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Footprints Reveal Direct Evidence of Group Behavior and Locomotion in Homo Erectus

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Footprints Reveal Direct Evidence of Group Behavior and Locomotion in Homo Erectus

Kevin G Hatala et al. Sci Rep.

Abstract

Bipedalism is a defining feature of the human lineage. Despite evidence that walking on two feet dates back 6-7 Ma, reconstructing hominin gait evolution is complicated by a sparse fossil record and challenges in inferring biomechanical patterns from isolated and fragmentary bones. Similarly, patterns of social behavior that distinguish modern humans from other living primates likely played significant roles in our evolution, but it is exceedingly difficult to understand the social behaviors of fossil hominins directly from fossil data. Footprints preserve direct records of gait biomechanics and behavior but they have been rare in the early human fossil record. Here we present analyses of an unprecedented discovery of 1.5-million-year-old footprint assemblages, produced by 20+ Homo erectus individuals. These footprints provide the oldest direct evidence for modern human-like weight transfer and confirm the presence of an energy-saving longitudinally arched foot in H. erectus. Further, print size analyses suggest that these H. erectus individuals lived and moved in cooperative multi-male groups, offering direct evidence consistent with human-like social behaviors in H. erectus.

Figures

Figure 1
Figure 1. 1.5 Ma hominin tracks from Ileret, Kenya.
Representative images of hominin tracks uncovered in the Ileret area between 2007 and 2014. These tracks come from five different sites within about 1.5 km of each other. Some tracks show deterioration and overprinting, while many preserve fine detail, indicating that they were rapidly hardened and covered with sediment. No two sites represent the same continuous surface, as all come from different stratigraphic levels within the Ileret tuff complex. The total sample includes 97 hominin tracks produced by at least 20 different individuals.
Figure 2
Figure 2. Comparisons of external footprint dimensions and body mass predictions from fossil hominin footprints.
In all boxplots, the box encloses the 25–75% interquartile range, the bold line represents the median, and the upper and lower whiskers extend to the largest and smallest observations within a distance of 1.5 times the interquartile range above and below the limits of the box. (A) The 1.5 Ma Ileret hominin footprints are comparable in size (length and breadth) to the prints of habitually barefoot modern humans. The 3.7 Ma prints from Laetoli are considerably shorter in length and only somewhat narrower. In the figure, data are averaged by trackway (fossil tracks) or subject (human tracks). Total sample sizes are—human (n = 41 subjects, 490 footprints), Ileret (lengths: n = 28 trackways, 46 footprints; breadth: n = 36 trackways, 68 footprints), Laetoli (n = 1 trackway, 5 footprints). (B) Predictions of body mass from fossil track dimensions (left), are compared with body masses estimated from postcranial skeletal material of hominin species living near Ileret around 1.5 Ma (center) and measured body masses of the habitually barefoot modern human experimental sample (right). The Ileret hominin prints (n = 23 trackways) suggest much larger body masses than the prints from Laetoli (n = 1 trackway). They are more comparable in predicted mass to modern humans (n = 41 subjects) and skeletally derived estimates for H. erectus (n = 4) than they are to estimates for fossils attributed to P. boisei (n = 1) or H. habilis (n = 2), consistent with preliminary analyses. See Supplementary Table 3 for details on fossil skeletal sample. (C) Photograph of an exceptionally large (>30 cm long) Ileret track, estimated to be a 58.8 kg male. Scale at left is 15 cm.
Figure 3
Figure 3. Forefoot depth profiles of modern human and fossil hominin footprints.
Boxplots compare regional depth profiles of modern human footprints (n = 490 footprints from 41 individuals) to those of the 1.5 Ma Ileret (n = 11 footprints from 8 trackways) and 3.7 Ma Laetoli (n = 5 footprints from 1 trackway) hominin tracks. Top row represents depths across the toes while bottom row represents depths across the metatarsal heads. In each plot, medial is left and lateral is right. The image at far left shows the distribution of pressure including the path of the center of pressure, plotted as a dashed black line, during a typical human walking step. The overall forefoot morphology of the Ileret tracks closely resembles that of human tracks and provides evidence of a human-like medial weight transfer. The Laetoli tracks are distinct from those of humans and the Ileret hominins, and reflect a different pattern of foot biomechanics. Note that scales differ only for the purpose of better visualizing the variation within the relatively smaller fossil samples. In all boxplots, the box encloses the 25–75% interquartile range, the bold line represents the median, and the upper and lower whiskers extend to the largest and smallest observations within a distance of 1.5 times the interquartile range above and below the limits of the box.
Figure 4
Figure 4. Schematic maps of excavated footprint surfaces at sites FE3 and FwJj14E.
Map of the Ileret area (lower left) shows the locations of sites FE3 and FwJj14E, marked by black stars. Schematic maps of the excavated surfaces at FE3 (top left) and the FwJj14E Upper Footprint Layer (right) show the presence of multiple trackways across each of these surfaces. Print size analyses indicate that the groups of individuals represented at each site consist of predominantly males. Multiple trackways at FwJj14E show parallel directional movement and similar preservation states, suggesting that they could represent a group traveling together. Note that the schematic map of the FwJj14E surface has been rotated relative to North for visualization purposes. Solid red lines mark borders of the current excavations, and the same geological layers that preserve footprints are known to extend beyond these borders. Dashed red lines indicate the finite edge of the preserved surface, as areas beyond these lines have been lost due to erosion. The schematic map of the Ileret area was created by N.T.R., using a map generated in ArcGIS software version 10.2 (https://www.arcgis.com).

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