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The relationship between plantar pressure and footprint shape
Journal of Human Evolution 65 (2013) 21e28
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Journal of Human Evolution journal homepage: www.elsevier.com/locate/jhevol
The relationship between plantar pressure and footprint shape Kevin G. Hatala a, b, *, Heather L. Dingwall b, c, Roshna E. Wunderlich d, Brian G. Richmond b, e a
Hominid Paleobiology Doctoral Program, The George Washington University, 2110 G St., NW, Washington, DC 20052, USA Center for the Advanced Study of Hominid Paleobiology, Department of Anthropology, The George Washington University, 2110 G St., NW, Washington, DC 20052, USA c Department of Human Evolutionary Biology, Harvard University, 11 Divinity Avenue, Cambridge, MA 02138, USA d Department of Biology, James Madison University, 951 Carrier Drive, MSC 7801, Harrisonburg, VA 22807, USA e Human Origins Program, National Museum of Natural History, Smithsonian Institution, 10th St. and Constitution Ave., NW, Washington, DC 20560, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 August 2012 Accepted 1 March 2013 Available online 28 May 2013
Fossil footprints preserve the only direct evidence of the external foot morphologies and gaits of extinct hominin taxa. However, their interpretation requires an understanding of the complex interaction among foot anatomy, foot function, and soft sediment mechanics. We applied an experimental approach aimed at understanding how one measure of foot function, the distribution of plantar pressure, influences footprint topography. Thirty-eight habitually unshod and minimally shod Daasanach individuals (19 male, 19 female) walked across a pressure pad and produced footprints in sediment directly excavated from the geological layer that preserves 1.5 Ma fossil footprints at Ileret, Kenya. Calibrated pressure data were collected and threedimensional models of all footprints were produced using photogrammetry. We found significant correlations (Spearman’s rank, p < 0.0001) between measurements of plantar pressure distribution and relative footprint depths at ten anatomical regions across the foot. Furthermore, plantar pressure distributions followed a pattern similar to footprint topography, with areas of higher pressure tending to leave deeper impressions. This differs from the results of experimental studies performed in different types of sediment, supporting the hypothesis that sediment type influences the relationship between plantar pressure and footprint topography. Our results also lend support to previous interpretations that the shapes of the Ileret footprints preserve evidence of a medial transfer of plantar pressure during late stance phase, as seen in modern humans. However, the weakness of the correlations indicates that much of the variation in relative depths within footprints is not explained by pressure distributions under the foot when walking on firm ground, using the methods applied here. This warrants caution when interpreting the unique foot anatomies and foot functions of extinct hominins evidenced by their footprint structures. Further research is necessary to clarify how anatomical, functional, and sedimentary variables influence footprint formation and how each can be inferred from footprint morphology. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Biomechanics Foot function Locomotion Ileret Human evolution
Introduction Fossil footprints are invaluable discoveries in paleoanthropology, as they contain anatomical and biomechanical information that cannot be obtained from fossil bones. A footprint provides not only an image of the surface anatomy of the foot but also a direct snapshot of foot function during locomotion. Thus, fossil footprints can provide unique anatomical and functional data to inform hypotheses about the evolution of hominin locomotion.
* Corresponding author. E-mail address: [email protected] (K.G. Hatala). 0047-2484/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jhevol.2013.03.009
One well-known set of fossil hominin footprints was discovered at Laetoli, Tanzania, in 1978 (Leakey and Hay, 1979). At the time of their discovery, the Laetoli footprints pushed back the antiquity of the earliest evidence for bipedalism in the hominin clade from about 3.0 to 3.7 Ma (million years ago) (White,1980). They provided critical evidence that early hominins, presumably of the genus Australopithecus (but see Tuttle et al.,1991), walked bipedally on the ground despite the retention of climbing adaptations in their upper limbs. Analyses of the Laetoli footprints have often included comparisons with actual footprints produced by modern humans or other great apes. Some have argued that modern human footprints are indistinguishable from those at Laetoli (e.g., Day and Wickens, 1980; White and Suwa, 1987; Tuttle et al.,1990), yet others have concluded that the Laetoli prints could not have been produced by a modern
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humanlike foot anatomy and gait (e.g., Stern and Susman, 1983; Susman et al., 1984; Deloison, 1991; Meldrum, 2004). More recently, fossilized hominin footprints from the early Pleistocene (1.51e1.53 Ma) were discovered at the site of Ileret, Kenya (Bennett et al., 2009), and continue to be unearthed (Richmond et al., 2010). Preliminary analyses have suggested a foot anatomy and gait that bear closer resemblance to those of modern humans than to those that created the Laetoli footprints. The Ileret footprints have been described as showing evidence of a humanlike longitudinal arch structure and also a humanlike distribution of pressure beneath the foot, including a transfer of pressure to the medial side of the foot prior to toe-off (Bennett et al., 2009). These footprints were interpreted as supportive of the hypothesis that by 1.5 Ma, at least one hominin taxon had undergone an adaptive shift that led to a postcranial body form similar to that of modern humans (Wood and Collard, 1999; McHenry and Coffing, 2000; Bramble and Lieberman, 2004). In their analyses, Bennett et al. (2009) also suggested that the Laetoli footprints represent a foot anatomy and gait different from, and more primitive than, those of modern humans, while others have more recently argued that the Laetoli footprints represent gait essentially equivalent to that of modern humans (e.g., Raichlen et al., 2010; Crompton et al., 2012). The accurate interpretation of human anatomy and gait from footprint morphology requires data on the relationships between footprint morphology and specific anatomical and functional variables. An understanding of how biomechanical variables influence footprint formation will allow us to interpret footprints that were made by early hominin feet for which we have no exact modern anatomical and/or functional analog (Richmond et al., 2012). Such an approach has proven fruitful in the study of dinosaur trackways. Through experimental analyses, Gatesy et al. (1999) developed a quantitative understanding of the ways in which the foot anatomy and hindlimb movements of extant avian taxa are preserved in the three-dimensional shapes of their footprints. The authors then used those data to analyze the unique shapes of fossilized footprints produced by theropod dinosaurs and draw inferences regarding their anatomy and locomotion. A number of studies have taken different approaches to infer aspects of human gait from footprint shape. Some have compared experimentally produced human footprints with fossil hominin footprints, and used those comparisons to draw conclusions about similarities or differences in gait (e.g., Day and Wickens, 1980; Charteris et al., 1981; Tuttle et al., 1990; Raichlen et al., 2010). Others have used records of plantar pressure distribution in human and nonhuman primates as hypothetical footprint shapes, and compared those with the morphologies of fossil hominin footprints (Crompton et al., 2012). However, we have lacked data on how such functional variables are actually recorded in footprints. Taking the first step in this direction, D’Août et al. (2010) conducted the first systematic experimental analysis of how dynamic plantar pressure is recorded in the morphology of footprints themselves. They found a positive correlation between plantar pressure and footprint depth when pressure was measured through an overlying layer of sand. However, they found no significant correlations between the morphology of footprints made in sand and plantar pressure measured directly at the interface between the foot and the pressure pad. In other words, the footprint shape made in sand differed from the ‘normal’ plantar pressure patterns when walking on firm ground, raising significant doubts about the reliability of inferences that have been drawn regarding ‘normal’ foot function and gait in the fossil hominins who made the Laetoli, or other, fossil footprints. The complexity of the results obtained by D’Août et al. (2010) highlighted the need to understand exactly how plantar pressure influences footprint morphology in soft sediments. Ultimately, fossilized footprints will be most useful to paleoanthropologists if
their analysis can be applied broadly to develop and test general hypotheses about the evolution of human locomotion, rather than simply comparing the ways in which different hominins coped with locomotion in pliable substrates. For example, efforts to interpret the Laetoli footprints have focused on whether or not footprint shape represented a humanlike manner of walking, rather than what gait was like when early hominins walked through mud. In order to test hypotheses about a species’ normal gait pattern, we must know whether or not footprinted sediments preserve accurate records of the way the foot functions on firmer ground. Most, if not all, modern biomechanical analyses that attempt to define ‘human foot function’ are conducted on solid surfaces, in part due to the nature of the available pedobarographic equipment. If a footprint in sediment preserves no discernible record of foot function as observed on firmer surfaces, then one can only draw limited inferences about the specific ways in which hominins moved on soft substrates. In this paper, we present an experimental approach for understanding how the distribution of plantar pressure is recorded in footprints produced by habitually unshod modern humans in the same sediments that preserve 1.5 Ma fossil hominin footprints. In the only other study to assess the relationship between plantar pressure and footprint depth quantitatively, D’Août et al. (2010) acknowledged that sediment mechanics almost certainly play a critical, highly variable role in determining the relationship between foot mechanical variables and footprint topography. Those authors, and several before them, studied footprints that were made in wet sand, which differs typologically from the wet ash that preserved the Laetoli footprints (Leakey and Hay, 1979), and from the clay- and silt-containing sediments that preserved the 1.5 Ma fossilized hominin footprints at Ileret, Kenya (Bennett et al., 2009). Importantly, silt and clay are, by definition, of a finer grain than sand (Garcia, 2008). Because cohesion increases as particle size decreases (Garcia, 2008), the mechanics of sediment deformation, and thus record of foot function preserved in footprint structure, may differ across these sediment types. During the process of footprint formation, the foot subjects the sediment to a combination of elastic and plastic (elasticeplastic) deformation (Allen, 1997). In the elastic phase, the sediment resists deformation but once its yield stress has been reached and the foot punches through the most superficial layer of the sediment, plastic deformation occurs and results in an impression. The elastic behaviors of clays and sands differ because values of Young’s modulus are highly variable throughout sand when pressure is applied to it, yet Young’s modulus for clays remains constant or only changes a negligible amount (Craig, 1992). As a result, vertical elastic forces are transmitted through these sediments in different ways. Clays provide uniform resistance to vertical loads across the foot during the elastic phase (which would also be the case when walking on a perfectly solid surface), while the resistance provided by sand can vary across different regions of the foot. As a result, the response of sand to the distributed load will likely affect downstream results in how closely a footprint’s shape matches the distribution of vertical forces, if the sand’s response to those forces varies between regions of the foot. These differences in sediment properties suggest that the topography of footprints in clay-containing sediments, such as those in which the Ileret fossil footprints were preserved, may reflect more directly the distribution of vertical forces (plantar pressure) during gait than footprints that are made in sand. Furthermore, the greater cohesion of finer-grained sediments, such as those from Ileret, suggests that they are more likely than sands to preserve a shape that reflects the transmission of forces during the plastic deformative process as they are less likely to undergo infilling or other post-deformational shape changes.
