Journal of Human Evolution 112 (2017) 93E104

Journal of Human Evolution 112 (2017) 93e104

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Journal of Human Evolution

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Hominin track assemblages from Okote Member deposits near Ileret, Kenya, and their implications for understanding fossil hominin paleobiology at 1.5 Ma

* Kevin G. Hatala a, b, , Neil T. Roach c, d, Kelly R. Ostrofsky b, Roshna E. Wunderlich e, Heather L. Dingwall c, Brian A. Villmoare f, David J. Green g, David R. Braun b, John W.K. Harris h, Anna K. Behrensmeyer i, Brian G. Richmond d, j a Department of Biology, Chatham University, Pittsburgh, PA 15232, USA b Center for the Advanced Study of Human Paleobiology, Department of Anthropology, The George Washington University, Washington, DC 20052, USA c Department of Human Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA d Division of Anthropology, American Museum of Natural History, New York, NY 10024, USA e Department of Biology, James Madison University, Harrisonburg, VA 22807, USA f Department of Anthropology, University of Nevada Las Vegas, Las Vegas, NV 89154, USA g Department of Anatomy, Midwestern University, Downers Grove, IL 60515, USA h Department of Anthropology, Rutgers University, New Brunswick, NJ 08901, USA i Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, DC 20013, USA j Humboldt Foundation Fellow at Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Leipzig D-04103, Germany article info abstract

Article history: Tracks can provide unique, direct records of behaviors of fossil organisms moving across their landscapes Received 1 September 2016 millions of years ago. While track discoveries have been rare in the human fossil record, over the last Accepted 7 August 2017 decade our team has uncovered multiple sediment surfaces within the Okote Member of the Koobi Fora Available online 13 September 2017 Formation near Ileret, Kenya that contain large assemblages of ~1.5 Ma fossil hominin tracks. Here, we provide detailed information on the context and nature of each of these discoveries, and we outline the Keywords: speciﬁc data that are preserved on the Ileret hominin track surfaces. We analyze previously unpublished Ichnology data to reﬁne and expand upon earlier hypotheses regarding implications for hominin anatomy and Trace fossils Footprints social behavior. While each of the track surfaces discovered at Ileret preserves a different amount of data Homo erectus that must be handled in particular ways, general patterns are evident. Overall, the analyses presented Koobi Fora here support earlier interpretations of the ~1.5 Ma Ileret track assemblages, providing further evidence of Pleistocene large, human-like body sizes and possibly evidence of a group composition that could support the emergence of certain human-like patterns of social behavior. These data, used in concert with other forms of paleontological and archaeological evidence that are deposited on different temporal scales, offer unique windows through which we can broaden our understanding of the paleobiology of hominins living in East Africa at ~1.5 Ma. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction paleoanthropological literature) and trackways (sequences of two or more consecutive tracks) have been used to address a wide array Across the field of paleontology, ichnological studies have of questions related to the biology of fossil organisms including figured prominently in the developments of major evolutionary questions related to social behavior (e.g., Ostrom, 1972; Lockley and hypotheses. Tracks (often referred to as ‘footprints’ in Meyer, 1994, 2006; Cotton et al., 1998; Matsukawa et al., 2001; Lingham-Soliar et al., 2003; Bibi et al., 2012), paleoecology (e.g., Lockley et al., 2007; Dentzien-Dias et al., 2008; Smith et al., 2009; Kukihara and Lockley, 2012), body size (e.g., Lockley, 1994; * Corresponding author. E-mail address: [email protected] (K.G. Hatala). Henderson, 2003), and locomotion (e.g., Alexander, 1976; Gatesy http://dx.doi.org/10.1016/j.jhevol.2017.08.013 0047-2484/© 2017 Elsevier Ltd. All rights reserved. 94 K.G. Hatala et al. / Journal of Human Evolution 112 (2017) 93e104 et al., 1999; Wilson and Carrano, 1999; Ezquerra et al., 2007; and can help to inform long-standing questions relating to the Castanera et al., 2013). evolution of human anatomy, locomotion, and behavior. Compared with their frequency in other paleontological records, Here, we present new evidence from multiple fossil hominin the known sample of track sites from the early parts of the human track sites discovered in recent years within ~1.5 Ma fossil deposits fossil record is sparse. Tracks attributed to modern Homo sapiens are near Ileret, Kenya. In the years since the track surfaces at site known at sites all around the world from the late Pleistocene (e.g., FwJj14E near Ileret were first described, expansions of these earlier Mountain, 1966; Roberts and Berger, 1997; Webb et al., 2006; Kim excavations combined with new surveys have led to discoveries of et al., 2009; Liutkus-Pierce et al., 2016) and from the Holocene new tracks and trackways and multiple additional ~1.5 Ma track (e.g., Rector, 1979; Roberts et al., 1996; Meldrum, 2004; surfaces in nearby areas (Dingwall et al., 2013; Richmond et al., Mastrolorenzo et al., 2006; Morse et al., 2013). Sites earlier than 2013; Hatala et al., 2016a; Roach et al., 2016, 2017). The total these are much rarer. One famous site preserving Pliocene assemblage of 1.5 Ma hominin tracks known from the Ileret area has (~3.66 Ma; Deino, 2011) hominin trackways was discovered at grown in size from the initially published sample of 20 tracks at a Laetoli, Tanzania, in 1978 (Leakey and Hay,1979). At the time of their single site (FwJj14E; Bennett et al., 2009) to a currently known discovery, the oldest known hominin skeletal fossils were younger sample of 97 hominin tracks across five distinct sites (Hatala et al., than 3.66 Ma, so these tracks provided indisputable evidence of the 2016a). The size and nature of this assemblage of ~1.5 Ma hominin earliest known appearance of fossil hominins and also showed that tracks now permits the exploration of an entirely new series of these earliest known hominins were bipeds. These tracks are typi- questions that could not be addressed in the initial publication. cally assumed to have been produced by Australopithecus afarensis, Recently published analyses (Hatala et al., 2016a) have focused on which is still the only hominin known to have inhabited the Laetoli the implications of these tracks and trackways for locomotion and area at this time (but see Tuttle et al., 1991). The Laetoli footprints social behavior. We have also assessed the implications of the offered direct evidence to support interpretations from fossil skel- hominin tracks for patterns of land use and the overall paleoenvir- etal morphology that early hominins with small brains and large onmental context of the track surfaces (Roach et al., 2016, 2017). In teeth walked bipedally (Leakey and Hay, 1979; Day and Wickens, the present study, we aim to 1) provide detail on the discoveries and 1980; White, 1980; Leakey, 1981), definitively contradicting much excavations of each of the five ~1.5 Ma hominin track sites at Ileret, earlier hypotheses (e.g., Darwin, 1871). Around the same time (in including their geological and sedimentological contexts, to aid 1978), an assemblage of early Pleistocene (~1.4 Ma) hominin tracks other researchers who may encounter or wish to search for similar was uncovered at Koobi Fora, Kenya (Behrensmeyer and Laporte, sites, 2) analyze external dimensions of Ileret hominin tracks within 1981). Because that site was less extensive, involving only seven a broad comparative framework, to further evaluate their taxo- tracks from one individual with variable preservation, and because nomic affiliation, 3) provide estimates of traveling speed for all of their younger age, these tracks from Koobi Fora have historically excavated Ileret trackways, and incorporate those results into past received less attention than those from Laetoli. inferences regarding social behavior, 4) explore, in detail, behavioral For two decades, these two were the only known hominin track hypotheses consistent with the evidence found within the Ileret sites that predated the emergence of anatomically modern humans. track sites, and 5) provide all raw data relevant to these analyses. As a result, despite an initial flurry of analyses regarding the Laetoli trackways, fossil hominin tracks have received considerably less 2. Discoveries and excavations of Ileret hominin track attention from paleoanthropologists than hominin skeletal fossils. surfaces Within the past 10e15 years, however, several sites that preserve pre-H. sapiens (>315 ka; Hublin et al., 2017) tracks have been 2.1. Initial discoveries of hominin track surfaces at FwJj14E discovered in both Europe and Africa, and these have provided important new data for the human fossil record. In 2001, re- The possibility that track surfaces might be preserved in Pleis- searchers conducted a detailed investigation of tracks long known tocene sedimentary deposits near Ileret, Kenya was recognized to exist on the slopes of Roccamonfina, a volcano in Italy. This study during paleontological surveys (by A.K.B.) in the late 1970s, but it concluded that these tracks were between 325 and 385,000 years was several decades before any tracks were discovered in this area. old and therefore must have been produced by a pre-H. sapiens In 2005, paleontological excavations were started at site FwJj14E taxon (Mietto et al., 2003; Avanzini et al., 2008). From 2006 to 2008, after the discovery of fossil skeletal material from a presumed multiple stratigraphic layers dating to ~1.5 Ma at Ileret, Kenya, were P. boisei upper limb (Richmond et al., 2009). In order to understand found with preserved hominin trackways likely attributable to the geological/sedimentary context of that skeletal fossil discovery, either Homo erectus sensu lato or Paranthropus boisei (Bennett et al., a stepped trench was dug at the site of the subsequent excavation. 2009). In 2013, a site at Happisburgh, UK, was uncovered by tidal While examining the stratigraphy of the site, Dr. Gail M. Ashley erosion and found to contain hominin tracks dating to between recognized that one sedimentary layer (later named the Lower 0.78 and 1.0 Ma, preliminarily attributed to Homo antecessor Footprint Surface [LFS]) likely preserved animal tracks and multiple (Ashton et al., 2014). Most recently, in 2015, two additional 3.66 Ma other layers were also identified as possible track surfaces. In 2006, hominin trackways were uncovered at Laetoli (Masao et al., 2016). a section of the LFS was uncovered and found to preserve a large This growing sample of hominin tracks from a variety of times and assemblage of bovid tracks. In 2007, the excavation of the LFS was geographical locations suggests that these new data, and additional expanded, and new excavations were focused on a second potential discoveries that may follow, can provide valuable contributions track surface (later named the Upper Footprint Surface [UFS]) that that will help us better understand our evolutionary history. had been identified at a higher (younger) position in the strati- These discoveries also have sparked the development of new graphic sequence. This initial excavation of the UFS in 2007 methodological approaches for digitally recording fossil hominin revealed the first hominin tracks discovered at FwJj14E. Through track assemblages (Bennett et al., 2009, 2013; Hatala et al., 2016a) 2007 and 2008, the LFS and UFS were further excavated and and new experimental approaches for interpreting aspects of loco- analyzed, along with several less extensive track surfaces between motor biomechanics from hominin track morphologies (D’Août these two layers. These initial discoveries and preliminary analyses et al., 2010; Raichlen et al., 2010; Crompton et al., 2011; Bates et al., of the track surfaces at FwJj14E were published in 2009 and 2013; Hatala et al., 2013, 2016a, b, c). These new techniques bypass revealed a total of 20 hominin tracks across both the LFS and UFS certain limitations of other paleontological and archaeological data (Bennett et al., 2009). K.G. Hatala et al. / Journal of Human Evolution 112 (2017) 93e104 95

