Trabecular Evidence for a Human-Like Gait in Australopithecus africanus

Abstract

Although the earliest known hominins were apparently upright bipeds, there has been mixed evidence whether particular species of hominins including those in the genus Australopithecus walked with relatively extended hips, knees and ankles like modern humans, or with more flexed lower limb joints like apes when bipedal. Here we demonstrate in chimpanzees and humans a highly predictable and sensitive relationship between the orientation of the ankle joint during loading and the principal orientation of trabecular bone struts in the distal tibia that function to withstand compressive forces within the joint. Analyses of the orientation of these struts using microCT scans in a sample of fossil tibiae from the site of Sterkfontein, of which two are assigned to Australopithecus africanus, indicate that these hominins primarily loaded their ankles in a relatively extended posture like modern humans and unlike chimpanzees. In other respects, however, trabecular properties in Au africanus are distinctive, with values that mostly fall between those of chimpanzees and humans. These results indicate that Au. africanus, like Homo, walked with an efficient, extended lower limb.

Full text

Trabecular Evidence for a Human-Like Gait in
Australopithecus africanus
Meir M. Barak
1,2
*
.
¤
, Daniel E. Lieberman
2
*
.
, David Raichlen
3
, Herman Pontzer
4
, Anna G. Warrener
2
, Jean-
Jacques Hublin
1
1Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany, 2Department of Human Evolutionary Biology, Harvard
University, Cambridge, Massachusetts, United States of America, 3School of Anthropology, University of Arizona, Tucson, Arizona, United States of America, 4Department
of Anthropology, Hunter College, New York, New York, United States of America
Abstract
Although the earliest known hominins were apparently upright bipeds, there has been mixed evidence whether particular
species of hominins including those in the genus Australopithecus walked with relatively extended hips, knees and ankles
like modern humans, or with more flexed lower limb joints like apes when bipedal. Here we demonstrate in chimpanzees
and humans a highly predictable and sensitive relationship between the orientation of the ankle joint during loading and
the principal orientation of trabecular bone struts in the distal tibia that function to withstand compressive forces within the
joint. Analyses of the orientation of these struts using microCT scans in a sample of fossil tibiae from the site of Sterkfontein,
of which two are assigned to Australopithecus africanus, indicate that these hominins primarily loaded their ankles in a
relatively extended posture like modern humans and unlike chimpanzees. In other respects, however, trabecular properties
in Au africanus are distinctive, with values that mostly fall between those of chimpanzees and humans. These results
indicate that Au. africanus, like Homo, walked with an efficient, extended lower limb.
Citation: Barak MM, Lieberman DE, Raichlen D, Pontzer H, Warrener AG, et al. (2013) Trabecular Evidence for a Human-Like Gait in Australopithecus africanus. PLoS
ONE 8(11): e77687. doi:10.1371/journal.pone.0077687
Editor: David Carrier, University of Utah, United States of America
Received June 17, 2013; Accepted August 31, 2013; Published November 5, 2013
Copyright: ß2013 Barak et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: For funding, the authors thank the American School of Prehistoric Research (Harvard University), the Hintze Family Charitable Foundation, the Minerva
Stiftung Gesellschaft fu
¨r die Forschung mbH, and the LSB Leakey Foundation. The human and chimpanzee microCT scans were performed in the Center for
Nanoscale Systems, Harvard University, a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the National Science
Foundation under NSF award no. ECS-0335765. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the
manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: barakm@winthrop.edu (MMB); danlieb@fas.harvard.edu (DEL)
¤ Current address: Department of Biology, Winthrop University, Rock Hill, South Carolina, United States of America
.These authors contributed equally to this work.
Introduction
The earliest hominins, Sahelanthropus,Ardipithecus and Orrorin, all
have adaptations for upright posture [1–3], thus supporting
Darwin’s conjecture that bipedalism was a key initial derived
feature of the hominin lineage [4]. These early hominins, however,
may have been facultative bipeds, and the oldest evidence for
obligate, non-facultative bipedalism does not appear until 4.2
million years ago in the genus Australopithecus [3,5–14]. The nature
of australopith bipedalism, however, remains disputed, with most
focus on the two best-sampled species: Au. afarensis and Au.
africanus. Some paleoanthropologists infer that these australopiths
walked with an efficient, modern gait characterized by relatively
extended hips and knees (EHEK) rather than a more bent-hip and
bent-knee gait (BHBK) similar to the way chimpanzees walk
bipedally (Fig. 1a,1b) [5,6,9,15–18]. This view is partly based on
simulations and experimental studies of bipedal locomotion, which
indicate that EHEK gaits are considerably less energetically costly
than BHBK gaits [16,18–20]. Additional support for the
hypothesis that australopiths used EHEK gaits comes from an
extensive array of anatomical features that are indicative of
extended lower limb postures, such as a tibial plateau oriented
parallel to the tibiotalar joint surface, the flattened distal contour of
femoral condyles, a pronounced lumbar lordosis, and a high
femoral carrying angle (i.e., valgus knee) [5,21,22].
Two recent studies (which included the fossil distal tibiae we
present in the current study, StW 358, 389 and 567) revealed that
the distal tibia and ankle joint external morphologies of the genus
Australopithecus were within the range of the genus Homo but
different from chimpanzees and gorillas [23,24]. DeSilva (2009)
demonstrated that australopiths resemble humans and differ from
chimpanzees and gorillas in having a perpendicularly oriented
tibia relative to the horizontal plane of the ankle joint, a square-
shaped articular surface of the distal tibia that lacks the wide
anterior rim which is found in climbing apes, and a low angle
between the axis of rotation and the horizontal plane of the ankle,
indicating that these individuals probably possessed a perpendic-
ularly oriented tibia [23]. DeSilva and Throckmorton (2010) also
showed that Australopithecus possessed a tibial arch angle similar to
humans and different from other non-human primates [24]. Other
paleoanthropologists, however, consider that retained features
which benefit arboreal locomotion in apes such as relatively short
hindlimbs, long and curved pedal phalanges, and less coronally-
oriented iliac blades (for a complete list see Stern 2000 [25])
compromised australopith walking performance, causing Au.
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afarensis and Au. africanus to use a BHBK gait [7,8,25–28]. Stride
lengths from the Laetoli trackway are compatible with either type
of gait [29], and while footprint morphology is more consistent
with EHEK gaits [9,12], no single skeletal feature so far
documented can reliably and definitively distinguish between
EHEK and BHBK gaits.
