The longitudinal arch of the human foot is viewed as a pivotal adaptation for bipedal walking and running. Fossil footprints from Laetoli, Tanzania, and Ileret, Kenya, are believed to provide direct evidence of longitudinally arched feet in hominins from the Pliocene and Pleistocene, respectively. We studied the dynamics of track formation using biplanar X-ray, three-dimensional animation and discrete element particle simulation. Here, we demonstrate that longitudinally arched footprints are false indicators of foot anatomy; instead they are generated through a specific pattern of foot kinematics that is characteristic of human walking. Analyses of fossil hominin tracks from Laetoli show only partial evidence of this walking style, with a similar heel strike but a different pattern of propulsion. The earliest known evidence for fully modern human-like bipedal kinematics comes from the early Pleistocene Ileret tracks, which were presumably made by members of the genus Homo. This result signals important differences in the foot kinematics recorded at Laetoli and Ileret and underscores an emerging picture of locomotor diversity within the hominin clade.
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Raw data from biplanar X-ray experiments are publicly available through the XMAPortal at the following link: https://xmaportal.org/webportal/larequest.php?request=CollectionView&StudyID=43&instit=BROWN&collectionID=20.
Source data and code used to generate the figures in this manuscript are publicly available at the following address: https://doi.org/10.6084/m9.figshare.20736697.
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We thank D. Baier, B. Brainerd, S. Cheleden, F. Drury, K. Fiske, K. Huffman, B. Knörlein, D. Laidlaw, K. Tani Little, S. Megherhi, J. Novotny, D. North, M. Turner and the students of CS137 for assistance directly related to the design and implementation of this project. We thank the anonymous volunteers who participated in biplanar X-ray experiments. We are grateful to A. Manafzadeh for feedback at many stages of analysis. Discrete element simulations were made possible through a PRACE allocation of supercomputer resources (project 2021250007, Irene-Rome). This study received funding support from the National Science Foundation (BCS-1825403 to K.G.H. and P.L.F.; BCS-1824821 to S.M.G.) and from the Chatham University Research & Sabbatical Committee (to K.G.H.).
The authors declare no competing interests.
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Extended Data Fig. 1 Track RAV and navicular height.
Extended Data Fig. 2 Origins of kinematic hypotheses.
(a) Maya visualization of the foot at midstance directly above the track that this foot produced during a walking trial. (b) An ‘animation snapshot’ positioned at the same height as the foot in A, directly above the 3-D model of the track that this foot and its motion produced. The foot at midstance is relatively flat compared with the longitudinally arched track. However, the animation snapshot and the track are similarly arched. A sequence of similar observations led us to hypothesize that track arch morphology was a product of foot kinematics and not foot anatomy.
Extended Data Fig. 3 RAVs and morphologies of human and chimpanzee tracks.
(a) As an example we focus on one Laetoli track, one Walvis Bay track and one experimental chimpanzee track, with similar RAV and relative depth measurements (red circle). (b) The morphologies of the two hominin tracks and their respective arch models are similar to each other and readily distinguished from those of the bipedal chimpanzee (all models from right feet). The arch volume of hominin tracks is concentrated beneath the medial midfoot, while that of the bipedal chimpanzee track is concentrated distally, in between the first and second rays. Thus, the hominin tracks are arched longitudinally, while the bipedal chimpanzee track is not. Two different colour scales are applied to map relative heights, one for tracks and one for arch models, to optimize visualization of each set. Scale bar at right is 10 cm.
Extended Data Fig. 4 Photograph of trackway setup used for biplanar X-ray experiments.
Two X-ray emitters (foreground) project overlapping collimated X-rays that are received by two circular image intensifiers (background) equipped with video cameras. At the intersection of the biplanar X-ray beams is a container that is filled with mud (‘wet 5’ variety pictured here). Atop the remainder of the trackway is a deformable foam whose thickness matches the depth of mud within the container, which therefore allows subjects to sink to a similar extent with each step.
Extended Data Fig. 5 Interobserver variation in RAV measurements from track and foot 3-D models.
Paired observations and line of identity are plotted. Average interobserver difference was 0.42 (95% confidence interval of −1.00 to 0.15). RAV measurements were more consistent between observers for deeper tracks.
Supplementary Notes 1–3.
Supplementary Video 1
Oblique isometric view of an animated foot (animated using biplanar X-ray experimental data) moving through a DEM-simulated mud. The foot is semitransparent, allowing for observation of foot–substrate interactions. Playback of simulation allows for visualization of continuous track arch formation throughout stance phase.
Supplementary Video 2
Cross-sectional view of simulated track formation. Same animation and simulation as presented in Supplementary Video 1 but with an opaque foot and with the simulated mud sectioned from heel-to-hallux, as in Fig. 2. This provides a more direct perspective for visualizing track arch formation over time. The track’s arch begins to form soon after heel strike and is continually shaped through push-off.
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Hatala, K.G., Gatesy, S.M. & Falkingham, P.L. Arched footprints preserve the motions of fossil hominin feet. Nat Ecol Evol 7, 32–41 (2023). https://doi.org/10.1038/s41559-022-01929-2