Journal of Human Evolution

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1 Journal of Human Evolution 64 (2013) 556e568 Contents lists available at SciVerse ScienceDirect Journal of Human Evolution journal homepage: Hominin stature, body mass, and walking speed estimates based on 1.5 million-year-old fossil footprints at Ileret, Kenya Heather L. Dingwall a, *,1, Kevin G. Hatala a,b, Roshna E. Wunderlich c, Brian G. Richmond a,d, * a Center for the Advanced Study of Hominid Paleobiology, Department of Anthropology, The George Washington University, 2110 G St. NW, Washington, DC 20052, USA b Hominid Paleobiology Doctoral Program, The George Washington University, 2110 G St. NW, Washington, DC 20052, USA c Department of Biology, James Madison University, MSC 7801 Harrisonburg, VA 22807, USA d Human Origins Program, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA article info abstract Article history: Received 30 April 2012 Accepted 11 February 2013 Available online 22 March 2013 Keywords: Bipedalism Body size Homo Human evolution Pleistocene The early Pleistocene marks a period of major transition in hominin body form, including increases in body mass and stature relative to earlier hominins. However, because complete postcranial fossils with reliable taxonomic attributions are rare, efforts to estimate hominin mass and stature are complicated by the frequent albeit necessary use of isolated, and often fragmentary, skeletal elements. The recent discovery of 1.52 million year old hominin footprints from multiple horizons in Ileret, Kenya, provides new data on the complete foot size of early Pleistocene hominins as well as stride lengths and other characteristics of their gaits. This study reports the results of controlled experiments with habitually unshod Daasanach adults from Ileret to examine the relationships between stride length and speed, and also those between footprint size, body mass, and stature. Based on significant relationships among these variables, we estimate travel speeds ranging between 0.45 m/s and 2.2 m/s from the fossil hominin footprint trails at Ileret. The fossil footprints of seven individuals show evidence of heavy (mean ¼ 50.0 kg; range: 41.5e60.3 kg) and tall individuals (mean ¼ cm; range: 152.6e185.8 cm), suggesting that these prints were most likely made by Homo erectus and/or male Paranthropus boisei. The large sizes of these footprints provide strong evidence that hominin body size increased during the early Pleistocene. Ó 2013 Elsevier Ltd. All rights reserved. Introduction The late Pliocene and early Pleistocene mark a major transitional stage in hominin evolution, with derived anatomical changes within the genus Homo including increased brain and body size, potentially with decreased sexual dimorphism, reduced tooth size suggesting dietary shifts, and elongated lower limbs that likely improved speed and energetic efficiency (Wood and Collard, 1999; McHenry and Coffing, 2000; Wood and Richmond, 2000). The increased body size and relative lower limb length (but see Pontzer, 2012) that distinguished some early Homo from Australopithecus resulted in a more derived hominin body shape that falls within the range of variation exhibited by modern humans (Richmond et al., 2002; Ruff, 2002). * Corresponding authors. addresses: hdingwall@fas.harvard.edu (H.L. Dingwall), kevin.g.hatala@ gmail.com (K.G. Hatala), wunderre@jmu.edu (R.E. Wunderlich), brich@gwu.edu (B.G. Richmond). 1 Present address: Department of Human Evolutionary Biology, Harvard University, 11 Divinity Ave., Cambridge, MA 02138, USA. Australopiths are estimated to have been relatively small compared with early Homo erectus or Homo ergaster (hereafter H. erectus ). Based on hindlimb joint size, which is arguably one of the best means of predicting body mass (e.g., Jungers, 1988a; Ruff, 2003; Gordon, 2004), Australopithecus afarensis specimens have been estimated at 45 kg for inferred males and 29 kg for inferred females (McHenry, 1992). Average stature for A. afarensis is also small, with male and female estimates averaging about 151 cm and 105 cm, respectively (McHenry, 1991; McHenry and Coffing, 2000). From these mass and stature predictions, as well as studies of size and shape variation, it is clear that A. afarensis exhibited substantial sexual dimorphism with regard to body size (Richmond and Jungers, 1995; Lockwood et al., 1996; Plavcan et al., 2005; Scott and Stroik, 2006; Gordon et al., 2008, 2010; contra Reno et al., 2003). Homo habilis estimates are also small, with male mean body mass and stature estimated at 37 kg and 131 cm, respectively. Average female mass is estimated at 32 kg and stature at 100 cm (McHenry, 1991; McHenry, 1992; re-analyzed in McHenry and Coffing, 2000 excluding KNM-ER 1472 and 1481). It should be noted that although these body size calculations are influenced by the dearth of postcrania that can be reliably attributed to H. habilis, /$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.

