Independent evolution of knuckle-walking in African
apes shows that humans did not evolve from
a knuckle-walking ancestor
Tracy L. Kivella,1,2and Daniel Schmitta
aDepartment of Evolutionary Anthropology, Duke University, P.O. Box 90383 Science Drive, Durham, NC 27708-0383
Edited by Alan Walker, Pennsylvania State University, University Park, PA, and approved June 30, 2009 (received for review February 5, 2009)
Despite decades of debate, it remains unclear whether human
bipedalism evolved from a terrestrial knuckle-walking ancestor or
from a more generalized, arboreal ape ancestor. Proponents of the
knuckle-walking hypothesis focused on the wrist and hand to find
These studies, however, have not examined variation or develop-
ment of purported knuckle-walking features in apes or other
primates, data that are critical to resolution of this long-standing
debate. Here we present novel data on the frequency and devel-
opment of putative knuckle-walking features of the wrist in apes
and monkeys. We use these data to test the hypothesis that all
knuckle-walking apes share similar anatomical features and that
these features can be used to reliably infer locomotor behavior in
our extinct ancestors. Contrary to previous expectations, features
long-assumed to indicate knuckle-walking behavior are not found
in all African apes, show different developmental patterns across
species, and are found in nonknuckle-walking primates as well.
However, variation among African ape wrist morphology can be
clearly explained if we accept the likely independent evolution of
2 fundamentally different biomechanical modes of knuckle-walk-
ing: an extended wrist posture in an arboreal environment (Pan)
versus a neutral, columnar hand posture in a terrestrial environ-
ment (Gorilla). The presence of purported knuckle-walking fea-
tures in the hominin wrist can thus be viewed as evidence of
arboreality, not terrestriality, and provide evidence that human
bipedalism evolved from a more arboreal ancestor occupying the
ecological niche common to all living apes.
bipedalism ? development ? hominoid ? homoplasy ? wrist
rancorous debate over the nature of locomotion in our prebi-
pedal human ancestor. The debate can be summarized with 2
competing models. One model envisions the prehuman ancestor
as a terrestrial knuckle-walker, a behavior frequently used by our
closest living relatives, the African apes (1–6). In the alternative
model, early human bipedalism is seen as having evolved from
a more generalized arboreal, climbing-oriented ancestor, a mode
has important and profoundly different implications for under-
standing the evolution of ape and human locomotion.
If early human bipedalism evolved from an arboreal ancestor,
current ape-human phylogeny showing chimpanzees and bono-
S1) logically implies that knuckle-walking evolved independently
in both African ape lineages (Gorilla and Pan). In contrast,
proponents of a terrestrial knuckle-walking hypothesis of human
locomotor evolution hypothesize that African apes and humans
share a common knuckle-walking ancestor. Advocates for this
model support their claims by arguing that there are specific
morphological features, particularly of the wrist and hand, that
reflect this behavior in all living African apes and that can also
be found in fossil and living humans (2, 4–6).
ince Darwin first discussed pathways of human evolution in
The Descent of Man, there has been an ongoing and often
Studies advocating a terrestrial knuckle-walking ancestor and
arguing for a clear form-function relationship in the primate
wrist have neglected to consider variation within and across
species and age classes. Previous analyses of African ape hand
morphology have documented variation in metacarpal knuckle-
walking features (11–13) and allometric growth of the phalanges
(14) and carpals (15) between Pan and Gorilla. Building upon
this work, we test the specific hypotheses that posited knuckle-
walking features of the wrist are consistently found in both Pan
and Gorilla using novel ontogenetic data. The presence of such
morphology across all African apes would provide strong evi-
dence that these features are indeed knuckle-walking adapta-
tions. However, variation in the developmental timing, fre-
quency, and/or expression of these features between Pan and
Gorilla may suggest a more complicated pathway for the evolu-
tion of knuckle-walking behavior which, in turn, has profound
implications for the locomotor origin of human bipedalism. This
study provides detailed data on variation in putative knuckle-
walking features of the wrist in both adult and juvenile African
apes and other primates and we interpret our results in both a
biomechanical and behavioral context.
The features most commonly thought to reflect knuckle-
walking behavior in the African ape wrist are listed in Table 1.
