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Penn State
Date: 
  
2004-09-03
      
Chimp-sized Hominid Walked Upright On Two Legs Six 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 6 million years ago --- about 3 million years earlier than "Lucy," 
the most famous early biped in our lineage.

Left: Reconstruction of the fossil bone from CT slices. The vertical 
line indicates the plane of the view shown on the right. (Image courtesy 
of Penn State)
  
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Dr. Robert Eckhardt, professor of developmental genetics and 
evolutionary morphology, Laboratory of Comparative Morphology and 
Mechanics (LCMM), Department of Kinesiology at Penn State, led the U.S. 
research team responsible for analysis of the CT scans of the internal 
structure of the fossil bone. Eckhardt says, "We have solid evidence of 
the earliest upright posture and bipedalism securely dated to six 
million years."

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.