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In the experimental work presented in this study, subjects produced footprints in sediment taken directly from layers in which the 1.5 Ma fossil footprints were formed at Ileret, Kenya (Bennett et al., 2009; Behrensmeyer, 2011). By conducting our experiments in this sediment, and making our best attempt to recreate the sedimentary conditions that existed at 1.5 Ma, we directly test the hypothesis that the shape of footprints produced in the Ileret fossil footprint sediment preserves detectable records of the ‘normal’ distribution of plantar pressure as measured on a firm surface. By comparing our results to those of D’Août et al. (2010) from their experiments in sand, we were also able to test the hypothesis that sediment type influences the relationship between plantar pressure and footprint topography. Materials and methods Experimental subjects Data were collected from 38 habitually unshod or minimally shod adults (19 men, 19 women) from the Daasanach tribe, living in and around the town of Ileret, Kenya. All subjects had either never worn any footwear or had not worn footwear until adulthood, after their feet had fully developed. Given that unshod human feet can develop differently from shod feet in both anatomical and functional respects (Hoffmann, 1905; Wells, 1931; Sim-Fook and Hodgson, 1958; Barnett, 1962; Ashizawa et al., 1997; D’Aout et al., 2009), this experimental sample is close to ‘ideal’ for understanding fossil footprints that pre-date the earliest evidence of footwear in the fossil record (Trinkaus and Shang, 2008). Subjects were recruited, and provided their informed consent, in accordance with the policies of The George Washington University’s Institutional Review Board (#031030). Experimental setup The experiments were conducted on a large, flat, open space adjacent to the site of fossil footprint excavations at Ileret, Kenya. A 1-m RSscan International Footscan pressure pad (RSscan International, Olen, Belgium) was placed on the ground and leveled. One meter from the end of the pressure pad, a pit was dug that measured 150 cm long, 50 cm wide, and 15 cm deep. This pit was filled with sediment excavated directly from a fossil layer containing hominin footprints (Fig. 1). In 2010, A1 layer sediment was used, and Lower Layer sediment was used in 2011 (due to the availability of a necessary volume that did not include any fossil footprints). These layers are typologically very similar, as they were formed from the same source sediments (Bennett et al., 2009; Behrensmeyer, 2011), and there is no reason to suspect that their mechanical properties would be substantially different from each other. There are currently no methods for calculating the exact saturation and/or compaction levels of the sediment when the fossil footprints were formed, so we acknowledge that these
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variables may have differed between the sediments used in our experiments and those that existed at 1.5 Ma. However, we made our best attempt to cover the range of sedimentary conditions that could have existed when the fossilized prints were formed. We varied the amount of water added, as well as the degree of sediment compaction, and asked subjects to create ‘test’ footprints until the overall depths of the test prints were consistent with the range of depths observed in the fossil footprints. Naturally, saturation and compaction levels varied to a small degree between trials and between subjects. However, because there is similar variation in the depths of the fossil trackways, a sample of experimental prints that spanned the range of depths seen in the fossil footprint layers should include the sedimentary conditions under which the fossil prints were formed. Until methods are developed to determine saturation and compaction levels from fossilized sediments, such approximations will be necessary. A video camera positioned 8 m from (and perpendicular to) the trackway was used to record all trials (recorded at 60 Hz in 2010 and 210 Hz in 2011). Distance was calibrated using stakes positioned 3 m apart, spanning the pressure mat and the sediment patch.
Experimental protocol Biometric measurements were taken from each subject, including stature, weight, functional leg length (height at the level of the greater trochanter of the femur while standing), and foot length. Each subject performed at least six walking trials, three while maintaining their comfortable walking pace (mean speed ¼ 1.43 m/s, SD ¼ 0.23, mean Froude number ¼ 0.23, SD ¼ 0.07) and three at a fast pace (mean speed ¼ 1.86 m/s, SD ¼ 0.25, mean Froude number ¼ 0.39, SD ¼ 0.10). These trials consisted of walking over the pressure pad and through the sediment patch. In 2010, the first twenty subjects conducted pressure and footprint trials separately, with pressure data collected in one trial and footprint data collected in a subsequent trial (thus requiring six trials at each walking speed). During the 2011 field season, the next eighteen subjects walked on the pressure pad and made footprints within the same trial (requiring only three trials at each walking speed). Start and finish markers were placed approximately 5 m from the beginning of the experimental apparatus and 5 m from the end. Allocating this distance, for a few steps to be taken both before and after the calibrated space in which measurements were taken, minimized the chance that acceleration or deceleration would occur while the subjects were traveling over the experimental trackway. Subjects were asked to repeat trials if they adjusted their pace along the trackway, or if they visibly altered their gait in order to target the pressure pad or sediment patch. All trials were recorded with 2-dimensional video so that they could later be digitized to quantify speed (see below). This is important because speed has been reported to be significantly correlated with pressure data such as peak pressure and pressuree
Figure 1. Schematic drawing showing an overhead view of the experimental trackway. Start line is about 5 m before the front edge of the pressure pad. Pressure pad is 1 m long and its end is 1 m prior to the sediment pit. The sediment pit is 1.5 m long and the finish line is about 5.0 m from the end of the sediment pit. Drawing is not to scale.
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Figure 2. Image from RSscan Footscan 7 software, showing the ten regions of the foot from which plantar pressure measurements were taken. Each rectangle is 7.62 mm 5.08 mm.
impulse (e.g., Vereecke et al., 2005; Pataky et al., 2008; but see; Kivell et al., 2010; Patel and Wunderlich, 2010). Pressure data were collected within RSscan International Footscan 7 software (RSscan International, Olen, Belgium). Following completion of each trial, the associated footprint(s) produced in the sediment trackway was/ were photographed at multiple camera positions and angles such that 3-dimensional models could later be produced using photogrammetric software (see below). Experimental data processing To quantify velocity for each walking trial, 2-dimensional videos from 2010 (recorded at 60 Hz) were digitized using Peak Motus software (Peak Performance Technologies, Inc., Englewood, CO, USA). Videos from 2011 were digitized using ImageJ (Rasband, 1997e2012) because Peak Motus could not support high-speed (210 Hz) video. For 48 videos, speed was measured using both techniques and a Wilcoxon signed-rank test revealed no significant differences between measurements obtained from the two programs (p ¼ 0.327). Velocity was measured as the time it took for a digitized marker on the sternum to travel a horizontal distance of 3 m, calibrated from the stakes in the experimental setup. The sternum was consistently visible throughout the experiments and horizontal translational movements of the trunk are typically small (<3 cm) during walking (Thorstensson et al., 1984; Stokes et al., 1989). Pressure data were extracted from 10 anatomical subdivisions across the foot (medial heel, lateral heel, lateral midfoot, metatarsal
heads 1e5, hallux, and second toe) (Fig. 2). These subdivisions represent the areas of the plantar surface of the foot that are typically in contact with the ground during walking. Pressure measurements were extracted from a 7.62 mm 5.08 mm rectangular sampling region placed in the center of each respective anatomical region. These regions could be easily identified because each registers a pressure point visually distinct from immediately surrounding areas. Measurements of maximum pressure and pressureeimpulse (the pressureetime integral, defined as the area beneath the curve of pressure over time) were taken from each of these regions for each trial. It has often been assumed that fossil hominin footprint morphology in some way represents foot kinetics (e.g., Day and Wickens, 1980; Berge et al., 2006; Bennett et al., 2009) and D’Août et al. (2010) found that both maximum pressure and pressureeimpulse are influential in determining footprint depths in certain regions beneath the foot. While maximum pressure describes the peak normal force applied in a certain area, pressureeimpulse also characterizes the time over which a load is applied, which may be particularly important when considering sediments that deform in an elasticeplastic manner. Photographs of footprints were used to generate 3-dimensional virtual models using PhotoModeler Scanner 2011 software (Eos Systems, Inc., Vancouver, BC, Canada; Fig. 3). This software matches identical pixel values across sets of photographs and, knowing the parameters of the camera and lens due to a calibration procedure, triangulates camera positions and calculates depth to render threedimensional models from those photos. Each model included not only the footprint, but also about 5e10 cm of undisturbed sediment around the perimeter of the print, so that it could later be oriented for measurement. These 3-dimensional models were then exported to Geomagic Qualify 2012 software (Geomagic, Inc., Research Triangle Park, NC, USA) for further analysis. Twenty evenly spaced points were randomly selected on the undisturbed sediment around the perimeter of each print, and best-fit planes were aligned to those points using a least-squares algorithm. This ‘ground’ plane was then configured to the XeY plane in coordinate space, such that the z-axis would directly represent depth. Footprint depth measurements were taken at each of the 10 anatomical regions from which pressure data were collected. For each region, five points were selected that represented the approximate locations of the corners and center of the 7.62 mm 5.08 mm sampling window from which pressure measurements were taken (Fig. 2). The measure of depth for that region of the footprint was then calculated as the centroid of those five points. This process was repeated for all 10 anatomical regions from which pressure measurements were taken. Analyzing the relationship between pressure and footprint topography All regional measurements of maximum pressure, pressuree impulse, and footprint depth (z-coordinate values) were
Figure 3. Example of a three-dimensional model of a footprint, created using PhotoModeler software.