It has been argued that these features reflect the need for
increased stability and limited extension* at the radiocarpal and
midcarpal joints during the stance phase of knuckle-walking
locomotion (2, 4–6, 16–19). However, these conclusions have
been based on small sample sizes, often of chimpanzee mor-
phology alone (16, 17), and have been made in the absence of
specific biomechanical data on hand posture or context of
locomotion across taxa. Not only is it imperative to analyze these
wrist features in all species of African apes, but an analysis of the
development of these features throughout ontogeny is also
necessary to understand fully the functional significance of
morphological variation between Pan and Gorilla. Bony mor-
phology is thought to reflect at least to some extent function
during development (20–22). For example, the degree of pha-
langeal curvature is positively correlated with increased arbo-
reality in African apes during ontogeny (23, 24). The absence of
some knuckle-walking features in hominins has been attributed
to lack of function during development (5). Thus, ontogenetic
Author contributions: T.L.K. designed research; T.L.K. performed research; T.L.K. and D.S.
analyzed data; and T.L.K. and D.S. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
1To whom correspondence should be addressed. E-mail: email@example.com.
2Present address: Max Planck Institute for Evolutionary Anthropology, Department of
Human Evolution, Deutscher Platz 6, D-04103 Leipzig, Germany.
*For clarity in this paper, we use ‘‘neutral’’ when referring to a posture in which the wrist
in held in line with the radius and ulna of the forearm. Deviations from neutral in which
the angle of the wrist relative to the anterior forearm is greater than 180 degrees is
referred to as ‘‘extension’’ (see Fig. 3).
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no. 34 ?
analyses can provide a critical insight into the homology and
function of morphological features that cannot be gained from
adult morphology alone (25, 26).
Knuckle-walking is rare among mammals and unique to
African apes among primates (1, 16). Previously researchers
have always treated knuckle-walking in Pan and Gorilla as a
tion persists despite well-known variation in substrate use among
African apes (27–30), as well as known differences in hand
posture across species (16, 31, 32) and substrates (33). When
considered without respect to important confounding variables
like substrate and hand posture, variation in morphology across
species of knuckle-walking apes is difficult to interpret and leads
to overly complex evolutionary scenarios. However, when con-
sidered in an appropriate biomechanical and ecological context,
that same variation between Gorilla and Pan is more interpret-
able and the insights derived from such analyses provide a novel
foundation for comparative tests concerning the functional
significance of posited knuckle-walking morphology.
As adults, all African apes engage in the same amount of
knuckle-walking (27, 30), but they do not do so in the same
ecological setting; chimpanzees and bonobos spend significantly
more time in the trees than do gorillas (28, 29). In addition,
juvenile chimpanzees and bonobos engage in less knuckle-
walking than do juvenile gorillas (34, 35).
Therefore, if features of the African ape wrist are to be
considered reflective of knuckle-walking behavior as some have
suggested (2, 4–6, 18, 19), we should find the following patterns.
First, these features should be common to all African apes.
Second, these features should be equally, if not more, pro-
nounced in gorillas given their more frequent knuckle-walking
behavior and increased load associated with a larger body mass.
Finally, compared to Pan, these features should appear relatively
earlier in ontogeny in gorillas given their faster growth rate in
body mass (36) and the fact that they engage in an adult-like
frequency of knuckle-walking behavior at a much earlier age as
a consequence of their larger size (28, 34, 35). If the hypothesis
that features of the wrist are functionally related to knuckle-
walking cannot be supported by these types of analyzes, then the
walking ancestor must also be reevaluated and possibly rejected.
Results and Discussion
Analysis of the timing of appearance, frequency, and expression of
apes and monkeys (Table S1) supports previous analyses (5, 6, 18)
that African ape carpal morphology on the whole can be distin-
guished from that of Asian apes, arboreal, and terrestrial monkeys,
and humans. However, this analysis also reveals a previously
unrecognized and surprisingly high amount of morphological vari-
ation in adult African ape carpal features that casts doubt on the
assumed functional link between specific aspects of wrist morphol-
ogy and knuckle-walking behavior. We detected significant (P ?
0.05 using a ?2goodness-of-fit test; Table S2) differences in the
presence (defined generously to include even weakly developed
features) or absence of knuckle-walking features among African
bonobos in their low frequency of all but 2 of the knuckle-walking
features analyzed here. For example, the dorsal concavity and beak
of the scaphoid, are found in almost all (96%) adult Pan troglodytes
in this sample (n ? 32) and are fully developed in 76% of Pan
paniscus (n ? 21). Yet these same features are found together in
only 6% of Gorilla (n ? 45), regardless of sex or size (Table 2; Fig.
