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

Journal of Human Evolution xxx (2017) 1e12

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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 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 b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 September 2016 Accepted 7 August 2017 Available online xxx

Tracks can provide unique, direct records of behaviors of fossil organisms moving across their landscapes millions of years ago. While track discoveries have been rare in the human fossil record, over the last decade our team has uncovered multiple sediment surfaces within the Okote Member of the Koobi Fora 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 specific data that are preserved on the Ileret hominin track surfaces. We analyze previously unpublished data to refine and expand upon earlier hypotheses regarding implications for hominin anatomy and social behavior. While each of the track surfaces discovered at Ileret preserves a different amount of data that must be handled in particular ways, general patterns are evident. Overall, the analyses presented here support earlier interpretations of the ~1.5 Ma Ileret track assemblages, providing further evidence of 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.

Keywords: Ichnology Trace fossils Footprints Homo erectus Koobi Fora Pleistocene

1. Introduction Across the field of paleontology, ichnological studies have figured prominently in the developments of major evolutionary hypotheses. Tracks (often referred to as ‘footprints’ in

* Corresponding author. E-mail address: [email protected] (K.G. Hatala).

paleoanthropological literature) and trackways (sequences of two or more consecutive tracks) have been used to address a wide array of questions related to the biology of fossil organisms including questions related to social behavior (e.g., Ostrom, 1972; Lockley and 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; 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.

Please cite this article in press as: Hatala, K.G., et al., Hominin track assemblages from Okote Member deposits near Ileret, Kenya, and their implications for understanding fossil hominin paleobiology at 1.5 Ma, Journal of Human Evolution (2017), http://dx.doi.org/10.1016/ j.jhevol.2017.08.013

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K.G. Hatala et al. / Journal of Human Evolution xxx (2017) 1e12

et al., 1999; Wilson and Carrano, 1999; Ezquerra et al., 2007; Castanera et al., 2013). Compared with their frequency in other paleontological records, the known sample of track sites from the early parts of the human fossil record is sparse. Tracks attributed to modern Homo sapiens are known at sites all around the world from the late Pleistocene (e.g., Mountain, 1966; Roberts and Berger, 1997; Webb et al., 2006; Kim et al., 2009; Liutkus-Pierce et al., 2016) and from the Holocene (e.g., Rector, 1979; Roberts et al., 1996; Meldrum, 2004; Mastrolorenzo et al., 2006; Morse et al., 2013). Sites earlier than these are much rarer. One famous site preserving Pliocene (~3.66 Ma; Deino, 2011) hominin trackways was discovered at Laetoli, Tanzania, in 1978 (Leakey and Hay, 1979). At the time of their discovery, the oldest known hominin skeletal fossils were younger than 3.66 Ma, so these tracks provided indisputable evidence of the earliest known appearance of fossil hominins and also showed that these earliest known hominins were bipeds. These tracks are typically assumed to have been produced by Australopithecus afarensis, which is still the only hominin known to have inhabited the Laetoli area at this time (but see Tuttle et al., 1991). The Laetoli footprints offered direct evidence to support interpretations from fossil skeletal morphology that early hominins with small brains and large teeth walked bipedally (Leakey and Hay, 1979; Day and Wickens, 1980; White, 1980; Leakey, 1981), definitively contradicting much earlier hypotheses (e.g., Darwin, 1871). Around the same time (in 1978), an assemblage of early Pleistocene (~1.4 Ma) hominin tracks was uncovered at Koobi Fora, Kenya (Behrensmeyer and Laporte, 1981). Because that site was less extensive, involving only seven tracks from one individual with variable preservation, and because of their younger age, these tracks from Koobi Fora have historically received less attention than those from Laetoli. For two decades, these two were the only known hominin track sites that predated the emergence of anatomically modern humans. As a result, despite an initial flurry of analyses regarding the Laetoli trackways, fossil hominin tracks have received considerably less attention from paleoanthropologists than hominin skeletal fossils. Within the past 10e15 years, however, several sites that preserve pre-H. sapiens (>315 ka; Hublin et al., 2017) tracks have been discovered in both Europe and Africa, and these have provided important new data for the human fossil record. In 2001, researchers conducted a detailed investigation of tracks long known to exist on the slopes of Roccamonfina, a volcano in Italy. This study concluded that these tracks were between 325 and 385,000 years old and therefore must have been produced by a pre-H. sapiens taxon (Mietto et al., 2003; Avanzini et al., 2008). From 2006 to 2008, multiple stratigraphic layers dating to ~1.5 Ma at Ileret, Kenya, were found with preserved hominin trackways likely attributable to either Homo erectus sensu lato or Paranthropus boisei (Bennett et al., 2009). In 2013, a site at Happisburgh, UK, was uncovered by tidal erosion and found to contain hominin tracks dating to between 0.78 and 1.0 Ma, preliminarily attributed to Homo antecessor (Ashton et al., 2014). Most recently, in 2015, two additional 3.66 Ma hominin trackways were uncovered at Laetoli (Masao et al., 2016). This growing sample of hominin tracks from a variety of times and geographical locations suggests that these new data, and additional discoveries that may follow, can provide valuable contributions that will help us better understand our evolutionary history. These discoveries also have sparked the development of new methodological approaches for digitally recording fossil hominin track assemblages (Bennett et al., 2009, 2013; Hatala et al., 2016a) and new experimental approaches for interpreting aspects of locomotor biomechanics from hominin track morphologies (D’Août et al., 2010; Raichlen et al., 2010; Crompton et al., 2011; Bates et al., 2013; Hatala et al., 2013, 2016a, b, c). These new techniques bypass certain limitations of other paleontological and archaeological data

