A new fossil analysis bolsters the theory that a chimp-size primate that lived in Kenya's Tugen Hills some six million years ago walked on two legs, researchers say. As such, the creature, known as Orrorin tugenensis, may be one of the earliest human ancestors on record.

Orrorin discoverers Martin Pickford and Brigitte Senut of the National Museum of Natural History in Paris have argued for some time that the fossils belonged to a biped, based on certain features of the upper leg bone, or femur--namely an elongated neck connecting the femur head to the shaft and the presence of a groove carved by the obturator externus muscle. They also noted in a report published several years ago that computed tomography (CT) scans through the femoral neck of the most complete Orrorin thigh bone revealed a humanlike bone structure--an assertion that met with criticism.

In the new work, Senut and Pickford, along with Robert Eckhardt of Pennsylvania State University and other collaborators, again analyzed CT scans of the femoral neck, this time with the help of a new software program. Describing the results in the current issue of Science, the team reports that the distribution of cortical bone--an indicator of the load placed on the femur during locomotion--looks more like that of humans than chimps or gorillas. This, they contend, "constitutes direct evidence for frequent bipedal posture and locomotion."

Other scientists aren't so sure. Owen Lovejoy of Kent State University observes that the CT data seem ambiguous. "The new scans just don't show anything that a simple x-ray or a photograph of the neck at the point where it's broken and glued back together would," he remarks. Based on the obturator externus groove and the long femoral neck, "I think the Orrorin femora are anatomically biped and therefore probably hominid," Lovejoy concedes. But just how much time Orrorin spent walking upright remains to be determined.