In what may be the most startling fossil find in decades, scientists in central Africa say they have unearthed the oldest trace of a pre-human ancestor a remarkably intact skull of an apelike species that walked upright as far back as 7 million years ago.

The thick-browed, flat-faced skull was found in Chad, 1,500 miles west of pre-human discoveries in east Africa.

The Toumai skull, found in central Africa, is 6 to 7 million years old.

Exactly where the skull fits into man’s family tree is not clear. But the skull’s age, shape and location challenge basic beliefs about the evolution of man’s earliest ancestors.

Among other things, the find could push back the date at which humans are believed to have diverged from apes. And it suggests that upright-walking human ancestors evolved not just on the grasslands, as anthropologists have long thought, but in forests as well.

Scientists nicknamed the specimen Toumai, or “hope of life” in Africa’s Goran language.

It was uncovered a year ago by a 40-person research team led by French paleontologist Michel Brunet in a remote, wind-scoured stretch of desert that was a lush forest long ago. Details appear in today’s issue of the journal Nature.

It is the culmination of a lonely, dangerous search by Brunet that began 25 years ago, far from the most celebrated excavations in east Africa. Besides having to deal with the harsh climate, the researchers had to dig carefully around land mines left from decades of war.

“It’s a lot of emotion to have in my hand the beginning of the human lineage,” Brunet said. “I have been looking for this for so long.”

Others who have examined the skull said it was a landmark specimen.

“It’s not what we thought we would find,” said Bernard Wood of George Washington University. “If you look at the back of it, it looks like a chimpanzee, and if you look at the front of it, it looks like something in the human fossil record unambiguously a couple of million years ago.”

Toumai is the oldest addition to a growing array of recently discovered hominid, or pre-human, fossils that show human evolution was not an orderly progression.

Instead, Africa appears to have been widely populated with many different transitional species.

Toumai was classified as a member of the newly dubbed species Sahelanthropus tchadensis and is thought to be between 6 million and 7 million years old.

That means it could be as much as 1 million years older than previously found hominid fossils, and at least 3 million years older than the next-oldest hominid skulls.

The specimen’s age falls near a critical point when the human lineage split from apes. Relying on genetic evidence, scientists have assumed the split occurred 5 million to 7 million years ago.