and can help to inform long-standing questions relating to the evolution of human anatomy, locomotion, and behavior. Here, we present new evidence from multiple fossil hominin track sites discovered in recent years within ~1.5 Ma fossil deposits near Ileret, Kenya. In the years since the track surfaces at site FwJj14E near Ileret were first described, expansions of these earlier excavations combined with new surveys have led to discoveries of new tracks and trackways and multiple additional ~1.5 Ma track surfaces in nearby areas (Dingwall et al., 2013; Richmond et al., 2013; Hatala et al., 2016a; Roach et al., 2016, 2017). The total assemblage of 1.5 Ma hominin tracks known from the Ileret area has grown in size from the initially published sample of 20 tracks at a single site (FwJj14E; Bennett et al., 2009) to a currently known sample of 97 hominin tracks across five distinct sites (Hatala et al., 2016a). The size and nature of this assemblage of ~1.5 Ma hominin tracks now permits the exploration of an entirely new series of questions that could not be addressed in the initial publication. Recently published analyses (Hatala et al., 2016a) have focused on the implications of these tracks and trackways for locomotion and social behavior. We have also assessed the implications of the hominin tracks for patterns of land use and the overall paleoenvironmental context of the track surfaces (Roach et al., 2016, 2017). In the present study, we aim to 1) provide detail on the discoveries and excavations of each of the five ~1.5 Ma hominin track sites at Ileret, including their geological and sedimentological contexts, to aid other researchers who may encounter or wish to search for similar sites, 2) analyze external dimensions of Ileret hominin tracks within a broad comparative framework, to further evaluate their taxonomic affiliation, 3) provide estimates of traveling speed for all excavated Ileret trackways, and incorporate those results into past inferences regarding social behavior, 4) explore, in detail, behavioral hypotheses consistent with the evidence found within the Ileret track sites, and 5) provide all raw data relevant to these analyses. 2. Discoveries and excavations of Ileret hominin track surfaces 2.1. Initial discoveries of hominin track surfaces at FwJj14E The possibility that track surfaces might be preserved in Pleistocene sedimentary deposits near Ileret, Kenya was recognized during paleontological surveys (by A.K.B.) in the late 1970s, but it was several decades before any tracks were discovered in this area. In 2005, paleontological excavations were started at site FwJj14E after the discovery of fossil skeletal material from a presumed P. boisei upper limb (Richmond et al., 2009). In order to understand the geological/sedimentary context of that skeletal fossil discovery, a stepped trench was dug at the site of the subsequent excavation. While examining the stratigraphy of the site, Dr. Gail M. Ashley recognized that one sedimentary layer (later named the Lower Footprint Surface [LFS]) likely preserved animal tracks and multiple other layers were also identified as possible track surfaces. In 2006, a section of the LFS was uncovered and found to preserve a large assemblage of bovid tracks. In 2007, the excavation of the LFS was expanded, and new excavations were focused on a second potential track surface (later named the Upper Footprint Surface [UFS]) that had been identified at a higher (younger) position in the stratigraphic sequence. This initial excavation of the UFS in 2007 revealed the first hominin tracks discovered at FwJj14E. Through 2007 and 2008, the LFS and UFS were further excavated and analyzed, along with several less extensive track surfaces between these two layers. These initial discoveries and preliminary analyses of the track surfaces at FwJj14E were published in 2009 and revealed a total of 20 hominin tracks across both the LFS and UFS (Bennett et al., 2009).

Please cite this article in press as: Hatala, K.G., et al., Hominin track assemblages from Okote Member deposits near Ileret, Kenya, and their implications for understanding fossil hominin paleobiology at 1.5 Ma, Journal of Human Evolution (2017), http://dx.doi.org/10.1016/ j.jhevol.2017.08.013

K.G. Hatala et al. / Journal of Human Evolution xxx (2017) 1e12

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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 FwJj14E. Both the LFS and UFS were further exposed and several layers between these two surfaces in the stratigraphic sequence were identified as potential track surfaces (Fig. 1). Excavators cleared small test squares (~1 m2) on several of these intermediate layers to determine whether they might also preserve hominin tracks. Through these continued excavations, a total of 53 additional hominin tracks (48 on the UFS, three on the LFS, two on an intermediate layer named Layer A2) were uncovered at the site (Table 1). These additional discoveries provide a wealth of new data and clarify earlier interpretations of the track surfaces (Fig. 2). For instance, it became clear that a trackway originally thought to represent a single individual moving across the UFS (FUT1 sensu Bennett et al., 2009) actually consisted of the trackways of two individuals, where one made prints that overlapped those of another individual (Dingwall et al., 2013; Richmond et al., 2013). The similar step lengths, and states of preservation of these trackways, suggest that these individuals walked at similar speeds and may have traveled across the surface at or around the same time. It also became clear upon further excavation and detailed examination that certain impressions formerly classified as potential hominin tracks in the initial description of the site (FUI2, FUI5, FUI7 within Bennett et al., 2009) could not be confidently attributed to hominins, although these impressions did not influence any previous interpretations made by those authors. More detailed excavations also led previously unidentified impressions to be recognized as hominin tracks. In total, these ongoing excavations have now exposed 54 m2 of the UFS, about 65 m2 of the LFS, and approximately 2 m2 of the intermediate Layer A2. These surfaces at the site of FwJj14E together preserve a total of 72 hominin tracks, including nine distinct sequences of tracks that have been identified as continuous trackways (Table 1).

In 2013, expanded surveys for track-bearing sedimentary surfaces were conducted in East Turkana Collecting Area 1A, where site FwJj14E is located. Paleontological Collecting Areas are delineated by natural features such as rivers and were defined during the initial rounds of research at Koobi Fora (Feibel, 2011). Surveys aimed to identify additional track surfaces within the well-defined ~1.5 Ma Ileret Tuff Complex (ITC), which is part of the Okote Member of the Koobi Fora Formation. The ITC is capped by the Northern Ileret Tuff, which is dated to 1.51e1.52 Ma, and it is bounded below by the Lower Ileret Tuff, which is dated to 1.53 Ma (Brown et al., 2006; McDougall and Brown, 2006). At an intermediate position between these two tuffs is the Ileret Tuff, which has been dated to 1.52 Ma (Bennett et al., 2009). Within the ITC is a ~8.5 m sequence of massive laminated silts with intervening layers of fine-grained, stratified and cross-stratified sands that were likely deposited by intermittent low energy water transport processes within a delta margin environment, near a lakeshore (Roach et al., 2016). On June 28, 2013, three additional surfaces were identified while surveying the ITC exposures in Collecting Area 1A. The sediments overlying these track surfaces had eroded near the edges, such that portions of the track surface could be exposed simply by sweeping away loose surface sand, and tracks were immediately visible. Two of the surfaces discovered in this manner preserved hominin tracks. Upon discovery, and prior to excavation, these surfaces were assigned field names of ET-2013-1A-FE1 and ET-2013-1A-FE3 (ET for East Turkana, 2013 to designate the year of discovery, 1A to designate the East Turkana Collecting Area, and FE for footprint excavation; these names are abbreviated hereafter as FE1 and FE3). FE1 was discovered by K.R.O. and FE3 by B.G.R. These surfaces were excavated in 2013, leading to the exposure of 8 m2 of the track surface at site FE1 and 18 m2 of the track surface at site FE3. These excavations revealed many additional hominin tracks in situ, including five 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.