1). Furthermore, both features are also commonly found in non-
knuckle-walking arboreal palmigrade (80%, n ? 15) monkeys and
terrestrial palmigrade (73%, n ? 11) and digitigrade (57%, n ? 14)
monkeys (Table S2).
Thus, these results show (i) that most gorillas lack key features
that have been assumed to be critical for limiting extension of the
wrist during knuckle-walking (4, 17), and (ii) these features are
found in monkeys that use a variety of different hand postures
and substrates. In addition, the ontogenetic analysis shows that
the features of the scaphoid that are assumed to be essential for
knuckle-walking (2, 4–6) are not only inconsistently developed
in Gorilla, but, when present, do not appear relatively earlier in
development in gorillas (Table 2). Therefore, using the tradi-
tional functional interpretation of these features (17), it would
appear that the Gorilla radiocarpal joint may be actually less,
rather than more, stable in extension throughout ontogeny
compared to Pan.
behavior and the ontogeny of scaphoid-centrale fusion (37). This
feature is the most commonly discussed knuckle-walking feature
in African ape and human wrists (2, 4–6). The strong hetero-
chronic pattern found in the increasingly earlier timing of this
fusion among hominoids, in addition to the occurrence of fusion
in several nonknuckle-walking, highly arboreal strepsirrhine
primates, suggests that this trait may be more clearly linked to
phylogeny than to function (37).
The same pattern is found repeatedly in other carpal bones as
well. For example, the morphology of the capitate and hamate
has also traditionally been used to diagnose knuckle-walking
behavior (5, 6, 18, 19). Compared to other primates, the capitate
and hamate in African apes are said to have extension-limiting
ridges and concavities that have been interpreted as important
features that enhance stability during weight-bearing in knuckle-
walking postures (5, 6, 19) (Fig. 2). As expected, these features
are common in adult Pan (81–100%). But they are also fully
developed in several nonknuckle-walking monkeys (17, 19, 38)
(Table S2). However, these ridges and concavities are found at
a much lower frequency in Gorilla (as low as 39%) and, when
present, are less accentuated than in Pan (Table 2).
Narrowing or ‘‘waisting’’ of the capitate body, a feature that has
been interpreted as limiting extension at the midcarpal joint, is also
Table 1. List of commonly discussed knuckle-walking features of the African ape wrist and their proposed adaptive function
Scaphoid dorsal concavity &
Capitate distal concavity &
Capitate dorsal ridge &
Hamate dorsal ridge
Hamate distal concavity
together accommodate the dorsal extension of the distal radius,
limiting extension at the radiocarpal joint (4–6,17)
receives scaphoid between concavity and trapezoid and together
they limit extension of capitate-scaphoid joint (12, 19)
together limit extension of the proximal carpal row on the distal
carpal row (5)
limits extension at the triquetro-hamate joint (19)
the timing of appearance, frequency and expression of these features on the scaphoid, capitate and hamate was best assessed qualitatively (rather than
quantitatively) throughout juvenile and adult ontogeny.
www.pnas.org?cgi?doi?10.1073?pnas.0901280106Kivell and Schmitt
less accentuated in Gorilla compared to Pan (14) and even com-
apes and monkeys reveals that Gorilla has significantly less waisting
of the capitate compared to P. paniscus (P ? 0.00), P. troglodytes
scaphoid, many of these capitate and hamate features also develop
later and are less accentuated or absent throughout ontogeny in
Gorilla compared to Pan (Table 2; Fig. 2). Thus, the absence or
minimal expression of these features in Gorilla reflects more, not
less, extension and mobility at the midcarpal joint compared to that
of other knuckle-walking African apes.
Table 2. Qualitative analysis of the ontogenetic timing, frequency and morphology of putative knuckle-walking carpal features in
African apes. Frequency of posited knuckle-walking features in African apes evaluated as either present (including poorly developed
features) or absent. Juvenile stages divided into stages based on dental eruption and roughly correspond to the following
chronological ages (in years) in Pan and Gorilla, respectively (see Table S1): Stage 3: 0.8–4 and 1–3.5; Stage 4: 4–6 and 3.5–4.75;
Stage 5: 6–8 and 4.75–7; Stage 6: 8–10 and 7–9; Stage 7: 10–12 and 9–11. (*) Juvenile morphology describes both P. troglodytes and
Hypothesized knuckle-walking features
Beak Dorsal concavity Distal concavityDorsal ridge Distal concavityDorsal ridge
P. troglodytes Juv.* Appears at Stage 5
in 38% of
n ? 21
Appears at Stage 6
in 21% of
n ? 29
Appears at Stage 4
and in 90% of
(n ? 61)
Appears at Stage 4
(n ? 15) and
(n ? 60)
present in 81%
(n ? 32)
present in 81%
(n ? 21)
Appears at Stage 6
in 14% of
n ? 7
Appears at Stage 4
in 90% of
n ? 10
Appears at Stage 5
and found in
(n ? 74)
Adult Both traits well-developed and present in 96% of adults
(n ? 32)
present in all
deeper than Gorilla
and present in
present in 84%
(n ? 32)
present in 86%
(n ? 21)
Appears at Stage 4
and is found in
20% of juvenile
stages (n ? 30)
AdultBoth traits present in 76% of adults (n ? 21)well-developed and
present in all
similar to P.