K.G. Hatala et al. / Journal of Human Evolution 65 (2013) 21e28
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Standardized footprint depth Figure 4. Scatter plot of regional standardized maximum plantar pressure and corresponding regional standardized footprint depth. Average values across all anatomical regions, for both fast and normal walking speeds, are plotted for all subjects. The correlation between these variables was significant (r ¼ 0.3796, p < 0.0001).
standardized by dividing each individual measurement by the maximum of all measurements across the foot for each trial. For example, the overall maximum pressure measurement was taken from measures of maximum pressures within the regions of the hallux, second toe, metatarsal heads, lateral midfoot, and heel. Each of the original measurements was then divided by the overall maximum pressure measurement for a given trial. The same procedure was followed for measurements of pressureeimpulse and also footprint depth. Doing so allowed us to compare the distribution of pressure across the foot during walking to the ‘distribution of depth’ across a footprint. We favored this approach over a direct comparison of absolute pressure or depth measurements because some individuals may have absolutely higher plantar pressures or make absolutely deeper footprints than others due to variables not directly relevant to foot function, such as body mass or soft tissue thickness. Measurements were averaged within each speed category for each subject (i.e., one average pressure distribution represented the average of one subject walking at one categorical speed). Although this procedure may reduce variation within an individual subject, it is also improbable that a step on the pressure pad and a step creating a footprint, even in the same sequence, would be identical. As a result, we chose to compare a subject’s average pressure distribution at a given speed to their average footprint topography at that same speed. Spearman’s rank correlation was used to determine the relationship between standardized measurements of pressure and pressureeimpulse across the plantar surface of feet and standardized measurements of depth across footprints. Additional tests were conducted, using Spearman’s rank correlation, to determine relationships between plantar pressure and footprint depth at different walking speeds, and within various anatomical regions of the foot. The correlations between plantar pressure and footprint depth were then calculated within each individual subject, to determine how many showed a significant relationship between these two variables.
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Standardized footprint depth Figure 5. Scatter plot of regional standardized pressureeimpulses and corresponding regional standardized footprint depths. Average values across all anatomical regions, for both fast and normal walking speeds, are plotted for all subjects. The correlation between these variables was significant (r ¼ 0.2527, p < 0.0001) but weaker than the correlation between regional maximum pressures and regional footprint depths (shown in Fig. 4).
pressure measurements and average standardized footprint depths across all subjects at all speeds) revealed that the two were significantly correlated. Regional maximum pressure explained a greater amount of variance in footprint depth (Spearman’s r ¼ 0.3796) than did regional pressureeimpulse (r ¼ 0.2527), although both measures of plantar pressure were significantly correlated with measures of print depth (p < 0.0001 in both cases; Figs. 4 and 5). Since some aspects of data collection changed between field seasons (e.g., collecting pressure and footprint data within the same versus separate trials), data from the two seasons were also analyzed separately. In doing so, we found that the correlation between pressure and footprint depth was consistent between both procedures for data collection (Year 1 maximum pressure to depth r ¼ 0.4510, Year 2 r ¼ 0.3693, p < 0.0001 for both; Year 1 pressureeimpulse to depth r ¼ 0.3217, Year 2 r ¼ 0.2146, p < 0.0001 for both). Therefore, we pooled the data for all further analyses. When data collected at each walking speed were analyzed separately, we found that the relationship between plantar pressure and footprint depth changed with walking speed. A stronger relationship between maximum pressure and depth existed at fast walking speeds (r ¼ 0.4527) than at normal speeds (r ¼ 0.2924) but both relationships were significant (p < 0.0001; Table 1). The same was true of the correlations between pressureeimpulse and depth at fast (r ¼ 0.3413, p < 0.0001) and normal (r ¼ 0.1691, p ¼ 0.0009) walking speeds. Relationships between plantar pressure and footprint depth differed across the various anatomical regions. Maximum pressure and footprint depth were significantly correlated (p < 0.05) at the Table 1 Speed-specific correlations between maximum pressure and pressureeimpulse and footprint depth, over the entire foot and footprint.
Results
Relationship
Walking speed
Spearman’s r
p-Value
Correlations between plantar pressure and footprint depth
Maximum pressure e footprint depth Maximum pressure e footprint depth Pressureeimpulse e footprint depth Pressureeimpulse e footprint depth
Normal Fast Normal Fast
0.2924 0.4527 0.1691 0.3413
<0.0001 <0.0001 0.0009 <0.0001
The cumulative assessment of the correlation between pressure and depth (the correlation between the average standardized
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Table 2 Region-specific correlations between maximum pressure and footprint depth, and pressureeimpulse and footprint depth (* denotes significance, p < 0.05). Region Medial heel Lateral heel Lateral midfoot Metatarsal 1 Metatarsal 2 Metatarsal 3 Metatarsal 4 Metatarsal 5 Hallux Second digit
Table 3 Descriptive statistics of mean standardized plantar pressure and footprint depths, presented in Fig. 5.
Maximum pressure and depth (r)
p-Value
Pressureeimpulse and depth (r)
p-Value
Region
0.0168 0.0159 0.1030 0.1799 0.3208 0.2496 0.0761 0.0181 0.1976 0.0382
0.8854 0.8912 0.3758 0.1199 0.0047* 0.0297* 0.5133 0.8765 0.0871 0.7431
0.0170 0.0548 0.1521 0.2789 0.4284 0.3934 0.1045 0.0487 0.2596 0.0792
0.8840 0.6381 0.1896 0.0147* 0.0001* 0.0004* 0.3689 0.6760 0.0235* 0.4966
Medial heel Lateral heel Lateral midfoot Metatarsal 1 Metatarsal 2 Metatarsal 3 Metatarsal 4 Metatarsal 5 Hallux Second digit
Mean standardized maximum pressure (SD) 0.811 0.825 0.183 0.470 0.538 0.587 0.481 0.478 0.676 0.390
(0.136) (0.114) (0.148) (0.187) (0.163) (0.183) (0.164) (0.219) (0.195) (0.182)
Mean standardized footprint depth (SD) 0.915 0.922 0.786 0.858 0.850 0.847 0.829 0.804 0.926 0.894
(0.068) (0.071) (0.096) (0.071) (0.080) (0.081) (0.083) (0.082) (0.052) (0.056)
In each trial, individual regional measurements were standardized by dividing each one by the overall maximum (across all regions) for that trial.
second and third metatarsal heads, but not at any other regions (Table 2). Correlations between pressureeimpulse and depth were significant (p < 0.05) at the hallux and the first, second, and third metatarsal heads. Further analyses examined the correlations between pressure and depth within each subject independently, with all anatomical regions pooled. Significant correlations were found between maximum pressure and depth within 23 out of the 38 subjects. Correlations between impulse and print depth were only found within 10 of the 38 subjects. Distribution patterns of plantar pressure and footprint depth While the correlations between regional standardized maximum plantar pressure and regional standardized footprint depth were highly significant, these relationships were weak. However, we found that certain aspects of the distribution patterns of maximum plantar pressure and footprint depth were actually quite similar to each other (Fig. 6, Table 3). Maximum pressures and footprint depths were both greatest at the heel and hallux. The lateral midfoot experienced the lowest maximum plantar pressures and also left the shallowest impressions. The metatarsal heads
occupied an intermediate position for both pressures and footprint depths. The second toe was the only exception, leaving a relatively deep impression despite experiencing lower pressures.
Discussion The results of this study establish that barefoot plantar pressure is related to the topography of footprints created in the sediment that preserves the 1.5 Ma fossil footprints at Ileret. However, the fact that the correlation between plantar pressure and footprint depth was weak highlights the complexity of the footprint formation process, along with the need to continue exploring (1) the ways in which specific anatomical and functional variables can be preserved in footprints, and (2) the mechanisms by which human foot function can be measured in soft substrates. It is very likely that other anatomical and/or functional variables that were not measured in this study contributed to the large amount of unexplained variation in footprint shape. For example, the bony architecture of the foot and kinematic factors such as joint angles are among many variables that probably directly influence footprint
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Hallux
Lateral midfoot
Metatarsal 5
Metatarsal 4
Metatarsal 3
Metatarsal 2
Metatarsal 1
Lateral heel
Medial heel
0.0 0.5
Figure 6. Regional standardized maximum plantar pressures (white, left axis) and standardized footprint depths (gray, right axis) across 10 anatomical regions of the foot, for all subjects walking at both normal and fast speeds. Lines correspond to median values, boxes represent interquartile range (25e75%), whiskers represent non-outlier range.