An alternative approach to assess whether early hominins
walked with EHEK or BHBK gait is to use the orientation of
trabecular struts deep to the articular surface of the hindlimb joints
(see Fig. 2a). This strategy takes advantage of Wolff’s Law of
trabecular orientation, first proposed in 1892, that trabecular
struts within joints respond to external loads by preferentially
aligning their long axes along the trajectories of peak principal
stresses [30]. Despite equivocal results from some comparative
studies [31,32], numerous studies support Wolff’s Law both in sub-
adults and skeletally mature animals and humans (to name a few
[33–43]). More importantly, two controlled experiments demon-
strated that the relationship between principal trabecular orien-
tation (PTO) and the orientation of peak compressive forces in
limb joints during loading is sufficiently accurate and precise to
distinguish between individuals that load their joints in slightly
different orientations. In one experiment, bipedal birds run for
10 minutes a day (6 days/week) on a 20uinclined treadmill, flexed
their knee joints on average 13.7umore than birds run on a flat
treadmill (76.361.33uand 62.663.52ufor the birds run on flat
and inclined treadmills, respectively, P,0.01), causing a 13.6ushift
in the sagittal plane 2D-PTO within the distal femur (P,0.01)
[44]. In another experiment, sheep exercised on a flat and 7u
inclined treadmill (15 min/day, 6 days/week), altered the angle of
the ankle (tibiotalar) joint by 3.6u(124.365.3uand 127.964.7ufor
the sheep exercised on flat and inclined treadmills, respectively,
P,0.01), leading to a 4.3ushift (P,0.05) in sagittal plane 2D-PTO
of the distal tibia medial side [45]. Therefore, even subtle
differences in limb orientation during loading can be detected in
trabecular bone in the ankle of medium-sized mammals.
Although discussions of BHBK versus EHEK gaits have focused
mostly on the hip and knee (two exceptions are [21,23]), we focus
here on the distal tibia of humans and chimpanzees. We do so
because trabecular bone in the distal tibia has been shown to be
very sensitive to subtle variations in ankle angle during loading in
sheep [45], and because the ankle is more extended (plantarflexed)
during midstance in humans walking bipedally than chimpanzees
walking quadrupedally (Fig. 1), which means that the direction of
forces close to the joint surface in the distal tibia should differ
between these two species. Since humans have a more extended
ankle compared to chimpanzees at midstance during walking
(bipedal and quadrupedal for humans and chimpanzees respec-
tively), we postulate that PTO would differ significantly between
humans and chimpanzees. We therefore predict that the difference
in the sagittal 2D-PTO of the distal tibia between humans and
chimpanzees will represent accurately the difference in their ankle
joint angle at the midstance phase of their walking cycle. We also
predict a significant difference between humans and chimpanzees
in their 3D-PTO. Finally, if this hypothesis is not refuted, we
would be able to test if Au. africanus walked with an EHEK or
BHBK gait by comparing the distal tibia PTO from Au. africanus
and humans.
Results
Ankle angles and trabecular orientation in humans and
chimpanzees
In order to test the relationship between 2D-PTO and ankle
angles we first analyzed data on vertical ground reaction force
(GRFv) and tibia orientation in adult chimpanzees (Pan troglodytes,
n = 3) and a similar-sized sample of adult humans (Homo sapiens,
Figure 1. Differences in ankle angle (dashed line) at midstance in humans walking normally (a), with a bent-hip bent-knee gait (b)
and chimpanzees walking quadrupedally (c). Note that the ankle is more extended (plantarflexed) during midstance in humans walking
normally than chimpanzees walking quadrupedally. The bottom part of the figure shows representative vertical ground reaction force traces plotted
as a percentage of body weight over stance duration.
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n = 6). Both species walked at preferred speeds on a level force
plate; chimpanzees walked quadrupedally, and humans were
asked to walk with both EHEK and BHBK gaits (Fig. 1). Ankle
angle was measured in lateral view as the angle between two lines:
from the lateral epicondyle of the femur to the lateral malleolus,
and from the posterior tuber calcaneus to the distal head of the 5th
metatarsal (Fig. 1, upper part). Because GRFv traces from some
chimpanzee and all human trials had a double force peak (Fig. 1,
lower part), tibia orientation relative to the long axis of the foot
(ankle angle) was averaged over the period of stance when GRFv
was greater than 75% of body weight (hereafter termed ‘peak
loading’). Mean ankle angle in humans was significantly more
extended by 16uwhen they walked with an EHEK compared to a
BHBK gait (85.6u63.3 and 69.6u64.3 respectively, P,0.05).
When chimpanzees walked quadrupedally, mean ankle angle was
75.2u63.0, not significantly different to bipedal humans walking
with a BHBK gait.
Although chimpanzees sometimes climb and occasionally adopt
bipedal postures, quadrupedal walking comprises more than 98%
of their locomotor behavior [46]. Therefore trabecular bone in the
distal tibiae of chimpanzees is predicted to respond to these
external loads by preferentially aligning the long axes of struts
along the trajectories of peak principal stresses that are generated
during quadrupedal walking around midstance. Thus, if PTO in
the distal tibia accurately reflects differences in locomotor posture,
we predict an approximately 10udifference in PTO in the distal
tibia between humans and chimpanzees.
We compared microCT scans of distal tibiae from a sample of
adult humans (n = 6) and chimpanzees (n = 6). In order to
correlate ankle angle to the corresponding PTO in the joint’s
plane of motion, we determined 2D-PTO in the parasagittal plane
from 2D projections using the mean intercept length technique
(MIL, see materials and methods for a description of the
technique). As predicted by the kinematic data, the 2D-PTO in
the distal tibia sagittal plane was inclined significantly more
obliquely by 7.7u(P,0.05) in chimpanzees (82.3u610.7, Fig. 2c)
than in humans (90.0u62.3, Fig. 2b). To further test the
correlation between ankle joint loading and PTO, we employed
the MIL technique to measure the 3D-PTO in two volumes of
interest (VOIs) in the medial and lateral side of the distal tibiae,
just deep to the cortex of the joint surface where the talar trochlea
contact the distal tibia (the tibial plafond, Fig. 3). As with the 2D
analysis, chimpanzees and humans differ significantly in both the
medial and lateral VOIs (P,0.05). For a detailed account of the
3D-PTO coordinates see Table S1. Furthermore, Figure 3 reveals
a much higher variability in chimpanzees’ 3D-PTO, especially in
the lateral VOI (see Fig. 3b); these results demonstrate the greater
variability in chimpanzee loading of the tibiotalar joint.
PTO in Sterkfontein tibiae
Given the predictive relationship between ankle angle at peak
loading and 2D-PTO in the distal tibia of humans and
chimpanzees, we obtained microCT scans of the distal tibia in
two Australopithecus africanus specimens from Member 4 of
Sterkfontein, dated to 2.6–2.8 Ma (StW 358 and StW 389) [23],
and one distal tibia from Member 5 of Sterkfontein dated to 1.4–
1.7 Ma (StW 567) putatively assigned to early Homo [23,47]. As
Figure 4 shows, these bones are well preserved both externally and
internally with intact 3D trabecular structure that is detectable
using an X-ray source. Using the methods described above for
humans and chimpanzees, the 2D-PTO in the sagittal plane is
97.1uin StW 358 (Fig. 2d), 86.3uin StW 389 (Fig. 2e) and 87.5uin
StW 567 (Fig. 2f). These values are not significantly different from
the orientation in humans (90.0u62.3, Fig. 2b; P = 0.40; permu-
tation test), but due to the low sample size (n = 3) are also not
significantly different from chimpanzees (P = 0.22; permutation
test). Because our goal is to test if Au.africanus walked with an
EHEK like modern humans or BHBK, and in order to overcome
the fossil small sample size, we tested the combined human and
fossil samples (n = 9) versus the chimpanzee samples (n = 6), and
the combined chimpanzee and fossil samples (n = 9) versus the
human samples (n = 6). While the combined human and fossil
samples differed significantly from chimpanzees (P = 0.04; permu-
tation test), the combined chimpanzee and fossil samples were not
significantly different from humans (P = 0.22; permutation test).