2.2. Continued excavations of hominin track surfaces at FwJj14E 2.3. Discoveries of sites ET-2013-1A-FE1 and ET-2013-1A-FE3

From 2010 to 2014, excavations were continued at the site of In 2013, expanded surveys for track-bearing sedimentary sur- FwJj14E. Both the LFS and UFS were further exposed and several faces were conducted in East Turkana Collecting Area 1A, where site layers between these two surfaces in the stratigraphic sequence FwJj14E is located. Paleontological Collecting Areas are delineated were identified as potential track surfaces (Fig. 1). Excavators by natural features such as rivers and were defined during the cleared small test squares (~1 m2) on several of these intermediate initial rounds of research at Koobi Fora (Feibel, 2011). Surveys aimed layers to determine whether they might also preserve hominin to identify additional track surfaces within the well-defined ~1.5 Ma tracks. Through these continued excavations, a total of 53 addi- Ileret Tuff Complex (ITC), which is part of the Okote Member of the tional hominin tracks (48 on the UFS, three on the LFS, two on an Koobi Fora Formation. The ITC is capped by the Northern Ileret Tuff, intermediate layer named Layer A2) were uncovered at the site which is dated to 1.51e1.52 Ma, and it is bounded below by the (Table 1). Lower Ileret Tuff, which is dated to 1.53 Ma (Brown et al., 2006; These additional discoveries provide a wealth of new data and McDougall and Brown, 2006). At an intermediate position be- clarify earlier interpretations of the track surfaces (Fig. 2). For tween these two tuffs is the Ileret Tuff, which has been dated to instance, it became clear that a trackway originally thought to 1.52 Ma (Bennett et al., 2009). Within the ITC is a ~8.5 m sequence of represent a single individual moving across the UFS (FUT1 sensu massive laminated silts with intervening layers of fine-grained, Bennett et al., 2009) actually consisted of the trackways of two stratified and cross-stratified sands that were likely deposited by individuals, where one made prints that overlapped those of intermittent low energy water transport processes within a delta another individual (Dingwall et al., 2013; Richmond et al., 2013). margin environment, near a lakeshore (Roach et al., 2016). The similar step lengths, and states of preservation of these track- On June 28, 2013, three additional surfaces were identified while ways, suggest that these individuals walked at similar speeds and surveying the ITC exposures in Collecting Area 1A. The sediments may have traveled across the surface at or around the same time. It overlying these track surfaces had eroded near the edges, such that also became clear upon further excavation and detailed examina- portions of the track surface could be exposed simply by sweeping tion that certain impressions formerly classified as potential away loose surface sand, and tracks were immediately visible. Two hominin tracks in the initial description of the site (FUI2, FUI5, FUI7 of the surfaces discovered in this manner preserved hominin tracks. within Bennett et al., 2009) could not be confidently attributed to Upon discovery, and prior to excavation, these surfaces were hominins, although these impressions did not influence any pre- assigned field names of ET-2013-1A-FE1 and ET-2013-1A-FE3 (ET for vious interpretations made by those authors. More detailed exca- East Turkana, 2013 to designate the year of discovery, 1A to desig- vations also led previously unidentified impressions to be nate the East Turkana Collecting Area, and FE for footprint excava- recognized as hominin tracks. In total, these ongoing excavations tion; these names are abbreviated hereafter as FE1 and FE3). FE1 was have now exposed 54 m2 of the UFS, about 65 m2 of the LFS, and discovered by K.R.O. and FE3 by B.G.R. These surfaces were exca- approximately 2 m2 of the intermediate Layer A2. These surfaces at vated in 2013, leading to the exposure of 8 m2 of the track surface at the site of FwJj14E together preserve a total of 72 hominin tracks, site FE1 and 18 m2 of the track surface at site FE3. These excavations including nine distinct sequences of tracks that have been identi- revealed many additional hominin tracks in situ, including five fied as continuous trackways (Table 1). tracks at site FE1 and 21 tracks at site FE3 (Figs. 3 and 4).