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morphology, and indirectly affect the relationship between plantar pressure and footprint depth. Furthermore, previous studies have found disparities between the bony structure of the foot and patterns of foot function, suggesting that the foot’s soft tissue anatomy plays a significant role in both the distribution and application of forces between the foot and the substrate (e.g., Griffin and Richmond, 2005). Therefore, soft tissue anatomy and its functions likely also affect footprint shape in ways that were not measured in this study. Still, the results of this study can give us new insights into interpretations of fossil hominin footprints. Bennett et al. (2009) hypothesized that the Ileret footprints preserve evidence of foot anatomy and function essentially similar to those seen in modern humans. Those authors pointed to the depth of the Ileret footprints in the hallux and medial forefoot as providing evidence for the medial transfer of pressure that typifies modern human locomotion. In the present study, the correlation between maximum pressure and depth was not significant at the first metatarsal head but the correlation between pressureeimpulse and depth within this region was, in fact, significant (Table 2). Furthermore, despite the fact that, in this study, maximum pressure was not typically highest beneath the first metatarsal head compared with the others, footprints were typically deeper in this region than they were beneath any of the other metatarsals (Fig. 6). This suggests that the Ileret footprints, being deeper under the first compared with the other metatarsal heads, have shapes that are generally similar to the footprints of modern humans produced in the same sediment. In sum, these experimental results do lend some support to the functional hypotheses of Bennett et al.’s (2009) interpretation, inferring modern humanlike aspects of foot anatomy and gait from the 1.5 Ma footprints. However, our results also reveal that variables other than the distribution of plantar pressure explain more than 80% of the overall variation in footprint topography. In light of these findings, it is necessary to proceed with caution in drawing direct interpretations of foot function from footprint topography. While we have shown here that one variable related to foot function is significantly correlated with footprint topography, this correlation is weaker than might be expected and underscores the fact that many variables, not measured in this study, simultaneously influence the dynamic deformative process through which footprints are created. Future research would benefit from a multivariate approach, to gain a full appreciation of the many factors related to foot anatomy, foot function, gait, and sediment properties, which simultaneously influence footprint shape. While we did not directly quantify mechanical properties of the sediment used in this study, it seems likely that the mechanistic differences between sand and the sediment from Ileret might explain why the Ileret sediments preserve footprints whose topography is correlated to the distribution of plantar pressure at the footesubstrate interface, but sand does not. As mentioned previously, the mechanical behavior of the fine-grained clay- and silt-containing sediments found in the Ileret footprint layers likely differed from that of the coarser-grained sand used in D’Août et al.’s (2010) experiments. First, the elastic behavior of sand, unlike clay, is not uniform within a given sample. As a result, its mechanics of deformation may actually differ across various regions of a footprint. Second, the finer grain size of the Ileret footprint layer sediment makes it more cohesive (Garcia, 2008) and therefore more likely to retain the shape produced by the deformative process. Sand, on the other hand, is less cohesive, leaving the deepest points of a footprint vulnerable to some degree of infill after their initial deformation. This means that a footprint in sand may be less likely to preserve the pattern in which it was plastically deformed. No methods currently exist for quantifying the relationships between sediment mechanical properties and kinetic variables
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such as plantar pressure. For example, there are no available means for adjusting experimental results obtained in sand so that they would reflect the same relationship between plantar pressure and footprint topography if the experiments were performed in mud. This is unfortunate for the interpretation of fossilized footprints because we may not be able to apply experimental results obtained using commonly-available sediments. Rather, until methods are developed for thoroughly quantifying the relationships among sediment mechanical properties, footprint topography and kinetic variables, the most viable approach may be to perform experiments in sediments that directly represent the conditions in which fossil hominin footprints were formed. Fine-grained sand is commonly used in experimentally-based analyses of the Laetoli footprints (e.g., Day and Wickens, 1980; Raichlen et al., 2010; Crompton et al., 2012) based upon similarity in grain size to the Laetoli ash. Given the results of our study, before drawing direct conclusions about kinesiological variables derived from Laetoli footprint topography, it may be important to assess quantitatively the mechanical behavior of sand in order to determine whether or not it can serve as an accurate mechanical substitute for the phonolitic ash that preserves the Laetoli footprints. Conclusions We conclude that the distribution of plantar pressure is correlated with the topography of footprints made in the sediment that preserves 1.5 Ma fossil hominin footprints at Ileret, Kenya. Furthermore, the relationship between plantar pressure and footprint morphology is not constant between different sediment types and likely varies according to the sediment’s mechanical properties. While the results of this study begin to shed light on how specific aspects of foot function are, or are not, preserved within footprint morphology, they also demonstrate that many questions remain unanswered about how we can accurately interpret footprints in the human fossil record. The correlation between the distribution of plantar pressure and footprint topography is not strong and the process of footprint formation is clearly a multivariate scenario in which anatomical, functional, and sedimentary variables simultaneously influence footprint shape. It will be necessary to gain an understanding of how specific variables influence footprint morphology, both independently and together, before we can develop accurate functional interpretations of fossil hominin footprints, particularly those made by extinct taxa whose foot anatomy and function almost certainly differ from those of extant humans and apes. Only within this framework can we use these unique sources of data to better inform hypotheses regarding the evolution of human foot anatomy and locomotion. Acknowledgments We thank David Green, David Braun, Jack Harris, Purity Kiura, Emmanuel Ndiema, the Koobi Fora Field School, the National Museums of Kenya, the town of Ileret, Kenya, and our Daasanach subjects for their contributions to this project. We also thank three anonymous reviewers who provided insightful and constructive comments and advice during the preparation of this manuscript. This study was funded by the National Science Foundation, grants BCS-0924476, BCS-1128170, and DGE-0801634. References Allen, J.R.L., 1997. Subfossil mammalian tracks (Flandrian) in the Severn Estuary, S.W. Britain: mechanics of formation, preservation and distribution. Phil. Trans. R. Soc. B 352, 481e518.
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Ashizawa, K., Kumakura, C., Kusumoto, A., Narasaki, S., 1997. Relative foot size and shape to general body size in Javanese, Filipinas and Japanese with special reference to habitual footwear types. Annls. Hum. Biol. 24, 117e129. Barnett, C.H., 1962. The normal orientation of the human hallux and the effect of footwear. J. Anat. 96, 489e494. Behrensmeyer, A.K., 2011. Conversations with Glynn’s ghost: the evolution of paleolandscape research at East Turkana. In: Sept, J., Pilbeam, D. (Eds.), Casting the Net Wide: Papers in Honor of Glynn Isaac and His Approach to Human Origins Research. Oxbow Books, Oxford, pp. 21e39. Bennett, M.R., Harris, J.W.K., Richmond, B.G., Braun, D.R., Mbua, E., Kiura, P., Olago, D., Kibunjia, M., Omuombo, C., Behrensmeyer, A.K., Huddart, D., Gonzalez, S., 2009. Early hominin foot morphology based on 1.5-million-yearold footprints from Ileret, Kenya. Science 323, 1197e1201. Berge, C., Penin, X., Pellé, É., 2006. New interpretation of Laetoli footprints using an experimental approach and procrustes analysis: preliminary results. C. R. Palevol 5, 561e569. Bramble, D.M., Lieberman, D.E., 2004. Endurance running and the evolution of Homo. Nature 432, 345e352. Charteris, J., Wall, J.C., Nottrodt, J.W., 1981. Functional reconstruction of gait from the Pliocene hominid footprints at Laetoli, northern Tanzania. Nature 290, 496e498. Craig, R.F., 1992. Soil Mechanics, fifth ed. Chapman & Hall, London. Crompton, R.H., Pataky, T.C., Savage, R., D’Août, K., Bennett, M.R., Day, M.H., Bates, K., Morse, S., Sellers, W.I., 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, 707e719. Day, M.H., Wickens, E.H., 1980. Laetoli Pliocene hominid footprints and bipedalism. Nature 286, 385e387. Deloison, Y., 1991. Les australopitheques marchaient-ils comme nous? In: Coppens, Y., Senut, B. (Eds.), Origine(s) de la Bipedie Chez les Hominides. Editions du CNRS, Paris, pp. 177e186. D’Aout, K., Pataky, T.C., De Clercq, D., Aerts, P., 2009. The effects of habitual footwear use: foot shape and function in native barefoot walkers. Footwear Sci. 1, 81e94. D’Août, K., Meert, L., Van Gheluwe, B., De Clercq, D., Aerts, P., 2010. Experimentally generated footprints in sand: analysis and consequences for the interpretation of fossil and forensic footprints. Am. J. Phys. Anthropol. 141, 515e525. Garcia, M.H., 2008. Sedimentation Engineering: Processes, Measurements, Modeling, and Practice. American Society of Civil Engineers, Reston. Gatesy, S.M., Middleton, K.M., Jenkins Jr., F.A., Shubin, N.H., 1999. Three-dimensional preservation of foot movements in Triassic theropod dinosaurs. Nature 399, 141e144. Griffin, N.L., Richmond, B.G., 2005. Cross-sectional geometry of the human forefoot. Bone 37, 253e260. Hoffmann, P., 1905. Conclusions drawn from a comparative study of the feet of barefooted and shoe-wearing peoples. Am. J. Orthoped. Surg. 3, 105e136. Kivell, T.L., Schmitt, D., Wunderlich, R.E., 2010. Hand and foot pressures in the ayeaye (Daubentonia madagascariensis) reveal novel biomechanical trade-offs required for walking on gracile digits. J. Exp. Biol. 213, 1549e1557. Leakey, M.D., Hay, R.L., 1979. Pliocene footprints in the Laetolil Beds at Laetoli, northern Tanzania. Nature 278, 317e323.
