Human ancestor gets leg up on walking

One of the earliest known hominids, a 6-million-year-old member of humanity’s evolutionary family, walked upright with nearly the same facility as do people today, according to a new fossil analysis.

In 2001, a French team recovered teeth and limb-bone fragments of Orrorin tugenensis in Kenya. A disagreement quickly arose about whether the fossil teeth more closely resembled those of even-older apes or of later hominids (SN: 7/14/01, p. 20: Earliest Ancestor Emerges in Africa). Anthropologists leaning toward the hominid interpretation argue that the upper-leg bones from O. tugenensis support a two-legged gait, a prime characteristic of hominids.

An inside look at one of those upper-leg bones, which preserves the knob-topped shaft that connected it to the pelvis, confirms O. tugenensis‘ upstanding status, say Robert B. Eckhardt of Pennsylvania State University in University Park and his colleagues. Computerized tomography (CT) scans reveal a markedly thicker layer of bone on the shaft’s underside, compared with its topside, Eckhardt’s team reports in the Sept. 3 Science. This pattern of bone distribution, an accommodation to support upright posture, occurs to an even greater extent in people, the researchers say.

In contrast, ancient and modern apes display roughly equal bone thickness on the top and bottom of the same upper-leg section.

O. tugenensis‘ upper-leg shaft contains two other humanlike features, the investigators add. First, it’s long relative to the shafts of chimps and other apes. Second, it possesses a groove for a muscle that stabilizes the hip during walking.

While those surface features indicate that O. tugenensis walked upright, the CT scans published by Eckhardt’s group lack enough resolution to provide a reliable picture of bone thickness, asserts C. Owen Lovejoy of Kent (Ohio) State University. He awaits further analyses of the O. tugenensis fossil with higher-resolution CT scans, other X rays, or even photographs taken through naturally caused breaks in the fossil.