Figure 1. Photograph showing a cross-section of the sedimentary layers overlying the FwJj14E LFS. Evident in this photograph are multiple layers of bedded silts, variable in thickness and not always continuous, which tend to be overlain by ﬁne or silty sands. This depositional couplet is repeated multiple times between the FwJj14E LFS and UFS. During the excavations at site FwJj14E, multiple bedded silt layers within the stratigraphic sequence were identiﬁed as having the potential to preserve tracks. One of these (designated Layer A2) preserved hominin tracks. Scale bar ¼ 10 cm. 96 K.G. Hatala et al. / Journal of Human Evolution 112 (2017) 93e104

Table 1 (continued )

Track Trackway Track Reported by Bennett Table 1 surface et al. (2009)? Catalog of hominin tracks discovered at site FwJj14E from 2007 to 2014.a FUT3-1 FUT3 UFS X Track Trackway Track Reported by Bennett FUT3-2 FUT3 UFS X surface et al. (2009)? A2-H2 Layer A2 A2-H3 Layer A2 FLT1-1 FLT1 LFS X a FLT1-2 FLT1 LFS X If a given track could be linked to a continuous trackway produced by the same fi FLT1-3 FLT1 LFS individual, the eld name of that associated trackway is listed. Indications are FLT1-4 FLT1 LFS provided if a given track was reported in the initial description of the site (Bennett FLT1-6 FLT1 LFS et al., 2009). FUI3 UFS X FU-A FU-O UFS FU-AA UFS 2.4. Discoveries of additional hominin track surfaces through FU-AB UFS random spatial sampling FU-AC UFS FU-AD FU-AD UFS In 2014, surveys were conducted at randomly selected locations FU-AE FU-AD UFS FU-B FU-O UFS within the ITC exposures in Collecting Areas 1A and 3 as part of a FU-C UFS protocol designed to systematically (and in an unbiased fashion) FU-D UFS sample track surfaces in order to assess the broader paleoecological FU-E/FUI6 FU-E UFS X context of the hominin track sites (Roach et al., 2016, 2017). Briefly, a FU-F FU-E UFS map of the ITC exposures in these areas was gridded into sequen- FU-G FU-E UFS FU-H FU-E UFS tially numbered 20 m 20 m squares using ArcGIS (v. 10.2). A FU-I FU-E UFS random number generator (Microsoft Excel v. 14.4.8) was used to FU-J UFS choose 20 m 20 m grid squares to survey for potential track sur- FU-K FU-E UFS faces. If a surface with an identifiable track occurred at one of the FU-L FU-O UFS FU-M FU-O UFS randomly selected locations, then a 1 m 1 m test square on that FU-N UFS surface was excavated (Roach et al., 2016). Through this protocol, two FU-O FU-O UFS additional hominin track surfaces were discovered. The field names FU-P UFS of these sites were ET-2014-3-FE8 and ET-2014-1A-FE16 (abbrevi- FU-S UFS ated here as FE8 and FE16). K.R.O. and N.T.R. discovered site FE8 and FU-T UFS FU-W UFS K.G.H. discovered site FE16. The excavations at these sites are less FU-X FU-X UFS extensive than the others listed above because they were part of the FU-Y FU-X UFS systematic footprint survey. FE8 consists of only a 1 m 1 m square FU-Z UFS (Fig. 5). The FE16 excavation was slightly expanded when it was FUT1-1 FUT1B UFS X FUT1-2 FUT1B UFS X realized that a potential hominin track lay on the edge of the random FUT1-3 FUT1A UFS X square and further excavation was needed to confirm its identity. FUT1-4 FUT1A UFS X That site consists of a 2 m 1 m rectangle of exposed track surface FUT1-4i FUT1B UFS (Fig. 6). At site FE8, one hominin track has been identified, and three FUT1-4ii FUT1B UFS hominin tracks have been identified at site FE16 (Figs. 5 and 6). FUT1-5 FUT1A UFS X FUT1-5i FUT1B UFS FUT1-6 FUT1A UFS X 2.5. Preservation of data from track surfaces FUT1-7A FUT1A UFS X FUT1-7B FUT1B UFS X Following direct measurements of tracks and trackways (see FUT1-8 FUT1A UFS FUT1-8i FUT1B UFS Methods), the global three-dimensional positions of all tracks were FUT1-9 FUT1A UFS logged using a total station (Leica Builder 505; Trimble Nomad 900 FUT1-10 FUT1A UFS LE data collector with EDMce for Windows Mobile software). Entire FUT1-11 FUT1B UFS track surfaces were then recorded using photogrammetry, as a FUT1-12 FUT1A UFS method of digitally quantifying but also preserving the original FUT1-13 FUT1A UFS FUT1-14 FUT1A UFS paleontological data (Falkingham, 2012). This method was deemed FUT1-14i FUT1B UFS the most useful approach for permanently recording the footprint FUT1-15 FUT1A UFS surfaces at Ileret, since they are located in a very remote location, FUT1-16 FUT1A UFS preserved in unconsolidated sediment, and are at relatively high FUT1-17 FUT1B UFS FUT1-18 FUT1A UFS risk of damage through natural erosion processes (Bennett et al., FUT1-18i FUT1B UFS 2013). Further, these digital records of the footprint surfaces will FUT1-19 FUT1A UFS be curated as original records of the site that accompany total FUT1-20 FUT1B UFS station data, field notes, and other paper records. FUT2–3 FUT2 UFS Even though track surfaces were digitally documented imme- FUT2–2 FUT2 UFS FUT2–1 FUT2 UFS diately following their excavation, considerable efforts were made FUT2-0 FUT2 UFS to rebury track surfaces in a manner that would aid their long-term FUT2-1 FUT2 UFS X preservation. The sterile sand that had previously overlain the track FUT2-2 FUT2 UFS X surfaces and helped to preserve them for the past 1.5 million years FUT2-3 FUT2 UFS X FUT2-4 FUT2 UFS X was sieved and used to rebury each site. Nylon tarpaulins were placed on top of the sand layers and were subsequently secured by rocks. The underlying sand was graded in a manner that would direct water off the edge of the excavation site and prevent it from K.G. Hatala et al. / Journal of Human Evolution 112 (2017) 93e104 97