McHenry, H.M., Coffing, K., 2000. Australopithecus to Homo: transformations in body and mind. A. Rev. Anthropol. 29, 125e146. Meldrum, D.J., 2004. Midfoot flexibility, fossil footprints, and sasquatch steps: new perspectives on the evolution of bipedalism. J. Sci. Explor. 18, 65e79. Pataky, T.C., Caravaggi, P., Savage, R., Parker, D., Goulermas, J.Y., Sellers, W.I., Crompton, R.H., 2008. New insights into the plantar pressure correlates of walking speed using pedobarographic statistical parametric mapping (pSPM). J. Biomech. 41, 1987e1994. Patel, B.A., Wunderlich, R.E., 2010. Dynamic pressure patterns in the hands of olive baboons (Papio anubis) during terrestrial locomotion: implications for cercopithecoid primate hand morphology. Anat. Rec. 293, 710e718. Raichlen, D.A., Gordon, A.D., Harcourt-Smith, W.E.H., Foster, A.D., Haas, W.R., 2010. Laetoli footprints preserve earliest direct evidence of human-like bipedal biomechanics. PLoS One 5, e9769. Rasband, W.S., 1997e2012. ImageJ. U. S. National Institutes of Health, Bethesda, Maryland, USA. http://imagej.nih.gov/ij/. Richmond, B.G., Bennett, M.R., Harris, J.W.K., Behrensmeyer, A.K., Braun, D.R., Carnation, S., Chirchir, H., Green, D.J., Kiura, P., Mbua, E., 2010. The anatomy of footprints from Koobi Fora, Kenya. Am. J. Phys. Anthropol. (S50), 197. Richmond, B.G., Hatala, K.G., Dingwall, H.L., Wunderlich, R.E., 2012. Using modern taxa to understand biomechanical variables: interpreting function from fossil footprints. Am. J. Phys. Anthropol. (S54), 249. Sim-Fook, L., Hodgson, A.R., 1958. A comparison of foot forms among the non-shoe and shoe-wearing Chinese population. J. Bone Jt. Surg. 40A, 1058e1062. Stern, J.T., Susman, R.L., 1983. The locomotor anatomy of Australopithecus afarensis. Am. J. Phys. Anthropol. 60, 279e317. Stokes, V.P., Andersson, C., Forssberg, H., 1989. Rotational and translational movement features of the pelvis and thorax during adult human locomotion. J. Biomech. 22, 43e50. Susman, R.L., Stern, J.T., Jungers, W.L., 1984. Arboreality and bipedality in the Hadar hominids. Folia Primatol. 43, 283e306. Thorstensson, A., Nilsson, J., Carlson, H., Zomlefer, M.R., 1984. Trunk movements in human locomotion. Acta Physiol. Scand. 121, 9e22. Trinkaus, E., Shang, H., 2008. Anatomical evidence for the antiquity of human footwear: Tianyuan and Sunghir. J. Archaeol. Sci. 35, 1928e1933. Tuttle, R.H., Webb, D.M., Baksh, M., 1991. Laetoli toes and Australopithecus afarensis. Hum. Evol. 6, 193e200. Tuttle, R.H., Webb, D.M., Weidl, E., Baksh, M., 1990. Further progress on the Laetoli trails. J. Archaeol. Sci. 17, 347e362. Vereecke, E., D’Août, K., Van Elsacker, L., De Clercq, D., Aerts, P., 2005. Functional analysis of the gibbon foot during terrestrial bipedal walking: plantar pressure distributions and three-dimensional ground reaction forces. Am. J. Phys. Anthropol. 128, 659e669. Wells, L.H., 1931. The foot of the South African native. Am. J. Phys. Anthropol. 15, 185e289. White, T.D., 1980. Evolutionary implications of Pliocene hominid footprints. Science 208, 175e176. White, T.D., Suwa, G., 1987. Hominid footprints at Laetoli: facts and interpretations. Am. J. Phys. Anthropol. 72, 485e514. Wood, B.A., Collard, M., 1999. The human genus. Science 284, 65e71.
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Journal of Human Evolution journal homepage: www.elsevier.com/locate/jhevol
The relationship between plantar pressure and footprint shape Kevin G. Hatala a, b, *, Heather L. Dingwall b, c, Roshna E. Wunderlich d, Brian G. Richmond b, e a
Hominid Paleobiology Doctoral Program, The George Washington University, 2110 G St., NW, Washington, DC 20052, USA Center for the Advanced Study of Hominid Paleobiology, Department of Anthropology, The George Washington University, 2110 G St., NW, Washington, DC 20052, USA c Department of Human Evolutionary Biology, Harvard University, 11 Divinity Avenue, Cambridge, MA 02138, USA d Department of Biology, James Madison University, 951 Carrier Drive, MSC 7801, Harrisonburg, VA 22807, USA e Human Origins Program, National Museum of Natural History, Smithsonian Institution, 10th St. and Constitution Ave., NW, Washington, DC 20560, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 August 2012 Accepted 1 March 2013 Available online 28 May 2013
Fossil footprints preserve the only direct evidence of the external foot morphologies and gaits of extinct hominin taxa. However, their interpretation requires an understanding of the complex interaction among foot anatomy, foot function, and soft sediment mechanics. We applied an experimental approach aimed at understanding how one measure of foot function, the distribution of plantar pressure, influences footprint topography. Thirty-eight habitually unshod and minimally shod Daasanach individuals (19 male, 19 female) walked across a pressure pad and produced footprints in sediment directly excavated from the geological layer that preserves 1.5 Ma fossil footprints at Ileret, Kenya. Calibrated pressure data were collected and threedimensional models of all footprints were produced using photogrammetry. We found significant correlations (Spearman’s rank, p < 0.0001) between measurements of plantar pressure distribution and relative footprint depths at ten anatomical regions across the foot. Furthermore, plantar pressure distributions followed a pattern similar to footprint topography, with areas of higher pressure tending to leave deeper impressions. This differs from the results of experimental studies performed in different types of sediment, supporting the hypothesis that sediment type influences the relationship between plantar pressure and footprint topography. Our results also lend support to previous interpretations that the shapes of the Ileret footprints preserve evidence of a medial transfer of plantar pressure during late stance phase, as seen in modern humans. However, the weakness of the correlations indicates that much of the variation in relative depths within footprints is not explained by pressure distributions under the foot when walking on firm ground, using the methods applied here. This warrants caution when interpreting the unique foot anatomies and foot functions of extinct hominins evidenced by their footprint structures. Further research is necessary to clarify how anatomical, functional, and sedimentary variables influence footprint formation and how each can be inferred from footprint morphology. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Biomechanics Foot function Locomotion Ileret Human evolution
Introduction Fossil footprints are invaluable discoveries in paleoanthropology, as they contain anatomical and biomechanical information that cannot be obtained from fossil bones. A footprint provides not only an image of the surface anatomy of the foot but also a direct snapshot of foot function during locomotion. Thus, fossil footprints can provide unique anatomical and functional data to inform hypotheses about the evolution of hominin locomotion.