Please cite this article in press as: Hatala, K.G., et al., Hominin track assemblages from Okote Member deposits near Ileret, Kenya, and their implications for understanding fossil hominin paleobiology at 1.5 Ma, Journal of Human Evolution (2017), http://dx.doi.org/10.1016/ j.jhevol.2017.08.013

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K.G. Hatala et al. / Journal of Human Evolution xxx (2017) 1e12 Table 1 (continued )

Table 1 Catalog of hominin tracks discovered at site FwJj14E from 2007 to 2014.a Track

Trackway

FLT1-1 FLT1-2 FLT1-3 FLT1-4 FLT1-6 FUI3 FU-A FU-AA FU-AB FU-AC FU-AD FU-AE FU-B FU-C FU-D FU-E/FUI6 FU-F FU-G FU-H FU-I FU-J FU-K FU-L FU-M FU-N FU-O FU-P FU-S FU-T FU-W FU-X FU-Y FU-Z FUT1-1 FUT1-2 FUT1-3 FUT1-4 FUT1-4i FUT1-4ii FUT1-5 FUT1-5i FUT1-6 FUT1-7A FUT1-7B FUT1-8 FUT1-8i FUT1-9 FUT1-10 FUT1-11 FUT1-12 FUT1-13 FUT1-14 FUT1-14i FUT1-15 FUT1-16 FUT1-17 FUT1-18 FUT1-18i FUT1-19 FUT1-20 FUT2–3 FUT2–2 FUT2–1 FUT2-0 FUT2-1 FUT2-2 FUT2-3 FUT2-4

FLT1 FLT1 FLT1 FLT1 FLT1 FU-O

FU-AD FU-AD FU-O

FU-E FU-E FU-E FU-E FU-E FU-E FU-O FU-O FU-O

FU-X FU-X FUT1B FUT1B FUT1A FUT1A FUT1B FUT1B FUT1A FUT1B FUT1A FUT1A FUT1B FUT1A FUT1B FUT1A FUT1A FUT1B FUT1A FUT1A FUT1A FUT1B FUT1A FUT1A FUT1B FUT1A FUT1B FUT1A FUT1B FUT2 FUT2 FUT2 FUT2 FUT2 FUT2 FUT2 FUT2

Track surface LFS LFS LFS LFS LFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS UFS

Reported by Bennett et al. (2009)? X X

Track

Trackway

FUT3-1 FUT3-2 A2-H2 A2-H3

FUT3 FUT3

Track surface UFS UFS Layer A2 Layer A2

Reported by Bennett et al. (2009)? X X

a If a given track could be linked to a continuous trackway produced by the same individual, the field name of that associated trackway is listed. Indications are provided if a given track was reported in the initial description of the site (Bennett et al., 2009).

X

2.4. Discoveries of additional hominin track surfaces through random spatial sampling

X

X X X X

X X X X

X X X X

In 2014, surveys were conducted at randomly selected locations within the ITC exposures in Collecting Areas 1A and 3 as part of a protocol designed to systematically (and in an unbiased fashion) sample track surfaces in order to assess the broader paleoecological context of the hominin track sites (Roach et al., 2016, 2017). Briefly, a map of the ITC exposures in these areas was gridded into sequentially numbered 20 m  20 m squares using ArcGIS (v. 10.2). A random number generator (Microsoft Excel v. 14.4.8) was used to choose 20 m  20 m grid squares to survey for potential track surfaces. If a surface with an identifiable track occurred at one of the randomly selected locations, then a 1 m  1 m test square on that surface was excavated (Roach et al., 2016). Through this protocol, two additional hominin track surfaces were discovered. The field names of these sites were ET-2014-3-FE8 and ET-2014-1A-FE16 (abbreviated here as FE8 and FE16). K.R.O. and N.T.R. discovered site FE8 and K.G.H. discovered site FE16. The excavations at these sites are less extensive than the others listed above because they were part of the systematic footprint survey. FE8 consists of only a 1 m  1 m square (Fig. 5). The FE16 excavation was slightly expanded when it was realized that a potential hominin track lay on the edge of the random square and further excavation was needed to confirm its identity. That site consists of a 2 m  1 m rectangle of exposed track surface (Fig. 6). At site FE8, one hominin track has been identified, and three hominin tracks have been identified at site FE16 (Figs. 5 and 6). 2.5. Preservation of data from track surfaces Following direct measurements of tracks and trackways (see Methods), the global three-dimensional positions of all tracks were logged using a total station (Leica Builder 505; Trimble Nomad 900 LE data collector with EDMce for Windows Mobile software). Entire track surfaces were then recorded using photogrammetry, as a method of digitally quantifying but also preserving the original paleontological data (Falkingham, 2012). This method was deemed the most useful approach for permanently recording the footprint surfaces at Ileret, since they are located in a very remote location, preserved in unconsolidated sediment, and are at relatively high risk of damage through natural erosion processes (Bennett et al., 2013). Further, these digital records of the footprint surfaces will be curated as original records of the site that accompany total station data, field notes, and other paper records. Even though track surfaces were digitally documented immediately following their excavation, considerable efforts were made to rebury track surfaces in a manner that would aid their long-term preservation. The sterile sand that had previously overlain the track surfaces and helped to preserve them for the past 1.5 million years 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