Figure 2. Schematic map of site FwJj14E UFS. Figure is adapted from a previously published version (Hatala et al., 2016a), with copyright retained by K.G.H. The left and right images show the extent of the excavations following the 2009 and 2014 field seasons, respectively. The solid red lines denote the excavation borders (i.e., they do not symbolize the limits of the surface, as the surface may continue beyond these). The dashed red line indicates the edge of the track surface and areas west of this line have been lost due to erosion of the outcrop. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) pooling on top of the tarpaulin. The tarpaulins were subsequently understood in the greatest detail. Earlier interpretations of the buried with additional sterile, sieved sand excavated from the stratigraphy at site FwJj14E (Bennett et al., 2009) have been fol- stratified layers that had overlain each track surface prior to exca- lowed by more detailed reconstructions of the sedimentary context vation. These overlying sediments were also graded in a manner of the FwJj14E track surfaces (Behrensmeyer, 2011; Roach et al., that would divert surface water flow away from the site of the 2016). At the other hominin track sites at Ileret (FE1, FE3, FE8, excavated track surface. Continuous work in the Koobi Fora area and FE16), our understanding of site depositional processes is each year will allow for monitoring of these sites, and intervention derived from detailed studies of the sedimentary layers and will be possible if any appear to be at risk of damage. bedding structures exposed by the track surface excavations. These sites have also been tied in to the overall geology of the area and 3. Geological/sedimentological contexts of Ileret hominin linked to the stratigraphic sequence exposed at site FwJj14E by track assemblages measuring relative positions within the sequence of three volcanic tuffs (the Lower Ileret, Ileret, and Northern Ileret Tuffs) that Studies of the geological and sedimentological contexts of track comprise the ITC. It should be noted, however, that lateral variation surfaces have differed methodologically between the various track in the track-bearing lithofacies makes it difficult to confirm the sites. Because site FwJj14E has been the focus of continued study for exact correlations of depositional surfaces at different sites. Hom- more than a decade, the geology and sedimentology of this site is inin track surfaces were found on multiple bedded silt layers within

Figure 3. Overhead view of a three-dimensional photogrammetric model of site ET-2013-1A-FE1. Hominin tracks are circled in dashed white. Hominin tracks that form a continuous trackway are circled in solid white. Scale bar ¼ 1 m (displayed in the bottom left corner). The approximate direction of magnetic north is indicated by the black arrow in the top left corner. 98 K.G. Hatala et al. / Journal of Human Evolution 112 (2017) 93e104

Figure 4. Overhead view of a three-dimensional photogrammetric model of site ET-2013-1A-FE3. Hominin tracks are circled in dashed white. Scale bar ¼ 1 m (displayed on the left side of the image). The approximate direction of magnetic north is indicated by the black arrow in the top left corner.

the ITC, with one of these surfaces (the FwJj14E UFS) lying between the Ileret and Northern Ileret Tuffs and the other six (FwJj14E LFS, FwJj14E Layer A2, FE1, FE3, FE8, FE16) lying between the Lower Ileret and Ileret Tuffs (Fig. 7). The hominin track surfaces were buried by ﬁne silty sand after tracks were produced on the bedded silt layers, and this likely occurred very rapidly. The lack of mud cracking on any of these surfaces is consistent with a high, stable water table, and the absence of root traces or other evidence of pedogenesis in track-bearing sediments indicates that they were not sub-aerially exposed (Roach et al., 2016). Paleosols also occur within the ITC and provide evidence for the temporary development of stable land surfaces, but the intervening periods of low energy deposition of interbedded silt and sand on the margin of a delta or lake allowed tracks to be produced and track surfaces to be preserved repeatedly in this area during the approximately 20 ka spanned by the ITC (Fig. 7).

4. Methods

Figure 5. Overhead view of a three-dimensional photogrammetric model of site ET- 2014-3-FE8. The single hominin track on this surface is circled in dashed white. Scale 4.1. Measurement and documentation of fossil hominin tracks and bar ¼ 1 m (shown on the left side of the image). The approximate direction of magnetic trackways north is indicated by the black arrow in the top left corner. Following excavation and exposure of track surfaces, multiple techniques were used to measure and record the track and trackway data. First, the linear dimensions of individual tracks (lengths and widths) were measured directly. The exact linear measurements taken from each track were dependent upon the specific nature of its preservation. Some tracks represented only parts of the foot, and in some cases, the anatomical definition of the track was somewhat distorted. As such, multiple measurements of track length and breadth were attempted. A tape measure was used to measure the track's length along one or more (where possible) of three different trajectoriesdfrom the most proximal part of the outline of the heel impression to the tips of the impressions for the hallux and/or second and/or third digits. In some tracks, the impressions for certain digits were unclear and the measurements were estimated (and recorded as such) or excluded. Track breadths were measured using digital calipers at two different locations. The first measurement was taken between the impressions created by Figure 6. Overhead view of a three-dimensional photogrammetric model of site ET- the first and fifth metatarsal heads, and the second was taken 2014-1A-FE16. The three definitive hominin tracks discovered on this surface are across the widest part of the heel impression. circled in dashed white. Because of the small size of this excavation, it is unclear at this Based on visual observations of track locations and track mor- point whether these tracks form a continuous trackway. As a result, they have not been phologies, and consideration of individual track dimensions, assigned to a trackway. Scale bar ¼ 1 m (shown on the left side of the image). The approximate direction of magnetic north is indicated by the black arrow in the top left certain tracks could immediately be linked to others within track- corner. ways consisting of multiple steps by the same individual. In these K.G. Hatala et al. / Journal of Human Evolution 112 (2017) 93e104 99

estimated traveling speeds from each of the Ileret hominin trackways. To do so, we used a linear regression relationship between stride length and speed that was derived in an earlier human experimental study (speed ¼1.39 þ (0.48*(stride length/average footprint length)); Dingwall et al., 2013). Traveling speed estimates for some of these trackways have been published previously (Dingwall et al., 2013), but the estimates that we present here incorporate new data (including newly excavated tracks and new measurements of stride lengths) that were obtained during our expansions of the site excavation.