* Corresponding author. E-mail address: [email protected] (K.G. Hatala). 0047-2484/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jhevol.2013.03.009
One well-known set of fossil hominin footprints was discovered at Laetoli, Tanzania, in 1978 (Leakey and Hay, 1979). At the time of their discovery, the Laetoli footprints pushed back the antiquity of the earliest evidence for bipedalism in the hominin clade from about 3.0 to 3.7 Ma (million years ago) (White,1980). They provided critical evidence that early hominins, presumably of the genus Australopithecus (but see Tuttle et al.,1991), walked bipedally on the ground despite the retention of climbing adaptations in their upper limbs. Analyses of the Laetoli footprints have often included comparisons with actual footprints produced by modern humans or other great apes. Some have argued that modern human footprints are indistinguishable from those at Laetoli (e.g., Day and Wickens, 1980; White and Suwa, 1987; Tuttle et al.,1990), yet others have concluded that the Laetoli prints could not have been produced by a modern
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humanlike foot anatomy and gait (e.g., Stern and Susman, 1983; Susman et al., 1984; Deloison, 1991; Meldrum, 2004). More recently, fossilized hominin footprints from the early Pleistocene (1.51e1.53 Ma) were discovered at the site of Ileret, Kenya (Bennett et al., 2009), and continue to be unearthed (Richmond et al., 2010). Preliminary analyses have suggested a foot anatomy and gait that bear closer resemblance to those of modern humans than to those that created the Laetoli footprints. The Ileret footprints have been described as showing evidence of a humanlike longitudinal arch structure and also a humanlike distribution of pressure beneath the foot, including a transfer of pressure to the medial side of the foot prior to toe-off (Bennett et al., 2009). These footprints were interpreted as supportive of the hypothesis that by 1.5 Ma, at least one hominin taxon had undergone an adaptive shift that led to a postcranial body form similar to that of modern humans (Wood and Collard, 1999; McHenry and Coffing, 2000; Bramble and Lieberman, 2004). In their analyses, Bennett et al. (2009) also suggested that the Laetoli footprints represent a foot anatomy and gait different from, and more primitive than, those of modern humans, while others have more recently argued that the Laetoli footprints represent gait essentially equivalent to that of modern humans (e.g., Raichlen et al., 2010; Crompton et al., 2012). The accurate interpretation of human anatomy and gait from footprint morphology requires data on the relationships between footprint morphology and specific anatomical and functional variables. An understanding of how biomechanical variables influence footprint formation will allow us to interpret footprints that were made by early hominin feet for which we have no exact modern anatomical and/or functional analog (Richmond et al., 2012). Such an approach has proven fruitful in the study of dinosaur trackways. Through experimental analyses, Gatesy et al. (1999) developed a quantitative understanding of the ways in which the foot anatomy and hindlimb movements of extant avian taxa are preserved in the three-dimensional shapes of their footprints. The authors then used those data to analyze the unique shapes of fossilized footprints produced by theropod dinosaurs and draw inferences regarding their anatomy and locomotion. A number of studies have taken different approaches to infer aspects of human gait from footprint shape. Some have compared experimentally produced human footprints with fossil hominin footprints, and used those comparisons to draw conclusions about similarities or differences in gait (e.g., Day and Wickens, 1980; Charteris et al., 1981; Tuttle et al., 1990; Raichlen et al., 2010). Others have used records of plantar pressure distribution in human and nonhuman primates as hypothetical footprint shapes, and compared those with the morphologies of fossil hominin footprints (Crompton et al., 2012). However, we have lacked data on how such functional variables are actually recorded in footprints. Taking the first step in this direction, D’Août et al. (2010) conducted the first systematic experimental analysis of how dynamic plantar pressure is recorded in the morphology of footprints themselves. They found a positive correlation between plantar pressure and footprint depth when pressure was measured through an overlying layer of sand. However, they found no significant correlations between the morphology of footprints made in sand and plantar pressure measured directly at the interface between the foot and the pressure pad. In other words, the footprint shape made in sand differed from the ‘normal’ plantar pressure patterns when walking on firm ground, raising significant doubts about the reliability of inferences that have been drawn regarding ‘normal’ foot function and gait in the fossil hominins who made the Laetoli, or other, fossil footprints. The complexity of the results obtained by D’Août et al. (2010) highlighted the need to understand exactly how plantar pressure influences footprint morphology in soft sediments. Ultimately, fossilized footprints will be most useful to paleoanthropologists if
their analysis can be applied broadly to develop and test general hypotheses about the evolution of human locomotion, rather than simply comparing the ways in which different hominins coped with locomotion in pliable substrates. For example, efforts to interpret the Laetoli footprints have focused on whether or not footprint shape represented a humanlike manner of walking, rather than what gait was like when early hominins walked through mud. In order to test hypotheses about a species’ normal gait pattern, we must know whether or not footprinted sediments preserve accurate records of the way the foot functions on firmer ground. Most, if not all, modern biomechanical analyses that attempt to define ‘human foot function’ are conducted on solid surfaces, in part due to the nature of the available pedobarographic equipment. If a footprint in sediment preserves no discernible record of foot function as observed on firmer surfaces, then one can only draw limited inferences about the specific ways in which hominins moved on soft substrates. In this paper, we present an experimental approach for understanding how the distribution of plantar pressure is recorded in footprints produced by habitually unshod modern humans in the same sediments that preserve 1.5 Ma fossil hominin footprints. In the only other study to assess the relationship between plantar pressure and footprint depth quantitatively, D’Août et al. (2010) acknowledged that sediment mechanics almost certainly play a critical, highly variable role in determining the relationship between foot mechanical variables and footprint topography. Those authors, and several before them, studied footprints that were made in wet sand, which differs typologically from the wet ash that preserved the Laetoli footprints (Leakey and Hay, 1979), and from the clay- and silt-containing sediments that preserved the 1.5 Ma fossilized hominin footprints at Ileret, Kenya (Bennett et al., 2009). Importantly, silt and clay are, by definition, of a finer grain than sand (Garcia, 2008). Because cohesion increases as particle size decreases (Garcia, 2008), the mechanics of sediment deformation, and thus record of foot function preserved in footprint structure, may differ across these sediment types. During the process of footprint formation, the foot subjects the sediment to a combination of elastic and plastic (elasticeplastic) deformation (Allen, 1997). In the elastic phase, the sediment resists deformation but once its yield stress has been reached and the foot punches through the most superficial layer of the sediment, plastic deformation occurs and results in an impression. The elastic behaviors of clays and sands differ because values of Young’s modulus are highly variable throughout sand when pressure is applied to it, yet Young’s modulus for clays remains constant or only changes a negligible amount (Craig, 1992). As a result, vertical elastic forces are transmitted through these sediments in different ways. Clays provide uniform resistance to vertical loads across the foot during the elastic phase (which would also be the case when walking on a perfectly solid surface), while the resistance provided by sand can vary across different regions of the foot. As a result, the response of sand to the distributed load will likely affect downstream results in how closely a footprint’s shape matches the distribution of vertical forces, if the sand’s response to those forces varies between regions of the foot. These differences in sediment properties suggest that the topography of footprints in clay-containing sediments, such as those in which the Ileret fossil footprints were preserved, may reflect more directly the distribution of vertical forces (plantar pressure) during gait than footprints that are made in sand. Furthermore, the greater cohesion of finer-grained sediments, such as those from Ileret, suggests that they are more likely than sands to preserve a shape that reflects the transmission of forces during the plastic deformative process as they are less likely to undergo infilling or other post-deformational shape changes.
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In the experimental work presented in this study, subjects produced footprints in sediment taken directly from layers in which the 1.5 Ma fossil footprints were formed at Ileret, Kenya (Bennett et al., 2009; Behrensmeyer, 2011). By conducting our experiments in this sediment, and making our best attempt to recreate the sedimentary conditions that existed at 1.5 Ma, we directly test the hypothesis that the shape of footprints produced in the Ileret fossil footprint sediment preserves detectable records of the ‘normal’ distribution of plantar pressure as measured on a firm surface. By comparing our results to those of D’Août et al. (2010) from their experiments in sand, we were also able to test the hypothesis that sediment type influences the relationship between plantar pressure and footprint topography. Materials and methods Experimental subjects Data were collected from 38 habitually unshod or minimally shod adults (19 men, 19 women) from the Daasanach tribe, living in and around the town of Ileret, Kenya. All subjects had either never worn any footwear or had not worn footwear until adulthood, after their feet had fully developed. Given that unshod human feet can develop differently from shod feet in both anatomical and functional respects (Hoffmann, 1905; Wells, 1931; Sim-Fook and Hodgson, 1958; Barnett, 1962; Ashizawa et al., 1997; D’Aout et al., 2009), this experimental sample is close to ‘ideal’ for understanding fossil footprints that pre-date the earliest evidence of footwear in the fossil record (Trinkaus and Shang, 2008). Subjects were recruited, and provided their informed consent, in accordance with the policies of The George Washington University’s Institutional Review Board (#031030). Experimental setup The experiments were conducted on a large, flat, open space adjacent to the site of fossil footprint excavations at Ileret, Kenya. A 1-m RSscan International Footscan pressure pad (RSscan International, Olen, Belgium) was placed on the ground and leveled. One meter from the end of the pressure pad, a pit was dug that measured 150 cm long, 50 cm wide, and 15 cm deep. This pit was filled with sediment excavated directly from a fossil layer containing hominin footprints (Fig. 1). In 2010, A1 layer sediment was used, and Lower Layer sediment was used in 2011 (due to the availability of a necessary volume that did not include any fossil footprints). These layers are typologically very similar, as they were formed from the same source sediments (Bennett et al., 2009; Behrensmeyer, 2011), and there is no reason to suspect that their mechanical properties would be substantially different from each other. There are currently no methods for calculating the exact saturation and/or compaction levels of the sediment when the fossil footprints were formed, so we acknowledge that these
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variables may have differed between the sediments used in our experiments and those that existed at 1.5 Ma. However, we made our best attempt to cover the range of sedimentary conditions that could have existed when the fossilized prints were formed. We varied the amount of water added, as well as the degree of sediment compaction, and asked subjects to create ‘test’ footprints until the overall depths of the test prints were consistent with the range of depths observed in the fossil footprints. Naturally, saturation and compaction levels varied to a small degree between trials and between subjects. However, because there is similar variation in the depths of the fossil trackways, a sample of experimental prints that spanned the range of depths seen in the fossil footprint layers should include the sedimentary conditions under which the fossil prints were formed. Until methods are developed to determine saturation and compaction levels from fossilized sediments, such approximations will be necessary. A video camera positioned 8 m from (and perpendicular to) the trackway was used to record all trials (recorded at 60 Hz in 2010 and 210 Hz in 2011). Distance was calibrated using stakes positioned 3 m apart, spanning the pressure mat and the sediment patch.
Experimental protocol Biometric measurements were taken from each subject, including stature, weight, functional leg length (height at the level of the greater trochanter of the femur while standing), and foot length. Each subject performed at least six walking trials, three while maintaining their comfortable walking pace (mean speed ¼ 1.43 m/s, SD ¼ 0.23, mean Froude number ¼ 0.23, SD ¼ 0.07) and three at a fast pace (mean speed ¼ 1.86 m/s, SD ¼ 0.25, mean Froude number ¼ 0.39, SD ¼ 0.10). These trials consisted of walking over the pressure pad and through the sediment patch. In 2010, the first twenty subjects conducted pressure and footprint trials separately, with pressure data collected in one trial and footprint data collected in a subsequent trial (thus requiring six trials at each walking speed). During the 2011 field season, the next eighteen subjects walked on the pressure pad and made footprints within the same trial (requiring only three trials at each walking speed). Start and finish markers were placed approximately 5 m from the beginning of the experimental apparatus and 5 m from the end. Allocating this distance, for a few steps to be taken both before and after the calibrated space in which measurements were taken, minimized the chance that acceleration or deceleration would occur while the subjects were traveling over the experimental trackway. Subjects were asked to repeat trials if they adjusted their pace along the trackway, or if they visibly altered their gait in order to target the pressure pad or sediment patch. All trials were recorded with 2-dimensional video so that they could later be digitized to quantify speed (see below). This is important because speed has been reported to be significantly correlated with pressure data such as peak pressure and pressuree
Figure 1. Schematic drawing showing an overhead view of the experimental trackway. Start line is about 5 m before the front edge of the pressure pad. Pressure pad is 1 m long and its end is 1 m prior to the sediment pit. The sediment pit is 1.5 m long and the finish line is about 5.0 m from the end of the sediment pit. Drawing is not to scale.