Please cite this article in press as: Hatala, K.G., et al., Hominin track assemblages from Okote Member deposits near Ileret, Kenya, and their implications for understanding fossil hominin paleobiology at 1.5 Ma, Journal of Human Evolution (2017), http://dx.doi.org/10.1016/ j.jhevol.2017.08.013

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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 buried with additional sterile, sieved sand excavated from the stratified layers that had overlain each track surface prior to excavation. These overlying sediments were also graded in a manner that would divert surface water flow away from the site of the excavated track surface. Continuous work in the Koobi Fora area each year will allow for monitoring of these sites, and intervention will be possible if any appear to be at risk of damage. 3. Geological/sedimentological contexts of Ileret hominin track assemblages Studies of the geological and sedimentological contexts of track surfaces have differed methodologically between the various track sites. Because site FwJj14E has been the focus of continued study for more than a decade, the geology and sedimentology of this site is

understood in the greatest detail. Earlier interpretations of the stratigraphy at site FwJj14E (Bennett et al., 2009) have been followed by more detailed reconstructions of the sedimentary context of the FwJj14E track surfaces (Behrensmeyer, 2011; Roach et al., 2016). At the other hominin track sites at Ileret (FE1, FE3, FE8, and FE16), our understanding of site depositional processes is derived from detailed studies of the sedimentary layers and bedding structures exposed by the track surface excavations. These sites have also been tied in to the overall geology of the area and linked to the stratigraphic sequence exposed at site FwJj14E by measuring relative positions within the sequence of three volcanic tuffs (the Lower Ileret, Ileret, and Northern Ileret Tuffs) that comprise the ITC. It should be noted, however, that lateral variation in the track-bearing lithofacies makes it difficult to confirm the exact correlations of depositional surfaces at different sites. Hominin 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.

Please cite this article in press as: Hatala, K.G., et al., Hominin track assemblages from Okote Member deposits near Ileret, Kenya, and their implications for understanding fossil hominin paleobiology at 1.5 Ma, Journal of Human Evolution (2017), http://dx.doi.org/10.1016/ j.jhevol.2017.08.013

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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 ET2014-3-FE8. The single hominin track on this surface is circled in dashed white. 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 corner.

Figure 6. Overhead view of a three-dimensional photogrammetric model of site ET2014-1A-FE16. The three definitive hominin tracks discovered on this surface are circled in dashed white. Because of the small size of this excavation, it is unclear at this point whether these tracks form a continuous trackway. As a result, they have not been 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 corner.

4.1. Measurement and documentation of fossil hominin tracks and trackways 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 the first and fifth metatarsal heads, and the second was taken across the widest part of the heel impression. Based on visual observations of track locations and track morphologies, and consideration of individual track dimensions, certain tracks could immediately be linked to others within trackways consisting of multiple steps by the same individual. In these

Please cite this article in press as: Hatala, K.G., et al., Hominin track assemblages from Okote Member deposits near Ileret, Kenya, and their implications for understanding fossil hominin paleobiology at 1.5 Ma, Journal of Human Evolution (2017), http://dx.doi.org/10.1016/ j.jhevol.2017.08.013

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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 ¼ 1.39 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

Figure 7. Representative stratigraphic section showing the positions of Ileret hominin track surfaces within the ITC. Figure is derived from a previously published version (Roach et al., 2016), with copyright retained by N.T.R. Scale at bottom indicates 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 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 finegrain sand layers deposited by low energy processes, probably within a delta or lake margin. These well-bedded intervals are separated by paleosols representing intervening periods of subaerial delta plain (fluvial) deposition.

cases, where trackways were evident, step and/or stride lengths were directly measured with a tape measure. A field name was assigned to each trackway after its identification. Following direct measurements, hundreds of digital photographs were taken in order to render high-resolution photogrammetric 3D models of entire track surfaces and all individual hominin tracks. These 3D records are important for long-term data preservation (see above), but high resolution 3D models of hominin tracks have also enabled quantitative comparative analyses with tracks of other fossil and modern taxa (Hatala et al., 2016a). 4.2. Analyses of fossil hominin tracks and trackways 4.2.1. Analyses of external track dimensions Linear measurements of hominin tracks from the Ileret track surfaces were compared with a compilation of similar measurements from a variety of extant and extinct samples. Comparative data from modern taxa included measurements of tracks from footprint formation experiments conducted by K.G.H. with habitually barefoot Daasanach people from near Ileret, Kenya (Hatala et al., 2016b) and with modern chimpanzees in the Primate Locomotion Lab at Stony Brook University (Hatala et al., 2016c). Fossil data consisted of a compilation of published linear measurements from other fossil hominin track sites, including 325e385 ka tracks from Roccamonfina, Italy (Avanzini et al., 2008), 0.78e1.0 Ma tracks from Happisburgh, UK (Ashton et al., 2014), and ~3.66 Ma tracks from Laetoli, Tanzania (Bennett et al., 2016; Hatala et al., 2016c; Masao et al., 2016). 4.2.2. Estimates of traveling speed After taking linear measurements of stride lengths (or in some cases step lengths), we

