Postural bipedalism may have preceded and not causally connected to walking.

American Journal of Physical Anthropology
Early View: Published Online 15 Nov 2005

Arboreal bipedalism in wild chimpanzees: Implications for the
evolution of hominid posture and locomotion (p NA)
Craig B. Stanford

>From abstract
"
Field observations of bipedal posture and locomotion in wild
chimpanzees (Pan troglodytes) can serve as key evidence for
reconstructing the likely origins of bipedalism in the last
prehominid human ancestor.
"

"
In 246.7 hr of observation from 2001-2003, 179 instances of
bipedal posture lasting 5 sec or longer were recorded, for a
rate of 0.73 bouts per observation hour. Bipedalism was
observed only on arboreal substrates, and was almost all
postural, and not locomotor. Bipedalism was part of a complex
series of positional behaviors related to feeding, which
included two-legged standing, one-legged standing with arm
support, and other intermediate postures. Ninety-six percent
of bipedal bouts occurred in a foraging context, always as a
chimpanzee reached to pluck fruit from tree limbs.
"

"
Most observations of bipedalism were made when the animals
were in treetops and the observer at eye-level across narrow
ravines. This suggests that wild chimpanzees may engage in
bipedal behavior more often than is generally appreciated.
Models of the likely evolutionary origins of bipedalism are
considered in the light of Bwindi bipedalism data. Bipedalism
among Bwindi chimpanzees suggests the origin of bipedal
posture in hominids to be related to foraging advantages in
fruit trees. It suggests important arboreal advantages in
upright posture. The origin of postural bipedalism may have
preceded and been causally disconnected from locomotor
bipedalism. Am J Phys Anthropol, 2005. © 2005 Wiley-Liss, Inc.
"

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