facial features bring the status of Australopithecus into doubt.

Another possibility is that Toumai is related to humans and chimpanzees, but the ancestor of neither.

External link: http://www.nature.com/nsu/020708/020708-12

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