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. They are longer and narrower than a collection of tracks produced experimentally by modern chimpanzees. On average, the Ileret tracks are larger than those preserved at the 0.78e1.0 Ma site of Happisburgh. They are generally longer but similarly wide to the ~3.66 Ma tracks from Laetoli, Tanzania. 5.2. Estimates of traveling speeds Most of the identified trackways, and hence most of these traveling speed estimates (eight of 10), come from our most expansive excavations on the LFS and UFS at site FwJj14E. All of the 10 trackways that we have identified almost certainly represent walking speeds, with estimates ranging from 0.45 to 1.58 m/s (Table 3). One isolated trackway on the LFS at FwJj14E had been thought to represent a speed within the range at which modern Daasanach people tend to transition from a walk to a run (2.0e2.3 m/s; Dingwall et al., 2013), but our revised estimate here falls squarely in the observed range of modern human walking speeds. 6. Discussion 6.1. Implications of analyses of external track dimensions The Ileret tracks are human-like in their external sizes. Any differences between these and more recent hominin tracks are almost certainly attributable to differences in the demographics of the hominins who produced them. For example, at Happisburgh there are many smaller tracks that are hypothesized to have been produced by children (Ashton et al., 2014). The larger Happisburgh tracks, presumably produced by adults, are generally comparable in size to the tracks at Ileret. Foot skeletal fossils are known for the two taxa proposed as most likely responsible for the Roccamonfina and Happisburgh tracks e Homo heidelbergensis and Homo antecessor, respectively. Analyses of these H. heidelbergensis and H. antecessor foot bones have suggested overall foot sizes similar to those of modern humans (Lu et al., 2011; Pablos et al., 2015). Here, our results suggest that modern humanlike foot sizes may have extended back even further in the hominin clade, to at least the time of the Ileret tracks at ~1.5 Ma. The ~3.66 Ma tracks from Laetoli, Tanzania are approximately as wide as the Ileret tracks, but generally are not as long. The average length of tracks from one Laetoli trackway exceed the pooled

Please cite this article in press as: Hatala, K.G., et al., Hominin track assemblages from Okote Member deposits near Ileret, Kenya, and their implications for understanding fossil hominin paleobiology at 1.5 Ma, Journal of Human Evolution (2017), http://dx.doi.org/10.1016/ j.jhevol.2017.08.013

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

Footprint length (cm)

Modern human (Daasanach) Modern chimpanzeesb Roccamonfinac Happisburghd Laetolie Ileret (all sites pooled) Ileret FwJj14E UFS Ileret FE1 Ileret FE3 Ileret FE8 Ileret FwJj14E LFS

Footprint breadth (cm)

Mean

Range

SD

Mean

Range

SD

25.4 (n ¼ 41) 20.3 (n ¼ 2) 24 (n ¼ ?) 19.1 (n ¼ 12) 22.8 (n ¼ 4) 25.3 (n ¼ 28) 25.2 (n ¼ 18) 23.6 (n ¼ 3) 26.3 (n ¼ 5) 27.0 (n ¼ 1) 24.9 (n ¼ 1)

20.0e29.5 19.9e20.7 e 14.0e26.0 19.1e26.1 20.5e30.5 20.5e30.5 21.0e26.8 23.5e29.5 e e

2.1 0.6 e 4.2 2.9 2.2 2.0 2.9 2.9 e e

9.7 (n ¼ 41) 11.3 (n ¼ 2) 12 (n ¼ ?) 7.5 (n ¼ 12) 9.6 (n ¼ 2) 9.6 (n ¼ 36) 9.8 (n ¼ 19) 8.0 (n ¼ 3) 9.9 (n ¼ 12) 9.5 (n ¼ 1) 8.0 (n ¼ 1)

7.4e11.8 11.0e11.6 e 5.0e11.0 8.7e10.4 7.5e12.7 8.25e12.7 7.5e9.0 7.5e12.0 e e

0.9 0.5 e 1.8 1.2 1.2 1.4 0.9 1.2 e e

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 FLT1 FUT1A FUT1B FUT2 FUT3 FU-E FU-O FU-X FU-AD FE1-HT1

Surface FwJj14E FwJj14E FwJj14E FwJj14E FwJj14E FwJj14E FwJj14E FwJj14E FwJj14E FE1

LFS UFS UFS UFS UFS UFS UFS UFS UFS

Step length (cm)

83.3 42.9

Stride length (cm)

Estimated speed (m/s)

110.0 86.5 73.3 124 130 156.5 131.5 114.0

0.73 0.65 0.45 1.05 1.23 1.52 1.32 1.29 1.58 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 length ¼ 26.1 cm; Masao et al., 2016) but the average lengths of all other Laetoli trackways fall below this average (average lengths of G1, G3, S2 range from 19.1 to 23.1 cm; Bennett et al., 2016; Hatala et al., 2016c; Masao et al., 2016). It has been proposed that the Laetoli track assemblage represents considerable body size variation, and possibly a high degree of sexual dimorphism in overall size with trackway S1 representing a large male (Masao et al., 2016). Assuming that track size is an accurate predictor of overall body size for fossil hominins (e.g., Dingwall et al., 2013; Hatala et al., 2016a), our results here show that the Laetoli track makers were still, on average, smaller in overall size than those from Ileret. This result agrees well with a recent analysis by Grabowski et al. (2015), which predicted fossil hominin body masses from lower limb skeletal fossils. That analysis included a range of body mass predictions from fossils attributed to both A. afarensis, the taxon most commonly assumed to have created the Laetoli tracks (e.g., Masao et al., 2016 but see; Tuttle et al., 1991), and H. erectus, the presumed maker 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 average, smaller than H. erectus. These comparisons with other fossil skeletal and track data rely, to some extent, upon accurate taxonomic attribution of the Ileret hominin tracks. Using the relationships between track size and

body size determined through experiments with modern Daasanach people, body mass predictions were generated for many Ileret hominin tracks and trackways (Hatala et al., 2016a). These body mass estimates from the Ileret hominin tracks were largely similar to the observed body masses of modern Daasanach people, and closer to skeletally based estimates from H. erectus fossils than they were to estimates derived from confidently attributable P. boisei or Homo habilis skeletal fossils (Grabowski et al., 2015). This line of evidence was used to support attribution of the tracks to H. erectus (Hatala et al., 2016a).2 The data in Table 2 demonstrate that large, human-like track size is consistent across all of the excavated Ileret track surfaces. While some Ileret track surfaces include relatively smaller tracks, none fall outside of the range of sizes observed in tracks made by adult modern humans. Because we have not observed differences in external track dimensions (this study) or internal track morphologies (Hatala et al., 2016a) between Ileret track sites, or between the large and small tracks within the assemblages, it is most parsimonious for us to hypothesize that all of

2 We acknowledge here that these previously published body mass predictions rely upon a reference sample of modern humans who have a very linear build compared with other modern populations (Ruff, 1994). If H. erectus were characterized 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 individuals (Ruff and Burgess, 2015).