2 H.L. Dingwall et al. / Journal of Human Evolution 64 (2013) 556e cranial size variation is consistent with small overall body size in H. habilis s.s. On the other hand, mean estimates of H. erectus stature (male: 180 cm, female: 160 cm; Ruff and Walker, 1993) and mass (male: 66 kg, female: 56 kg; Ruff et al., 1997) suggest that the substantial increase in East African hominin body size that took place during the early Pleistocene occurred with the appearance of the earliest H. erectus around 1.9 Ma. These estimates suggest an especially significant increase in female hominin body size and a concomitant decrease in the level of sexual dimorphism in H. erectus relative to earlier hominins. However, several recent discoveries of small cranial remains belonging to H. erectus, for example from Olorgesailie (Potts et al., 2004) and Ileret, Kenya (Spoor et al., 2007), and postcranial remains from Dmanisi, Georgia (Lordkipanidze et al., 2007) and Gona, Ethiopia (Simpson et al., 2008) suggest that H. erectus may have shown considerably more size variation than previously thought. Furthermore, the presence of these small H. erectus specimens raises questions about whether body size dimorphism in H. erectus was appreciably different than that of earlier hominins (Antón, 2012). The recent discoveries at Dmanisi include postcranial material from three adults and one adolescent attributed to H. erectus. Stature and body mass have been estimated for two of the adults, which imply smaller adult body sizes than those predicted from other H. erectus material. Stature estimates based on humeral, femoral, and tibial measurements for a large adult individual from Dmanisi averaged cm, while estimates of stature derived from the first metatarsal of a smaller adult individual yielded an estimate of 143 cm (Lordkipanidze et al., 2007). Body mass estimates for the larger individual based on joint dimensions of the humerus, femur, and tibia, averaged 48.8 kg. The smaller individual s mass was estimated at 40.2 kg based on first metatarsal joint surface dimensions (Lordkipanidze et al., 2007). A pelvis from Gona, attributed to an adult female H. erectus individual, has produced stature estimates between 123 cm and 146 cm based on an estimated femur length using the three major articular surfaces preserved in the pelvis (Simpson et al., 2008). These estimates from Dmanisi and Gona are all smaller than the male and female averages predicted for H. erectus prior to these discoveries (Ruff and Walker, 1993). Some researchers have questioned the estimated body size of male H. erectus, notably those estimates derived from the 1.53 Ma (millions of years ago) associated juvenile skeleton KNM-WT (the Turkana Boy ). Ruff and Walker (1993) estimated for KNM-WT a stature at death of 160 cm and mass of 48 kg. The same authors predicted adult long bone lengths for KNM-WT 15000, which they used to estimate adult stature (185 cm) and mass (68 kg) for the juvenile specimen assuming a human-like life history for H. erectus. More recently, Ruff (2007) reexamined KNM-WT and proposed new at-death body size estimates based on a juvenile comparative sample. Stature at death was estimated at about 157 cm and body mass at 50e53 kg (Ruff, 2007). Although these estimates are slightly different, they are similar to Ruff and Walker s (1993) earlier calculations of KNM-WT s body size and would still result in similarly large adult stature estimates given a relatively human-like growth trajectory. If correct, these estimates provide evidence that the shift to a larger body size and stature, comparable with the sizes of modern humans (at least in males), occurred with the emergence of H. erectus. However, skepticism regarding the accuracy of these size estimates has arisen based on questions about KNM-WT s age at death, growth, development, life history (Graves et al., 2010), and the possibility of spinal pathologies such as scoliosis (Lovejoy, 2005) and vertebral dysplasia resulting in disproportionate axial and appendicular elements (Ohman et al., 2002; but see; Haeusler et al., 2011). Based on several assumptions, including a younger age at death and a shorter and earlier period of growth relative to modern humans, Graves et al. (2010) argue that KNM-WT was 154 cm tall at his time of death and would have attained an adult stature of only 163 cm. This estimate is lower than that based on Ruff s (2007) analysis, closer to Ruff and Walker s (1993) average stature estimates for female H. erectus and not much larger than McHenry s (1991) male stature estimates of A. afarensis. Ohman et al. (2002) similarly claim that Ruff and Walker (1993) overestimated KNM- WT s stature at death. They propose a new estimate of 147 cm, which they argue accounts for axial/appendicular proportions in H. erectus that differ from those of modern reference populations. Based on newly identified rib and vertebral fragments, Haeusler et al. (2011) conclude that the rib cage of KNM-WT was symmetrical and question previous interpretations that scoliosis or other pathologies affected the skeleton. This, in turn, suggests that disease may not have affected KNM-WT s proportions and supports Ruff and Walker s (1993) original size estimates. It is clear from these and other studies that stature and mass in H. erectus are the subject of ongoing debate. Body size estimates in other early Pleistocene hominins are even more uncertain given the scarcity of well-preserved long bones with confident attributions to H. habilis or Paranthropus boisei (McHenry and Coffing, 2000). Conclusions about temporal trends in hominin body size have implications for the formulation of hypotheses about other aspects of hominin evolution, many of which are concerned with the shift to more xeric climatic conditions in East Africa throughout the early Pleistocene (McHenry and Coffing, 2000; Antón, 2003; Bobe, 2011). Evidence of increased mass and stature lies at the heart of hypotheses regarding behavioral and physiological changes in early Homo (McHenry, 1994; McHenry and Coffing, 2000; Aiello and Key, 2002; Aiello and Wells, 2002; Lieberman et al., 2009; Pontzer, 2012). The early Pleistocene falls temporally between the Pliocene, characterized by smaller-bodied hominins (australopiths and H. habilis s.s.), and the later Pleistocene, with species of Homo that certainly had larger body sizes (Grine et al.,1995; Carretero et al., 2012). Because of the limited sample of hominin fossils known from the early Pleistocene (between 2.0 and 1.0 Ma), and the further paucity of fossils from which it is appropriate to derive estimates of body size, new information from this time period is critical to clarify the tempo and mode of hominin body size evolution. The recent discovery of three stratigraphically distinct hominin footprint assemblages dating to c Ma at the site of FwJj14E in Ileret, Kenya, in the Koobi Fora Formation (Bennett et al., 2009; Richmond et al., 2010) not only increases the previously-known sample of hominin footprints from the Plio-Pleistocene, it also provides opportunities to test hypotheses about hominin body size during the early Pleistocene, a time in hominin evolution about which we know very little. Further, footprint trackways provide the only means for directly observing locomotor behaviors in the fossil record. In this study, we conducted controlled experiments with habitually unshod Daasanach subjects from Ileret, Kenya, in order to establish relationships between stride length and speed, as well as between footprint size, stature, and body mass. Habitually unshod subjects are critical to such an analysis given that early Pleistocene hominins almost certainly lacked shoes (Trinkaus and Shang, 2008), and the 1.52 Ma prints are distinctly barefoot prints. Furthermore, there exists substantial evidence that habitual shoe use influences foot development and possibly biomechanics (Hoffman, 1905; Wells, 1931; Sim-Fook and Hodgson, 1958; Barnett, 1962; Ashizawa et al., 1997; D Août et al., 2009; Lieberman et al., 2010; Hatala et al., 2013). More prints continue to be discovered within previously described trackways at FwJj14E that are preserved in at least three

