Laetoli footprints preserve earliest direct evidence of human-like bipedal mechanics

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Laetoli Footprints Preserve Earliest Direct Evidence of
Human-Like Bipedal Biomechanics
David A. Raichlen
1
*, Adam D. Gordon
2
, William E. H. Harcourt-Smith
3,4
, Adam D. Foster
1
, Wm. Randall
Haas, Jr.
1
1School of Anthropology, University of Arizona, Tucson, Arizona, United States of America, 2Department of Anthropology, University at Albany–SUNY, Albany, New York,
United States of America, 3Department of Anthropology, Lehman College, Bronx, New York, United States of America, 4Division of Vertebrate Paleontology, American
Museum of Natural History, New York, New York, United States of America
Abstract
Background:
Debates over the evolution of hominin bipedalism, a defining human characteristic, revolve around whether
early bipeds walked more like humans, with energetically efficient extended hind limbs, or more like apes with flexed hind
limbs. The 3.6 million year old hominin footprints at Laetoli, Tanzania represent the earliest direct evidence of hominin
bipedalism. Determining the kinematics of Laetoli hominins will allow us to understand whether selection acted to decrease
energy costs of bipedalism by 3.6 Ma.
Methodology/Principal Findings:
Using an experimental design, we show that the Laetoli hominins walked with weight
transfer most similar to the economical extended limb bipedalism of humans. Humans walked through a sand trackway
using both extended limb bipedalism, and more flexed limb bipedalism. Footprint morphology from extended limb trials
matches weight distribution patterns found in the Laetoli footprints.
Conclusions:
These results provide us with the earliest direct evidence of kinematically human-like bipedalism currently
known, and show that extended limb bipedalism evolved long before the appearance of the genus Homo. Since extended-
limb bipedalism is more energetically economical than ape-like bipedalism, energy expenditure was likely an important
selection pressure on hominin bipeds by 3.6 Ma.
Citation: Raichlen DA, Gordon AD, Harcourt-Smith WEH, Foster AD, Haas WR, Jr. (2010) Laetoli Footprints Preserve Earliest Direct Evidence of Human-Like Bipedal
Biomechanics. PLoS ONE 5(3): e9769. doi:10.1371/journal.pone.0009769
Editor: Karen Rosenberg, University of Delaware, United States of America
Received November 22, 2009; Accepted February 28, 2010; Published March 22, 2010
Copyright: ß2010 Raichlen 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: Funding was provided from the University of Arizona. 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: raichlen@email.arizona.edu
Introduction
Ever since Darwin [1], bipedal walking has been considered the
defining feature of the human lineage. However, how and why this
unique form of locomotion evolved remains the subject of
considerable debate. In particular, debates over the origins and
evolution of bipedalism revolve around whether early bipeds
walked with energetically economical human-like extended limb
biomechanics, or with more costly ape-like bent-knee, bent-hip
(BKBH) kinematics [2]. If early hominins used a BKBH gait, then
we must account for the persistence of an energetically costly form
of bipedal walking until the evolution of the genus Homo. The
Laetoli footprints may help resolve this debate, since they record
the footsteps of at least two, and possibly three individuals who
walked bipedally across wet ashfall approximately 3.6 million years
ago [3,4]. These prints represent the earliest direct evidence of
bipedalism in the fossil record, yet no study to date has
demonstrated exactly how these hominins walked.
For decades, researchers have argued over whether the Laetoli
hominins walked with a modern human-like extended limb gait
[3,5–10], or a more ape-like form of bipedalism [11–14]. Most
recently, the discovery of 1.5 Ma footprints from Kenya provides
new evidence that the Laetoli prints were not completely modern
in weight transfer and morphology [15]. If true, then an
energetically costly form of bipedalism evolved and persisted in
early hominins until the evolution of the genus Homo [16].
Alternatively, if the Laetoli prints were made by extended-limb
bipeds, then, by 3.6 Ma, selection acted to reduce energy costs of
locomotion in hominins.
Most previous studies of the Laetoli prints, however, were
qualitative and did not test specific biomechanical hypotheses
about early hominin gait. Kullmer et al. [17] performed a
quantitative comparison of one Laetoli footprint (G1-36) with
three footprints made by a single human subject walking with
different postures. Their results suggest that Laetoli hominins did
not walk like modern humans, however their sample size was very
small (one human print for each posture studied) and there is not
enough kinematic information to generalize their results to the
Laetoli trackways. More recent quantitative work using geometric
morphometrics techniques [15,18] may conflate differences in foot
anatomy and shape with differences in locomotor biomechanics.