These results indicate that the 2D-PTO of the fossil hominins and
humans are similar, but unlike chimpanzees. The same is true for
the three-dimensional comparisons in the medial and lateral VOIs
(Fig. 3), which reveal no significant difference between fossil
hominin and modern human samples (P = 0.19 and P = 0.20 for
the medial and lateral VOIs respectively; permutation test), but
are significantly different between the combined human and fossil
samples chimpanzee samples (P,0.05 for both medial and lateral
VOIs).
Additional trabecular bone properties
MicroCT scans provide the opportunity to compare additional
trabecular bone properties between humans, chimpanzees and the
fossil hominins, summarized in Table 1. Compared to humans,
Figure 2. Mid-sagittal views of 2D-PTO in the distal tibia
(anterior corresponds to the left side of each bone). Black lines
represent the long axis of the bone. The 2D-PTO for each bone was
measured as the angle (a) between the 2D-PTO and the normal plane
to the long axis of the bone (represented as a horizontal dashed line in
Fig. 2a). Red arrows represent the average 2D-PTO for chimpanzees
(Fig. 2c, a= 92.3610.7u) and for humans (Fig 2b, a= 90.062.3u)) or the
specific 2D-PTO for the fossil samples StW 358 (Fig. 2d), StW 389
(Fig. 2e), and StW 567 (Fig. 2f). Scale bar, 1 cm.
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chimpanzees have significantly more trabeculae per mm (Tb.N),
which are less separated (Tb.Sp), thinner (Tb.Th), have a higher
connectivity density (ConnD), and a lower degree of anisotropy
(DA) in both the lateral and medial VOIs (P,0.01 see Table 1 for
details). Although sample sizes are small, the three fossil hominins
reveal a distinctive trabecular structure from both humans and
chimpanzees (in regards to Tb.N, Tb.Sp, ConnD and DA) and
values for their trabecular structural parameters are mostly
between those of humans and chimpanzees (Table 1). Remark-
ably, in all the hominoids taxa (humans, chimpanzees and fossils
hominins) the lateral VOI has consistently higher bone volume
fraction (BV/TV), Tb.N, Tb.Th and DA (stronger orientation),
and lower Tb.Sp compared to the medial VOI. As previous studies
have shown that the lateral aspect of the distal tibia in humans is
the main load-bearing structure in the tibiotalar joint [48,49] we
would expect to see a corresponding higher trabecular bone
volume and a more robust architecture in the lateral VOI. This
similarity between chimpanzees and early hominins suggests that
the lateral aspect dominancy in tibiotalar joint load-bearing is a
primitive trait.
Discussion
The major objective of this study is to test whether trabecular
structure in the distal tibia (both 2D-PTO in the sagittal plane and
3D-PTO) reliably predicts known differences in ankle joint angle
at the time of peak loading from GRFv during walking in
chimpanzees and humans, and to use this signal of loading to infer
ankle angles at peak loading in fossil hominin tibiae from
Sterkfontein. Our results show that humans’ ankle joint angles at
peak GRFv are more extended by 10.3ucompared to chimpan-
zees, which corresponds to a 7.7udifference in the sagittal 2D-
PTO (Fig. 2b and 2c). Furthermore, 3D measurements of the
medial and lateral aspects of the distal tibia demonstrate that the
3D-PTO in humans differs significantly from chimpanzees. These
results combined with those from controlled experiments on other
species [44,45] indicate that differences in sagittal 2D- and 3D-
PTO in the distal tibia are useful and reliable predictors of joint
angle during peak GRFv. In addition, PTO in the distal tibia
among the three Sterkfontein fossil hominins is comparable to
humans but significantly different from chimpanzees in the 2D
sagittal plane as well as in the 3D medial and lateral VOIs (Fig. 3).
Although trabecular orientation in these australopith fossils was
possibly influenced by loading during climbing, these hominins
were unlikely to have climbed more than chimps, which climb
only about 100 meters a day [50], and they probably had less
dorsiflexed ankles when climbing [23]. The most likely interpre-
tation of these data is that the Sterkfontein hominins loaded their
distal tibiae using human-like ankle angles, hence a relatively
Figure 3. Measurements of 3D-PTO in the distal tibia. (a) Schematic showing location of the lateral and medial VOIs in the distal tibia and how
the 3D spheres were visualized in 2D using an equal-angle stereoplot. A stereoplot is a 2D map which is created by projecting points from a surface of
a sphere to a tangential plane. (b) The stereoplot projections of the lateral VOI. (c) The stereoplot projections of the medial VOI. Filled circles,
chimpanzees; open circles, humans; grey circle 1, StW 358; grey circle 2, StW 389; grey circle 3, StW 567. Angles 0u,90u, 180uand 270ucorrespond to
the anatomical directions: posterior, lateral, anterior and medial respectively (as given in Fig. 3a). For a detailed account of the 3D-PTO coordinates
see Table S1.
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extended lower limb posture. This interpretation is also supported
by previous studies of the external morphology of StW 358, 389
and 567 [23,24], which showed that in these individuals the
loading of the ankle, the angle between the long axis of the tibia
and the ankle joint surface, and the ankle range of motion were all
humanlike, thus implying humanlike kinematics of the lower
extremity during walking.
One limitation of our study was that we measured tarsal joint
angles only while walking and not during running, which produces
higher stresses and could contribute to the signal affecting the
PTO. Chimpanzees, however, are almost solely knuckle walkers
and rarely locomote bipedally, let alone run [46]. Similarly, there
is no evidence or indication that australopiths ran habitually [51].
Therefore, out of the 3 species we studied, only humans sometimes
run long distances. We had 6 human distal tibiae (Table S1).
