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Published online on July 16, 2007, 10.1073/pnas.0703267104
PNAS | July 24, 2007 | vol. 104 | no. 30 | 12265-12269


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ANTHROPOLOGY
Chimpanzee locomotor energetics and the origin of human bipedalism

Michael D. Sockol*, David A. Raichlen{dagger}, and Herman Pontzer{ddagger},§

*Department of Anthropology, University of California, Davis, CA 95616; {dagger}Department of Anthropology, University of Arizona, Tucson, AZ 85721; and {ddagger}Department of Anthropology, Washington University, St. Louis, MO 63130

Edited by David Pilbeam, Harvard University, Cambridge, MA, and approved June 12, 2007 (received for review April 9, 2007)

Bipedal walking is evident in the earliest hominins [Zollikofer CPE, Ponce de Leon MS, Lieberman DE, Guy F, Pilbeam D, et al. (2005) Nature 434:755–759], but why our unique two-legged gait evolved remains unknown. Here, we analyze walking energetics and biomechanics for adult chimpanzees and humans to investigate the long-standing hypothesis that bipedalism reduced the energy cost of walking compared with our ape-like ancestors [Rodman PS, McHenry HM (1980) Am J Phys Anthropol 52:103–106]. Consistent with previous work on juvenile chimpanzees [Taylor CR, Rowntree VJ (1973) Science 179:186–187], we find that bipedal and quadrupedal walking costs are not significantly different in our sample of adult chimpanzees. However, a more detailed analysis reveals significant differences in bipedal and quadrupedal cost in most individuals, which are masked when subjects are examined as a group. Furthermore, human walking is {approx}75% less costly than both quadrupedal and bipedal walking in chimpanzees. Variation in cost between bipedal and quadrupedal walking, as well as between chimpanzees and humans, is well explained by biomechanical differences in anatomy and gait, with the decreased cost of human walking attributable to our more extended hip and a longer hindlimb. Analyses of these features in early fossil hominins, coupled with analyses of bipedal walking in chimpanzees, indicate that bipedalism in early, ape-like hominins could indeed have been less costly than quadrupedal knucklewalking.

biomechanics | human evolution | locomotion | limb length | inverse dynamics

Author contributions: M.D.S., D.A.R., and H.P. contributed equally to this work; M.D.S., D.A.R., and H.P. designed research; M.D.S., D.A.R., and H.P. performed research; M.D.S., D.A.R., and H.P. analyzed data; and M.D.S., D.A.R., and H.P. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

This article contains supporting information online at www.pnas.org/cgi/content/full/0703267104/DC1.

§To whom correspondence should be addressed at: Department of Anthropology, Washington University, 119 McMillan Hall, St. Louis, MO 63130. E-mail: hpontzer{at}artsci.wustl.edu

© 2007 by The National Academy of Sciences of the USA


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