3 558 H.L. Dingwall et al. / Journal of Human Evolution 64 (2013) 556e568 distinct horizons (Richmond et al., 2010) representing different instances in time within a few thousand years of each other at c Ma (Bennett et al., 2009; Behrensmeyer, 2011). Here, we present new measurements of stride lengths and use experimentally derived relationships to estimate walking speeds of the hominin printmakers. We also use our experimentally produced footprints as the basis for inferring the statures and body masses of the hominin individuals who made the fossil footprints at Ileret in order to evaluate the hypothesis that, relative to earlier taxa, body size increased in early Pleistocene hominins. Materials and methods Controlled field experiments were conducted in 2010 and 2011 with 38 adult Daasanach subjects (19 males, 19 females) who live near the northeast shore of Lake Turkana, Kenya. Subjects were recruited and their informed consent was obtained in accordance with the policies of The George Washington University s Institutional Review Board (#031030). The Daasanach are habitually unshod throughout ontogeny and into adulthood, with only some of the males wearing minimal footwear inconsistently beginning at the time of adolescence (personal observation and personal communication, A.K. Behrensmeyer, J.W.K. Harris). Relevant biometric data, including mass, stature, and foot length, were recorded for each subject. Subjects were asked to walk and run along a cleared, flat, open-ended 15 m-long natural surface trackway containing a 3 m-long calibrated space. In order to obtain a wide range of speeds, we asked the subjects to travel at a comfortable walk, fast walk, slow run, and fast run (each speed was later measured, see below). A pit measuring 150 cm long, 50 cm wide, and 15 cm deep was dug midway along the experimental trackway and was filled with sediment taken directly from a fossil footprint layer at the FwJj14E site (A1 layer sediment was used in 2010, sediment from the lower level was used in 2011; Bennett et al., 2009; Richmond et al., 2010). The sediment was rehydrated to approximate the conditions under which the 1.5 Ma fossil hominin footprints were made by adding water to the sediment until test footprint depths were comparable with those of the fossil footprints. Each subject performed as many trials as necessary until at least two usable trials were completed for each gait category. Trials in which the subjects visibly adjusted their gait or targeted the sediment patch were discarded from the dataset and repeated. Subjects were instructed to focus on a distant point on the landscape and most subjects showed no evidence of adjusting their gait. Many subjects missed the sediment patch during their running trials, suggesting that they were not paying close attention to its location. During each trial, the subjects created at least one footprint in the rehydrated sediment patch, which was photographed in high resolution with a Canon 5D Mark II (21 megapixel) camera, using a standard 50 mm lens to minimize radial distortion. The length and breadth of each experimental print was measured from these photographs using ImageJ (Rasband, 1997e2012). Footprint length was measured as the linear distance from the posterior-most point of the heel impression to the distal extent of the impression left by the longest digit (usually the hallux, but occasionally the second digit). A Wilcoxon signed rank test revealed no significant interobserver error for these footprint measurements (p ¼ 0.299; n ¼ 20). Recording and digitization differed slightly for the data collected in 2010 and The trials completed during the 2010 field season were filmed using a 60-Hz digital video recorder (Canon ZR50MC) from a lateral view in order to capture 2-dimensional kinematic data for each trial. These video data were later imported, calibrated, and digitized using the Peak Motus motion analysis software (7.2.10). Using this program, we measured the stride length and speed of travel for each experimental trial. Stride length was measured as the distance from the touchdown of one step to the touchdown of the next step made by the same foot. Each stride length was measured using the printmaking foot when possible. To account for any slight acceleration or deceleration from stride to stride, we measured velocity over the course of two gait cycles completed within the same trial. The 2011 experiments were completed using the same experiment design, but they were filmed using a high-speed (210-Hz) digital video recorder (Casio EX-FH20) and ImageJ (Rasband, 1997e2012) was used to measure stride lengths and speed in lieu of the Peak Motus system. A subset (n ¼ 48) of the video trials from 2010 were analyzed twice, once using each program, to ensure that there were no discrepancies between the results obtained from using the different software. We found no significant differences between the measurements taken using Peak Motus and those taken in ImageJ (p ¼ 0.327, Wilcoxon signed rank test). To address questions about how speed, body mass, and stature could be inferred from footprints, parametric regression statistics were used to assess the relationships between: 1) speed and stride length, 2) footprint length and stature, and 3) footprint area and body mass. The relationship between speed and stride length was evaluated using the speed of the experimental subjects for each trial and the value obtained from dividing the subject s stride length by footprint length. We used this value rather than the subject s stride length alone in order to adjust stride length relative to a measure of size that is preserved in the fossil print surface, following previous work (Charteris et al., 1981). An alternative method for calculating speed is based on the principle of dynamic similarity, which hypothesizes that two individuals move in a dynamically similar manner when they travel at equal Froude values. Froude is calculated as F ¼ v 2 /gl, where v is velocity, g is gravity, and l is characteristic length, resulting in a dimensionless value (Alexander, 1984a). Alexander (1984b) has demonstrated that dynamic similarity models are more appropriate in cases that involve individuals of different sizes and proportions. For example, dynamic similarity