A hip stance by an ancient ancestor

Fossil hunters discovered remains of a 6-million-year-old human ancestor, dubbed Orrorin tugenensis, at a Kenyan site in 2000. Their analysis of upper-leg fossils from Orrorin suggested that it walked upright in a surprisingly modern way, more like 2-million-year-old Homo erectus than the 3- to 4-million-year-old australopithecines, the group that includes the partial skeleton known as Lucy.

A new study of the most complete Orrorin leg bone, which includes the shaft and knob that connected the upper leg to the pelvis, reaches a different conclusion. Orrorin in fact shared a distinctive hip arrangement with australopithecines, as well as with a related line of fossil species (Paranthropus) that eventually died out, say Brian G. Richmond of George Washington University in Washington, D.C., and William L. Jungers of Stony Brook University in New York.

So, hips conducive to walking slowly with legs wide apart evolved in Orrorin and remained unchanged for almost 4 million years, until the demise of australopithecines, Richmond and Jungers propose in the March 21 Science. Around that time, Homo species evolved hips designed for a rapid stride with legs close together.

The scientists compared Orrorin’s upper leg to corresponding specimens from 130 modern humans, 49 common chimpanzees, 14 pygmy chimps, 59 gorillas, 32 orangutans, and nine fossil ancestors of people. Fossils came from australopithecines, Paranthropus, and early Homo species.

Earlier computerized tomography scans of the pattern of bone thickness in Orrorin’s hip connection raise the possibility that Orrorin also assumed apelike postures to climb trees, the researchers note.

The Kenyan fossil, which was found in several pieces and then glued together, should be unglued to examine its internal structure before drawing conclusions about how Orrorin moved about, remarks anthropologist Tim D. White of the University of California, Berkeley.