Three samples were Peruvian farmers (South America) and 3
samples are of unknown origin but have been in the Peabody
Museum collection for many years, long before recreational
running became common. The distal tibiae average PTO in all six
human samples was 90.0u62.3, indicating that loading patterns in
all six samples were nearly identical. Given the likelihood that our
comparative human samples did not come from individuals who
frequently ran long distances it is reasonable to hypothesize that
the day-to-day signals these bones were subjected to, and which
they reflect, were primarily walking. Furthermore, peak ground
reaction forces at the tarsal joint while running will be achieved in
a more flexed (bent) joint angle (by about 15u) [52]. If these peak
forces had a strong influence on the PTO, we should have seen a
much lower difference in the distal tibiae PTO between humans
and chimpanzees (i.e. the differences between humans and chimps
in tarsal joint angle and in the distal tibiae PTO would not
correspond to each other). Yet our results indicate a difference of
10.3uin tarsal joint angle and a 7.7udifference in PTO. This very
close overlap between the two parameters indicates that walking is
the main determinant of PTO in our samples. However, the small
difference in PTO in comparison to joint angle (,2.6u) may imply
Figure 4. The three Sterkfontein tibiae (StW 358, 389, 567)
(upper row) and their trabecular structure 1 mm (2
nd
row),
5mm(3
ed
row) and 10 mm (bottom row) below the cortex, as
reveled in transverse slices by the microCT scanning. Scale bar
for microCT scans is 1 cm. As can be seen, the fossils are in excellent
state of preservation with little and relatively loose deposition of
sediments; thus it did not affect our segmentation process (binarization
of CT slices). On the bottom right corner of the figure is an inset
showing two identical enlargements of an area in StW 567; the upper
image is the original, showing typical sedimentation and the bottom
image is the same area after segmentation. Note the distinct and clear
separation in appearance, consistency and X-ray absorption between
the sediments and the actual trabecular structure in the original image.
doi:10.1371/journal.pone.0077687.g004
Table 1. Trabecular bone properties means, standard deviations (6S.D.) and range (in parentheses).
H. sapiens
(n = 6)
P. troglodytes
(n = 6)
Au. africanus
(StW 358, 389) StW 567
Sagittal 2D-PTO 90.0u
*
62.3 (86.1–92.1) 82.3u
*
610.7 (72.5–97.3)
#
97.1u, 86.3u87.5u
BV/TV M 23.162.7 (18.9–26.4) 26.066.2 (17.3–32.4) 34.8, 26.5 30.8
(%) L 30.261.9 (28.1–32.5) 30.565.5 (23.5–36.6) 40.5, 27.9 36.7
Tb.N M 0.90
*
60.15 (0.73–1.1) 1.41
*
60.22 (1.1–1.64) 1.36, 1.31 1.23
(1/mm) L 1.11
*
60.13 (0.92–1.24) 1.56
*
60.20 (1.24–1.74) 1.34, 1.37 1.33
Tb.Th M 0.26
*
60.02 (0.24–0.29) 0.18
*
60.03 (0.14–0.21) 0.26, 0.20 0.25
(mm) L 0.27
*
60.03 (0.23–0.32) 0.19
*
60.02 (0.17–0.23) 0.30, 0.20 0.28
Tb.Sp M 0.82
*
60.12 (0.63–0.94) 0.55
*
60.06 (0.49–0.66) 0.58, 0.54 0.61
(mm) L 0.67
*
60.11 (0.53–0.81) 0.52
*
60.06 (0.44–0.62) 0.56, 0.51 0.54
DA M 2.67
*
60.55 (2.03–3.3) 1.88
*
60.14 (1.69–2.08) 2.34, 2.13 1.86
L 3.16
*
60.51 (2.61–3.88) 2.27
*
60.10 (2.13–2.4) 2.42, 2.52 1.98
ConnD M 4.3
*
61.4 (2.6–6.4) 12.5
*
63.1 (8.9–17.3) 4.8, 9.1 7.7
(1/mm
3
) L 4.5
*
61.3 (2.8–6.0) 11.5
*
63.3 (7.7–16.9) 4.0, 8.8 8
BV/TV stands for bone volume fraction, Tb.N for trabeculae per mm, Tb.Th for trabecular thickness in mm, Tb.Sp for trabecular separation in mm, DA for degree of
anisotropy and ConnD for connectivity density per mm
3
. L and M stand for lateral and medial VOIs respectively.
Trabecular bone properties differences between humans and chimpanzees were tested for statistical significance using the Wilcoxon rank-sum test. Statistically
significant differences between humans and chimps (P,0.01) indicated by *.
#Out of the six chimpanzees only two had 2D-PTO angles larger than 90u, the other four chimpanzees had 2D-PTO angles lower than 83uwhich is much lower than
human 2D-PTO angle range.
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that running did contribute some signal, which affected PTO as
well. Further research should study the relative contribution of
running and walking to the adaptation of trabecular bone.
Humans differ from chimpanzees not only in 2D- and 3D-PTO
but also by having significantly lower values of Tb.N and
connectivity density and significantly higher values of Tb.Th,
Tb.Sp and DA (i.e. in humans trabeculae are more oriented in one
direction) (Table 1). More trabeculae that are more connected and
anisotropic helps joints withstand high loads from multiple
directions [53]. This finding accords with evidence that chimpan-
zees load their ankles during climbing and other activities in a
much greater range of orientations than humans [23,54].
Interestingly, the trabecular bone properties of the distal tibiae
of Sterkfontein fossil hominins fall between human and chimpan-
zee values in terms of trabecular orientation (DA), which is
strongly affected by loading, but also in terms of parameters that
have both genetic and environmental influences such as Tb.N,
Tb.Sp, and ConnD (Table 1). These differences tentatively suggest
that, like chimpanzees, early hominins may have loaded their
ankles in more diverse and intensive ways than modern humans.
It is worth comparing our results to those of a recent study that
compared the trabecular architecture of the talus in humans,
several non-human primates and australopiths [55]. Since the
talus articulates with the distal tibia, one expects these two
components of the ankle joint to be similar in their trabecular
response to joint loading orientation. Although DeSilva and
Devlin [55] also found that humans have much higher degree of
anisotropy than chimpanzees and other non-human primates, they
did not measure and compare PTO among species. Further, while
DeSilva and Devlin found that chimps have significantly higher
BV/TV and that australopiths are human-like in most respects,
they did not find any other unique architectural differences such as
ConnD, Tb.Th or Tb.N between humans, non-human primates
and australopiths. Several differences between this study and
DeSilva and Devlin’s analysis likely account for the different
findings. First, while DeSilva and Devlin looked at the entire
trabecular volume of the talus, we analyzed VOIs just deep to the
joint surface, where the signal of loading orientation is the
strongest and clearest [44,45]. In addition, rather than dividing the
bone in to four quarters, our VOI’s were specifically located deep
to the cortical contact points with the talus, ensuring the
measurement of directly loaded trabeculae, and avoiding the
problem of averaging signals from other less relevant parts of the
joint that may diminish or cancel any signal from variations in how
the joint was loaded. Finally, DeSilva and Devlin used low
resolution medical CT scans of 1 mm for the fossils, but we used
high resolution microCT scans (32.8 mm), which is necessary to
accurately measure the thickness and orientation of trabeculae,
many of which are less than 0.2–0.25 mm thick. Future analyses of
trabecular orientation in VOIs just under the joint surface of the
talus using sufficiently high resolution are predicted to yield similar
results to those reported here for the distal tibia.