Appears at Stage 3
Juv. Appears at Stage 5
in 17% of
n ? 6
Appears at Stage 7
in 13% of
n ? 8
Appears at Stage 5
in 29% of
n ? 7 and 64%
from Stages 6–7
(n ? 11)
shallower than Pan
present in 75%
of n ? 36
Adult poorly developed,
alone in 11% of
n ? 45
alone in 16% of
n ? 45
Both traits found
together in only
n ? 45
or absent, found
in only 53% of
n ? 36
shallower than Pan
but present in all
or absent, found
in only 39% of
adults (n ? 36)
le-walking features in African ape scaphoids. Proxi-
molateral view of left scaphoids. Juvenile specimens
are labeled by developmental stage (Table S1) and all
other specimens are adult. Both the dorsal concavity
(A) and beak (B) of the scaphoid’s radial facet appear
earlier and are more common and accentuated
throughout ontogeny in P. troglodytes and P. panis-
cus. These features are rarely found in Gorilla and
instead the corresponding area is round and convex
(all specimens to 1 cm scale).
Ontogenetic morphology of putative knuck-
Kivell and SchmittPNAS ?
August 25, 2009 ?
vol. 106 ?
no. 34 ?
This research shows that none of the carpal features discussed
here that have traditionally been used to diagnose knuckle-
le-walking behavior in all African apes. Contrary to functional
predictions, the pattern of development, expression, and fre-
quency of the putative knuckle-walking features listed in Table
1 are not the same in all African apes; they are not more frequent
or accentuated in gorillas and are not correlated with increased
knuckle-walking behavior or body size. The difference between
Pan and Gorilla in the presence or absence of carpal bone
morphology thought to limit wrist motion is confirmed by
reported patterns of joint flexibility in the 2 genera. Gorilla have
a much larger range of wrist extension (58°) (40) compared to
that of P. troglodytes (30–42°) (19, 40). It remains challenging to
explain this difference if it is assumed that knuckle-walking in
Pan and Gorilla is biomechanically similar. The morphological
and range of motion data demand a new perspective on knuckle-
walking that leads to a reevaluation of long established models
of human evolution.
One possible explanation for the disparity in carpal morphology
among knuckle-walking apes is that Gorilla compensates for the
lack of extension-limiting osteological features with a stronger
same relative size of forearm flexor musculature among African
apes and the recruitment pattern of these muscles during knuckle-
walking locomotion (41, 42) suggest that soft tissues are not
counteracting this increased mobility in Gorilla.
The rejection of these possibilities leads to an alternative
hypothesis: that knuckle-walking is not the same biomechanical
phenomenon in Pan and Gorilla. Previous researchers have
noted postural (16, 31, 32) and biomechanical (13, 14) differ-
ences in knuckle-walking behavior among African apes and our
hypothesis expands upon this foundation of work to suggest that
knuckle-walking is a fundamentally different mode of locomo-
tion in Pan and Gorilla. This hypothesis is supported by existing
African ape locomotor data (17, 27–35). As with other forms of
locomotion, limb posture has a profound influence the load
experienced by the wrist and digits in a knuckle-walking animal.
We propose that Gorilla uses a relative ‘‘columnar’’ forelimb
posture during knuckle-walking in which the hand and wrist
joints are aligned in a relatively straight, neutral posture com-
pared to the more extended postures adopted by Pan (Fig. 3).
Animals using a relatively columnar wrist and hand posture
would have carpal joints that are in line with the hand and
forearm, similar to limb joint position in large graviportal
animals such as elephants (43). Supporting loads directly over
more vertically-oriented forelimb joints during support phase
explains the absence of posited bony adaptations to bending
loads in gorillas and also permits more mobility at the joint (43).