Please cite this article in press as: Hatala, K.G., et al., Hominin track assemblages from Okote Member deposits near Ileret, Kenya, and their implications for understanding fossil hominin paleobiology at 1.5 Ma, Journal of Human Evolution (2017), http://dx.doi.org/10.1016/ j.jhevol.2017.08.013

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the Ileret tracks were made by one hominin taxon and that taxon was most likely H. erectus. However, at this point we cannot rule out completely that another hominin species made some or all of the tracks: a paucity of postcranial skeletal fossils means we know very little about body size variation in P. boisei or H. habilis, two taxa that are known to have co-existed with H. erectus in the Ileret area around ~1.5 Ma, and even less about Homo rudolfensis. 6.2. Implications of estimates of traveling speeds Roach et al. (2016) evaluated whether certain Ileret hominin trackways on the FwJj14E UFS may have represented a group coordinating their movement and traveling together. To do so, they used total station data to quantitatively compare the compass orientations of hominin and non-hominin trackways. They found that the hominin tracks were predominantly and non-randomly oriented in a southeasterly direction, and the tracks of all other animals had a significantly different northwesterly orientation (Roach et al., 2016). These different directions of travel for the hominins and all other animals suggest that the movement patterns captured on the FwJj14E UFS were not constrained by features of the natural landscape, as can be observed when animals follow well-traveled game trails (e.g., Laporte and Behrensmeyer, 1980). Further, with the exception of two trackways (FUT1A and FUT1B) that overlap in a manner consistent with one individual following the other, the UFS preserves many sub-parallel hominin tracks that do not overlap with one another. Roach et al. (2016) hypothesized that these non-overlapping, sub-parallel tracks could represent multiple individuals simultaneously moving through this area together. Here, we are able to use traveling speeds estimated from the FwJj14E UFS trackways, combined with published data on hominin trackway orientations, in order to further evaluate the likelihood that particular trackways may represent groups of individuals moving together across the FwJj14E UFS. Among the trackways described in Table 3, for which traveling speeds could be estimated, six of the eight (FUT1A, FUT1B, FUT3, FU-E, FU-O, and FU-X) are oriented in a southeasterly direction while the other two (FUT2 and FU-AD) are oriented towards the northwest. These trackways suggest a range of walking speeds that would not preclude group travel, as individuals within a group may vary their speeds while still traveling together. It is notable that the FU-X individual created the smallest tracks within the entire Ileret sample (SOM Table S1), and in a previously published analysis, Hatala et al. (2016a) found that this trackway generated a body mass estimate that was an extreme outlier among the Ileret sample, being far smaller than average (26.5 kg). In this study, we estimate that this markedly smaller individual still traveled at a speed consistent with the other individuals who moved towards the southeast, perhaps making an effort to keep up with the rest of the group. At this point, it is still difficult to hypothesize exactly who may have traveled together across the FwJj14E UFS, but trackway orientations and traveling speeds estimates appear to be consistent with what would be expected from coordinated group movement. 6.3. Novel directions for testing hypotheses related to hominin paleobiology at ~1.5 Ma Evolutionary hypotheses related to early hominin group behavior and social structure have proven notoriously difficult to test due to inherent limitations of the most common types of human fossil data (Chapais, 2013). While certain aspects of skeletal morphology such as canine size dimorphism may be associated with broad patterns of social behavior in primates (Leutenegger and Shell, 1987), those links are often tenuous and not broadly applicable across diverse taxa (Plavcan, 2000). The Ileret track

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assemblages, however, represent immediate snapshots of fossil hominin behaviors and they are recorded at a level of spatiotemporal resolution that has the potential to directly inform hypotheses about group structures and interactions between fossil hominin individuals. The sedimentological contexts of each of the Ileret hominin track surfaces are consistent with a short duration of surface exposure, during which tracks and trackways were formed and then rapidly buried. Each hominin track surface was buried by watertransported, fine silty sand (Figs. 1 and 7) that infilled the tracks without scouring or erosion. Similar low energy cycles of silt and sand deposition occur today along the shoreline of Lake Turkana, suggesting similar circumstances during the time of the ITC in a delta or lake margin environment. Taphonomic experiments on the durations of modern human tracks and trackways along the shore of modern Lake Turkana suggest that, when left unburied, detailed morphologies within the tracks (as are seen in the fossil hominin track assemblages) normally have a lifespan of 1.3 days or less (Roach et al., 2016). Beyond this point, track morphologies become less well defined and less recognizable, typically due to weathering or trampling by other animals (Roach et al., 2016). Together, these lines of evidence suggest that the Ileret hominin track surfaces were created and buried within a narrow time span from a few days to a few hours. This very short timeframe of site formation and deposition suggests that any individuals or animals that made tracks on the same surface likely lived within sufficiently close proximity to each other that their ranges overlapped on a daily basis. On the FwJj14E UFS, we find multiple sub-parallel hominin trackways, traveling in a similar direction that is statistically distinct from the directions of travel of all other animal tracks and trackways (Roach et al., 2016). Coupled with the short windows of time during which all of the trackways were formed and buried, these data suggest that at least some of the FwJj14E UFS individuals moving in the same direction traveled together. Here, our estimates of traveling speeds for the FwJj14E UFS trackways, which all imply walking speeds, provide additional support for the hypothesis of coordinated group movement. Even if these individuals happened to move through the area at similar speeds but at slightly different times (if they did not travel as a group), the very limited time for site formation and burial evident from geological and sedimentological data means that the different individuals whose tracks are preserved on the same surface lived in immediate proximity and likely interacted with each other. Hatala et al. (2016a) used body mass estimates derived from external track dimensions in order to estimate the sexes of the individuals who created the two largest assemblages of tracks that we have excavated at Ileret. They estimated that at least 50% of the hominin trackways on the FwJj14E UFS, and at least 75% of the tracks at site FE3 were created by male H. erectus individuals (Hatala et al., 2016a). Based on the results of the current study, we further emphasize that it is not necessarily the case that the Ileret trackways were made by groups of exclusively males moving together. In fact, the group of sub-parallel southeast-oriented trackways on the FwJj14E UFS, which also imply similar walking speeds and therefore offer the strongest case for coordinated group movement, includes a set of trackways that were all estimated by Hatala et al. (2016a) as potentially representing females and/or subadults. But while a small group of females and/or subadults may have moved together, they represent only six of the estimated 18 different individuals who left tracks on the FwJj14E UFS (Table 2). What we find most striking and well-supported regarding social behavior is that within both the FwJj14E UFS and the FE3 track assemblages, there are several trackways with sizes that exceed the pooled Ileret mean (Table 2) and likely represent multiple male H. erectus individuals.

