Prehistoric World

The war of words over the significance of an ancient skull known as Toumai resurfaced on Wednesday.

The fossil specimen was described as possibly the most important find of its kind in living memory when it was first presented to the media in July.

French palaeontologist Michel Brunet, who unearthed Toumai in Chad, said it represented the earliest known hominid, or pre-human ancestor, yet found - it is almost seven million years old.

But some researchers voiced scepticism at the time and they have now gone into print with their criticism.

Brigette Senut, Milford Wolpoff, Martin Pickford and others argue in the journal Nature that the skull is not on the human branch of the evolutionary tree at all.

Instead, they say, the specimen (formally classified as Sahelanthropus tchadensis) is probably that of an early gorilla or a chimpanzee, or a species that has since become extinct.

Walking upright

They put very different interpretations on the skull's features to that of Brunet. Its short face and small canines are female characteristics and are not conclusive evidence that Sahelanthropus tchadensis was a hominid, they believe.

"I don't see how you can tell what it is," Wolpoff, a University of Michigan anthropologist, said. "But it is not human."

His team focus in particular on the "scarring" on the skull left by neck muscles which can indicate how the spine related to the head, giving clues as to whether the creature could have walked upright.

"In looking at the scars, they told us quite clearly that this animal did not habitually walk erect," Wolpoff said. "It did not have human posture, therefore it is not human."

Michel Brunet, of the University of Poitiers, has hit back at the criticism, and is clearly irritated by the comments of his detractors.

He accuses them of being "flippant", saying their statements are a "curious" attempt to "undermine... (and) misrepresent" his findings.

Peer review

Brunet says Wolpoff and colleagues provide no evidence that the skull is that of an ape, "nor have they disproved any derived feature that this species shares with later hominids".

This area of science is known to be fiercely competitive, with each new discovery received with cool scepticism by the rival research groups who are all digging in different parts of Africa.

In January last year, it was Senut and Pickford who had to face the doubters when they announced the discovery of Kenyan fossils - a piece of jaw, teeth, a fingertip, an arm, and a sturdy leg bone - they said came from a six-million-year-old hominid (Orrorin tugenensis).

It is the journals and the process of peer review that have to try to balance various claims and counter claims and steer the science towards a clearer understanding of the origins of the human race.

Chris Stringer, a hominids expert at London's Natural History Museum, said that whatever the truth about Sahelanthropus, it was still a find of great importance.

Patchy state

He said Toumai was the only relatively complete skull so far discovered from a time that has produced very few specimens.

And it was because of this "fossil gap" that he cautioned all researchers not to make grand claims for any find.

"In my opinion, it is still too early to say where either Sahelanthropus or Orrorin lie in relation to our evolutionary line.

"The ancestors of the gorilla and chimpanzees remain to be recognised or found from six million years ago, no doubt along with parallel side-branches that probably also existed at that time.

He added: "It is dangerous to assume that the present distinctive features of gorillas, chimpanzees or humans would necessarily have been present at the beginnings of their evolution, or were unique to their line only.

"In my opinion, it is premature to push the claims too far for any fossils to be the earliest members of the human family in the present patchy state of our knowledge."

And Mark Collard, an anthropologist from University College London, was of the same opinion.

"At this point in time I don't think there is any reason to accept either team's hypothesis," he said.

"We can be confident that the specimen is a member of Hominoidea (the group formed by hominids and apes), but at the moment we cannot say whether [Toumai] is a hominid or an ape with any certainty at all."

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