Chimp-sized hominid walked upright on two legs 6 million years ago

University Park, Pa. -- Recent fossil evidence suggests that a hominid, the size of a chimp, walked upright on two legs in Kenya's Tugen Hills, over six million years ago --- about 3 million years earlier than "Lucy," the most famous early biped in our lineage.

Robert Eckhardt, professor of developmental genetics and evolutionary morphology at Penn State's Laboratory of Comparative Morphology and Mechanics (LCMM) in the department of kinesiology, led the U.S. research team responsible for analysis of the CT scans of the internal structure of the fossil bone. "We have solid evidence of the earliest upright posture and bipedalism securely dated to 6 million years," Eckhardt says.

The evidence is detailed in the current (Sept. 3) issue of Science in a report, "External and Internal Morphology of the BAR 1002'00 Orrorin tugenensis Femur." Eckhardt's co-authors are Karl Galik, Orthopedic Biomechanics Laboratory, Allegheny General Hospital, Pittsburgh, Pa.; Brigitte Senut, Department of Earth History, National Museum of Natural History, Paris, France; Martin Pickford, who holds the chair of paleoanthropology and prehistory at the College of France and at the Department of Earth History of the National Museum of Natural History, Paris, France; Dominique Gommery, National Center for Scientific Research, Paris, France; Jacques Treil, Radiology Department, Pasteur Clinic, Toulouse, France; and Adam Kuperavage, a graduate student in Penn State's LCMM.

The fossil the team studied is part of a left thighbone unearthed nearly four years ago by Senut and Pickford at their dig in the Kenyan Lukeino Formation. The fragment includes the intact head of the left thighbone -- the ball that is inserted into the hip socket joint -- plus the bony neck that connects the ball to the thighbone shaft as well as part of the thighbone shaft.

Measurements, chiefly carried out by Dr. Galik, show that the fossil bone is about the same size as a chimpanzee's. However, CT scans of the interior of the bone reveal that the neck connecting the ball to the shaft is thinner on top than it is on the bottom, a sign the researchers say that the individual from which it came walked on two legs.

Eckhardt says, "In present day chimps and gorillas, the thicknesses in the upper and lower parts of that bone are approximately equal. In modern humans, the bone on top is thinner than on the bottom by a ratio of one to four or more. The ratio in this fossil is one to three."

The ratio in the fossil is evidence for transition to an upright posture and habitual bipedal gait the researchers argue. In addition, Eckhardt notes, because walking upright is the essential mark of a hominid, the ratio is functional evidence that the bones fossilized at Lukeino were from hominids.

The researchers write, "In known features, external and internal, BR1002'00 exhibits a total morphological pattern distinct from African apes, diagnostic of bipedal locomotion and appropriate for a population standing at the dawn of the human lineage."

The imaging research was supported, in part, by grants from the American Philosophical Society and from Sigma Xi.

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