By contrast, Pan, which exhibit extended wrist postures (Fig. 3),
will experience higher bending loads. Thus Pan carpal bones
have relatively prominent osteological features that have tradi-
tionally, but mistakenly, been interpreted simply as features
African ape capitates and hamates. Dorsal view of right capitate (Above) and
hamate (bBelow). Juvenile stages are labeled and all other specimens are
adult. Black dashed line shows less capitate waisting at all stages of ontogeny
in Gorilla compared to Pan (see Fig. S2). The capitate’s distal concavity (A) and
dorsal ridge (B) appear earlier and are more common and accentuated in Pan
is shallower and the dorsal ridge (D) appears earlier in ontogeny but is again
less frequent and accentuated in adults compared to Pan (all specimens to 1
Ontogenetic morphology of putative knuckle-walking features in
models describing morphological variation found be-
Lateral view of wrist and hand postures in Pan and
Gorilla adapted from ref. 4. In Pan, the wrist (and
carpometacarpal) joints are held in an extended pos-
ture (dotted line) such that extension-limiting mor-
phological features are required for stability. In con-
trast, we hypothesize that Gorilla uses a columnar,
neutral wrist and hand posture with axial loading
(dotted line) such that stabilizing features are not
necessary and generally absent (see text for discus-
sion). Radiocarpal and midcarpal joints are labeled in
lateral and dorsal views of Gorilla carpus. ‘‘S,’’ scaph-
oid; ‘‘C,’’ capitate; ‘‘H,’’ hamate.
Hypothesized biomechanical hand posture
www.pnas.org?cgi?doi?10.1073?pnas.0901280106 Kivell and Schmitt
associated with knuckle-walking in general rather than with a
specific posture. The notion that Pan and Gorilla use different
wrist postures is consistent with the morphometric data pre-
sented here. Although this hypothesis has yet to be explored in
detail with videographic data, this idea is further supported by
previous research showing that Gorilla exhibits increased wrist
mobility compared to Pan (40), a more hyperextended elbow
joint (31) and relatively equal length of rays 2 through 4, which
creates a larger, more stable area over which to disperse axial
The variation in African ape hand posture may be more
completely understood through a consideration of differences in
substrate use, which in turn may explain the high frequency of
walking quadrupedal primates reported in this study. Specifi-
cally, we propose that features traditionally associated with
knuckle-walking may actually reflect the habitual loading of the
wrist in an extended posture on arboreal substrates (Fig. 3).
Compared to gorillas, chimpanzees, and bonobos more fre-
quently use both knuckle-walking and palmigrady on arboreal
and security in an arboreal setting, primates may adopt a more
extended wrist posture (17), more flexed elbow joint (44), and
more variable hand postures (16, 31–33) compared to terrestrial
locomotion. Deeper limb joint angles lower the animal’s center
of mass relative to the substrate (45), improving balance, but will
also increase the moment arm on the wrist (and elbow) joints. In
addition, it is worth noting that Pan have significantly longer
metacarpals compared to Gorilla (46), and this increased length
could potentially further increase the bending load arm in Pan.
We hypothesize that the high frequency of posited extension-
limiting bony morphology shared between Pan and several other
primates that use relatively extended wrist postures (e.g., pal-
as simply reflecting habitual loading of the wrist in an extended
position, rather than features that limit extension per se. If, as
appears to be the case, wrist extension increases during arboreal
locomotion, then these bony features can be seen as reflecting
the arboreal hand postures of Pan rather than indication knuck-
Thus, a novel understanding of the functional significance of
what had previously been viewed as features reflecting knuckle-
walking comes to light when we (i) interpret them in only the taxa
for which they are present (Pan) and (ii) recognize the likelihood
of 2 biomechanically different types of knuckle-walking in Pan and
Gorilla. Functional interpretations of these features fail when
knuckle-walking among all African apes is considered a single,
unified behavior. Although there are other putative knuckle-
walking features of the wrist and forelimb (5, 6) that have not been
addressed in this report and that could potentially hold a stronger
functional signal of knuckle-walking behavior, the data presented
here reveal that the correlation between these features and knuck-
le-walking locomotion should not be presumed.