A recent step in this direction is Su et al. ’s (2013) [56] study of
trabecular bone structure just deep to the talar trochlea of humans,
non-human primates and a fossil sample of an extinct hominin
dated around 1.6 million years ago. This fossil, KNM-ER 1464,
although much younger than the two Au. africanus specimens
presented here, is still a useful comparison because both studies try
to correlate trabecular structure just deep to the joint surface to
locomotion behavior of extant primates and by that to infer the
locomotion behavior of extinct hominin taxa. Su et al. (2013)
found that the PTO of talar trabecular structure just deep to the
joint surface in the extinct hominin sample was similar to modern
humans but strikingly different from African great apes (namely
chimpanzee, gorilla and orangutan). Their results further support
our findings that PTO is a potent and sensitive parameter to
deduce locomotion behavior of extinct taxa.
There is no question that locomotor behavior must have varied
among different species of Australopithecus given evidence for
postcranial differences between the three best-known species, Au.
afarensis,Au. africanus, and Au sediba as well as other Pliocene
hominins such as the Burtele foot [57,58]. MicroCT data from the
distal tibia from these species as well as Ardipithecus and other early
hominin taxa are needed to gain a better understanding of the
range of variation in ankle angles during the evolution of hominin
locomotion. Even so, evidence for habitually extended hindlimb
postures in Au. africanus and whatever species is represented by
StW 567 is significant because BHBK walking incurs a substan-
tially higher cost of transport compared to the more extended
posture used by humans due to higher moments around the knee
and hip that must be countered by the large extensor muscles that
cross these joints [19,20,59]. It is already well established that
species of Australopithecus had some form of medial longitudinal
arch capable of stiffening the foot for efficient toe-off (evident from
the angle between the proximal and distal metatarsal ends to the
diaphysis), hip abductors with a high mechanical advantage, and
in some species, such as Au. afarensis, a calcaneus capable of
resisting the impact forces caused by heel strike during walking
[13,60,61]. In light of such adaptations, it is unsurprising that
efficient, humanlike walking evolved in Australopithecus prior to the
genus Homo.
Materials and Methods
Chimpanzee kinematics
Chimpanzee kinetics and kinematics were collected in 2005 and
described previously [19,20]. Three adult chimpanzees (two males
and one females; mean age, 12 years; range, 6–18 years) walked
quadrupedally at a Froude number of approximately 0.3 (1.2 m/s
60.1) down a 10-m track equipped with an embedded force-plate
(Kistler, Amherst, NY). We used data from chimpanzees during
quadrupedal walking because this type of locomotion comprises
more than 98% of their locomotion behavior [46] and hence will
be the key determining factor for trabecular orientation in the
distal tibiae. Vertical GRFs were measured using the force-plate at
1 kHz and normalized to body weight. Simultaneously, kinematic
data were collected via high-speed video (125 frames/s; Redlake)
with the hip, knee, ankle and foot marked on each subject using
nontoxic water-based white paint. Trials were accepted only if the
hindlimb contacted the force-plate cleanly and if fore-aft GRFv
traces indicated constant forward speed (,10% difference
between anterior and posterior impulse). Force-plate and kine-
matic data were smoothed using a zero-lag 4th order low pass
Butterworth filter (cut-off frequencies were 12 Hz and 200 Hz for
the kinematic and force-plate data, respectively). Ankle angle was
measured in lateral view as the angle between two lines: from the
lateral epicondyle of the femur to the lateral malleolus; from the
posterior tuber calcaneus to the distal head of the 5th metatarsal
(Fig. 1c). Because GRFv traces from some chimpanzee trials had a
double force peak, tibia orientation relative to the long axis of the
foot (ankle angle) was averaged over the period of stance when
GRFv was greater than 75% of body weight (Fig. 1c). The
chimpanzees were socially housed in large, outdoor enclosures at a
United States Department of Agriculture registered and approved
facility. Institutional Animal Care and Use Committee approval
was obtained before the beginning of the study, and institutional
animal care guidelines were followed throughout.
Evidence for a Human-Like Gait in Au. africanus
PLOS ONE | www.plosone.org 6 November 2013 | Volume 8 | Issue 11 | e77687

Human kinematics
Six adult humans (3 males and 3 females; mean age, 33 years;
range, 20–48 years) were measured while walking on custom-built,
dual-belt, force instrumented treadmill (Bertec Corporation,
Columbus OH, USA). Vertical GRFs were measured at
1000 Hz and normalized to body weight. Simultaneously,
kinematic data were collected with an 8-camera Oqus kinematics
system (Qualysis, Gothenburg, Sweden) at 500 Hz with markers
on the lateral aspect of the hip, knee, ankle, and 5th metatarsal
head. Subjects were recorded at a Froude number of 0.3 (1.3 m/s
60.2) while walking with a normal gait and after walking for 2–
4 minutes in a bent-hip bent-knee gait. We recorded humans
BHBK walking to test whether ankle joint angle at midstance
differs significantly from quadrupedal chimpanzees; these data
would be important in the case that Australopithecus africanus distal
tibia trabecular bone PTO differs significantly from humans but
not chimpanzees. Ankle angle was measured in lateral view as the
angle between two lines: from the lateral epicondyle of the femur
to the lateral malleolus, and from the posterior tuber calcaneus to
the distal head of the 5th metatarsal (Fig. 1a, b). Because GRFv
traces from human trails had a double force peak, tibia orientation
relative to the long axis of the foot (ankle angle) was averaged over
the period of stance when GRFv was greater than 75% of body
weight (Fig. 1). Experimental protocol was approved by Harvard
University Committee on the Use of Human Subjects, and prior
written informed consent was obtained from all subjects.
Human and chimpanzee distal tibiae microCT scanning
Tibiae from adult Homo sapiens (n = 6) bones were obtained from
the Peabody Museum of Archeology and Ethnology, Harvard
University, Cambridge MA, USA. Adult Pan troglodytes (n = 6)
tibiae were obtained from the Museum of Comparative Zoology,
Harvard University, Cambridge MA, USA (Table S2). Chimpan-
zee tibiae are from wild-shot individuals from populations in West
Africa. All bones had no traces of bone pathology. The distal part
of all tibiae were microCT scanned at the Center for Nanoscale
Systems, Harvard University using a Metris X-Tek HMX ST 225
scanner (Nikon Metrology Inc.) at 70 kV and 130 mA with no
filter. Scan resolutions are summarized in Table S2. The output
raw data (3142 projections, no frame averaging, and detector size
200062000 pixels) were imported into CT PRO software (Nikon
Metrology Inc.) and reconstructed into 3D volumes.