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Figure 2. Image from RSscan Footscan 7 software, showing the ten regions of the foot from which plantar pressure measurements were taken. Each rectangle is 7.62 mm 5.08 mm.
impulse (e.g., Vereecke et al., 2005; Pataky et al., 2008; but see; Kivell et al., 2010; Patel and Wunderlich, 2010). Pressure data were collected within RSscan International Footscan 7 software (RSscan International, Olen, Belgium). Following completion of each trial, the associated footprint(s) produced in the sediment trackway was/ were photographed at multiple camera positions and angles such that 3-dimensional models could later be produced using photogrammetric software (see below). Experimental data processing To quantify velocity for each walking trial, 2-dimensional videos from 2010 (recorded at 60 Hz) were digitized using Peak Motus software (Peak Performance Technologies, Inc., Englewood, CO, USA). Videos from 2011 were digitized using ImageJ (Rasband, 1997e2012) because Peak Motus could not support high-speed (210 Hz) video. For 48 videos, speed was measured using both techniques and a Wilcoxon signed-rank test revealed no significant differences between measurements obtained from the two programs (p ¼ 0.327). Velocity was measured as the time it took for a digitized marker on the sternum to travel a horizontal distance of 3 m, calibrated from the stakes in the experimental setup. The sternum was consistently visible throughout the experiments and horizontal translational movements of the trunk are typically small (<3 cm) during walking (Thorstensson et al., 1984; Stokes et al., 1989). Pressure data were extracted from 10 anatomical subdivisions across the foot (medial heel, lateral heel, lateral midfoot, metatarsal
heads 1e5, hallux, and second toe) (Fig. 2). These subdivisions represent the areas of the plantar surface of the foot that are typically in contact with the ground during walking. Pressure measurements were extracted from a 7.62 mm 5.08 mm rectangular sampling region placed in the center of each respective anatomical region. These regions could be easily identified because each registers a pressure point visually distinct from immediately surrounding areas. Measurements of maximum pressure and pressureeimpulse (the pressureetime integral, defined as the area beneath the curve of pressure over time) were taken from each of these regions for each trial. It has often been assumed that fossil hominin footprint morphology in some way represents foot kinetics (e.g., Day and Wickens, 1980; Berge et al., 2006; Bennett et al., 2009) and D’Août et al. (2010) found that both maximum pressure and pressureeimpulse are influential in determining footprint depths in certain regions beneath the foot. While maximum pressure describes the peak normal force applied in a certain area, pressureeimpulse also characterizes the time over which a load is applied, which may be particularly important when considering sediments that deform in an elasticeplastic manner. Photographs of footprints were used to generate 3-dimensional virtual models using PhotoModeler Scanner 2011 software (Eos Systems, Inc., Vancouver, BC, Canada; Fig. 3). This software matches identical pixel values across sets of photographs and, knowing the parameters of the camera and lens due to a calibration procedure, triangulates camera positions and calculates depth to render threedimensional models from those photos. Each model included not only the footprint, but also about 5e10 cm of undisturbed sediment around the perimeter of the print, so that it could later be oriented for measurement. These 3-dimensional models were then exported to Geomagic Qualify 2012 software (Geomagic, Inc., Research Triangle Park, NC, USA) for further analysis. Twenty evenly spaced points were randomly selected on the undisturbed sediment around the perimeter of each print, and best-fit planes were aligned to those points using a least-squares algorithm. This ‘ground’ plane was then configured to the XeY plane in coordinate space, such that the z-axis would directly represent depth. Footprint depth measurements were taken at each of the 10 anatomical regions from which pressure data were collected. For each region, five points were selected that represented the approximate locations of the corners and center of the 7.62 mm 5.08 mm sampling window from which pressure measurements were taken (Fig. 2). The measure of depth for that region of the footprint was then calculated as the centroid of those five points. This process was repeated for all 10 anatomical regions from which pressure measurements were taken. Analyzing the relationship between pressure and footprint topography All regional measurements of maximum pressure, pressuree impulse, and footprint depth (z-coordinate values) were
Figure 3. Example of a three-dimensional model of a footprint, created using PhotoModeler software.
K.G. Hatala et al. / Journal of Human Evolution 65 (2013) 21e28
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Standardized footprint depth Figure 4. Scatter plot of regional standardized maximum plantar pressure and corresponding regional standardized footprint depth. Average values across all anatomical regions, for both fast and normal walking speeds, are plotted for all subjects. The correlation between these variables was significant (r ¼ 0.3796, p < 0.0001).
standardized by dividing each individual measurement by the maximum of all measurements across the foot for each trial. For example, the overall maximum pressure measurement was taken from measures of maximum pressures within the regions of the hallux, second toe, metatarsal heads, lateral midfoot, and heel. Each of the original measurements was then divided by the overall maximum pressure measurement for a given trial. The same procedure was followed for measurements of pressureeimpulse and also footprint depth. Doing so allowed us to compare the distribution of pressure across the foot during walking to the ‘distribution of depth’ across a footprint. We favored this approach over a direct comparison of absolute pressure or depth measurements because some individuals may have absolutely higher plantar pressures or make absolutely deeper footprints than others due to variables not directly relevant to foot function, such as body mass or soft tissue thickness. Measurements were averaged within each speed category for each subject (i.e., one average pressure distribution represented the average of one subject walking at one categorical speed). Although this procedure may reduce variation within an individual subject, it is also improbable that a step on the pressure pad and a step creating a footprint, even in the same sequence, would be identical. As a result, we chose to compare a subject’s average pressure distribution at a given speed to their average footprint topography at that same speed. Spearman’s rank correlation was used to determine the relationship between standardized measurements of pressure and pressureeimpulse across the plantar surface of feet and standardized measurements of depth across footprints. Additional tests were conducted, using Spearman’s rank correlation, to determine relationships between plantar pressure and footprint depth at different walking speeds, and within various anatomical regions of the foot. The correlations between plantar pressure and footprint depth were then calculated within each individual subject, to determine how many showed a significant relationship between these two variables.
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Standardized footprint depth Figure 5. Scatter plot of regional standardized pressureeimpulses and corresponding regional standardized footprint depths. Average values across all anatomical regions, for both fast and normal walking speeds, are plotted for all subjects. The correlation between these variables was significant (r ¼ 0.2527, p < 0.0001) but weaker than the correlation between regional maximum pressures and regional footprint depths (shown in Fig. 4).
pressure measurements and average standardized footprint depths across all subjects at all speeds) revealed that the two were significantly correlated. Regional maximum pressure explained a greater amount of variance in footprint depth (Spearman’s r ¼ 0.3796) than did regional pressureeimpulse (r ¼ 0.2527), although both measures of plantar pressure were significantly correlated with measures of print depth (p < 0.0001 in both cases; Figs. 4 and 5). Since some aspects of data collection changed between field seasons (e.g., collecting pressure and footprint data within the same versus separate trials), data from the two seasons were also analyzed separately. In doing so, we found that the correlation between pressure and footprint depth was consistent between both procedures for data collection (Year 1 maximum pressure to depth r ¼ 0.4510, Year 2 r ¼ 0.3693, p < 0.0001 for both; Year 1 pressureeimpulse to depth r ¼ 0.3217, Year 2 r ¼ 0.2146, p < 0.0001 for both). Therefore, we pooled the data for all further analyses. When data collected at each walking speed were analyzed separately, we found that the relationship between plantar pressure and footprint depth changed with walking speed. A stronger relationship between maximum pressure and depth existed at fast walking speeds (r ¼ 0.4527) than at normal speeds (r ¼ 0.2924) but both relationships were significant (p < 0.0001; Table 1). The same was true of the correlations between pressureeimpulse and depth at fast (r ¼ 0.3413, p < 0.0001) and normal (r ¼ 0.1691, p ¼ 0.0009) walking speeds. Relationships between plantar pressure and footprint depth differed across the various anatomical regions. Maximum pressure and footprint depth were significantly correlated (p < 0.05) at the Table 1 Speed-specific correlations between maximum pressure and pressureeimpulse and footprint depth, over the entire foot and footprint.
Results
Relationship
Walking speed
Spearman’s r
p-Value
Correlations between plantar pressure and footprint depth
Maximum pressure e footprint depth Maximum pressure e footprint depth Pressureeimpulse e footprint depth Pressureeimpulse e footprint depth
Normal Fast Normal Fast
0.2924 0.4527 0.1691 0.3413
<0.0001 <0.0001 0.0009 <0.0001
The cumulative assessment of the correlation between pressure and depth (the correlation between the average standardized
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Table 2 Region-specific correlations between maximum pressure and footprint depth, and pressureeimpulse and footprint depth (* denotes significance, p < 0.05). Region Medial heel Lateral heel Lateral midfoot Metatarsal 1 Metatarsal 2 Metatarsal 3 Metatarsal 4 Metatarsal 5 Hallux Second digit
Table 3 Descriptive statistics of mean standardized plantar pressure and footprint depths, presented in Fig. 5.
