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The wind-blown Djurab Desert of Chad has opened a new window on early human evolution 鈥� a hominid skull six to seven million years old, at least two million years older than any skull previously discovered.

The stunning find was unearthed by Michel Brunet of the University of Poitiers in France and his team. 鈥淚t鈥檚 a lot of emotion to have in my hand the beginning of the human lineage. I鈥檝e been looking for 25 years.鈥�

Named Sahelanthropus, the new species is close to the common ancestor of humans and chimpanzees. It and other recent discoveries 鈥渟trongly shake our conceptions of the earliest steps of hominid history,鈥� Brunet says. 鈥淭he divergence between chimp and human must be even older than we thought.鈥�

Sahelanthropus shows the last common ancestor 鈥渄id not closely resemble any modern ape,鈥� said Tim White of the University of California at Berkeley. Although its body and brain were the size of a modern chimp鈥檚, its face was quite different, with large brow ridges and much smaller canine teeth.

From the back, the skull 鈥渓ooks like a chimpanzee, whereas from the front it could pass for a 1.75 million year old advanced australopithicine鈥� says Bernard Wood of the George Washington University. Such a mosaic of features is also evident in a newly discovered 1.8 million year old skull from Georgia.

Far and wide

The discovery in Chad also shows early hominids had spread at least 2500 kilometres from the east African rift valleys that have so far yielded the best fossils. Brunet鈥檚 team began digging in Chad in 1994 with a team from the University of N鈥橠jamena and the Centre National d鈥橝ppui a la Recherche of Chad.

Ahounta Djimdourmalbaye, an undergraduate at the University of N鈥橠jamena, found the skull a year ago, but although word leaked out, Brunet kept quiet until publication of a paper describing the find in Nature on Wednesday. The team have also found two lower-jaw fragments and three isolated teeth from other individuals

The site is expected to yield more bones and researchers hope these will reveal more about how Sahelanthropus lived. At that time the climate in Chad was moist, leading the team to nickname the skull Toumai, a local word for babies born before the dry season.

Whether Toumai walked upright will be of key interest. No leg bones have yet been found at the site, but Brunet says of the skull: 鈥淭he position of where his spine enters his head doesn鈥檛 prove that he is bipedal 鈥� but it shows he could be.鈥�

Mix and match

Brunet adds that, apart from its more primitive canine teeth, Sahelanthropus is close to another recently discovered hominid, Ardipithecus, which lived 5.8 to 4.4 million years ago in Kenya and Ethiopia. But Sahelanthropus is further from Orrorin or 鈥淢illennium Man鈥�, a second early hominid that lived six million years ago in Kenya, but known only from leg bones and a tooth.

The blend of features in the new fossil further challenges the old theory that hominids evolved each key trait only once in a line of descent. 鈥淚t鈥檚 a great example of how the fossil record keeps showing how wrong our inferences are,鈥� said Susan Ant贸n at Rutgers University.

Wood says it lends weight to the idea that hominid evolution is a series of diversifications, 鈥渋n which anatomical features are 鈥榤ixed and matched鈥� in ways that we are only beginning to comprehend.鈥�

Journal reference: Nature (vol 418, p 145)