Fossil hominin distal tibiae microCT scanning
Three Sterkfontein tibiae (StW 358, 389, 567) were obtained
from a collaborative project between the Department of Human
Evolution, Max Planck Institute for Evolutionary Anthropology
and the University of the Witwatersrand, South Africa, through its
Institute for Human Evolution (Table S2), (we thank the Institute
for Human Evolution at Witwatersrand University (Johannesburg)
for allowing CT-scanning of the fossil material). These fossils are in
an excellent state of preservation. Sample StW 389 has almost
4 cm of its diaphysis intact, StW 358 has around 1 cm of his
diaphysis intact and StW 567 comprises only the most distal part
of the tibia (Fig. 4). The entire 3D trabecular structure deep to the
joint surface of all three tibiae is intact and detectable using an X-
ray source (see Fig. 2d, 2e and 2f for sagittal views and Fig. 4). StW
358 has a crack running from the middle of the lateral edge to the
middle of the anterior edge of the bone. Sample StW 389 is
missing its medial condyle. Sample StW 567 is missing the postero-
lateral corner of the distal tibia (Fig. 4). None of these missing or
damaged areas were in the VOIs we analyzed. The fossils were
microCT scanned in Johannesburg by the Department of Human
Evolution, Max Planck Institute for Evolutionary Anthropology
using a BIR ACTIS 225/300 high resolution scanner at 130 kV
and 100 mA using a 0.5 brass filter. Scan resolutions are given in
Table S2. The scans (2500 projections, three-frame averaging,
and detector size 204862048 pixels) were reconstructed directly
into 16-bit TIFF image stacks.
Image processing
All reconstructed scans were imported into VGStudio Max 2.1
(Volume Graphics GmbH, Heidelberg Germany) and were
reoriented along the long axis of the bone using the tibiae distal
diaphysis and additional anatomical landmarks. All scans from the
same species were superimposed to ensure identical orientation for
all the bones (Fig. S1). The reconstructed scans were then cropped
and saved as 16-bit TIFF image stacks. Each scan was saved as 16-
bit TIFF image stacks twice, along two different axes: along the
transverse plane (proximodistal) and along the sagittal plane
(craniocaudally). The transverse image stacks were used to
quantify principal orientations of trabeculae in 3D; the sagittal
image stacks were used to quantify principal orientations of
trabeculae in 2D in the sagittal plane (Figure 2). After cropping,
image stacks were segmented (binarized) to differentiate bone from
non-bone pixels using an edge-detection ray-casting algorithm
(RCA) [62]. The RCA algorithm is advantageous over other
conventional threshold detection techniques because it uses the
gray level gradient of the image rather than the absolute gray-level
values. Finally, images were converted into 8-bit TIFF image
stacks (black pixels equal to ‘‘0’’ and white pixels equal to ‘‘255’’).
Trabecular bone properties and orientation calculation
Analyses of trabecular bone properties and PTO were
performed using CTAn (CTAnalyzer; SkyScan, Belgium) [63].
Two spherical VOIs were selected within the trabecular bone of
each tibia, one at the medial and one at the lateral distal articular
surface of the tibia (tibial plafond), just deep to the cortex of the
joint surface (i.e. proximal to the joint cortex and distal to the
growth plate). The VOI’s were positioned distally to the growth
plate, in the epiphyses (secondary ossification center). Exact VOIs
locations were chosen just deep to the contact points with the distal
tibia (Fig. 3). VOI diameter was 200 pixels (for PTO calculation)
and 400 pixels (for all other trabecular bone properties calcula-
tions) and varied in absolute size between species (i.e. VOI’s
absolute size was larger in humans; see Table S2 for scan
resolutions). For orientation detection, VOI size was determined
to be big enough to optimize the number of trabeculae that are
near the joint surface following Harrigan [64], but not too big to
avoid trabeculae more than 5–7 mm deep to the joint surface,
which are less affected by the orientation of stresses in the joint.
The following trabecular bone parameters were measured in 3D
for the distal tibiae VOIs [63]: bone volume fraction (BV/TV),
trabecular number (Tb.N), trabecular thickness (Tb.Th), trabec-
ular separation (Tb.Sp), degree of anisotropy (DA) and connec-
tivity density (ConnD). DA measures trabecular alignment along a
preferred axis; a larger value indicates a stronger tendency of the
trabecular structure to align itself along a preferred orientation.
ConnD defines how many connections per mm
3
between different
trabeculae can be severed before the trabecular tissue will be
divided into two separate parts. PTO was determined by CTAn
software using the mean intercept length (MIL) technique. The
MIL technique superimposes a linear grid over a selected area (in
2D) or volume (in 3D) and counts the number of intersections
between the grid and the bone/non-bone interface. The ‘‘mean
intercept length’’ is defined as total line length divided by the
number of intersections [65]. By rotating the grid’s orientation by
a constant angle (v) and measuring the MIL at each angle, it is
Evidence for a Human-Like Gait in Au. africanus
PLOS ONE | www.plosone.org 7 November 2013 | Volume 8 | Issue 11 | e77687

possible to determine the orientation at which the MIL is the
largest (i.e., has the fewest intersections between bone and non-
bone pixels). The output eigenvector values (x,y and z coordinates
of the principle orientation vector situated on the surface of the
spherical VOI) were imported into stereographic projection
software (StereoNett, Institute of Geology, Ruhr University,
Bochum, Germany) and were visualized using an equal-angle
stereoplot (A stereoplot is a 2D map which is created by projecting
points from a surface of a sphere to a tangential plane, Fig. 3a).
2D-PTO in the sagittal plane was also determined for each bone
using the MIL technique from 10 sagittal slices in the middle of
each VOI (Fig. 2). Note that 2D-PTO and ankle angle are shifted
consistently by approximately 8ubecause 2D-PTO was measured
relative to a plane perpendicular to the long axis of the tibia, not
the surface of the joint; for similar shifts, see also Pontzer 2006
[44], Barak 2011 [45].
Statistical Analyses
Statistical analyses were performed using R, version 2.15.0 (R
Foundation for Statistical Computing, Vienna, Austria; www. r-
project.org). Values given are mean and standard deviations (S.D)
unless indicated differently. Statistical significance was determined
using 95% confidence intervals. Statistically significant differences
between species for trabecular bone parameters other than
orientation were determined using Wilcoxon rank-sum test
(Table 1). In order to test if PTO differed significantly between
groups we ran a permutation test. This method allocates the data
points into two new groups, and then uses a non-parametric t-test
to test if the original groups differ significantly or not. This cascade
is repeated until the entire possible population of groups were
created and tested (we thank the Institute for Quantitative Social
Science at Harvard University (and especially Steven Worthing-
ton) for help in performing the permutation tests). For the sagittal
plane measurements of 2D-PTO, we determined the medians of
the groups and tested them against the median of all other possible
allocated groups using the same original datapoints. For the 3D-
PTO, we calculated each group’s centroid on the surface of the
sphere (VOI) using the haversine formula, which calculates the
shortest distance between two points on a surface of a sphere. We
then measured the distance between the centroids of the two
original groups. Finally, we executed a permutation test, checking
the measured distance against the distance between centroids of all
other possible allocated groups (using the same datapoints). A P-
value #0.05 indicates that the distance between the two group
centroids is significant. In one permutation test (comparing the
chimpanzees to humans and early hominins in the medial 3D
VOI) we removed one chimpanzee outlier (see Fig. 3c; point MCZ
10736: longitude 91.5 and latitude 69.3), including this outlier
yields a P-value of 0.07.
