https://www.pnas.org/content/pnas/104/30/12265.full.pdf

Abstract
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 @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.

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