Comment on "Origin of Human Bipedalism As an
Adaptation for Locomotion on Flexible Branches"
D. R. Begun, et al.
Science 318, 1066d (2007);
DOI: 10.1126/science.1146446
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Comment on “Origin of Human
Bipedalism As an Adaptation for
Locomotion on Flexible Branches”
D. R. Begun,1*† B. G. Richmond,2* D. S. Strait3*
Thorpe et al. (Reports, 1 June 2007, p. 1328) concluded that human bipedalism evolved from a type of
bipedal posture they observed in extant orangutans with seemingly human-like extended knees.
However, humans share knuckle-walking characters with African apes that are absent in orangutans.
These are most parsimoniously explained by positing a knuckle-walking precursor to human bipedalism.
horpe et al. (1) presented an interesting
model to account for the origin of human
bipedalism from an arboreal “bipedal”
ancestor, but their model is problematic and they
neglected many important facts that point to a
knuckle-walking ancestry.
Features related to knuckle-walking in extant
African apes are preserved in the fossils of some
early hominins (2, 3). These hominins were bipedal, meaning that the only plausible explanation
for the presence of these traits is that they represent
primitive characters retained from a knucklewalking ancestor. In contrast, the Thorpe et al.
hypothesis requires that knuckle-walking features
evolved independently (and unparsimoniously) in
three separate lineages (chimpanzees, gorillas, and
humans) and that in the human lineage, knucklewalking anatomy evolved in the absence of knucklewalking behavior. Such a scenario seems highly
improbable.
Other evidence favoring a knuckle-walking
ancestry for humans derives from the comparative
anatomy of living hominoids. Many attributes
of the wrists, hands, and feet of African apes
and humans relate to terrestrial plantigrady and
extended wrist and hand postures loaded in
compression, as happens during knuckle-walking
(2–8). In contrast, the upper limbs of Pongo are
more strongly modified in response to the
demands of below-branch locomotion and posture. A non–knuckle-walking ancestry for humans
would have to account for the evolution of more
than 30 wrist and hand stabilizing features
independently three times in each of the African
ape and human lineages (3, 4). In contrast, there
are no unambiguous characters of which we are
aware related to bipedalism shared between
Homo and Pongo.
T
1
Department of Anthropology, University of Toronto, Toronto,
ON, M5S 2S2, Canada. 2Department of Anthropology, Center
for the Advanced Study of Hominid Paleobiology, George
Washington University, 2110 G Street, NW, Washington, DC
20052, USA. 3Department of Anthropology, University at
Albany, 1400 Washington Avenue, Albany, NY 12222, USA.
*These authors contributed equally to this work.
†To whom correspondence should be addressed. E-mail:
[email protected]
1066d
Thorpe et al.’s interpretation of orangutan
kinematics (i.e., movement) is also problematic.
The nature of the loading and movement of the
lower limb in Pongo “hand-assisted arboreal bipedalism” is so different from hominin bipedalism that it raises the question as to whether these
modes of locomotion are homologous. Indeed, it
is not even clear that the behaviors observed by
Thorpe et al. (1) can be truly called bipedalism.
Orangutans and hylobatids often extend their
lower limbs when suspending from their hands
even when their feet hang freely [see figure 1 in
(1)]. Thorpe et al. have not distinguished the
relative weight support borne by forelimbs versus
hindlimbs, so they cannot determine whether
orangutans employ true “hand-assisted arboreal
bipedalism” or whether their behavior is better
described as “foot-assisted forelimb suspension”.
