Estimates of hominin body size are typically made from a handful of measurements taken from just a few individuals separated in space and time. Calculations of body mass derived from the Ileret track assemblage offer a rare opportunity to examine this important measure at the population scale from individuals living in close proximity to each other. The Ileret footprints are generally comparable in size (length and breadth) to those of footprints produced in the same substrate by habitually barefoot Daasanach people living near Ileret today and differ markedly from the earlier 3.7 Ma Laetoli hominin tracks (Fig. 2A, Supplementary Table 1). Using experimental footprint and anthropometric data, a machine learning algorithm was built that accurately predicts an individual’s body mass from their footprint’s external dimensions (more accurately than past linear regression techniques–see Methods). Body mass estimates were calculated for well-defined footprints from 23 Ileret trackways, where the whole foot outline was visible and not distorted by depositional/taphonomic factors. Based on stratigraphic positions, depositional contexts and quantifications of footprint sizes and morphologies, we estimate that these 23 Ileret trackways from which we could estimate body mass were produced by as many as 23 or as few as 15 unique individuals (in some cases we can neither exclude nor confirm the possibility that the same individual may have produced multiple trackways within the same site). For now, we present here 23 distinct predictions. The mean predicted mass from these 23 Ileret hominin trackways was 48.9 kg (standard deviation: 9.6 kg), which is generally comparable to the body sizes of adult Daasanach individuals (mean = 52.6 kg, standard deviation = 5.9 kg for a sample of 29 adults including 15 males and 14 females). In some cases, estimates derived from the fossil tracks are on the largest end of the Daasanach range (Fig. 2B,C, Supplementary Table 2). It is also important to note that these predicted body masses from the Ileret tracks are similar to the estimates of 48–52 kg that were derived from presumed H. antecessor footprints at the European site of Happisburgh21. One outlying Ileret track implies a small body mass and could represent a child. The length of this track, measured from the end of the heel to the tip of the hallux, is 20.5 cm and this length, according to the data collected by Ashton and colleagues21, is roughly equivalent to the foot size of a 9-year-old modern human.

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 analyses44. 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. Full size image

Estimates of body size derived from the Ileret tracks support their attribution to H. erectus. In Fig. 2B, body mass predictions are compared with recently published estimates based on skeletal material that is most reliably attributable to the three hominin taxa known to have lived in East Africa close to 1.5 Ma22 (Supplementary Table 3). The majority of the Ileret tracks produced body mass estimates that fall comfortably within the interquartile range of skeletally-based estimates for H. erectus. All but two outliers exceeded estimates derived from H. habilis specimens and the majority of mass estimates from the Ileret tracks are larger than that of the only confidently attributed P. boisei specimen, which has been described as an extremely robust male individual23. Small sample sizes of confidently attributed H. habilis and P. boisei postcrania lead to unavoidable complications in these comparisons, as does the total lack of postcrania that can be confidently attributed to Homo rudolfensis, a species also known from the Ileret area but with a poorly known temporal range. It remains possible that any of these sites could preserve the prints of more than one taxon but, because each of the five excavated footprint sites contains at least one set of prints with a predicted body mass that exceeds all estimates of either P. boisei or H. habilis (and falls squarely within the H. erectus range), this would require a level of sympatry and habitat overlap that included tolerance of multiple species traversing the same c.1–20 m2 areas within a short time span. Multiple lines of evidence regarding the anatomy, diet and reconstructed paleoenvironments of these taxa would also complicate a hypothesis of extreme habitat overlap (Supplementary Note 1). While the presence of multiple species cannot be ruled out, it is most parsimonious at this time to infer that H. erectus individuals are responsible for most, if not all, of the Ileret hominin prints.