models have been used to make predictions based on modern human data about walking speeds of hominin taxa with significantly different proportions and smaller size, including A. afarensis (Alexander, 1984b) and Homo floresiensis (Vaughan and Blaszcyzk, 2008). Because the most appropriate methods for the current study depend on the extent to which the fossil hominin print-makers at Ileret were geometrically similar to modern humans, we report the results using both approaches. Here we use effective limb length (i.e., height of the greater trochanter) as the characteristic length for Froude calculation. Froude is then regressed against dimensionless stride length in log space to determine the relationship between these values (after Alexander, 1984b; Raichlen et al., 2008). Stature estimates were based on regressions of stature by footprint length. Body mass was regressed against both footprint length and footprint area, calculated as the product of footprint length and maximum forefoot breadth. The resulting equations from the least squares regression for each set of relationships (see Results, below) were subsequently used to predict: 1) speed, 2) stature, and 3) mass for the 1.52 Ma Ileret hominins based on stride length, print length, and print area measured from the fossil footprints at FwJj14E. Body mass data were restricted to those collected during the 2011 field season due to equipment malfunction in These experiments demonstrated that there is variation in footprint size for the same individual between different experimental trials, which should be taken into account in predictions for the hominin prints. Experimental footprints were found to range in lengths that were both longer (potentially due to foot movements) and shorter (e.g., in shallow prints when the foot may not sink

4 H.L. Dingwall et al. / Journal of Human Evolution 64 (2013) 556e completely into the sediment) than the actual foot length for a given subject. On average, footprint length measurements differed from the measured foot length by 0.4 cm (mean ¼ 1.7% error; maximum difference ¼ 4 cm, or 14.5% error). Furthermore, since gait dynamics change between walking and running, we completed the regression analysis for each relationship noted above in two ways: 1) with all data pooled, and 2) with data divided by gait category. For our predictions, we used the equation (i.e., walk, run, or combined) most appropriate for each individual trackway based on the speed estimates for each trackway. Results Daasanach experiment results Speed in Daasanach sample Digital measurement from video of the travel speed for all subjects across all trials yielded a range of speeds from 0.73 m/s to 6.35 m/s (Froude 0.05e4.23). The walk trials (n ¼ 148) were characterized by speeds ranging from 0.73 m/s to 2.54 m/s (Froude 0.05e0.61), while the run trials (n ¼ 124) yielded speeds ranging from 1.79 m/s to 6.35 m/s (Froude 0.31e4.23). The overlap of walking and running speeds between 1.79 and 2.54 m/s may imply a wide range of variation (0.8 m/s) in the transitional speeds between walking and running. However, it should be noted that 99.97% of the running trials measured were at speeds greater than Froude 0.5, the walkerun transition theoretically determined by the dynamic similarity hypothesis (Alexander, 1984a), and 99.95% of the walking trials measured were at speeds less than Froude 0.5. Thus, for the majority of the subjects in this study, the transition from a walk to a run occurred around speeds of 2.2 m/s (Froude 0.5), which corresponds with that predicted by the dynamic similarity hypothesis. Speeds of each experimental trial were plotted against the ratio of stride length to footprint length for each corresponding subject. Regression analysis of the experimental speed data shows a significant relationship between stride length and speed (r 2 ¼ 0.91, p < ; Fig. 1). This relationship is slightly stronger for running speeds (r 2 ¼ 0.80, p < ) than for walking speeds (r 2 ¼ 0.73, p < ) when speed categories are evaluated separately, but all relationships are highly statistically significant (see Table 1). Based on the principle of dynamic similarity, we also tested this relationship using Froude numbers. The regression of Froude on dimensionless stride length is also both strong and significant (r 2 ¼ 0.93, p < ), slightly stronger than the relationship for the raw velocities. When evaluated by gait category, this relationship is equally strong for walking and running speeds (r 2 ¼ 0.85, p < ), although slightly weaker than the regression that includes the entire dataset. Stature of Daasanach sample Stature measurements for all subjects (n ¼ 38) ranged from 154 cm to 184 cm. Male stature was found to range from 163 cm to 184 cm while female stature ranged from 154 cm to 177 cm. The stature measurement for the tallest female is an outlier (see below and Fig. 7a). Evaluation of the mean stature for each sex (male ¼ cm, female ¼ cm) shows that Daasanach males are significantly taller than females (Student s t-test; p < ). Based on these data, the Daasanach show significant sexual dimorphism in stature. Regression statistics were applied to these biometric data to determine the relationship between the lengths of the experimental footprints and the statures of the print-makers. For all data (i.e., combined data from both walking and running trials), a statistically significant relationship was found between stature and footprint length (r 2 ¼ 0.60, p < ; Fig. 2). Because they represent biomechanically distinct gaits, data from running and walking were evaluated separately. Results show a statistically significant relationship between footprint length and stature for both the walk-only (r 2 ¼ 0.61, p < ; Fig. 2) and run-only trials (r 2 ¼ 0.58, p < ; see Table 2). Mass of Daasanach sample Mass for Daasanach subjects from the 2011 experiments (n ¼ 19) ranged from 43.0 kg to 62.0 kg, with a mean of 51.1 kg. Just as for stature, females (n ¼ 9; mean ¼ 49.4 kg) were found to be lighter than males (n ¼ 10; mean ¼ 52.6 kg), although this difference is not statistically significant (two-sample t test, p ¼ 0.2). There is considerable overlap in the range of mass measurements for both groups (43.0e60.0 kg for females and 45.0e62.0 kg for males) as well as in the 95% confidence intervals of the means for the sex specific means (females: 45.9e53.0 kg, males: 48.5e56.7 kg; see below and Fig. 8a). Therefore, although the Daasanach are sexually dimorphic with regard to stature, the limited data here do not show significant dimorphism in mass. The relationships between footprint dimensions and body mass were also investigated via regression analysis. We tested the ability of both footprint length and footprint area to predict mass and, for each case, we tested regression models for the combined range of speeds as well as for the walk-only and run-only data. All of the linear regression models were statistically significant (p < ) but overall, body mass was correlated more closely with footprint area than with footprint length (see Table 3) and thus area was used to estimate body mass from the fossil footprints. While the regression based on the combined dataset of footprint area measured from both walking and running prints is somewhat more tightly correlated with mass (r 2 ¼ 0.58) than are the footprint areas measured from only walking prints (r 2 ¼ 0.52), for our predictions, we chose to use the walking print model for the fossil prints characterized by stride lengths and inferred speeds that fall within the experimentally determined walking speed range (Fig. 3). New fossil footprints Figure 1. Linear fit of stride length-to-footprint length ratio to speed for the full range of speeds measured demonstrating a significant relationship between stride length (adjusted for limb proportions) and speed. p < , r 2 ¼ 0.91, n ¼ 272. Several new footprints have been recognized (Richmond et al., 2010) since the initial announcement (Bennett et al., 2009) of fossil hominin footprints at FwJj14E. Excavations of the print surface known as the upper level, conducted in July 2011, focused on