Please cite this article in press as: Hatala, K.G., et al., Hominin track assemblages from Okote Member deposits near Ileret, Kenya, and their implications for understanding fossil hominin paleobiology at 1.5 Ma, Journal of Human Evolution (2017), http://dx.doi.org/10.1016/ j.jhevol.2017.08.013

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The presence and presumed interactions of multiple H. erectus males within the same local group has interesting implications that are relevant to major evolutionary hypotheses. Direct tests of hypothesized social behaviors have been particularly difficult to achieve due to a lack of pertinent data from the human fossil record. While there are certainly limitations to the data preserved in these track assemblagesdfor example, the exact types of associations or genetic relationships between individuals and the activities that led to their interactions will never be knowndwe can examine the evidence of group composition and look for consistencies with, or deviations from, existing hypotheses related to the evolution of human social structure. We present below a series of behavioral hypotheses, each of which could be supported by evidence from the Ileret track assemblages. This is not an exhaustive list of possibilities, but provides multiple scenarios that can be explored in future work to refine methods for inferring behavior from assemblages of tracks and trackways. First, it is possible that the Ileret track assemblages were produced by group behavior patterns that have deep evolutionary roots and may have characterized the Pan-Homo last common ancestor (LCA). Among modern chimpanzees, multi-male groups are known to assemble and travel together while conducting patrols of their borders as a method of mitigating intergroup competition (Watts and Mitani, 2001; Mitani and Watts, 2005; Watts et al., 2006). A similar type of behavior may be evidenced by the large assemblages of presumed male tracks at the FwJj14E UFS and site FE3. Many of the hominin trackways on the FwJj14E UFS appear to represent individuals moving along the water's edge (Roach et al., 2016), which may or may not be consistent with a group actively surveying a territorial perimeter. Multi-male groups of chimpanzees are also known to engage in cooperative hunting expeditions, although this behavior may not be generalizable across the species as a whole because it has been observed at only a single study site (Boesch, 2002). At this point, we do not have sufficient modern or fossil data to rule out the possibility that the Ileret track assemblages could represent a behavior analogous to that observed in modern chimpanzees. Future analyses of how specific chimpanzee behaviors are recorded in tracks and trackways are necessary to test these hypotheses. The Ileret track assemblages do meet the predictions of the more general hypothesis that multi-male groups with some level of cooperation were present in the LCA (Wrangham and Pilbeam, 2001; Duda and Zrzavy, 2013). Yet, in order for human and chimpanzee maleemale cooperation to be homologous, this strategy must have been inherited from the LCA and persisted in the lineages leading to both modern humans and chimpanzees. The high levels of size dimorphism in A. afarensis (Plavcan et al., 2005; Gordon et al., 2008; but see also Reno and Lovejoy, 2015), a species widely regarded as a stem hominin (Kimbel and Delezene, 2009), suggest high levels of maleemale competition. This hypothesized competition, plus little to no information on the existence of cooperative behaviors such as group defense and/or hunting in A. afarensis, do not provide substantial support for the homology hypothesis at this time. However, given the growing number of contemporaneous taxa recognized in the Pliocene (Haile-Selassie et al., 2016), the poor preservation of many of them, and the inherent difficulties in identifying ancestoredescendant relationships in the fossil record, it is premature to rule out the hypothesis that the maleemale cooperation observed in modern chimpanzees and humans may be homologous. At the very least, the evidence from the Ileret track surfaces of multiple H. erectus males walking across the same landscape, and possibly even traveling together, is consistent with a level of maleemale cooperation similar to that observed in modern chimpanzees.

Alternatively, the Ileret track assemblages may preserve direct evidence of derived behaviors that led to the emergence of a social structure that, among living primates, uniquely characterizes modern humans. Maleemale cooperation is a key feature that underlies the maintenance of multilevel social structures, which describe nearly all modern human societies and have been hypothesized, based on a variety of other archaeological and paleontological evidence, to have first emerged in H. erectus (Swedell and Plummer, 2012). This model suggests a tiered hierarchy of differently sized social unitsdincluding, from smallest to largest, the polygynous one-male unit, the clan, the band, and the troopdand coordination at all but the lowest levels of this hierarchy depends upon, at the very least, mutual tolerance among males (Swedell and Plummer, 2012). Among modern human huntergatherers, it has been demonstrated that this nested, hierarchical pattern of social structure may represent an ‘optimal’ solution for the distribution of resources both among and within groups (Hamilton et al., 2007). As H. erectus emerged and evolved within a dynamically changing environment that required wider dispersal and provided less reliable resources (Potts, 1998; DeMenocal, 2004;  n et al., 2014; Potts and Faith, 2015), this social structure could Anto have played an essential adaptive role in promoting foraging success. In modern human hunter-gatherers, multiple males from within the same band cooperate to hunt for game and then share the calorically rich profits of that hunt among all members of their band (Hill, 2002; Gurven, 2004; Marlowe, 2005; Hill et al., 2009). Cooperative foraging and the sharing of resources seems likely if H. erectus regularly foraged for animal resources, as has been suggested by collections of cut-marked bone from 1.5 Ma deposits in Collecting Area 1A (Pobiner et al., 2008) and potentially by the Ileret tracks themselves (Roach et al., 2017). Cooperation improves the probability of a successful kill in modern hunter-gatherers and may have been even more important at the time of H. erectus, when weapons were likely much less lethal than those used by huntergatherers today (Hill, 2002). Further, the sharing of food resources acquired through cooperative hunting across all members of the band buffers the relatively high probability of failure that is associated with any given hunt (Lee, 1968; Hill, 2002; Gurven, 2004). If a multilevel social structure was present in H. erectus, mediated by maleemale cooperation and perhaps including patterns of cooperative foraging, then this structure could have promoted the emergence of the number of unique behavioral traits and the “deep social structure” that are considered to define modern humans (Hill et al., 2009; Chapais, 2011). Ultimately, comprehensive tests of these types of hypotheses about social behavior and social structure require more modern and fossil data than are currently available. Determining the presence or absence of specific patterns of social behavior will rely upon continued work to refine our abilities and better understand our limitations for testing hypotheses about hominin social interactions from track and trackway data. Experimental research is necessary in order to determine how behaviors of humans, and of nonhuman primates, are recorded in assemblages of their tracks. The continued pursuit of such methods is valuable, as tracks and trackways offer unique opportunities to directly observe, in deep time, snapshots of groups of our fossil relatives and draw inferences regarding the compositions and social behaviors of those groups. Track sites may allow for types of inferences regarding group behavior that have been notoriously difficult to gain through other forms of archaeological and paleontological data (Chapais, 2013). Already, we have been able to draw some conservative conclusions from the Ileret track assemblages. Regardless of the specifics of the social structure and social behaviors of H. erectus, our analyses do suggest the presence and interactions of multiple H. erectus males living and traveling on the same landscapes. This evidence is