Maximum pressure and depth (r)
p-Value
Pressureeimpulse and depth (r)
p-Value
Region
0.0168 0.0159 0.1030 0.1799 0.3208 0.2496 0.0761 0.0181 0.1976 0.0382
0.8854 0.8912 0.3758 0.1199 0.0047* 0.0297* 0.5133 0.8765 0.0871 0.7431
0.0170 0.0548 0.1521 0.2789 0.4284 0.3934 0.1045 0.0487 0.2596 0.0792
0.8840 0.6381 0.1896 0.0147* 0.0001* 0.0004* 0.3689 0.6760 0.0235* 0.4966
Medial heel Lateral heel Lateral midfoot Metatarsal 1 Metatarsal 2 Metatarsal 3 Metatarsal 4 Metatarsal 5 Hallux Second digit
Mean standardized maximum pressure (SD) 0.811 0.825 0.183 0.470 0.538 0.587 0.481 0.478 0.676 0.390
(0.136) (0.114) (0.148) (0.187) (0.163) (0.183) (0.164) (0.219) (0.195) (0.182)
Mean standardized footprint depth (SD) 0.915 0.922 0.786 0.858 0.850 0.847 0.829 0.804 0.926 0.894
(0.068) (0.071) (0.096) (0.071) (0.080) (0.081) (0.083) (0.082) (0.052) (0.056)
In each trial, individual regional measurements were standardized by dividing each one by the overall maximum (across all regions) for that trial.
second and third metatarsal heads, but not at any other regions (Table 2). Correlations between pressureeimpulse and depth were significant (p < 0.05) at the hallux and the first, second, and third metatarsal heads. Further analyses examined the correlations between pressure and depth within each subject independently, with all anatomical regions pooled. Significant correlations were found between maximum pressure and depth within 23 out of the 38 subjects. Correlations between impulse and print depth were only found within 10 of the 38 subjects. Distribution patterns of plantar pressure and footprint depth While the correlations between regional standardized maximum plantar pressure and regional standardized footprint depth were highly significant, these relationships were weak. However, we found that certain aspects of the distribution patterns of maximum plantar pressure and footprint depth were actually quite similar to each other (Fig. 6, Table 3). Maximum pressures and footprint depths were both greatest at the heel and hallux. The lateral midfoot experienced the lowest maximum plantar pressures and also left the shallowest impressions. The metatarsal heads
occupied an intermediate position for both pressures and footprint depths. The second toe was the only exception, leaving a relatively deep impression despite experiencing lower pressures.
Discussion The results of this study establish that barefoot plantar pressure is related to the topography of footprints created in the sediment that preserves the 1.5 Ma fossil footprints at Ileret. However, the fact that the correlation between plantar pressure and footprint depth was weak highlights the complexity of the footprint formation process, along with the need to continue exploring (1) the ways in which specific anatomical and functional variables can be preserved in footprints, and (2) the mechanisms by which human foot function can be measured in soft substrates. It is very likely that other anatomical and/or functional variables that were not measured in this study contributed to the large amount of unexplained variation in footprint shape. For example, the bony architecture of the foot and kinematic factors such as joint angles are among many variables that probably directly influence footprint
1.1 1.0
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Lateral midfoot
Metatarsal 5
Metatarsal 4
Metatarsal 3
Metatarsal 2
Metatarsal 1
Lateral heel
Medial heel
0.0 0.5
Figure 6. Regional standardized maximum plantar pressures (white, left axis) and standardized footprint depths (gray, right axis) across 10 anatomical regions of the foot, for all subjects walking at both normal and fast speeds. Lines correspond to median values, boxes represent interquartile range (25e75%), whiskers represent non-outlier range.
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morphology, and indirectly affect the relationship between plantar pressure and footprint depth. Furthermore, previous studies have found disparities between the bony structure of the foot and patterns of foot function, suggesting that the foot’s soft tissue anatomy plays a significant role in both the distribution and application of forces between the foot and the substrate (e.g., Griffin and Richmond, 2005). Therefore, soft tissue anatomy and its functions likely also affect footprint shape in ways that were not measured in this study. Still, the results of this study can give us new insights into interpretations of fossil hominin footprints. Bennett et al. (2009) hypothesized that the Ileret footprints preserve evidence of foot anatomy and function essentially similar to those seen in modern humans. Those authors pointed to the depth of the Ileret footprints in the hallux and medial forefoot as providing evidence for the medial transfer of pressure that typifies modern human locomotion. In the present study, the correlation between maximum pressure and depth was not significant at the first metatarsal head but the correlation between pressureeimpulse and depth within this region was, in fact, significant (Table 2). Furthermore, despite the fact that, in this study, maximum pressure was not typically highest beneath the first metatarsal head compared with the others, footprints were typically deeper in this region than they were beneath any of the other metatarsals (Fig. 6). This suggests that the Ileret footprints, being deeper under the first compared with the other metatarsal heads, have shapes that are generally similar to the footprints of modern humans produced in the same sediment. In sum, these experimental results do lend some support to the functional hypotheses of Bennett et al.’s (2009) interpretation, inferring modern humanlike aspects of foot anatomy and gait from the 1.5 Ma footprints. However, our results also reveal that variables other than the distribution of plantar pressure explain more than 80% of the overall variation in footprint topography. In light of these findings, it is necessary to proceed with caution in drawing direct interpretations of foot function from footprint topography. While we have shown here that one variable related to foot function is significantly correlated with footprint topography, this correlation is weaker than might be expected and underscores the fact that many variables, not measured in this study, simultaneously influence the dynamic deformative process through which footprints are created. Future research would benefit from a multivariate approach, to gain a full appreciation of the many factors related to foot anatomy, foot function, gait, and sediment properties, which simultaneously influence footprint shape. While we did not directly quantify mechanical properties of the sediment used in this study, it seems likely that the mechanistic differences between sand and the sediment from Ileret might explain why the Ileret sediments preserve footprints whose topography is correlated to the distribution of plantar pressure at the footesubstrate interface, but sand does not. As mentioned previously, the mechanical behavior of the fine-grained clay- and silt-containing sediments found in the Ileret footprint layers likely differed from that of the coarser-grained sand used in D’Août et al.’s (2010) experiments. First, the elastic behavior of sand, unlike clay, is not uniform within a given sample. As a result, its mechanics of deformation may actually differ across various regions of a footprint. Second, the finer grain size of the Ileret footprint layer sediment makes it more cohesive (Garcia, 2008) and therefore more likely to retain the shape produced by the deformative process. Sand, on the other hand, is less cohesive, leaving the deepest points of a footprint vulnerable to some degree of infill after their initial deformation. This means that a footprint in sand may be less likely to preserve the pattern in which it was plastically deformed. No methods currently exist for quantifying the relationships between sediment mechanical properties and kinetic variables
27
such as plantar pressure. For example, there are no available means for adjusting experimental results obtained in sand so that they would reflect the same relationship between plantar pressure and footprint topography if the experiments were performed in mud. This is unfortunate for the interpretation of fossilized footprints because we may not be able to apply experimental results obtained using commonly-available sediments. Rather, until methods are developed for thoroughly quantifying the relationships among sediment mechanical properties, footprint topography and kinetic variables, the most viable approach may be to perform experiments in sediments that directly represent the conditions in which fossil hominin footprints were formed. Fine-grained sand is commonly used in experimentally-based analyses of the Laetoli footprints (e.g., Day and Wickens, 1980; Raichlen et al., 2010; Crompton et al., 2012) based upon similarity in grain size to the Laetoli ash. Given the results of our study, before drawing direct conclusions about kinesiological variables derived from Laetoli footprint topography, it may be important to assess quantitatively the mechanical behavior of sand in order to determine whether or not it can serve as an accurate mechanical substitute for the phonolitic ash that preserves the Laetoli footprints. Conclusions We conclude that the distribution of plantar pressure is correlated with the topography of footprints made in the sediment that preserves 1.5 Ma fossil hominin footprints at Ileret, Kenya. Furthermore, the relationship between plantar pressure and footprint morphology is not constant between different sediment types and likely varies according to the sediment’s mechanical properties. While the results of this study begin to shed light on how specific aspects of foot function are, or are not, preserved within footprint morphology, they also demonstrate that many questions remain unanswered about how we can accurately interpret footprints in the human fossil record. The correlation between the distribution of plantar pressure and footprint topography is not strong and the process of footprint formation is clearly a multivariate scenario in which anatomical, functional, and sedimentary variables simultaneously influence footprint shape. It will be necessary to gain an understanding of how specific variables influence footprint morphology, both independently and together, before we can develop accurate functional interpretations of fossil hominin footprints, particularly those made by extinct taxa whose foot anatomy and function almost certainly differ from those of extant humans and apes. Only within this framework can we use these unique sources of data to better inform hypotheses regarding the evolution of human foot anatomy and locomotion. Acknowledgments We thank David Green, David Braun, Jack Harris, Purity Kiura, Emmanuel Ndiema, the Koobi Fora Field School, the National Museums of Kenya, the town of Ileret, Kenya, and our Daasanach subjects for their contributions to this project. We also thank three anonymous reviewers who provided insightful and constructive comments and advice during the preparation of this manuscript. This study was funded by the National Science Foundation, grants BCS-0924476, BCS-1128170, and DGE-0801634. References Allen, J.R.L., 1997. Subfossil mammalian tracks (Flandrian) in the Severn Estuary, S.W. Britain: mechanics of formation, preservation and distribution. Phil. Trans. R. Soc. B 352, 481e518.
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