5 560 H.L. Dingwall et al. / Journal of Human Evolution 64 (2013) 556e568 Table 1 Regression relationship between speed and stride length/average footprint length (SL/avgFPL). Linear fit for speed estimation Gait category Linear fit n R 2 adj. S.E.E. Prob. > F Walk only Speed ¼ 0.38 þ 0.30*(SL/avgFPL) < Run only Speed ¼ 0.63 þ 0.41*(SL/avgFPL) < Walk and run Speed ¼ 1.39 þ 0.48*(SL/avgFPL) < collecting new data from the known print surface. These renewed excavations resulted in the identification of four previously unrecognized hominin prints within the FUT1 trackway (Bennett et al., 2009). The FUT1 trackway was originally interpreted as a trail made by a single individual (Bennett et al., 2009). However, the position of the prints relative to each other, including the overlap of two left footprints (FUT1-7A and B; Fig. 4), indicates that prints of at least two individuals are preserved in the trail. Based on comparable distances between certain prints, similarities in print orientation, and subtle differences in morphology (e.g., slightly more abducted hallucal impression), we have developed a new hypothesis regarding which prints are most likely to belong in the same trackway and thus represent the same individual. Fig. 4 provides a schematic representation of this new working hypothesis for the FUT1 trackway. We use this hypothesis in our calculations of speed, stature, and mass predictions for the 1.52 Ma hominins. The accuracy of the speed estimates depends upon accurate stride lengths, which in turn depend upon correct footprint association. Stature and mass estimates, however, can be calculated for individual prints, so they are not influenced by the accuracy of the trackway hypothesis, particularly as the prints in this trail are comparable in size with one another. Two additional hominin prints (A2-I2 and A2-I3) have been unearthed in a sedimentary level between the lower level and upper level (Richmond et al., 2010). This level is designated as the A level ( A was preferred over naming it middle level to follow a naming scheme that allows us to name additional levels as we unearth them at this site). These prints are preserved in isolation; they provide no data regarding speed. However, their lengths and breadths (Table 4) provide data regarding hominin foot size from which stature and mass can be estimated. Footprint lengths and breadths are reported in Table 4 for all measurable prints from the upper, lower, and A levels. Estimates from fossil hominin footprints The stature, mass, and speed estimates derived directly from the Daasanach regressions are based on the assumption that foot size and body size proportions of the Ileret printmakers were comparable with those of our modern human sample. However, three hominin species, H. erectus, H. habilis, and P. boisei, are known from c. 1.5 Ma sediments at Koobi Fora, within a few kilometers of FwJj14E (Feibel et al., 1989; Spoor et al., 2007). Therefore, any of these taxa could have made prints in sediments of this age and it is possible that multiple species were active at FwJj14E, especially given the fact that footprints are found in three temporally distinct strata (Fig. 4). To assess the validity of assuming foot proportions like those of modern humans, we investigated the pedal proportions of H. erectus. This was not possible for H. habilis and P. boisei since securely attributed postcranial fossils relevant to the assessment of foot-body size proportions are not known for these species (but see below). The site of Dmanisi yielded an adult H. erectus partial skeleton (D4166) that preserves an associated femur and third metatarsal (MT3; Lordkipanidze et al., 2007). While foot length and stature cannot be directly measured on this, or any other, early H. erectus fossil, the lengths of the MT3 and femur provide the best available measure of foot length, leg length, and stature as they are among the longest bones in their respective anatomical regions. The MT3:femur length proportions of D4166 fall securely in the range of modern humans (from the Terry collection, Smithsonian Institution s National Museum of Natural Figure 2. Linear fit of footprint length to stature for the walk speeds only showing a significant relationship. p < , r 2 ¼ 0.61, n ¼ 38. This relationship is used to predict stature for the FwJj14E hominins. Figure 3. Linear fit of footprint area to mass for the walk speeds only showing a significant relationship. p < , r 2 ¼ 0.52, n ¼ 19. This relationship is used to predict body mass for the FwJj14E hominins.

6 H.L. Dingwall et al. / Journal of Human Evolution 64 (2013) 556e Figure 4. a) Schematic representation of the stratigraphic orientation of the three footprint surfaces. The FLT (i.e., Lower Trackway ) and FUT (i.e., Upper Trackway ) surfaces have been published previously (Bennett et al., 2009). The A level contains two new isolated prints from different individuals. Stratigraphic section adapted from Behrensmeyer (2011). Sediment texture scale: C ¼ clay, Z ¼ silt, S ¼ sand, G ¼ gravel. b) Diagram showing our current working hypothesis for the FUT1 trackway, which is now believed to represent at least two individuals. Red footprints (dark grey in print version) represent the FUT1A trackway (individual 2) and blue footprints (light grey in print version) represent the FUT1B trackway (individual 1). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Figure 5. Comparison of third metatarsal length-to-femur length proportions measured for eastern gorillas (G. beringei, n ¼ 4), western gorillas (G. gorilla, n ¼ 13), the Dmanisi large adult (H. erectus; Lordkipanidze et al., 2007), modern humans (H. sapiens, n ¼ 20), and chimpanzees (P. troglodytes, n ¼ 20). The Dmanisi specimen falls squarely within the range of modern human proportions and has a distinct pattern from that of the African great apes. Figure 6. Estimated speeds for each hominin trackway from FwJj14E based on the Daasanach regression using raw speed. Dashed box represents the range of speeds at which our subjects were observed to transition from a walk to a run (2.0e2.3 m/s). Note that the majority of these estimated speeds are certainly walking speeds, with the exception of the estimate for the FLT1 trackway, which may represent a slow jogging speed. Error bars represent 95% confidence intervals.

