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Finder of key hominid fossil disputes 7-million-year dating

Beauvilain says it is clear that the soil around the find, and possibly the find itself, had been shifted by wind or erosion, a phenomenon that can happen swiftly and frequently in the desert.

So carbon-dating the soil and attributing that to the skull was a perilous exercise, he says.

"How many times was it exposed and reburied by shifting sands before being picked up?" he asks in the commentary.

Beauvilain also takes issue with the soil samples used for the PNAS study and analysed by experts from France's National Centre for Scientific Research (CNRS).

He says these samples were taken selectively and did not give a full picture of the depth and range of topography in which the find was made. He describes some of the collection choices as "astonishing."

On the same grounds, Beauvilain attacks Brunet's dating of an ancient Chadian jawbone, dubbed Abel and estimated to be between three million and 3.5 million years old.

"Abel," too was picked up on the surface in 1995, and was not embedded in the soil, he says, showing photos of both finds on their day of discovery.

The debate is important because of its implications for anthropology.

Toumai -- the name means "hope of life" in the local Goran language -- was found 2,500 kilometers (1,500 miles) west of the Great Rift Valley, until now considered the cradle of humanity.

So if the skull's dating is right, it implies the early hominids ranged far wider from East Africa, and far earlier, than previously thought.

The discovery also implies hominids evolved quickly from apes after they split from a common primate ancestry.

Hominids are considered the forerunners of anatomically modern humans, who appeared on the scene about 200,000 years ago.

Still unclear, though, is the exact line of genealogy from these small, rather ape-like creatures to the rise of the powerfully-brained Homo sapiens.