An international team of scientists led by French anthropologists has unearthed the oldest skull ever found in the human family, a seven-million-year-old fossil in the central African nation of Chad. The discovery opens up what experts say is a potential treasure trove of information about the evolution of mankind.

Paleontologists who found the seven-million-year-old skull have given it the almost unpronounceable name of Sahelanthropus tchadensis. But it also goes by the nickname Toumai, which means "Hope of Life" in the Goran language. They describe their discovery in two articles published in the journal Nature. "It's really one of the most exciting discoveries in paleontology in the last 100 years, and especially exciting for anybody interested in the evolution of humans," said Daniel Lieberman, who teaches anthropology at Harvard University. He says Toumai is four million years older than the next oldest skull in the human family, which is about three million years old.

Scientists believe humans and chimpanzees, which are closely related, went their separate ways between five and eight million years ago. Scientists hope Toumai might offer clues to what happened during the period before divergence and the development of ancient humans.

Toumai, according to Professor Leiberman, looks like a combination of man and chimp. "In most respects it looks very much like a chimpanzee," he said. "It's got a very chimpanzee sized head and many chimpanzee-like bits of anatomy. Its teeth are very chimpanzee-like. But in a few important ways, it looks like a human ancestor. It has smaller canines [teeth], it has smaller cheek teeth, and its face is very, very modern. It's an amazing face. In fact, its face is much more modern than any of the other early humans that have been discovered in eastern Africa."

Anthropologists, including Daniel Lieberman, are surprised that the skull was unearthed in central Africa, whereas previous ancient skulls were discovered in eastern and southern Africa. The latest discovery suggests that human evolution was occurring throughout the continent, possibly at different rates.

Professor Lieberman says Toumai is going to make anthropologists rethink how mankind evolved. "Even though it's seven million years old, its face is very much like species found in east Africa that is two million years old," he said. "So it means that we don't really know how to draw the right evolutionary tree at this point."

One unanswered question is whether modern humans are directly descended from Toumai. The paleontologists who dug up the skull think we might be.

Bernard Wood, an anthropologist at George Washington University in Washington, D.C., is skeptical and wrote about his misgivings in an editorial in Nature. "The thought that the first time you find something that's well preserved in that time it's going to be our ancestor is probably unlikely," he said. "And the notion that there is probably one creature and we just happen to have found the right creature is just a little too easy."

The director of human origins at the Natural History Museum in London, Professor Chris Stringer agrees. Evolution, he says, is messy.

"Nature, if you like, keeps experimenting with how to evolve humans, and this was going on certainly two to three million years ago in Africa," he explained. He added this was probably also going on six million years ago. "And I think it's too soon to say whether this new find from Chad lies on the human line, as is being claimed, whether it might be related to the chimpanzee or gorilla, or might indeed be a parallel side branch that isn't actually on the line of the evolution of any of the living great apes or us," he added.

Meanwhile, other experts say the primate features of Toumai, notably the prominent brow ridge, represent the first ancient chimpanzee skull.

Scientists say the discovery provides the first hard evidence confirming modern DNA tests that show the close biological relationship between humans and chimpanzees.