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doi 10.4436/jass.94035 JASs Proceeding Paper Journal of Anthropological Sciences Vol. 94 (2016), pp. 19-27

Early hominin diversity and the emergence of the Homo

William E.H. Harcourt-Smith

Department of , Lehman College CUNY, 250 Bedford Park Blvd W, Bronx, N.Y. 10468, USA; Division of Paleontology, American Museum of Natural , CPW & 79th St., New York, N.Y. 10024, USA; Department of Anthropology, CUNY Graduate Center, 365 5th Avenue, New York, N.Y. 10016, USA; New York Consortium in Evolutionary Primatology, N.Y., USA. e-mail: [email protected]

Summary - is a defining trait of hominins, as all members of the clade are argued to possess at least some characters indicative of this unusual form of locomotion. Traditionally the evolution of bipedalism has been treated in a somewhat linear way. This has been challenged in the last decade or so, and in this paper I consider this view in light of the considerable new fossil hominin discoveries of the last few . It is now apparent that there was even more locomotor diversity and experimentation across hominins than previously thought, and with the discovery of taxa such as H. floresiensis and H. naledi, that diversity continues well into the genus Homo. Based on these findings, we need to reevaluate how we define members of the genus Homo, at least when considering postcranial morphology, and accept that the evolution of hominin bipedalism was a complex and messy affair. It is within that context that the modern form of bipedal locomotion emerged.

Keywords - Hominins, Bipedalism, Homo, Locomotor diversity.

Introduction highly unusual form of locomotion within the context of the increasingly diverse and complex The symposium at which this paper was hominin clade. This will be done with par- given was entitled “What made us human”. ticular emphasis on the foot, the only structure The answers to that question are of course that comes into contact with the ground in myriad and multifaceted, and entirely depend bipeds, and thus one that is extremely reflec- on whether one is considering strictly our own tive of locomotor behaviour. , Homo sapiens, or the hominin line- The contemporary view of the hominin age as a whole. Look at our own species and tree is that it is distinctly bushy. There are cur- one has to consider complex attributes linked rently at least 23 taxa that are widely accepted exclusively to our relatively large brains, such as as belonging to the . Bipedalism is or symbolic behaviour. Look at the arguably the first major adaptation we see in entire hominin lineage and the picture becomes hominins (Senut et al., 2000; Zollikofer et far cloudier. That having been said, one major al., 2005; White et al., 2009), and yet until behavioural attribute that arguably character- about ten years ago it’s evolution had largely izes the entire lineage is bipedal locomotion been treated as a relatively linear affair. Some and its associated anatomical modifications. researchers, such as Robinson (1972) did All hominins possess features that show at least argue that different hominin taxa living at some indication of bipedality. The purpose the same time ( africanus and of this paper is to discuss the evolution of this robustus) had differing degrees of

