Scientists have concluded that the world's oldest primate fossil known as the Millennium Ancestor belongs to a group that includes prehistoric humans rather than apes. Investigators studying the remains have determined that the six-million-year-old species walked upright, a characteristic that separated human ancestors from apes. VOA's Jessica Berman reports.

Ever since the fossil of the six million year old Millennium Ancestor was discovered in Kenya by French paleoanthropologists in 2000, scientists have debated whether it belonged to the family of prehistoric apes or humans.

An independent team of researchers led by Brian Richmond of George Washington University in Washington, D.C. studied the upper thighbone of the Millennium Ancestor.

Shape of fossil femur (thigh bone, center) from Kenya shows our early ancestors were already adapted to upright walking by 6 million years ago

Investigators used a statistical method to compare the fossil to 300 thighbones from great apes and modern humans as well as fossilized human ancient remains.

Richmond says it appears the Millenneum Ancestor walked upright, a characteristic possessed by prehistoric humans and one that marked the divergence with apes some two million years ago.

Richmond says the evidence can be seen in the hip bone of the Millennium Ancestor.

"It doesn't look anything like a modern or a fossil ape," he said. "So we know that this thigh bone was adapted to upright walking on two legs all the way six million years ago."

But Richmond says the Millennium Ancestor is not a direct ancestor of prehistoric humans.

Rather, Richmond says it appears to be related to a diverse family of human-like primates that arose in Africa much later.

"At six million years ago, the fossil looked so similar to things that are two and three million years ago that it suggests when our early ancestors started walking upright, they evolved a hip anatomy that remained stable for about four million years for the majority of human evolutionary," he explained. " And there weren't further changes until two million years with the origin of our own genus homo."

Richmond's paper on the Millennium Ancestor is published in the journal Science.