5. Results

5.1. Analyses of external track dimensions

The measurements of the lengths and breadths of each track in the Ileret assemblage are provided in Supplementary Online Material (SOM) Table S1. Comparisons of heel to hallux length and breadth across the forefoot among the Ileret track surfaces and other fossil hominin track sites are given in Table 2. The tracks from each of the ~1.5 Ma Ileret track surfaces are generally comparable in size to the tracks produced in footprint formation experiments by modern Daasanach people (Hatala et al., 2016a, b) and to the 325e385 ka tracks preserved at Roccamonfina. Theyare longerand narrower than a collection of tracks produced experimentally by modern chimpanzees. On average, the Ileret tracks are larger than those preserved Figure 7. Representative stratigraphic section showing the positions of Ileret hominin e track surfaces within the ITC. Figure is derived from a previously published version at the 0.78 1.0 Ma site of Happisburgh. They are generally longer but (Roach et al., 2016), with copyright retained by N.T.R. Scale at bottom indicates similarly wide to the ~3.66 Ma tracks from Laetoli, Tanzania. approximate grain sizes: C ¼ clay, Z ¼ silt, S ¼ sand, G ¼ gravel. One surface, the FwJj14E UFS, lies between the Ileret and Northern Ileret Tuffs, while the other six 5.2. Estimates of traveling speeds surfaces lie between the Lower Ileret and Ileret Tuffs. The depositional contexts of each of these surfaces is similar, as they lie within stratified and interbedded silt and fine- grain sand layers deposited by low energy processes, probably within a delta or lake Most of the identified trackways, and hence most of these trav- margin. These well-bedded intervals are separated by paleosols representing inter- eling speed estimates (eight of 10), come from our most expansive vening periods of subaerial delta plain (fluvial) deposition. excavations on the LFS and UFS at site FwJj14E. All of the 10 trackways that we have identified almost certainly represent walking cases, where trackways were evident, step and/or stride lengths speeds, with estimates ranging from 0.45 to 1.58 m/s (Table 3). One were directly measured with a tape measure. A field name was isolated trackway on the LFS at FwJj14E had been thought to assigned to each trackway after its identification. represent a speed within the range at which modern Daasanach Following direct measurements, hundreds of digital photo- people tend to transition from a walk to a run (2.0e2.3 m/s; graphs were taken in order to render high-resolution photogram- Dingwall et al., 2013), but our revised estimate here falls squarely in metric 3D models of entire track surfaces and all individual the observed range of modern human walking speeds. hominin tracks. These 3D records are important for long-term data preservation (see above), but high resolution 3D models of hominin 6. Discussion tracks have also enabled quantitative comparative analyses with tracks of other fossil and modern taxa (Hatala et al., 2016a). 6.1. Implications of analyses of external track dimensions

4.2. Analyses of fossil hominin tracks and trackways The Ileret tracks are human-like in their external sizes. Any differences between these and more recent hominin tracks are 4.2.1. Analyses of external track dimensions Linear measurements almost certainly attributable to differences in the demographics of of hominin tracks from the Ileret track surfaces were compared the hominins who produced them. For example, at Happisburgh with a compilation of similar measurements from a variety of there are many smaller tracks that are hypothesized to have been extant and extinct samples. Comparative data from modern taxa produced by children (Ashton et al., 2014). The larger Happisburgh included measurements of tracks from footprint formation exper- tracks, presumably produced by adults, are generally comparable in iments conducted by K.G.H. with habitually barefoot Daasanach size to the tracks at Ileret. people from near Ileret, Kenya (Hatala et al., 2016b) and with Foot skeletal fossils are known for the two taxa proposed as most modern chimpanzees in the Primate Locomotion Lab at Stony likely responsible for the Roccamonﬁna and Happisburgh tracks e Brook University (Hatala et al., 2016c). Fossil data consisted of a Homo heidelbergensis and Homo antecessor, respectively. Analyses of compilation of published linear measurements from other fossil these H. heidelbergensis and H. antecessor foot bones have suggested hominin track sites, including 325e385 ka tracks from overall foot sizes similar to those of modern humans (Lu et al., 2011; Roccamonﬁna, Italy (Avanzini et al., 2008), 0.78e1.0 Ma tracks Pablos et al., 2015). Here, our results suggest that modern human- from Happisburgh, UK (Ashton et al., 2014), and ~3.66 Ma tracks like foot sizes may have extended back even further in the homi- from Laetoli, Tanzania (Bennett et al., 2016; Hatala et al., 2016c; nin clade, to at least the time of the Ileret tracks at ~1.5 Ma. Masao et al., 2016). The ~3.66 Ma tracks from Laetoli, Tanzania are approximately as 4.2.2. Estimates of traveling speed After taking linear measure- wide as the Ileret tracks, but generally are not as long. The average ments of stride lengths (or in some cases step lengths), we length of tracks from one Laetoli trackway exceed the pooled 100 K.G. Hatala et al. / Journal of Human Evolution 112 (2017) 93e104

Table 2 Linear dimensions of the tracks from various Ileret hominin track surfaces and multiple comparative samples.a

Sample Footprint length (cm) Footprint breadth (cm)

Mean Range SD Mean Range SD

Modern human (Daasanach) 25.4 (n ¼ 41) 20.0e29.5 2.1 9.7 (n ¼ 41) 7.4e11.8 0.9 Modern chimpanzeesb 20.3 (n ¼ 2) 19.9e20.7 0.6 11.3 (n ¼ 2) 11.0e11.6 0.5 Roccamonﬁnac 24 (n ¼ ?) ee12 (n ¼ ?) ee Happisburghd 19.1 (n ¼ 12) 14.0e26.0 4.2 7.5 (n ¼ 12) 5.0e11.0 1.8 Laetolie 22.8 (n ¼ 4) 19.1e26.1 2.9 9.6 (n ¼ 2) 8.7e10.4 1.2 Ileret (all sites pooled) 25.3 (n ¼ 28) 20.5e30.5 2.2 9.6 (n ¼ 36) 7.5e12.7 1.2 Ileret FwJj14E UFS 25.2 (n ¼ 18) 20.5e30.5 2.0 9.8 (n ¼ 19) 8.25e12.7 1.4 Ileret FE1 23.6 (n ¼ 3) 21.0e26.8 2.9 8.0 (n ¼ 3) 7.5e9.0 0.9 Ileret FE3 26.3 (n ¼ 5) 23.5e29.5 2.9 9.9 (n ¼ 12) 7.5e12.0 1.2 Ileret FE8 27.0 (n ¼ 1) ee9.5 (n ¼ 1) ee Ileret FwJj14E LFS 24.9 (n ¼ 1) ee8.0 (n ¼ 1) ee

a The comparative samples include experimentally produced tracks made by modern humans and chimpanzees, and fossil tracks from the sites of Roccamonfina (325e385 ka), Happisburgh (0.78e1.0 Ma), and Laetoli (~3.66 Ma). For each sample, the average footprint dimensions are calculated by first computing means within each trackway (i.e., for each individual), and then using those trackway/individual means to calculate the average across the entire sample. SD ¼ standard deviation. b These measurements represent unpublished data from K.G.H. The experiments in which these data were produced are described in Hatala et al. (2016a). c Data are from Avanzini et al. (2008). Sample size is listed as a question mark because, although 56 tracks are described to exist at the site, it is unclear exactly how many were measured to arrive at the average measurements included in the publication. d Data are from Ashton et al. (2014). e Data included from four Laetoli trackways e G1, G3, S1, and S2. Data for Laetoli G1 were collected by K.G.H. Measurements of Laetoli G3 are from Bennett et al. (2016). Laetoli S1 and S2 data are from Masao et al. (2016). Average lengths from each of the four trackways were used to derive the ‘population’ average footprint length, but confident width measurements were only available for the G1 and S1 trackways.