Without using experimental analyses, it is difficult to determine
which landmark differences between Laetoli and more recent fossil
footprints are due simply to differences in foot morphology, and
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which are, in fact, due to differences in kinematics. Thus, a
thorough experimental-based analysis of the Laetoli prints may
resolve the debate over hominin biomechanics, and therefore help
clarify the importance of selection for reduced energy costs of
locomotion prior to the evolution of the genus Homo.
Here, we present the results of the first experimental analysis of
footprints in a sample of humans walking with different gaits and
compare our results to the Laetoli prints. Eight human subjects
(mean body mass [SD] = 65.6 [6.1] kg) walked through a 5 m
trackway filled with 15 cm of fine grained sand (0.075 mm–
0.70 mm diameter). Although our subjects are likely heavier than
the earlier hominins that generated the Laetoli prints, body mass
does not appear to have an effect on footprint morphology [19].
We compared footprints made by subjects walking with a normal,
extended limb gait, and with a bent-knee, bent-hip (BKBH) ape-
like gait at their preferred speeds with sand water content of 6–8%
(see supplementary materials Text S1 for hind limb kinematic data
from these experiments). These substrate conditions match those
of Laetoli, which are described as similar to damp, fine to medium
grained sand [20]. We also examined the effects of increased speed
and increased substrate moisture (10–12% water) on footprint
morphology. We tested the hypothesis that a BKBH gait alters
body weight transfer and produces significantly different footprint
morphology than an extended limb gait. We predicted that Laetoli
footprint morphology would match humans walking with one of
these gaits. To test this prediction, we scanned each human print
using a 3D laser scanner (MicrosribeHMLX 6DOF digitizer with
attached Microscan laser sensor system), and compared the
maximum depths of the fore- (toe) and aft- (heel) sections of the
prints. These values were calculated after leveling each print, since
prints were made over a substrate that had a very small grade
(mean grade = 1.49%60.16%). We compared our experimental
data with Laetoli footprint depths calculated from contour maps of
the footprint trails [21] (Table S1). Additionally, we captured
kinematic and kinetic data to determine how walking biomechan-
ics influence footprint morphology (see supplementary materials
Text S1).
Results and Discussion
There is a significant difference in the relative depth of the toes
in BKBH compared to extended limb prints (Figs. 1 and 2;
Table 1). BKBH footprints from humans walking at preferred
speeds have toe depressions that are 76.69%68.35% lower than
the heel (calculated as toe depth as a percentage of heel depth;
Table 1). When walking with an extended hind limb at preferred
speeds, toe depressions are, on average, 22.36%64.28% below the
heel. Thus, BKBH gaits generate significantly greater toe relative
to heel depths compared to extended limb gaits. Speed influences
print morphology, with faster speeds leading to deeper toe
depressions in both gaits, however between-gait differences are
not significant (Table 1). Substrate moisture content also alters
footprint morphology (Table 1). Wetter substrates lead to greater
toe depths, regardless of gait, however, BKBH gaits have toe
depths that are significantly greater than extended limb postures
(Table 1).
The difference in print morphology is related to a fundamental
difference in body weight transfer between extended limb and
BKBH gaits. The center of pressure (COP) is the point of ground
reaction force application under the foot. As the COP travels from
the heel to the toe during stance phase, its path, and the
magnitude of ground force at any given moment, determines the
amount of substrate displacement at a given position under the
foot. During walking, as the COP shifts anterior to the metatarsal
heads, the metatarso-phalangeal joint flexes, and because of the
stiff longitudinal arch, the entire foot posterior to the metatarsal
heads lifts off the ground [22]. In a BKBH gait, the COP passes
the metatarsal heads significantly earlier in the step (i.e., closer to
heel-strike; Paired t-test p,0.001), and the ground reaction force
impulse after the COP passes the metatarsal heads is significantly
greater compared to extended limb gaits (Fig. 3; Paired t-test
p,0.001). The larger impulse when the body is supported by the
forefoot explains the increased toe-depth in BKBH footprints.