Please cite this article in press as: Hatala, K.G., et al., Hominin track assemblages from Okote Member deposits near Ileret, Kenya, and their implications for understanding fossil hominin paleobiology at 1.5 Ma, Journal of Human Evolution (2017), http://dx.doi.org/10.1016/ j.jhevol.2017.08.013

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consistent with previous hypotheses that have suggested that the social structure of H. erectus supported the emergence of modern human-like patterns of social behavior. 7. Conclusions These discoveries of multiple hominin track sites in Okote Member deposits near Ileret, Kenya, provide a unique window that can further our knowledge of various aspects of hominin paleobiology at ~1.5 Ma. Initial analyses of these sites explored the implications of these track sites for hominin anatomy, locomotion, land use patterns, and behavior (Bennett et al., 2009; Hatala et al., 2016a; Roach et al., 2016, 2017). Here, we provide new comparative assessments of external track dimensions, and estimates of traveling speed derived from hominin trackways, to refine earlier interpretations of foot size, taxonomic attribution, and group movement. These new analyses, combined with those published previously, highlight the utility of trace fossil data in developing and testing major hypotheses regarding the biology and behavior of fossil hominins. We hope that the details we provide on how we discovered and excavated the Ileret hominin track sites, our descriptions of the geological/sedimentological contexts in which we find these sites, and the initial analyses and hypotheses laid out in this study will help motivate continued research in hominin ichnology. Continued discoveries of new hominin trace fossil data, and the development of new approaches for interpreting them, will help us use these data in concert with other parts of the paleontological and archaeological records to better inform understandings of our evolutionary past. Acknowledgements  Bobe, Andrew Du, Matt Ferry, Purity Kiura, We thank Rene Emma Mbua, Emmanuel Ndiema, Jonathan Reeves, Erin Marie Williams-Hatala, students of the Koobi Fora Field School, the National Museums of Kenya, the town of Ileret, Kenya, and the local Daasanach volunteers for their contributions to this research. This study was conducted under a research permit granted by the Kenyan National Council for Science and Technology and an excavation license granted by the Ministry of Higher Education, Science and Technology. This study was funded by the Leakey Foundation, the National Science Foundation (BCS-1232522, BCS-0924476, BCS1128170, BCS-1515054, BCS-0935321, DGE-080163, SMA-1409612), the Wenner-Gren Foundation (Grant 8592), and The George Washington University's Research Enhancement Fund. Supplementary Online Material Supplementary online material related to this article can be found at http://dx.doi.org/10.1016/j.jhevol.2017.08.013. References Alexander, R.M., 1976. Estimates of speeds of dinosaurs. Nature 261, 129e130. n, S.C., Potts, R., Aiello, L.C., 2014. Evolution of early Homo: An integrated Anto biological perspective. Science 345, 45. Ashton, N., Lewis, S.G., De Groote, I., Duffy, S.M., Bates, M., Bates, R., Hoare, P., Lewis, M., Parfitt, S.A., Peglar, S., Williams, C., Stringer, C., 2014. Hominin footprints from early Pleistocene deposits at Happisburgh, UK. PLoS One 9, e88329. Avanzini, M., Mietto, P., Panarello, A., De Angelis, M., Rolandi, G., 2008. The devil's trails: Middle Pleistocene human footprints preserved in a volcanoclastic deposit of southern Italy. Ichnos 15, 179e189. Bates, K.T., Savage, R., Pataky, T.C., Morse, S.A., Webster, E., Falkingham, P.L., Ren, L., Qian, Z., Collins, D., Bennett, M.R., McClymont, J., Crompton, R.H., 2013. Does footprint depth correlate with foot motion and pressure? J. R. Soc. Interface 10, 20130009. Behrensmeyer, A.K., 2011. Conversations with Glynn's ghost: The evolution of paleolandscape research at East Turkana. In: Sept, J., Pilbeam, D. (Eds.), Casting

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Please cite this article in press as: Hatala, K.G., et al., Hominin track assemblages from Okote Member deposits near Ileret, Kenya, and their implications for understanding fossil hominin paleobiology at 1.5 Ma, Journal of Human Evolution (2017), http://dx.doi.org/10.1016/ j.jhevol.2017.08.013

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Please cite this article in press as: Hatala, K.G., et al., Hominin track assemblages from Okote Member deposits near Ileret, Kenya, and their implications for understanding fossil hominin paleobiology at 1.5 Ma, Journal of Human Evolution (2017), http://dx.doi.org/10.1016/ j.jhevol.2017.08.013