7 562 H.L. Dingwall et al. / Journal of Human Evolution 64 (2013) 556e568 Figure 7. a) Daasanach measured stature (separated by sex) compared with hominin stature estimates. b) Hominin stature estimates based on the Daasanach regression separated by individual trackway. Error bars represent 95% confidence intervals. History) and are distinct from those of gorillas and chimpanzees (American Museum of Natural History collections; Fig. 5). Thus, if H. erectus individuals were indeed responsible for the FwJj14E footprints, our regression models are based on reasonable assumptions about foot to body proportions. To assess how the results would differ if the Ileret printmakers had more primitive foot:body size proportions, we examined the only available estimates of foot and stature lengths for an earlier hominin, namely A. afarensis, which may approximate the primitive condition from which H. habilis and P. boisei evolved (until good evidence of relative foot size is available for H. habilis and P. boisei,weworkwith the parsimonious assumption that they retain relatively primitive anatomy, such as that seen in A. afarensis). Fleshy foot length in AL has been estimated as 16.5 cm based on measurements of a composite foot skeleton including the scaled length of the AL toe and metatarsal head region, and the scaled length of much of the remaining foot based on OH 8 (White and Suwa, 1987). A larger foot length estimate of cm was reached based on the relationship between talus length and subtalar length (Jungers,1988b). Combined with AL s femur length (28.1 cm), the relative fleshy foot length (100 fleshy foot length/femur length) is 58.7e61.4, well below that of bonobos (73.9), chimpanzees (82.5), and LB1 (70.0; the type specimen of H. floresiensis), and slightly above the values observed for samples of modern humans (54.2, 54.5 for small-bodied pygmies; Jungers et al., 2009). When comparing fleshy foot length estimates with AL s estimated stature (106.7 cm, or ; Jungers,1988b), the foot:stature ratio for AL is 0.155e0.162, which is significantly smaller than that observed for bonobos (mean ¼ 0.188; range: 0.171e0.200; data from Coolidge and Shea,1982) and above the mean but within the range of our Daasanach sample (mean ¼ 0.150; range: 0.138e0.162). This suggests that the stature, mass, and speed estimates would not differ substantially if the Ileret printmakers had more primitive foot proportions. In light of the general similarity in foot:body size proportions of early hominins and modern humans, as well as the comparability between the sizes of the Ileret prints and those made by our Daasanach subjects, we prefer the geometric similarity models (e.g., stride length/foot length) in our analyzes. Nonetheless, we also report speed predictions based on a dynamic similarity model for comparison. The dynamic similarity prediction equation was derived by regressing Froude number on relative stride length in log space using effective lower limb length rather than stature for characteristic length, since the former yields a better relationship and is theoretically more appropriate (Alexander, 1984b; Raichlen et al., 2008). Below, we provide estimates derived directly from the Daasanach regressions (assuming modern human proportions for the Ileret printmakers) and an estimate of how those values would differ if the proportions of the Ileret hominin printmakers were more primitive (i.e., like those of A. afarensis). Speed estimates from fossil footprints The experimentally-derived regression equation representing the relationship between speed and the ratio of stride length to footprint length was used to estimate speeds from five partial trackways from two stratigraphic levels at the FwJj14E site. Stride lengths of fossil trackways were measured as the distance from the heel of one print to the heel of the next print made by the same foot where possible. For prints with poorly preserved heel impressions, stride length was measured as the distance from hallux to hallux. Where only the length of a single step was preserved, the step length was measured and doubled to approximate stride length. The trackway of two prints from the oldest stratigraphic level (the lower level) has a stride length/foot length that overlaps those Table 2 Regression relationship between stature and average footprint length (FPL). Linear fit for stature estimation Gait category Linear fit n R 2 adj. S.E.E. Prob. > F Walk only Stature ¼ þ 3.72*avgFPL < Run only Stature ¼ þ 3.87*avgFPL < Walk and run Stature ¼ þ 3.78*avgFPL <

8 H.L. Dingwall et al. / Journal of Human Evolution 64 (2013) 556e Figure 8. a) Daasanach measured mass (separated by sex) compared with hominin mass estimates. b) Hominin body mass estimates based on the Daasanach regression separated by individual trackway. Error bars represent 95% confidence intervals. observed for the walking and running trails in the Daasanach experiments. Using the equation that incorporates the full range of speeds yields an estimate of 2.69 m/s 0.41 m/s (95% prediction interval), compared with 2.21 m/s 0.20 m/s for the walk-only regression and 2.94 m/s 0.48 m/s for the run-only equation (Fig. 6). These speeds are consistent with a slow run or fast walk; the overlap between walking and running gaits established by our experiments occurs around 2.2 m/s. Because the FLT1 trackway only consists of two footprints, in other words a single step, more work is necessary to determine whether the morphology of these prints can conclusively establish whether the prints were made by a walking or running gait. The other four trackways are in the youngest stratigraphic level (the upper level). The walk-only regression equation was used to estimate the speeds at which these fossil prints were created because their stride length/footprint length values fell within the range of those observed during walking trials in the Daasanach experiments. Two of these trackways (FUT1A and B) are unique within the FwJj14E assemblage in that each trackway contains multiple prints (Fig. 4). Whereas all of the other trackways consist of only one or two useable prints, each of the FUT1 trackways contains six footprints. Estimates of both FUT1 trails show that these two individuals were traveling at comparable speeds (FUT1A: m/s; FUT1B: m/s), both of which are characteristic of a slow walking gait (Fig. 6). Based on our calculations, the remaining trackways in the upper level (FUT2 and FUT3) are also slow walking trails. We have estimated that FUT2 was made at a speed of m/s and FUT3 was made at a speed of m/s. Speed estimates derived from the regression of Froude speed on dimensionless stride length, based on the walking data, are similar to those reported above. The speeds of the FUT1 trails were predicted to be m/s (95% PI; Froude 0.02) for individual 1 and m/s (Froude 0.04) for individual 2. The dynamic similarity speed estimate for the FUT3 trackway was also slightly lower than the estimate based on the raw speeds at m/s (Froude 0.02). The FUT2 and FLT1 trackways, however, produced slightly larger estimated speeds than those reported above. Using this dimensionless model, we calculated speeds of m/s (Froude 