A seven-million-year-old human skull has been found in the African republic of Chad.

Christened 'Toumaï', which means 'hope of life' in the local Goran language, the find is said to be the most important fossil discovery in living memory.

It is twice the age of previously found ape-man skulls, making it the earliest known relic of recognisably human lineage.

The researchers report their discovery in the journal Nature.

Last July, a French team led by Poitiers University palaeontolgists Michel Brunet and Patrick Vignaud were excavating in sandstone beneath the sands of the Djurb Desert.

They unearthed six specimens including a cranium, jaw and teeth. Assembled, Toumaï's skull displays a mix of primitive and modern features. His braincase looks like that of an ape, but his short face and teeth are similar to those of humans. His prominent brow is of a shape found only in our Homo line.

The theory is that Toumaï lived on the perimeter of Lake Chad before the central African land became desert.

It is the first skull find from a critical time in human evolution — between 10 million years ago, when there were only apes, and five million years ago, when evidence suggests early human ancestors, or hominids, appeared.

Bernard Wood, of Goerge Washington University, Washington DC, wrote in Nature that although Toumaï is not a 'missing link', it does suggest a diversity of ape-men lived even six to seven million years ago.

Indeed, the discovery strengthens the argument that humans did not evolve in a linear progression from apes that roamed the planet 10 million years ago. Rather, our lineage sprouted in many directions — the so-called 'bushy' model — before we evolved as Homo erectus and then modern humans, Homo sapiens.

According to Australian National University anthropologist Dr Alan Thorne, the Chad discovery is important because it "pushes things back a few million years".

"From this and previous discoveries of things like teeth and a few femurs … it's clear that there were upright people six to seven million years ago," he said.

"What this find really means is we've got a head for those people."

Professor Colin Groves, from the Australian National University's Department of Archaeology and Anthropology, was equally enthusiastic about the discovery, but less convinced the skull is human.

He places Toumaï at the point in evolution before humans and chimpanzees diverged, believed to be around five million years ago. One reason for doubt is Toumaï's small canine teeth.

"We have precedence for short canines in other things besides humans, such as the creature Graecopithecus, which is said to be an early proto-gorilla."

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