Experimental data collected from footprints of habitually unshod modern humans shows that three-dimensional track shapes preserve direct evidence of lower limb motion patterns24. Using only the best-preserved samples, we compared the morphologies of the Ileret footprints (n = 11 footprints from 8 trackways) and the c.3.7 Ma Laetoli footprints (n = 5 footprints from 1 trackway) to a large sample of habitually barefoot modern human tracks (n = 490 footprints from 41 individuals). Six of the eight Ileret hominin trackways have morphologies that are statistically indistinguishable from those of modern humans, whereas a morphology similar to that of the Laetoli footprints was never observed in the entire human sample (Supplementary Table 4). Due to the ambiguities of other fossil evidence described above, these similarities between the Ileret and modern human footprints support the initial analyses14 of the Ileret prints and provide the first direct fossil evidence of a human-like pattern of external foot motion in H. erectus and a bipedal gait that mirrored what is seen in humans today.

During human walking, forces are applied to the ground in a diagnostic pattern in which greater forces are concentrated beneath the medial forefoot while the foot acts as a rigid lever with a toe-off through the first and second digits at the end of each step25. This medial transfer of pressure differs from the pattern of foot function observed in non-human primates and has been consistently recognized as a defining characteristic of human foot function26,27. Modern human footprints show an overall medial-to-lateral gradient of decreasing depth across the forefoot region (metatarsal heads and toes) (Fig. 3). This reflects our unique pattern of external foot function, including a medial transfer of pressure and toe-off through the hallux and second toe. The Ileret footprints show a pattern that is slightly different but still generally similar. They may lack a clear medial-to-lateral gradient but they still show deeper impressions beneath the medial parts of the forefoot. This appears to demonstrate that a generally human-like pattern of foot function dates back to at least H. erectus (Supplementary Note 2). In comparison, the Laetoli prints lack this medial-to-lateral depth gradient and imply a pattern of foot mechanics and a bipedal gait that differed significantly from that of modern humans and the Ileret hominins (Supplementary Note 3). Evidence for this weight transfer pattern in the Ileret tracks, combined with shallower print depths in the medial midfoot, strongly supports the presence of a modern human-like longitudinally arched foot in H. erectus, which would allow considerable energy savings during long-distance walking and running28. The Ileret footprints thus provide new direct evidence for human-like foot anatomy and foot function in H. erectus and support hypotheses of human-like functional patterns derived from the small sample of isolated foot fossils possibly attributable to this species13.

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. Full size image

Multiple lines of geological, sedimentary and taphonomic evidence suggest that the hominin tracks on any of the Ileret footprint surfaces were formed and buried within the same day, perhaps within a few hours29. First, many of the Ileret fossil tracks and trackways show similarly fine-detailed preservation states and track surfaces lack evidence of soil development or root traces. The depositional context and the lack of mud cracks on any of the footprint layers suggest that these sites were rapidly buried by fine or silty sand before any drying occurred29. Further, modern taphonomic experiments conducted on the shores of Lake Turkana have demonstrated that human footprints retain the level of anatomical detail preserved in the fossil tracks for a maximum of 1.3 days, on average29. In addition to this evidence for rapid formation and burial, some of the Ileret trackways show parallel directional movement that contrasts from the directions of travel for other animals29. This suggests that movement was not constrained and guided by natural land features and that hominins elected to move in the same direction. While we can never know with certainty the precise events that transpired 1.5 million years ago, these multiple lines of evidence are consistent with the hypothesis that within any one site, parallel hominin trackways could represent a group that traveled together, or at the very least a collection of individuals who co-existed and moved across the same landscape within the same day or so. The data therefore offer a rare opportunity to examine and test hypotheses about hominin group composition, which cannot be assessed directly from any other form of fossil data.

In the two most spatially-expansive footprint surface excavations, ET-2013-1A-FE3 and the Upper Footprint Layer at site FwJj14E, the presence of multiple trackways (and individuals) is clearly evident (Fig. 4). Using the mean method30 on our body mass predictions generated from well-preserved fossil tracks (see above), presumed sex was attributed for each of the different individuals who walked across these surfaces (Supplementary Table 2). These sex predictions indicate that a large proportion of both assemblages were likely created by adult males (3 of 4 individuals at ET-2013-1A-FE3 and perhaps as many as 8 of 16 individuals on the FwJj14E Upper Footprint Layer). This approach could conservatively underestimate the number of males present, since certain fossil discoveries11,31,32 and morphological analyses22,33 have evidenced that body size dimorphism in H. erectus was considerably higher than is observed in modern humans. If this were the case and the size differences between male H. erectus and female H. erectus were greater than those between modern human males and females, then our use of the mean method could lead to smaller males being incorrectly classified as females30 (except in the seemingly unlikely scenario that the Ileret footprints were all created by female H. erectus, in which case male H. erectus body masses must have been much larger than the body masses of male modern humans and the masses that have been predicted based on H. erectus skeletal evidence).