Since most researchers agree that the Laetoli prints exhibit
evidence of a longitudinal arch [5,6,8,10,23–26] (but see [13–15]
and supplementary materials Text S1; Figs. S1, S2), an analysis of
relative toe depths should provide unequivocal evidence of limb
posture in these early hominins.
The Laetoli prints have toe depths that are generally shallower
than heel depths, however, the trackways were made on a very
slight grade (3.12%60.94%; see supplementary materials Text
S1). After adjusting for this grade with the same procedures used
for the experimental footprints (see supplementary materials Text
S1 and above), mean toe depths for the G1 set of prints are
generally equal to mean heel depths (0.11%61.61% shallower
than heels [0.00 mm60.02 mm]), resembling weight transfer in a
modern human-like extended limb gait more than a BKBH gait
(Figs. 1 and 2; Supporting Table S1). In fact, while Laetoli
proportional toe depths fall within the range of normal human
patterns, Laetoli data fall outside of the range of human
proportional toe depths made during BKBH trials (Fig. 2). We
did not analyze the G2/3 set of footprints, since they were likely
made by two individuals walking one behind the other, rendering
their morphology less suitable for biomechanical determinations
[9,11,27]. There are two other caveats that must be considered in
this analysis. First, bioturbation is evident in some of the Laetoli
G1 prints [23], and could impact our results. However, much of
the bioturbation occurred on the rims of the prints and does not
greatly impact the internal morphology of most prints [23,24].
Second, the thickness of the substrate at Laetoli varies along the
print trail [28], which may have an effect on footprint depths [19].
However, the similarity of proportional depths across the G1 trail
(Table S1) suggests that differences in substrate thickness did not
impact our analysis. Finally, the Laetoli prints offer no evidence
that these individuals were walking at fast speeds, since high speed
walking also produces prints with much larger toe compared to
heel depths for both gaits (see Table 1). Therefore, we conclude
that the Laetoli hominins walked with an extended limb gait at
speeds consistent with previous predictions (i.e., preferred or slow
speeds) [8–10,26,29–32]. The relative toe depths of the Laetoli
prints show that, by 3.6 Ma, fully extended limb bipedal gait had
evolved. Thus, our results provide the earliest unequivocal
evidence of human-like bipedalism in the fossil record.
Hypotheses for the origins of bipedalism often focus on selection
for energy economy in early hominins [16,33]. Energetic
hypotheses are based on the reduced locomotor costs of humans
compared to apes walking with BKBH gaits, and therefore,
compared to ape-like pre-hominin ancestors [16]. Human walking
is inexpensive primarily because extended hind limb joints reduce
external moments acting at the joints, and therefore, reduce the
amount of muscle force required to support body weight [16,34].
In addition, extended-limb walking reduces joint reaction forces
[35] and reduces total body heat loads compared to BKBH
walking [36]. By 3.6 Ma, hominins at Laetoli, Tanzania walked
with modern human-like hind limb biomechanics, suggesting that
selection for energetically economical bipedalism occurred prior to
the evolution of the genus Homo. It is likely that reduced energy
costs associated with extended limb bipedalism allowed early
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hominins to increase ranging distances during times of forest
fragmentation [37] without enduring greatly increased energy
costs.
While our results show that Laetoli hominins walked with
human-like kinematics, we still cannot be sure of which hominin
taxon made the footprints. Many researchers suggest that
Australopithecus afarensis made the footprint trails [6,7,11], although
this hypothesis is disputed by others based on differences between
print morphology and fossilized foot remains [10,38]. If Au.
afarensis did make the Laetoli footprints, then our results support
the hypothesis that this species walked with relatively human-like
hip and knee extension [39,40], and that kinematically human-like
bipedalism is compatible with adaptations for arboreality found
throughout the australopith skeleton [2]. Thus, settling the dispute
over the taxonomic identification of the makers of the Laetoli
prints will clarify debates surrounding fossil hominin post-cranial
material and locomotor behavior [11,39].