The fact that orangutans practice this behavior
more frequently on smaller supports suggests that
they are predominantly supporting themselves
with their forelimbs. Furthermore, the observation that Pongo may more frequently fully extend
their knees when adopting “bipedal” postures on
small, compliant supports compared with Pan
probably reflects the tremendous mobility of the
lower limb in Pongo, which in turn is related, as
Thorpe et al. suggest, to the need to distribute a
large body mass across as many supports as possible. All large primates use this strategy to varying degrees (9). Pongo excels in this behavior
and has long been called quadrumanous (fourhanded), given the handlike morphology of their
feet. The behavior observed by Thorpe et al. (1)
is perhaps best considered a component of this
locomotor repertoire. Moreover, Pongo does not
load the lower limbs in a manner even approaching the human condition (with extended lower
limbs positioned beneath the center of mass,
knees in sagittal plane, feet fully plantigrade) and,
not surprisingly, lacks all of the unique features of
the hominin lower limb that are related to loadbearing and joint motion essential for committed
bipedalism.
Key to the argument of Thorpe et al. (1) is
their claim that orangutans use human-like
extended hindlimbs when they are bipedal and
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that orangutans differ from the African apes in
this respect. Indeed, for orangutans to serve as a
better model for the origin of hominin bipedalism, they would need to walk bipedally more
like humans than do African apes. However,
Thorpe et al. do not provide or cite quantitative
kinematic data. Previous studies have reported
that orangutans, chimpanzees, and gibbons all
use flexed hindlimbs during bipedal walking
(10). These studies indicate that orangutans may
walk with slightly more extended knees compared with chimpanzees, but they still walk
bipedally with partly flexed knees, unlike humans. Recent studies of chimpanzee bipedalism
found considerable variability in limb kinematics,
with some individuals using more extended lower
limbs and, like humans and trained macaques
(11), enjoying energy benefits (12). Moreover, hip
and knee extension are not the only important
kinematic criteria. Orangutans are awkward bipeds, using partly abducted hips, with their knees
facing laterally, and walking on the sides of their
feet. In these respects, chimpanzees and bonobos
appear to be better bipeds.
Thorpe et al. (1) reject the broadly accepted
hypothesis that vertical climbing provides an
important biomechanical link to hominin bipedalism, despite the fact that multiple studies show
that both kinematics and muscle recruitment
during terrestrial bipedalism and vertical climbing are more similar to each other than either are
to quadrupedal locomotion (10, 13, 14). Nothing
in the knuckle-walking hypothesis precludes a
suspensory, vertically postured arboreal ancestry
for all great apes and humans or the importance
of orthograde and vertical climbing adaptations
in the origin of hominin bipedalism (3, 4).
Finally, Thorpe et al. cite “recent paleontological evidence” in support of their hypothesis,
but this evidence merely states that some Miocene apes were orthograde and that early hominins lived in wooded habitats and were bipeds
that retained some suspensory characteristics. As
previously noted (3), all of this evidence is also
consistent with a knuckle-walking ancestry for
hominins. After all, African apes are arboreal
orthograde, suspensory, and forest-dwelling,
while also having knuckle-walking adaptations
to move on the ground.
We applaud Thorpe et al.’s field observations
and statistical analysis of Pongo positional
behavior, which adds to our understanding of
this fascinating and highly endangered large, very
arboreal, and extremely specialized great ape.
However, we question whether these observations are relevant to understanding the origins of
human bipedalism, especially in light of the large
body of evidence for a climbing and knucklewalking ancestry. We agree with long-standing
evidence that orthograde and probably vertical
climbing (10, 13) adaptations of the last common
ancestor of chimpanzees and humans made
possible the origin of hominin bipedalism but
suggest that this ancestor employed terrestrial
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TECHNICAL COMMENT
TECHNICAL COMMENT
knuckle-walking behaviors as part of its locomotor repertoire. Why bipedalism was favored over
knuckle-walking in the hominin clade remains an
open question.
References
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13. J. G. Fleagle et al., Symp. Zool. Soc. London 48, 359
(1981).
14. J. H. Prost, Am. J. Phys. Anthropol. 52, 175
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13 June 2007; accepted 4 October 2007
10.1126/science.1146446
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1. S. K. S. Thorpe, R. L. Holder, R. H. Crompton, Science
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