Table 3 Estimates of traveling speed for Ileret hominin trackways.a

Trackway Surface Step length (cm) Stride length (cm) Estimated speed (m/s)

FLT1 FwJj14E LFS 110.0 0.73 FUT1A FwJj14E UFS 86.5 0.65 FUT1B FwJj14E UFS 73.3 0.45 FUT2 FwJj14E UFS 124 1.05 FUT3 FwJj14E UFS 130 1.23 FU-E FwJj14E UFS 156.5 1.52 FU-O FwJj14E UFS 131.5 1.32 FU-X FwJj14E UFS 114.0 1.29 FU-AD FwJj14E UFS 83.3 1.58 FE1-HT1 FE1 42.9 0.58

a Step and stride lengths are provided. Traveling speed estimates were produced using the equation provided by Dingwall et al. (2013). In cases where a stride length measurement was not possible, stride length was estimated as two times step length. average track length from Ileret (individual S1, average track body size determined through experiments with modern Daasa- length ¼ 26.1 cm; Masao et al., 2016) but the average lengths of all nach people, body mass predictions were generated for many Ileret other Laetoli trackways fall below this average (average lengths of hominin tracks and trackways (Hatala et al., 2016a). These body G1, G3, S2 range from 19.1 to 23.1 cm; Bennett et al., 2016; Hatala mass estimates from the Ileret hominin tracks were largely similar et al., 2016c; Masao et al., 2016). It has been proposed that the to the observed body masses of modern Daasanach people, and Laetoli track assemblage represents considerable body size varia- closer to skeletally based estimates from H. erectus fossils than they tion, and possibly a high degree of sexual dimorphism in overall size were to estimates derived from confidently attributable P. boisei or with trackway S1 representing a large male (Masao et al., 2016). Homo habilis skeletal fossils (Grabowski et al., 2015). This line of Assuming that track size is an accurate predictor of overall body size evidence was used to support attribution of the tracks to H. erectus for fossil hominins (e.g., Dingwall et al., 2013; Hatala et al., 2016a), (Hatala et al., 2016a).2 The data in Table 2 demonstrate that large, our results here show that the Laetoli track makers were still, on human-like track size is consistent across all of the excavated Ileret average, smaller in overall size than those from Ileret. This result track surfaces. While some Ileret track surfaces include relatively agrees well with a recent analysis by Grabowski et al. (2015),which smaller tracks, none fall outside of the range of sizes observed in predicted fossil hominin body masses from lower limb skeletal tracks made by adult modern humans. Because we have not fossils. That analysis included a range of body mass predictions from observed differences in external track dimensions (this study) or fossils attributed to both A. afarensis, the taxon most commonly internal track morphologies (Hatala et al., 2016a) between Ileret assumed to have created the Laetoli tracks (e.g., Masao et al., 2016 track sites, or between the large and small tracks within the as- but see; Tuttle et al., 1991), and H. erectus, the presumed maker of semblages, it is most parsimonious for us to hypothesize that all of the Ileret tracks (Hatala et al., 2016a). Grabowski et al. (2015) found that while male A. afarensis individuals may have had body sizes that fell within the range observed in H. erectus, the species was still, on 2 We acknowledge here that these previously published body mass predictions average, smaller than H. erectus. rely upon a reference sample of modern humans who have a very linear build These comparisons with other fossil skeletal and track data rely, compared with other modern populations (Ruff, 1994). If H. erectus were charac- to some extent, upon accurate taxonomic attribution of the Ileret terized by a relatively wide pelvis and a less linear build (Ruff, 2010), our experimental design may have produced underestimates of body mass for H. erectus hominin tracks. Using the relationships between track size and individuals (Ruff and Burgess, 2015). K.G. Hatala et al. / Journal of Human Evolution 112 (2017) 93e104 101 the Ileret tracks were made by one hominin taxon and that taxon assemblages, however, represent immediate snapshots of fossil was most likely H. erectus. However, at this point we cannot rule out hominin behaviors and they are recorded at a level of spatiotem- completely that another hominin species made some or all of the poral resolution that has the potential to directly inform hypothe- tracks: a paucity of postcranial skeletal fossils means we know very ses about group structures and interactions between fossil hominin little about body size variation in P. boisei or H. habilis, two taxa that individuals. are known to have co-existed with H. erectus in the Ileret area The sedimentological contexts of each of the Ileret hominin track around ~1.5 Ma, and even less about Homo rudolfensis. surfaces are consistent with a short duration of surface exposure, during which tracks and trackways were formed and then rapidly 6.2. Implications of estimates of traveling speeds buried. Each hominin track surface was buried by water- transported, fine silty sand (Figs. 1 and 7) that infilled the tracks Roach et al. (2016) evaluated whether certain Ileret hominin without scouring or erosion. Similar low energy cycles of silt and trackways on the FwJj14E UFS may have represented a group sand deposition occur today along the shoreline of Lake Turkana, coordinating their movement and traveling together. To do so, they suggesting similar circumstances during the time of the ITC in a used total station data to quantitatively compare the compass ori- delta or lake margin environment. Taphonomic experiments on the entations of hominin and non-hominin trackways. They found that durations of modern human tracks and trackways along the shore of the hominin tracks were predominantly and non-randomly ori- modern Lake Turkana suggest that, when left unburied, detailed ented in a southeasterly direction, and the tracks of all other ani- morphologies within the tracks (as are seen in the fossil hominin mals had a significantly different northwesterly orientation (Roach track assemblages) normally have a lifespan of 1.3 days or less et al., 2016). These different directions of travel for the hominins (Roach et al., 2016). Beyond this point, track morphologies become and all other animals suggest that the movement patterns captured less well defined and less recognizable, typically due to weathering on the FwJj14E UFS were not constrained by features of the natural or trampling by other animals (Roach et al., 2016). Together, these landscape, as can be observed when animals follow well-traveled lines of evidence suggest that the Ileret hominin track surfaces were game trails (e.g., Laporte and Behrensmeyer, 1980). Further, with created and buried within a narrow time span from a few days to a the exception of two trackways (FUT1A and FUT1B) that overlap in few hours. This very short timeframe of site formation and depo- a manner consistent with one individual following the other, the sition suggests that any individuals or animals that made tracks on UFS preserves many sub-parallel hominin tracks that do not over- the same surface likely lived within sufficiently close proximity to lap with one another. Roach et al. (2016) hypothesized that these each other that their ranges overlapped on a daily basis. non-overlapping, sub-parallel tracks could represent multiple in- On the FwJj14E UFS, we find multiple sub-parallel hominin dividuals simultaneously moving through this area together. trackways, traveling in a similar direction that is statistically Here, we are able to use traveling speeds estimated from the distinct from the directions of travel of all other animal tracks and FwJj14E UFS trackways, combined with published data on hominin trackways (Roach et al., 2016). Coupled with the short windows of trackway orientations, in order to further evaluate the likelihood time during which all of the trackways were formed and buried, that particular trackways may represent groups of individuals these data suggest that at least some of the FwJj14E UFS individuals moving together across the FwJj14E UFS. Among the trackways moving in the same direction traveled together. Here, our estimates described in Table 3, for which traveling speeds could be estimated, of traveling speeds for the FwJj14E UFS trackways, which all imply six of the eight (FUT1A, FUT1B, FUT3, FU-E, FU-O, and FU-X) are walking speeds, provide additional support for the hypothesis of oriented in a southeasterly direction while the other two (FUT2 and coordinated group movement. Even if these individuals happened FU-AD) are oriented towards the northwest. These trackways sug- to move through the area at similar speeds but at slightly different gest a range of walking speeds that would not preclude group times (if they did not travel as a group), the very limited time for travel, as individuals within a group may vary their speeds while site formation and burial evident from geological and sedimento- still traveling together. It is notable that the FU-X individual created logical data means that the different individuals whose tracks are the smallest tracks within the entire Ileret sample (SOM Table S1), preserved on the same surface lived in immediate proximity and and in a previously published analysis, Hatala et al. (2016a) found likely interacted with each other. that this trackway generated a body mass estimate that was an Hatala et al. (2016a) used body mass estimates derived from extreme outlier among the Ileret sample, being far smaller than external track dimensions in order to estimate the sexes of the average (26.5 kg). In this study, we estimate that this markedly individuals who created the two largest assemblages of tracks that smaller individual still traveled at a speed consistent with the other we have excavated at Ileret. They estimated that at least 50% of the individuals who moved towards the southeast, perhaps making an hominin trackways on the FwJj14E UFS, and at least 75% of the effort to keep up with the rest of the group. At this point, it is still tracks at site FE3 were created by male H. erectus individuals difficult to hypothesize exactly who may have traveled together (Hatala et al., 2016a). Based on the results of the current study, we across the FwJj14E UFS, but trackway orientations and traveling further emphasize that it is not necessarily the case that the Ileret speeds estimates appear to be consistent with what would be ex- trackways were made by groups of exclusively males moving pected from coordinated group movement. together. In fact, the group of sub-parallel southeast-oriented trackways on the FwJj14E UFS, which also imply similar walking 6.3. Novel directions for testing hypotheses related to hominin speeds and therefore offer the strongest case for coordinated group paleobiology at ~1.5 Ma movement, includes a set of trackways that were all estimated by Hatala et al. (2016a) as potentially representing females and/or Evolutionary hypotheses related to early hominin group subadults. But while a small group of females and/or subadults may behavior and social structure have proven notoriously difficult to have moved together, they represent only six of the estimated 18 test due to inherent limitations of the most common types of hu- different individuals who left tracks on the FwJj14E UFS (Table 2). man fossil data (Chapais, 2013). While certain aspects of skeletal What we find most striking and well-supported regarding social morphology such as canine size dimorphism may be associated behavior is that within both the FwJj14E UFS and the FE3 track with broad patterns of social behavior in primates (Leutenegger assemblages, there are several trackways with sizes that exceed the and Shell, 1987), those links are often tenuous and not broadly pooled Ileret mean (Table 2) and likely represent multiple male applicable across diverse taxa (Plavcan, 2000). The Ileret track H. erectus individuals. 102 K.G. Hatala et al. / Journal of Human Evolution 112 (2017) 93e104