Finally, although our results clearly demonstrate that human-
like bipedal kinematics had evolved by 3.6 Ma, the Laetoli prints
cannot provide detailed information regarding the locomotor
mechanics of earlier hominins. The recently published skeletal
material attributed to Ardipithecus ramidus raises the hypothesis that,
prior to 4.4 Ma, at least one lineage of hominins walked with
kinematics that differed greatly from those of modern humans and
other later hominins [41]. However, until further functional
morphological analyses are performed, it is difficult to assess the
Figure 1. Three dimensional scans of experimental footprints and a Laetoli footprint. Contours are 1 mm. A) Contour map of modern
human footprint (Subject 6) walking with a normal, extended limb gait and side view of normal, extended limb footprint. B) Contour map of modern
human footprint (Subject 6) walking with a BKBH gait and side view of BKBH print. C) Contour map of Laetoli footprint (G1-37) and side view of
Laetoli footprint (G1-37). Note the difference in heel and toe depths between modern humans walking with extended and BKBH gaits. Laetoli has
similar toe relative to heel depths as the modern human extended limb print.
doi:10.1371/journal.pone.0009769.g001
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likely biomechanics of Ar. ramidus. Thus, based on the results of this
study, kinematically human-like bipedalism clearly evolved within
the first three to four million years of hominin evolution. However,
we are left with two possible scenarios for the origins and evolution
of bipedalism. First, if a detailed functional analysis supports the
hypothesis that Ar. ramidus was a habitual bipedal hominin that
walked with flexed lower limbs, then energetic selection pressures
likely became strongest after the origins of bipedalism, but prior to
3.6 Ma. This finding would not necessarily rule out the energy
hypothesis for the origins of bipedalism [34], but would suggest
that early bipeds were less energetically economical than modern
humans. Second, if Ar. ramidus was not a bipedal hominin, then the
earliest bipeds may have walked with extended limb joints,
suggesting that early bipedalism was as energetically economical as
that of later hominins, including those that made the Laetoli
footprints. Testing these hypotheses requires more detailed fossil
evidence from the earliest hominins. However, our analysis of the
Laetoli prints refines the timing of the evolution of human-like
bipedal mechanics in the fossil record. Future analyses of fossil
remains, and future fossil discoveries, will no doubt improve our
understanding of just how early human-like bipedal mechanics
evolved, and therefore, help us determine the importance of
selection for energy economy during the early evolution of bipedal
walking.
Methods
Eight subjects participated in this study (see Table S2). All
methods were approved by the University of Arizona Human
Subjects Committee and subjects gave their written informed
consent prior to participation.
All subjects were asked to walk at their preferred walking
speed through a sand trackway (see Table S3). After making two
passes down the walkway at their preferred speed, subjects were
asked to walk with a bent-knee, bent-hip (BKBH) gait at their
preferred speed. Subjects also repeated the BKBH trial twice.
The trackway was 15 cm deep and5mlong.Itwasfilledwith
mostly fine sand, with coarser grained sand confined to the
beginning and end of the trackway. Grain sizes ranged from
,0.074 mm to 0.7 mm in diameter. After walking at their
preferred speeds, a subset of the sample was asked to walk at a
fast speed using both gaits. A different subset of the sample was
asked to walk in both gaits at their preferred speeds after the
moisture content of the sand was increased by ,4%. Moisture
contentofsandwasmeasuredusingaHydroSense(Campbell
Scientific CD 620) soil moisture system. For low moisture trials,
moisture content was 6–8%. For high moisture trials, moisture
content was 10–12%. To determine moisture content, the
moisture probe was inserted into the footprint after each scan
was taken.
Prior to trackway trials, reflective markers were affixed to the
hip, knee, ankle, heel, hallux, 1
st
metatarsal head, and shoulder.
Subjects were filmed walking through the trackway using a six-
camera Vicon motion analysis system (200 Hz). Maximum,
minimum, and average joint angles were calculated for the hip
and knee during all trials (Table S4).
After each pass through the trackway, footprints were digitized
using a 3-dimensional laser scanner (Microscribe MLX 6DOF
digitizer with attached Micorscan laser sensor system). At least two
prints (left and right) for each trial were scanned. For each scanned
print, four points were selected from the sand around the print:
two points medial and lateral to the toes, and two points medial
and lateral to the heel. A plane representing the sand substrate was
generated based on those four points using a least-squares fit in
Microscan Tools. The 3D coordinates of the print and plane were
then imported into R where the plane and print were reoriented
such that the plane was level. Finally, the depths of the deepest
point in the heel and in the toe were extracted from the leveled
print, and proportional toe depth was calculated as a fraction of
the depth of the heel.