Supporting Information
Figure S1 The tibiae distal surfaces of Chimpanzees,
humans and early hominin fossils visualized using
VGStudio Max 2.1. Using VGStudio Max 2.1 bones were
reoriented along their long axis. Next, all bone scan reconstruc-
tions from the same species were overlapped in 3D to ensure
identical orientation. Each illustration shows a combination of all
bones from the same group superimposed one on top of the other:
chimpanzees (a), humans (b) and early hominin fossils (c). In view
is the tibiae distal surfaces (tibial plafond), the medial malleolus is
at the upper right side of each illustration.
(TIF)
Table S1 Longitude and latitude coordinates for the 3D-
PTO presented in Fig. 3b and 3c.
(DOCX)
Table S2 A list of all samples used.
(DOCX)
Acknowledgments
We thank J. Chupasko, O. Herschensohn, T. Kivell, L. Miratrix, M.
Morgan, and R Tilgner for their help.
Author Contributions
Conceived and designed the experiments: DEL MMB JJH. Performed the
experiments: DEL MMB HP DR AGW. Analyzed the data: DEL MMB
HP DR AGW. Contributed reagents/materials/analysis tools: DEL JJH
HP DR. Wrote the paper: DEL MMB.
References
1. Zollikofer CPE, Ponce de Leo´n MS, Lieberman DE, Guy F, Pilbeam D, et al.
(2005) Virtual cranial reconstruction of Sahelanthropus tchadensis. Nature 434:
755–759.
2. Richmond BG, Jungers WL (2008) Orrorin tugenensis femoral morphology and
the evolution of hominin bipedalism. Science 319: 1662–1665.
3. White TD, Asfaw B, Beyene Y, Haile-Selassie Y, Lovejoy CO, et al. (2 009)
Ardipithecus ramidus and the paleobiology of early hominids. Science 326: 75–
86.
4. Darwin C (1871) The Descent of man. D. Appleton and Co. 430 p.
5. Ward CV (2002) Interpreting the posture and locomotion of Australopithecus
afarensis: where do we stand? Am J Phys Anthropol Suppl 35: 185–215.
6. Lovejoy CO, Meindl RS, Ohman JC, Heiple KG, White TD (2002) The Maka
femur and its bearing on the antiquity of human walking: applying
contemporary concepts of morphogenesis to the human fossil record.
Am J Phys Anthropol 119: 97–133.
7. Schmitt D (2003) Insights into the evolution of human bipedalism from
experimental studies of humans and other primates. J Exp Biol 206: 1437–1448.
8. Susman RL, Stern JT Jr, Jungers WL (1984) Arboreality and bipedality in the
Hadar hominids. Folia Primatol 43: 113–156.
9. Raichlen DA, Gordon AD, Harcourt-Smith WEH, Foster AD, Haas WR (2010)
Laetoli footprints preserve earliest direct evidence of human-like bipedal
biomechanics. PLoS ONE 5: e9769.
10. Leakey MG, Feibel CS, McDougall I, Walker A (1995) New four-million-year-
old hominid species from Kanapoi and Allia Bay, Kenya. Nature 376: 565–571.
11. Lovejoy CO, Suwa G, Spurlock L, Asfaw B, White TD (2009) The pelvis and
femur of Ardipithecus ramidus: the emergence of upright walking. Science 326:
71e1–6.
12. Crompton RH, Pataky TC, Savage R, D’Aouˆ t K, Bennett MR, et al. (2012)
Human-like external function of the foot, and fully upright gait, confirmed in the
3.66 million year old Laetoli hominin footprints by topographic statistics,
experimental footprint-formation and computer simulation. J R Soc Interface 9:
707–719.
13. Latimer B, Lovejoy CO (1989) The calcaneus of Australopithecus afarensis and
its implications for the evolution of bipedality. Am J Phys Anthropol 78: 369–
386.
14. Lovejoy CO, Heiple KG, Burstein AH (1973) The gait of Australopithecus.
Am J Phys Anthropol 38: 757–779.
15. Lovejoy CO, McCollum MA (2010) Spinopelvic pathways to bipedality: why no
hominids ever relied on a bent-hip-bent-knee gait. Philos Trans R Soc Lond, B,
Biol Sci 365: 3289–3299.
16. Crompton RH, Yu L, Weijie W, Gu¨nther M, Savage R (1998) The mechanical
effectiveness of erect and ‘‘bent-hip, bent-knee’’ bipedal walking in Australo-
pithecus afarensis. J Hum Evol 35: 55–74.
17. Wang WJ, Crompton RH, Li Y, Gunther MM (2003) Energy transformation
during erect and ‘‘bent-hip, bent-knee’’ walking by humans with implications for
the evolution of bipedalism. J Hum Evol 44: 563–579.
18. Carey TS, Crompton RH (2005) The metabolic costs of ‘‘bent-hip, bent-knee’’
walking in humans. J Hum Evol 48: 25–44.
19. Pontzer H, Raichlen DA, Sockol MD (2009) The metabolic cost of walking in
humans, chimpanzees, and early hominins. J Hum Evol 56: 43–54.
20. Sockol MD, Raichlen DA, Pontzer H (2007) Chimpanzee locomotor energetics
and the origin of human bipedalism. Proc Natl Acad Sci USA 104: 12265–
12269.
Evidence for a Human-Like Gait in Au. africanus
PLOS ONE | www.plosone.org 8 November 2013 | Volume 8 | Issue 11 | e77687

21. Latimer B, Ohman JC, Lovejoy CO (1987) Talocrural joint in African
hominoids: implications for Australopithecus afarensis. Am J Phys Anthropol 74:
155–175.
22. Tardieu C, Trinkaus E (1994) Early ontogeny of the human femoral bicondylar
angle. Am J Phys Anthropol 95: 183–195.
23. DeSilva JM (2009) Functional morphology of the ankle and the likelihood of
climbing in early hominins. Proc Natl Acad Sci USA 106: 6567–6572.
24. DeSilva JM, Throckmorton ZJ (2010) Lucy’s flat feet: the relationship between
the ankle and rearfoot arching in early hominins. PLoS ONE 5: e14432.
25. Stern JT Jr (2000) Climbing to the top: A personal memoir of Austra lopithecus
afarensis. Evol Anthropol 9: 113–133.
26. Stern JT Jr, Susman RL (1983) The locomotor anatomy of Australopithecus
afarensis. Am J Phys Anthropol 60: 279–317.
27. Berger LR, Tobias PV (1996) A chimpanzee-like tibia from Sterkfontein, South
Africa and its implications for the interpretation of bipedalism in Australopithe-
cus africanus. J Hum Evol 30: 343–348.
28. Rook L, Bondioli L, Ko¨hler M, Moya`-Sola` S, Macchiarelli R (1999)
Oreopithecus was a bipedal ape after all: Evidence from the iliac cancellous
architecture. Proc Natl Acad Sci USA 96: 8795–8799.
29. Raichlen DA, Pontzer H, Sockol MD (2008) The Laetoli footprints and early
hominin locomotor kinematics. J Hum Evol 54: 112–117.
30. Wolff J (1892) Das Gesetz der Transformation der Knochen. Berlin: Hirschwald
Verlag.