The presence and presumed interactions of multiple H. erectus Alternatively, the Ileret track assemblages may preserve direct males within the same local group has interesting implications that evidence of derived behaviors that led to the emergence of a social are relevant to major evolutionary hypotheses. Direct tests of hy- structure that, among living primates, uniquely characterizes pothesized social behaviors have been particularly difficult to modern humans. Maleemale cooperation is a key feature that achieve due to a lack of pertinent data from the human fossil re- underlies the maintenance of multilevel social structures, which cord. While there are certainly limitations to the data preserved in describe nearly all modern human societies and have been hy- these track assemblagesdfor example, the exact types of associa- pothesized, based on a variety of other archaeological and pale- tions or genetic relationships between individuals and the activities ontological evidence, to have first emerged in H. erectus (Swedell that led to their interactions will never be knowndwe can examine and Plummer, 2012). This model suggests a tiered hierarchy of the evidence of group composition and look for consistencies with, differently sized social unitsdincluding, from smallest to largest, or deviations from, existing hypotheses related to the evolution of the polygynous one-male unit, the clan, the band, and the human social structure. We present below a series of behavioral troopdand coordination at all but the lowest levels of this hierar- hypotheses, each of which could be supported by evidence from the chy depends upon, at the very least, mutual tolerance among males Ileret track assemblages. This is not an exhaustive list of possibil- (Swedell and Plummer, 2012). Among modern human hunter- ities, but provides multiple scenarios that can be explored in future gatherers, it has been demonstrated that this nested, hierarchical work to refine methods for inferring behavior from assemblages of pattern of social structure may represent an ‘optimal’ solution for tracks and trackways. the distribution of resources both among and within groups First, it is possible that the Ileret track assemblages were pro- (Hamilton et al., 2007). As H. erectus emerged and evolved within a duced by group behavior patterns that have deep evolutionary dynamically changing environment that required wider dispersal roots and may have characterized the Pan-Homo last common and provided less reliable resources (Potts, 1998; DeMenocal, 2004; ancestor (LCA). Among modern chimpanzees, multi-male groups Anton et al., 2014; Potts and Faith, 2015), this social structure could are known to assemble and travel together while conducting pa- have played an essential adaptive role in promoting foraging suc- trols of their borders as a method of mitigating intergroup cess. In modern human hunter-gatherers, multiple males from competition (Watts and Mitani, 2001; Mitani and Watts, 2005; within the same band cooperate to hunt for game and then share Watts et al., 2006). A similar type of behavior may be evidenced the calorically rich profits of that hunt among all members of their by the large assemblages of presumed male tracks at the FwJj14E band (Hill, 2002; Gurven, 2004; Marlowe, 2005; Hill et al., 2009). UFS and site FE3. Many of the hominin trackways on the FwJj14E Cooperative foraging and the sharing of resources seems likely if UFS appear to represent individuals moving along the water's edge H. erectus regularly foraged for animal resources, as has been sug- (Roach et al., 2016), which may or may not be consistent with a gested by collections of cut-marked bone from 1.5 Ma deposits in group actively surveying a territorial perimeter. Multi-male groups Collecting Area 1A (Pobiner et al., 2008) and potentially by the of chimpanzees are also known to engage in cooperative hunting Ileret tracks themselves (Roach et al., 2017). Cooperation improves expeditions, although this behavior may not be generalizable the probability of a successful kill in modern hunter-gatherers and across the species as a whole because it has been observed at only a may have been even more important at the time of H. erectus, when single study site (Boesch, 2002). At this point, we do not have weapons were likely much less lethal than those used by hunter- sufficient modern or fossil data to rule out the possibility that the gatherers today (Hill, 2002). Further, the sharing of food re- Ileret track assemblages could represent a behavior analogous to sources acquired through cooperative hunting across all members that observed in modern chimpanzees. Future analyses of how of the band buffers the relatively high probability of failure that is specific chimpanzee behaviors are recorded in tracks and track- associated with any given hunt (Lee, 1968; Hill, 2002; Gurven, ways are necessary to test these hypotheses. 2004). If a multilevel social structure was present in H. erectus, The Ileret track assemblages do meet the predictions of the mediated by maleemale cooperation and perhaps including pat- more general hypothesis that multi-male groups with some level terns of cooperative foraging, then this structure could have pro- of cooperation were present in the LCA (Wrangham and Pilbeam, moted the emergence of the number of unique behavioral traits 2001; Duda and Zrzavy, 2013). Yet, in order for human and and the “deep social structure” that are considered to define chimpanzee maleemale cooperation to be homologous, this modern humans (Hill et al., 2009; Chapais, 2011). strategy must have been inherited from the LCA and persisted in Ultimately, comprehensive tests of these types of hypotheses the lineages leading to both modern humans and chimpanzees. about social behavior and social structure require more modern The high levels of size dimorphism in A. afarensis (Plavcan et al., and fossil data than are currently available. Determining the pres- 2005; Gordon et al., 2008; but see also Reno and Lovejoy, 2015), ence or absence of specific patterns of social behavior will rely upon a species widely regarded as a stem hominin (Kimbel and continued work to refine our abilities and better understand our Delezene, 2009), suggest high levels of maleemale competition. limitations for