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). Full size image

It is important to note here that preservation bias can affect observed size variation in track assemblages, in that only individuals within a particular range of body masses leave discernible tracks on a substrate of a particular strength (i.e., the ‘Goldilocks’ effect)34. The morphologies of various tracks and trackways across a site can also be affected in disparate ways due to substrate variability across the same footprint surface35, such that size variation is affected in complex ways by substrate behavior. These factors may affect the diversity of sizes observed in the Ileret track assemblages, but this does not change the fact that we observe several sets of hominin footprints consistent with large overall body sizes. Regardless of the exact composition, the group of H. erectus individuals who at least lived in close proximity and possibly traveled together across the FwJj14E Upper Footprint Layer appears most likely to have included multiple males.

The observation of multiple H. erectus males interacting in close physical and temporal proximity and possibly even moving together, on these footprint surfaces provides the first direct evidence of hominin social group composition in deep time. Among primates, cooperative male-male alliances tend to form in situations where they can provide direct advantages for accessing mates or acquiring food resources36. Because male-male alliances are observed in both modern humans and several modern nonhuman primates, the presence of this behavior itself in fossil hominins may be expected and unsurprising. However, the Ileret footprint surfaces offer the first opportunity to directly observe such behavior in the human fossil record.

In modern human hunter-gatherers, male-male cooperation is a key component of foraging particularly when hunting animals and subsequently sharing highly-valued meat resources15. Furthermore, foraging for large mammals is a high-risk strategy that may not be possible without some degree of provisioning by other individuals, often females, who typically pursue more predictably obtained foods16. The orientations of the hominin trackways compared to those of other animals suggests that the other animals moved to and from the water shore while hominins moved along it and, based on observations of human and other animal behavior along the modern shore of Lake Turkana, this scenario suggests the possibility that the hominins may have been foraging. The sexual division of foraging behavior is known to distinguish modern human hunter-gatherer groups from great apes and all other mammalian social carnivores17. If H. erectus was characterized by relatively high levels of sexual dimorphism11,22,31,32,33 and the group of H. erectus individuals who possibly traveled together across the FwJj14E Upper Footprint Layer consisted of all or mostly adult males (i.e., the actual number of males is greater than our estimate because we applied the mean method to a sex-imbalanced sample of H. erectus footprints), then these data could be evident of sexually divided foraging behavior in H. erectus.

We do not exclude other possibilities, however, for the types of behavior that may be represented on these footprint surfaces. For example, chimpanzees are known to form predominantly male groups during border patrols37. Similarly directed movement by a (possibly) predominantly male group on the Ileret footprint surfaces could reflect some type of ‘patrol’ behavior that is not unique to modern humans but instead reflects patterns of behavior with much deeper roots in our evolutionary history. The snapshots of fossil hominin behavior that are preserved by the Ileret footprint surfaces can provoke a number of alternative interpretations and further work will be necessary in order to evaluate competing hypotheses.

Regardless of the exact behavior that was taking place, the data from multiple sites clearly show that groups of H. erectus individuals including multiple adult males walked together on the same landscape. These data are at the very least consistent with hypotheses that H. erectus had a group composition and dynamic that could have supported the emergence of human-like social behaviors such as patterns of increased cooperation and sexually-divided foraging behavior. Taken together, the data recorded within the Ileret footprint assemblages offer provocative evidence that is consistent with hypothesized grade-level shifts in the anatomy, locomotion and behavior of H. erectus compared to earlier hominin taxa8.