0.11) for the FUT2 trackway and m/s (Froude 0.74) for the FLT1 trackway (see Table 5), suggesting that the FLT1 individual may have been traveling at a slow run. Nonetheless, the differences between the two sets of estimates are only slight; in fact, the confidence intervals for the speeds calculated by the two different methods overlap with each other. If the makers of the FwJj14E footprints instead belonged to a different taxon that had proportions different from those of H. erectus and modern humans (e.g., longer feet relative to leg length and stature), our regression models would have underestimated the hominins traveling speeds. To test how such differences in limb proportions would affect these estimates, we have calculated the FwJj14E speeds using the limb proportions known Table 3 Regression relationships between mass and average footprint length (FPL) and area (FP area). Linear fit for mass estimation Gait category Measurement Linear fit n R 2 adj. S.E.E. Prob. > F Walk only Footprint length Mass ¼ 4.71 þ 1.82*avgFPL Run only Footprint length Mass ¼ 1.99 þ 1.97*avgFPL Walk and run Footprint length Mass ¼ 3.94 þ 1.87*avgFPL < Walk only Footprint area Mass ¼ þ 0.11*avgFP area Run only Footprint area Mass ¼ þ 0.13*avgFP area < Walk and run Footprint area Mass ¼ þ 0.12*avgFP area <0.0001

9 564 H.L. Dingwall et al. / Journal of Human Evolution 64 (2013) 556e568 Table 4 Hominin footprint lengths and breadths. * Individual Footprint Length (cm) Breadth (cm) FUT1A FUT FUT1-4 e 8.7 FUT FUT FUT1-7A FUT FUT1B FUT FUT1-2 e 9.9 FUT1-4i FUT1-4ii FUT1-7B e 9.62 FUT2 FUT e FUT FUT3 FUT FLT1 FLT FLT A2-I2 A2-I A2-I3 A2-I * More subtle prints that do not preserve reliable length and breadth measurements have been omitted. from AL (Jungers, 1982; Pontzer et al., 2010), the results of which are reported in Table 5. The reduced relative leg length estimate (about 7.3% shorter) based on AL s proportions resulted in slightly faster speed estimates, but do not change the conclusions. Thus any calculation method shows that these individuals were walking slowly, with the exception of the FLT1 trail with a stride length reflecting an individual traveling at a slow running or fast walking pace. Stature estimates from fossil footprints Living stature estimates for the Ileret hominins were calculated using the equation that describes the relationship between each Daasanach subject s stature and his or her experimental footprint lengths. For all fossil trackways made at walking speeds (see above), the regression data were limited to the footprints produced during walking trials; predictions for individual FLT1 used combined walking and running data. Stature was estimated for a total of 14 hominin prints from three different stratigraphic levels at FwJj14E: two prints from a single individual from the lower level, two isolated prints of two individuals from a middle level (A2), and 11 prints representing four individuals from the upper level that, all together, likely represent seven individuals (Table 4). Stature estimates were based on at least two prints for each individual except in the case of the two isolated prints, A2-I2 and A2-I3, which yield estimates of cm (95% PI) and cm, respectively, and thus are not likely to have been made by the same individual. The youngest level (upper level) contains the largest stature estimate of the assemblage, cm, which is attributed to the FUT2 trackway. The FUT3 trackway produced an estimate of cm, which is the smallest stature estimate from the upper level. The print lengths from the FUT1B trackway yield a stature estimate of cm for individual 1, while the estimate for individual 2 (FUT1A) is smaller at cm. We estimate a stature of cm for the individual in the oldest stratigraphic layer stature (Fig. 7b). In sum, the stature estimates for the seven individuals, based on 14 fossil prints, range from cm to cm, which is similar to the Daasanach stature range (154e184 cm; Fig. 7). The error in footprint length due to variation within an individual (see Methods ) results in a 3% error in stature estimation. Thus, incorporating this error into our estimates would expand the range of possible statures for the Ileret hominins to 148.4e191.4 cm. If the Ileret printmakers had AL s estimated foot:stature proportions (0.155e0.162), their stature estimates would range from cm for the smallest footprint to 192 cm for the individual with the largest prints (no prediction limits are available when using a simple ratio instead of a regression), indicating large statures for the Ileret printmakers regardless of assumptions about relative foot proportions (see Table 5). Mass estimates from fossil footprints The experimentally determined relationships between footprint area and body mass (see Table 4 for equation) were used to estimate body mass from measurements of the FwJj14E footprints. Only the equation derived from measurements of walking footprints was used because the fossil stride length/footprint lengths and inferred speeds were most similar to those of the walking footprint experiments, with the exception of individual FLT1, for which combined walking and running data were used. Estimated masses for the hominin individuals range from 41.5 kg to 60.3 kg (3.8 kg, 95% PI). These mass estimates fall within the range of Daasanach masses reported above (see Fig. 8a). The largest individual is again attributed to the FUT2 trackway. Likewise, the smallest mass estimate is associated with the A2-I2 individual, who also exhibited the smallest stature estimate. Overall, the stratigraphic levels containing tall individuals also produced heavy mass estimates-upper level: 44.5e60.3 kg; A level: 41.5e52.7 kg; lower level: 48.4 kg, all with estimate intervals of 3.8 kg. A 5.7% error in mass estimation resulted from the variation noted in experimental footprint size. With this source of error included, a conservative estimate of the possible mass for these hominins would widen to a range between 39.1 kg and 63.7 kg. If the Ileret printmakers had AL s estimated foot length:body mass proportions (ranging from 0.543e0.632, using the fleshy foot lengths estimates above and mass estimates of 27.3 kg and 30.4 kg from Jungers, 1982; Jungers, 1988c; McHenry, 1992), their mass estimates would range from 33.4 kg to 55.1 kg (no prediction intervals are available when using simple ratios). These estimates overlap with the range previously reported for A. afarensis (McHenry, 1992; McHenry and Coffing, 2000), but body Table 5 Comparison of estimates using proportions of H. sapiens versus A. afarensis. Hominin individual Speed (m/s) * Stature (cm) Mass (kg) H. sapiens estimate A. afarensis estimate H. sapiens estimate A. afarensis estimate H. sapiens estimate A. afarensis estimate FUT1A (Froude 0.04) 0.68e0.71 (Froude 0.06) e e46.8 FUT1B (Froude 0.02) 0.48e0.51 (Froude 0.02) e e49.4 FUT (Froude 0.11) 1.11e1.17 (Froude 0.13) e e55.1 FUT (Froude 0.02) 0.44e0.46 (Froude 0.02) e e43.3 FLT1 y (Froude 0.74) 2.83e2.98 (Froude 0.96) e e47.0 A2-I2 e e e e38.9 A2-I3 e e e e48.8 * Calculated using Froude speed equations. y Estimates calculated using the combined regression equations that include all gait categories.