testing hypotheses about hominin social interactions This hypothesized competition, plus little to no information on the from track and trackway data. Experimental research is necessary existence of cooperative behaviors such as group defense and/or in order to determine how behaviors of humans, and of nonhuman hunting in A. afarensis, do not provide substantial support for the primates, are recorded in assemblages of their tracks. The homology hypothesis at this time. However, given the growing continued pursuit of such methods is valuable, as tracks and number of contemporaneous taxa recognized in the Pliocene trackways offer unique opportunities to directly observe, in deep (Haile-Selassie et al., 2016), the poor preservation of many of time, snapshots of groups of our fossil relatives and draw inferences them, and the inherent difficulties in identifying ances- regarding the compositions and social behaviors of those groups. toredescendant relationships in the fossil record, it is premature Track sites may allow for types of inferences regarding group to rule out the hypothesis that the maleemale cooperation behavior that have been notoriously difficult to gain through other observed in modern chimpanzees and humans may be homolo- forms of archaeological and paleontological data (Chapais, 2013). gous. At the very least, the evidence from the Ileret track surfaces Already, we have been able to draw some conservative conclusions of multiple H. erectus males walking across the same landscape, from the Ileret track assemblages. Regardless of the specifics of the and possibly even traveling together, is consistent with a level of social structure and social behaviors of H. erectus, our analyses do maleemale cooperation similar to that observed in modern suggest the presence and interactions of multiple H. erectus males chimpanzees. living and traveling on the same landscapes. This evidence is K.G. Hatala et al. / Journal of Human Evolution 112 (2017) 93e104 103 consistent with previous hypotheses that have suggested that the the Net Wide: Papers in Honor of Glynn Isaac and His Approach to Human e social structure of H. erectus supported the emergence of modern Origins Research. Oxbow Books, Oakville, pp. 21 39. Behrensmeyer, A.K., Laporte, L., 1981. Footprints of a Pleistocene hominid in human-like patterns of social behavior. northern Kenya. Nature 289, 167e169. Bennett, M.R., Harris, J.W., Richmond, B.G., Braun, D.R., Mbua, E., Kiura, P., Olago, D., Kibunjia, M., Omuombo, C., Behrensmeyer, A.K., Huddart, D., Gonzalez, S., 2009. 7. Conclusions Early hominin foot morphology based on 1.5-million-year-old footprints from Ileret, Kenya. Science 323, 1197e1201. These discoveries of multiple hominin track sites in Okote Bennett, M.R., Falkingham, P., Morse, S.A., Bates, K., Crompton, R.H., 2013. Preserving Member deposits near Ileret, Kenya, provide a unique window that the impossible: conservation of soft-sediment hominin footprint sites and strategies for three-dimensional digital data capture. PLoS One 8, e60755. can further our knowledge of various aspects of hominin paleobi- Bennett, M.R., Reynolds, S.C., Morse, S.A., Budka, M., 2016. Laetoli's lost tracks: 3D ology at ~1.5 Ma. Initial analyses of these sites explored the impli- generated mean shape and missing footprints. Sci. Rep. 6, 21916. cations of these track sites for hominin anatomy, locomotion, land Bibi, F., Kraatz, B., Craig, N., Beech, M., Schuster, M., Hill, A., 2012. Early evidence for complex social structure in Proboscidea from a late Miocene trackway site in use patterns, and behavior (Bennett et al., 2009; Hatala et al., the United Arab Emirates. Biol. Lett. 8, 670e673. 2016a; Roach et al., 2016, 2017). Here, we provide new compara- Boesch, C., 2002. Cooperative hunting roles among Tai chimpanzees. Hum. Nat. 13, e tive assessments of external track dimensions, and estimates of 27 46. fi Brown, F.H., Haileab, B., McDougall, I., 2006. Sequence of tuffs between the KBS Tuff traveling speed derived from hominin trackways, to re ne earlier and the Chari Tuff in the Turkana Basin, Kenya and Ethiopia. J. Geol. Soc. Lond. interpretations of foot size, taxonomic attribution, and group 163, 185e204. 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Evidence for social behavior in ornith- find these sites, and the initial analyses and hypotheses laid out in opod dinosaurs from the Dakota group of northeastern New Mexico, U.S.A. e this study will help motivate continued research in hominin ich- Ichnos 6, 141 149. Crompton, R.H., Pataky, T.C., Savage, R., D’Août, K., Bennett, M.R., Day, M.H., nology. Continued discoveries of new hominin trace fossil data, and Bates, K.T., Morse, S., Sellers, W.I., 2011. Human-like external function of the the development of new approaches for interpreting them, will foot, and fully upright gait, confirmed in the 3.66 million year old Laetoli help us use these data in concert with other parts of the paleon- hominin footprints by topographic statistics, experimental footprint-formation and computer simulation. J. R. Soc. Interface 9, 707e719. tological and archaeological records to better inform un- D’Août, K., Meert, L., Van Gheluwe, B., De Clercq, D., Aerts, P., 2010. 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African climate change and faunal evolution during the Daasanach volunteers for their contributions to this research. This Pliocene-Pleistocene. Earth Planet. Sci. Lett. 220, 3e24. study was conducted under a research permit granted by the Dentzien-Dias, P.C., Schultz, C.L., Bertoni-Machado, C., 2008. Taphonomy and Kenyan National Council for Science and Technology and an exca- paleoecology inferences of vertebrate ichnofossils from Guara Formation (Up- per Jurassic), southern Brazil. J. S. Am. Earth Sci. 25, 196e202. vation license granted by the Ministry of Higher Education, Science Dingwall, H.L., Hatala, K.G., Wunderlich, R.E., Richmond, B.G., 2013. Hominin stat- and Technology. This study was funded by the Leakey Foundation, ure, body mass, and walking speed estimates based on 1.5 million-year-old the National Science Foundation (BCS-1232522, BCS-0924476, BCS- fossil footprints at Ileret, Kenya. J. Hum. Evol. 64, 556e568. Duda, P., Zrzavy, J., 2013. 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