Human ancestors walked upright 6 million years ago, at least 1 million years earlier than previously believed, according to an analysis of the oldest primate fossil known.

The fossil, known as the Millennium Ancestor because it was found in 2000 in the Tugen Hills of Kenya, has been highly controversial. Its French discoverers, Martin Pickford of the College of France and Brigitte Senut of the French National Museum of Natural History, claimed that the hominid was bipedal and that humans were descended directly from it, bypassing the Australopithecines and relegating them to an evolutionary dead end.

A new analysis of the femur, or upper thighbone of the fossil, technically known as Orrorin tugensis, confirms that the hominid walked upright but shows that it was, in fact, an ancestor of both Australopithecines and humans.

Paleoanthropologist Brian G. Richmond of George Washington University and one of his former graduate students, William L. Jungers of Stony Brook University in New York, carefully measured the dimensions of the O. tugensis femur, which is stored in a bank vault in Nairobi, Kenya.

They then compared the dimensions to a large number of other fossil femurs, as well as bones from modern humans and great apes. They reported Friday in the journal Science that the shape of the femur did, indeed, indicate that the creature walked on two legs and was distinct from other primates.

But they rejected the idea that it was a direct ancestor of humans, finding that it was very closely related to the Australopithecines, which are themselves human ancestors.

They concluded that early hominids used a similar method of locomotion for 4 million years, before early Homo species evolved a new hip and thigh configuration that gave them a gait like that of modern humans.