A nearly 6-million-year-old thigh bone may provide some of the earliest evidence for human ancestors walking on two legs.
New measurements of the bone, discovered in Kenya in 2000, confirm that the hip and upper leg were adapted to walking upright, researchers report in this week's issue of the journal Science.
Lead researcher Brian Richmond of George Washington University in Washington, D.C., reports the bone is from an early hominin called Orrorin tugenensis.
Richmond reports that the bone resembles thigh bones from early human ancestors known as Australopithecus and Paranthropus which lived 2 million to 3 million years ago, which also were adapted to walk upright.
The bone is adapted to attach to muscles that hold the hip to keep balance and is strengthened to handle the stress of repeated, regular motion, the researchers said. Thigh bones from ancient and modern apes are more rounded to handle stress in all directions because they are used in many different ways including climbing and even hanging upside down.
Anthropologists had speculated that O. tugenensis could walk upright but are divided about its place in the evolution of modern humans.