31. Ryan TM, Walker A (2010) Trabecular bone structure in the humeral and
femoral heads of anthropoid primates. Anat Rec 293: 719–729.
32. Shaw CN, Ryan TM (2012) Does skeletal anatomy reflect adaptation to
locomotor patterns? Co rtical and trabecular architecture in hu man and
nonhuman anthropoids. Am J Phys Anthropol 147: 187–200.
33. Lanyon LE (1974) Experimental support for the trajectorial theory of bone
structure. J Bone Joint Surg Br 56-B: 160–166.
34. Volpato V, Viola TB, Nakatsukasa M, Bondioli L, Macchiarelli R (2008)
Textural characteristics of the iliac-femoral trabecular pattern in a bipedally
trained Japanese macaque. Primates 49: 16–25.
35. Ryan TM, Ketcham RA (2002) The three-dimensional structure of trabecular
bone in the femoral head of strepsirrhine primates. J Hum Evol 43: 1–26.
36. MacLatchy L, Mu¨ ller R (2002) A comparison of the femoral head and neck
trabecular architecture of Galago and Perodicticus using micro-computed
tomography (mCT). J Hum Evol 43: 89–105.
37. Fajardo RJ, Mu¨ ller R (2001) Three-dimensional analysis of nonhuman primate
trabecular architecture using micro-computed tomography. Am J Phys Anthro-
pol 115: 327–336.
38. Saparin P, Scherf H, Hublin JJ, Fratzl P, Weinkamer R (2011) Structural
adaptation of trabecular bone revealed by position resolved analysis of proximal
femora of different primates. Anat Rec 294: 55–67.
39. Christen P, van Rietbergen B, Lambers FM, Mu¨ ller R, Ito K (2012) Bone
morphology allows estimation of loading history in a murine model of bone
adaptation. Biomech Model Mechanobiol 11: 483–492.
40. Kobayashi S, Takahashi HE, Ito A, Saito N, Nawata M, et al. (2003) Trabecular
minimodeling in human iliac bone. Bone 32: 163–169.
41. Schiff A, Li J, Inoue N, Masuda K, Lidtke R, et al. (2007) Trabecular angle of
the human talus is a ssociated with t he level of cartilage degeneration.
J Musculoskelet Neuronal Interact 7: 224–230.
42. Sugiyama T, Price JS, Lanyon LE (2010) Functional adaptation to mechanical
loading in both cortical and cancellous bone is controlled locally and is confined
to the loaded bones. Bone 46: 314–321.
43. Van der Meulen MCH, Morgan TG, Yang X, Baldini TH, Myers ER, et al.
(2006) Cancellous bone adaptation to in vivo loading in a rabbit model. Bone 38:
871–877.
44. Pontzer H, Lieberman DE, Momin E, Devlin MJ, Polk JD, et al. (2006)
Trabecular bone in the bird knee responds with high sensitivity to changes in
load orientation. J Exp Biol 209: 57–65.
45. Barak MM, Lieberman DE, Hublin JJ (2011) A Wolff in sheep’s clothing:
trabecular bone adaptation in response to changes in joint loading orientation.
Bone 49: 1141–1151.
46. Hunt KD (1992) Positional behavior of Pan troglodytes in the Mahale
Mountains and Gombe Stream National Parks, Tanzania. Am J Phys Anthropol
87: 83–105.
47. Kuman K, Clarke RJ (2000) Stratigraphy, artefact industries and hominid
associations for Sterkfontein, member 5. J Hum Evol 38: 827–847.
48. Kimizuka M, Kurosawa H, Fukubayashi T (1980) Load-bearing pattern of the
ankle joint. Contact area and pressure distribution. Arch Orthop Trauma Surg
96: 45–49.
49. Haraguchi N, Armiger RS, Myerson MS, Campbell JT, Chao EYS (2009)
Prediction of three-dimensional contact stress and ligament tension in the ankle
during stance determined from computational modeling. Foot Ankle Int 30:
177–185.
50. Pontzer H, Wrangham RW (2004) Climbing and the daily energy cost of
locomotion in wild chimpanzees: implications for hominoid locomotor
evolution. J Hum Evol 46: 317–335.
51. Bramble DM, Lieberman DE (2004) Endurance running and the evolution of
Homo. Nature 432: 345–352.
52. Novacheck TF (1998) The biomechanics of running. Gait Posture 7: 77–95.
53. Kabel J, Odgaard A, van Rietbergen B, Huiskes R (1999) Connectivity and the
elastic properties of cancellous bone. Bone 24: 115–120.
54. D’Aouˆ t K, Aerts P, De Clercq D, De Meester K, Van Elsacker L (2002) Segment
and joint angles of hind limb during bipedal and quadrupedal walking of the
bonobo (Pan paniscus). Am J Phys Anthropol 119: 37–51.
55. Desilva JM, Devlin MJ (2012) A comparative study of the trabecular bony
architecture of the talus in humans, non-human primates, and Australopithecus.
J Hum Evol 63: 536–551.
56. Su A, Wallace IJ, Nakatsukasa M (2013) Trabecular bone anisotropy and
orientation in an Early Pleistocene hominin talus from East Turkana, Kenya.
J Hum Evol 64: 667–677.
57. Zipfel B, DeSilva JM, Kidd RS, Carlson KJ, Churchill SE, et al. (2011) The Foot
and Ankle of Australopithecus sediba. Science 333: 1417–1420.
58. Haile-Selassie Y, Saylor BZ, Deino A, Levin NE, Alene M, et al. (2012) A new
hominin foot from Ethiopia shows multiple Pliocene bipedal adaptations. Nature
483: 565–569.
59. Sellers WI, Cain GM, Wang W, Crompton RH (2005) Stride lengths, speed and
energy costs in walking of Australopithecus afarensis: using evolutionary robotics
to predict locomotion of early human ancestors. J R Soc Interface 2: 431–441.
60. Harcourt-Smith WEH, Aiello LC (2004) Fossils, feet and the evolution of human
bipedal locomotion. J Anat 204: 403–416.
61. Ward CV, Kimbel WH, Johanson DC (2011) Complete fourth metatarsal and
arches in the foot of Australopithecus afarensis. Science 331: 750–753.
62. Scherf H, Tilgner R (2009) A new high-resolution computed tomography (CT)
segmentation method for trabecular bone architectural analysis. Am J Phys
Anthropol 140: 39–51.
63. Morphometric parameters measure d by SkyscanTM CT-analyzer software
(2009) Skyscan website. Available: http://www.skyscan.be/next/CTAn03.pdf.
Accessed 2013 Sept 25.
64. Harrigan TP, Jasty M, Mann RW, Harris WH (1988) Limitations of the
continuum assumption in cancellous bone. J Biomech 21: 269–275.
65. Odgaard A (1997) Three-dimensional methods for quantification of cancellous
bone architecture. Bone 20: 315–328.
Evidence for a Human-Like Gait in Au. africanus
PLOS ONE | www.plosone.org 9 November 2013 | Volume 8 | Issue 11 | e77687