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Prehistoric World

Sahelanthropus tchadensis: More Likely Human

Edited from BBC News, 6 April 2005

By 2005 experts were closing in on on answer about whether an ancient skull from Africa belonged to a possible human ancestor or to a creature which was closer to apes.

Fresh fossil finds from Chad in central Africa, as well as a new analysis of the skull, seemed to confirm that 'Toumaï' was closer to humans. The Toumaï specimen was unearthed in Chad in 2002 to international acclaim. But rival researchers attacked claims by the discovery team that it was the oldest hominid, or human-like creature.

The near-complete skull, pieces of jawbone and several teeth which were unveiled in 2002 were found in the desert of northern Chad by a team led by Michel Brunet, at the University of Poitiers, France. At six to seven million years old, Sahelanthropus tchadensis (better known by its nickname Toumaï) dates to about the time at which, according to genetic data, the ancestors of humans and the ancestors of chimpanzees went their separate evolutionary ways.

The find had a puzzling combination of modern and primitive features, with an ape-like brain size and skull shape, combined with a more human-like face and teeth. It also sported a remarkably large brow-ridge, more similar to that of hominids. But at least one anthropologist argued that the fossil could belong to a female forerunner of the gorilla.

Now in 2005 Brunet and colleagues reported the discovery of two new jaw fragments and the crown of a tooth in the same geographical area as the earlier fossils. The authors stated that their analysis revealed key similarities to hominid fossils and differences from African apes which supported the idea that Toumaï was a hominid.

Virtual reconstruction

In a separate paper, a team including Brunet and Christoph Zollikofer of the University of Zurich in Switzerland, presented a 3D computer reconstruction of the skull, which had been badly distorted in the ground.

The team had essentially 'unmangled' the skull, and the reconstruction apparently confirmed the claim that tchadensis shared key features with later hominids.

Sahelanthropus tchadensis
Shown here is the cranium of Sahelanthropus tchadensis, one of perhaps several species on the human side of the human-chimpanzee divide which may still have interbred with an early chimpanzee species at a time at which the two lines were still very similar - perhaps no more different than modern humans and Neanderthals who certainly did interbreed (a, facial view. b, lateral view. c, dorsal view. d, basal view)

In addition, the position of the foramen magnum - the hole through which the spinal cord enters the skull - is similar to that of humans but not apes. This suggests that Toumaï was bipedal; the creature walked upright like modern humans.

The scientists carried out a virtual reconstruction because the skull was heavily mineralised and distorted. It was impossible to do a reconstruction by physical means. The find was absolutely unique for several reasons. Firstly, because of its age. Secondly due to its geographical location. Thirdly, because it was incredibly complete.

Not swayed

Martin Pickford, of the National Museum of Natural History in Paris, was one of those scientists who was unconvinced by arguments that Toumaï was a hominid.

He was of the opinion that it was an ape-like animal. It may well have given rise to bipedal hominids, but it was not yet a bipedal hominid, according to Dr Pickford.

Professor Zollikofer commented about his feeling that most of the disagreement over the fossil came from the fact that it was distorted, so it was quite difficult to recognise the diagnostic hominid features.

If Toumaï really did belong on the human branch of the evolutionary tree, its discovery called into question certain assumptions about our prehistory. The fossils were found some 2,500km west of the African Great Rift Valley - traditionally seen as humankind's ancestral home due to the wealth of hominid fossils which have been discovered there.

The discovery of tchadensis implies that early hominids ranged far wider from East Africa, and far earlier, than previously thought. It also suggests that hominids evolved quickly when they set off on their own evolutionary path.



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