A second sample of subjects (n = 6) were recruited to determine
the kinetics of BKBH walking. Subjects walked over a forceplate
(AMTI; 4000 Hz) and were simultaneously filmed with a six-
camera Vicon motion analysis system (200 Hz). Prior to walking
trials, reflective markers were affixed to the hip, knee, ankle, heel,
hallux, 1
st
metatarsal head and shoulder. Subjects walked at
preferred speeds using normal extended limb bipedalism as well as
two BKBH trials (one with light and one with deep knee and hip
flexion; Table S5).
Center of pressure data were collected from force plate
recordings for each trial. Combing kinematic and kinetic data,
we calculated the time of heel strike (touchdown) and the time
(relative to heel strike) that the COP passed anterior to the
metatarsal heads. We also calculated the total impulse of
thegroundreactionforcefromthetimetheCOPpassedthe
metatarsal heads to toe-off. These values provide a biome-
chanical measure of how toe depths are generated. For
more information on methods see supplementary materials
Text S1.
Figure 2. Toe depths as a fraction of heel depth for BKBH,
normal walking and Laetoli prints. Laetoli values were calculated
using values from topographic maps. Note that the values for Laetoli
fall within the range of human normal prints but outside of the range of
human BKBH prints.
doi:10.1371/journal.pone.0009769.g002
Table 1. Proportional toe depths of human footprints.
Side Speed Moisture Normal SEM BKBH SEM p-value
Right p d 0.27 0.04 0.77 0.12 ,0.001
Left p d 0.19 0.06 0.76 0.12 ,0.001
Both p w 0.48 0.11 0.81 0.12 ,0.05
Right f d 0.98 0.33 0.51 0.12 0.12
Left f d 0.38 0.08 0.67 0.16 0.07
p is preferred walk; f is fast walk; d is dry (6–8% water); w is wet (10–12% water).
Values are toe depths as a fraction of heel depth. P-values are for T-Tests
comparing BKBH and extended (normal) datasets.
doi:10.1371/journal.pone.0009769.t001
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Supporting Information
Text S1 Supporting Text
Found at: doi:10.1371/journal.pone.0009769.s001 (0.02 MB
PDF)
Table S1 Laetoli footprint data
Found at: doi:10.1371/journal.pone.0009769.s002 (0.03 MB
DOC)
Table S2 Subject information
Found at: doi:10.1371/journal.pone.0009769.s003 (0.03 MB
DOC)
Table S3 Sample size information for footprint analyses
Found at: doi:10.1371/journal.pone.0009769.s004 (0.03 MB
DOC)
Table S4 Speed and joint angles for trackway trials. Values are
means (SEM) for all subjects.
Found at: doi:10.1371/journal.pone.0009769.s005 (0.03 MB
DOC)
Table S5 Hip and knee angles for force plate trials.
Found at: doi:10.1371/journal.pone.0009769.s006 (0.03 MB
DOC)
Figure S1 Proximal pressure ridge in a normal human footprint.
Found at: doi:10.1371/journal.pone.0009769.s007 (3.70 MB TIF)
Figure S2 Contour map of a footprint from a normal extended
limb step showing lack of discernable arch (compare with Fig. 1).
Found at: doi:10.1371/journal.pone.0009769.s008 (0.61 MB TIF)
Acknowledgments
We thank William Jungers, Daniel Lieberman, and Herman Pontzer for
helpful discussions of this project. Vija Garcia, Justin Higgins, Alicia Waltz,
Stephanie Reyes, and David Zahn provided assistance during data
collection. We thank Mitchell Pavao-Zuckerman for generously providing
us with equipment to measure soil moisture content. We are grateful to
Lars Fogelin for assistance in trackway construction. We also thank all of
our subjects for their time and effort.
Author Contributions
Conceived and designed the experiments: DR ADG WEHHS. Performed
the experiments: DR ADG. Analyzed the data: DR ADG. Contributed
reagents/materials/analysis tools: ADG WEHHS AF WRHJ. Wrote the
paper: DR ADG WEHHS AF.
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PLoS ONE | www.plosone.org 6 March 2010 | Volume 5 | Issue 3 | e9769