The results of this study show that researchers need to
reevaluate all posited knuckle-walking features and reconsider
their efficacy as indicators of knuckle-walking behavior in extant
and extinct primates. In this context, the absence of several
posited knuckle-walking features in extant knuckle-walkers (and
the presence of some of these features in nonknuckle-walkers)
makes it difficult to argue that there is unambiguous evidence
that bipedalism evolved from a terrestrial knuckle-walking an-
cestor. Instead, our data support the opposite notion, that
features of the hand and wrist found in the human fossil record
that have traditionally been treated as indicators of knuckle-
walking behavior are in fact evidence of arboreality and not
The data presented here and in other studies of variation in
African ape morphology (11, 12, 14, 15) and behavior (30–32,
35) support a hypothesis of independent evolution of knuckle-
walking behavior in the 2 African ape lineages. Our data cannot
reject the hypothesis that knuckle-walking evolved only once at
the base of the African ape and human clade and that these
differences evolved after the Gorilla and Pan split (Fig. S1).
Without fully understanding the evolutionary and ontogenetic
plasticity of these osteological features or the affect on wrist
morphology of other locomotor behaviors in which Pan and
Gorilla engage, it is difficult to be certain about the evolution of
nonhomologous knuckle-walking behavior in African apes.
However, in absence of clear evidence for a terrestrial knuckle-
walking origin for human bipedalism, we suggest that the inde-
pendent evolution of a generalized locomotor adaptation that
simply allows large-bodied apes to retain highly-arboreal mor-
phology while also moving effectively on the ground is a
reasonable and likely evolutionary scenario. The increasing
climatic and ecological instability that typified much of the
through several independent stages of terrestrial and arboreal
locomotor behaviors. The Miocene hominoid fossil record
strongly supports the independent evolution of specialized sus-
pensory adaptations (e.g., Morotopithecus, Oreopithecus, or
Pongo) (48) and the same may be true for knuckle-walking.
There are few, if any, wrist similarities shared among all African
apes and humans that can be directly related to terrestrial
knuckle-walking locomotion. Thus the independent evolution of
this behavior among African apes requires less homoplasy than
previously proposed (4–6). Features found in the hominin fossil
record that have traditionally been associated with a broad
definition of knuckle-walking are more likely reflecting the
habitual Pan-like use of extended wrist postures that are partic-
ularly advantageous in an arboreal environment. This, in turn,
suggests that human bipedality evolved from a more arboreal
ancestor occupying a generalized locomotor and ecological
niche common to all living apes (7, 8, 10).
Materials and Methods
Qualitative and quantitative comparisons were made to an ontogenetic sample
of largely wild-caught hominoids and cercopithecoids listed in Table S1. Sample
in some taxa and a general paucity of juvenile specimens in osteological collec-
tions. Juvenile stages (pooled sexes) were based on eruption of teeth into full
occlusion and were defined as the following: Stage 1, deciduous dentition with
less than dP3fully erupted; Stage 2, dP3fully erupted; Stage 3, dP4fully erupted;
alveolar surface to an almost fully erupted M2; Stage 6, mixed dentition ranging
between M3erupted just past alveolar surface to an almost fully erupted M3;
chronological ages (in years) for Gorilla (49) and wild Pan (50).
Frequency of putative knuckle-walking features were scored as a
‘‘present,’’ even when weakly developed, or ‘‘absent’’ throughout all onto-
genetic stages in all taxa, including cercopithecoids (Table S2b). Significant
differences (P ? 0.05) in frequency among African ape groups were tested
using a ?2goodness-of-fit test for all pairwise comparisons and results are
given in Table S2a.
calipers. Proximodistal length of the capitate was defined as the maximum
distance from the head of the capitate to the metacarpal articular surface.
Breadth of the capitate neck was measured in dorsal view and at the narrow-
est point just distal to the head. The degree of capitate waisting was quanti-
fied as a ratio of capitate neck breadth divided by the length of the capitate
body. Differences in the capitate waisting ratio between groups were tested
using Mann-Whitney U test with significance at P ? 0.05 (Fig. S2).
ACKNOWLEDGMENTS. We gratefully acknowledge the assistance of curators
at the numerous institutions that provided access to specimens in their care.
We also thank D. Begun, E. Fiume, J. Hanna, B. Hare, J. Horvath, C. Orr, B.
Richmond, M. Rose, M. Tocheri, C. Wall, R. Wunderlich, Animal Locomotion
Lab members, 2 anonymous reviewers and the editor for providing valuable
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comments and discussion on our manuscript. This research was supported by Download full-text
Natural Sciences and Engineering Research Council of Canada, General Mo-
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www.pnas.org?cgi?doi?10.1073?pnas.0901280106Kivell and Schmitt