JOHANNESBURG (Reuters) - South African fossils thought to be key links in the chain of human evolution may not actually be old enough to shed light on the ancestry of modern man, scientists said on Friday.

Research by paleoanthropologists at Johannesburg's University of the Witwatersrand showed the hominid fossils thought to hold clues to the origins of the species Homo sapiens might be too young to claim that role.

The work put the age of fossil skeletons found at the nearby Sterkfontein site at up to one million years younger than previously estimated.

That suggested the species were contemporaries of modern man's ancestors and had already diverged from the line that led to the Homo genus.

"This means we are almost certainly going to have to redraw the family tree," said Dr. Lee Berger, the lead author of the article, published in October's issue of the American Journal of Physical Anthropology.

The World Heritage Sterkfontein site, known as the 'Cradle of Humankind', is fondly referred to by South African President Thabo Mbeki to emphasize the modern world's roots in Africa.

But while the researchers said their work did not detract from the site's significance, they said the site's main fossil cache -- the "member 4" deposit -- was simply not ancient enough at about two million years to contain human ancestors' remains.

"It's just too young," researcher Daniel Lacruz said. "The genus Homo has already been around for several hundred thousand years by the time of Sterkfontein member 4."

The evolutionary chain that links modern man, chimpanzees, and a whole host of extinct fossil species to a common ancestor somewhere about 20 million years ago, has been subjected to fierce criticism since Charles Darwin first proposed the theory of evolution in the 19th century.

A scientific consensus view puts modern man's origins in Africa some 1.5 million years ago, but the route leading to the Homo genus is the focus of intense debate and the research is expected to fuel fresh controversy.

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