10 H.L. Dingwall et al. / Journal of Human Evolution 64 (2013) 556e mass estimates for most individuals are near or above the upper limit for A. afarensis, and are more comparable with the mass estimates of H. erectus and the upper end of the P. boisei range (see Tables 5 and 6). Discussion Speed The speeds of the hominin individuals differ between the two stratigraphic horizons that preserve trails (the upper and lower layers; prints A2-I2 and A2-I3 in the A layer are isolated). The lower level FLT1 prints show evidence of a large individual traveling at a slow run or fast walk (2.2e2.7 m/s, or 4.9 to 6.0 miles/hour). The four trails in the upper layer all have short stride lengths, which suggest that these hominins were traveling at very slow walking speeds (0.45e1.0 m/s, or 1.0 to 2.2 miles/hour). These speed estimates are robust, regardless of assuming modern human or primitive (A. afarensis-like) foot:hindlimb length proportions, or using direct regressions or dimensionless Froude numbers (Alexander, 1984b; Raichlen et al., 2008; Vaughan and Blaszcyzk, 2008). In this respect, the upper layer footprints show some similarity to the only other hominin footprints known from the early Pleistocene, those at the site of GaJi10 in Area 103 at Koobi Fora, Kenya (Behrensmeyer and LaPorte, 1981), dating to c Ma (Bennett et al., 2009). The single GaJi10 hominin trail preserves large (c. 26 cm length) footprints with a very short stride length (c. 80 cm). Using our walking speed regression (Table 1), this hominin would have been walking at a speed of approximately 0.55 m/s (or 1.2 miles/hour), falling at the low end of the speed estimates for the FwJj14E upper layer trails reported here (Fig. 6). Furthermore, the GaJi10 prints are oriented at an angle relative to the direction of travel, suggesting to Behrensmeyer and LaPorte (1981:169) a hesitant, somewhat sideways progression across a slippery surface. Body size The new stature and mass estimates for the FwJj14E hominins reported here provide evidence of large body size at 1.51e1.53 Ma. Because three hominin taxa e H. erectus, H. habilis, and P. boisei e are present in Okote Member deposits within a few kilometers of the FwJj14E site, it is unclear which species was responsible for making the footprints at this site. Of the three known taxa, it is unlikely that the small-bodied H. habilis (McHenry and Coffing, 2000) made such large footprints. The stature and mass estimates reported here are consistent with estimates based on H. erectus fossils (Ruff and Walker, 1993; Aiello and Wood, 1994; Kappelman, 1996; Ruff et al., 1997; McHenry and Coffing, 2000; Ruff, 2007); even the estimates that were calculated using A. afarensis proportions indicate larger body size. Indeed, they do overlap with australopith body size estimates, but most are larger than sizes predicted for australopiths such as A. afarensis or Australopithecus africanus, which is even smaller-bodied (McHenry, 1992). Furthermore, foot:leg proportions for A. afarensis are tenuous and should be treated with caution, as they are based on an amalgamation of specimens from multiple hominin taxa (White and Suwa, 1987) or on very fragmentary remains (Jungers, 1998a). We also note that stature estimates for A. afarensis (McHenry, 1991, Table 6) are likely to be overestimated because they are based on a femur:stature ratio observed for modern humans (Feldesman et al., 1989, 1990), which does not take into account the relatively short femur length in A. afarensis (Richmond et al., 2002) or the positively allometric relationship between femur length and stature in modern humans (Auerbach and Sylvester, 2011). Shorter stature estimates for australopithecines would further accentuate the stature differences of the Ileret printmakers. Estimated body size for the Ileret hominins may also be consistent with the body size of male P. boisei. Existing mass estimates for individual P. boisei specimens range from 32 kg to as large as 70 kg (Hartwig-Scherer, 1993; Aiello and Wood, 1994; Kappelman, 1996). Based on specimens with uncertain taxonomic attribution, McHenry and Coffing (2000) estimate P. boisei male mean mass of 49 kg and female mean of 34 kg. Given the body mass estimates from orbit sizes (Aiello and Wood, 1994; Kappelman, 1996) for likely male individuals of P. boisei, specimens KNM-ER 406 (60e70 kg) and OH 5 (40e58 kg), and the high degree of size dimorphism in this taxon (Kappelman, 1996; Silverman et al., 2001), the male mean of 49 kg suggested by McHenry and Coffing (2000) is probably an underestimate. Similarly, stature estimates for P. boisei (male mean ¼ 137 cm; female mean ¼ 124 cm; McHenry and Coffing, 2000) are considerably smaller (and probably overestimated without taking allometry into account; see above) than those for H. erectus and the estimates derived here from the Ileret footprints, but little confidence should be placed on earlier estimates for P. boisei given the lack of securely attributed postcranial fossils. Some fossils that differ from those of H. erectus, such as the very large humerus, KNM-ER 739, may belong to P. boisei (McHenry, 1978). If so, and if cranial-based body size estimates are reasonable, then male P. boisei almost certainly reached larger mass and stature than those typical of earlier gracile australopiths. Under either assumption of modern humanlike or more primitive foot and body proportions, the estimates based on the FwJj14E footprints (Figs. 7 and 8; Tables 5 and 6) are consistent with cranialbased mass estimates for male P. boisei (Aiello and Wood, 1994; Kappelman,1996), suggesting that we cannot rule out the possibility that larger P. boisei individuals, presumably males, were responsible Table 6 Hominin body size comparison. Species Dates (Ma) Stature (cm) Mass (kg) Male Female Average Male Female Average Australopithecus afarensis 3.9e Paranthropus boisei * 2.3e1.2 e e e Homo habilis 2.3e Homo erectus (Turkana) 1.9e (Dmanisi) Homo heidelbergensis 0.7e0.2 e e e e 67.0 Homo sapiens (Daasanach) Extant FwJj14E hominins 1.51e1.53 e e (153.0e185.8) e e 50.0 (41.5e60.3) Stature estimates from: McHenry 1991, McHenry and Coffing, 2000 (A. afarensis, P. boisei, H. habilis); Ruff and Walker 1993 (H. erectus); Carretero et al (H. heidelbergensis). Mass estimates from: Kappelman, 1996 (P. boisei); McHenry 1992, McHenry and Coffing 2000 (A. afarensis, H. habilis); Ruff and Walker 1993 (H. erectus); Carretero et al (H. heidelbergensis). FwJj14E hominin estimates are based on modern human proportions. * Estimates are based only on cranial remains.