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terrestriality in their locomotor repertoires, but et al., 2005). The overall femoral morphology the predominant view remained that that earlier, of tugenensis also indicates bipedal- small brained hominins, such as Australopithecus, ity (Richmond & Jungers, 2008) although the were bipedal much of time but retained some upper limb remains imply arboreality (Senut et level of arboreality, the first members of the genus al., 2001). The remains also come from a wide Homo has eschewed arboreality all together, and geographic area, perhaps making it difficult to that the modern human condition (obligate be sure that they all represent the same taxon. bipedalism) was simply a matter of subtle modi- The best represented of these early hominins, fications of the condition of basal Homo. ramidus, whose hominin status is By the 1990s this position was becoming now well established (Kimbel et al., 2014), pos- increasingly untenable, and in the following sesses a postcranial skeleton dominated by fea- decade it was evident that within the context of tures related to an arboreal lifestyle. However greatly increased hominin taxonomic diversity, the pelvis, although distorted, suggests bipedal- which was based entirely on craniodental fea- ity based on a number of features, including a tures, a reevaluation was needed. The first mod- prominent anterior inferior iliac spine, indicat- ern paper to challenge this linear view of biped- ing a human-like attachment and function of the alism argued, based on quantitative analyses of hip flexor m. rectus femoris and the iliofemoral hominin foot bones, that there were at least two ligament, a hip joint stabilizer distinct experiments with the foot, one typified Although important for understanding the by Australopithecus afarensis, and one typified emergence of hominin bipedality, these taxa do by Australopithecus africanus and not temporally overlap, and in the case of S. tch- (Harcourt-Smith & Aiello, 2004). It concluded adensis and O. tugenensis are too poorly repre- that there was likely more locomotor diversity sented in the postcranium to sufficiently address in the hominin clade than had been previously questions relating to locomotor diversity in early argued. It has been over ten years since that hominins. In that respect it cannot be argued that paper, and it is thus interesting to consider its there was locomotor diversity (or not) during the conclusions within the context of the significant 7 – 4 Ma time period. The one interesting con- number of new hominin taxa that have been sideration, though, is the relationship between named and described since. Ar. ramidus at 4.4 Ma and the earliest species of Australopithecus, Au. anamensis at 4.2 – 3.9 Ma. This latter taxon is argued to have had features in Locomotor diversity in early the lower limb indicative of habitual bipedalism hominins (Ward et al., 2001). With a first appearance in the fossil record only 200,000 years after the 4.4 Early hominins (here defined as hominins Ma date of Au. ramidus, it would have required living between approximately 7 and 4 Ma) are a very fast rate of morphogenesis for Au. rami- now well represented in the record thanks to dus to have speciated into Au. anamensis. The significant discoveries over the last twenty years. alternative is that Ar. ramidus and Au. anamensis The earliest, putative members of the clade, as represent distinct evolutionary lineages, and that one might expect, possess very few features that the two species may well have overlapped in time indicate bipedalism. At approximately 7 Ma in and space, thus indicating different experiments age, tchadensis has no published with bipedalism occurring in two different early postcrania, but the digital reconstruction of the hominin taxa. However, until further Au. ana- heavily crushed skull indicates an anteriorly mensis remains are discovered (and critically positioned and horizontally orientated fora- from the same anatomical regions as we have for men magnum, the latter feature being especially Ar. ramidus), this remains a somewhat specula- important in indicating bipedality (Zollikofer tive assertion. W.E.H. Harcourt-Smith 21

Locomotor diversity in australopiths Au. afarensis foot also support these findings (e.g. Berillon, 2003; DeSilva & Throckmorton, Between 4 and 3 Ma we see a wealth of homi- 2010). It follows, then, that the trails nin remains that are predominantly assigned to may well represent a hominin taxon with a more the genus Australopithecus. The best know taxon derived foot than either that of Au. afarensis or from this time period, Au. afarensis, is well- the Burtele foot, thus pointing to even more vari- represented throughout the skeleton. Although ation in hominin foot form and locomotion in scholars have differed in their functional inter- the 4 – 3 Ma time period. pretations of the postcranium (e.g. Lovejoy et al., The period between 3 and 2 Ma is extremely 1973; Lovejoy, 1988; contra Stern & Susman, meagre for East African material, although the 2.5 1983; Stern, 2000) the predominant view today Ma remains from Bouri, Ethiopia, indicate a hom- is that this taxon was habitually bipedal (some- inin that might have had slightly elongated hind times referred to as facultative bipedalism) but limbs (Asfaw et al., 1999). In South Africa, how- was not an obligate biped. Au. afarensis thus ever, the numerous well-known remains attributed retained an arboreal component to its locomo- to Au. africanus and Au. sediba (and possibly other tor repertoire, as indicated by its curved manual species of Australopithecus) indicate combinations and pedal phalanges and -like shoulder joint of postcranial morphologies that are distinct from (Stern, 2000; Alemseged et al., 2006). Recently, both those in east Africa and also from each other. new findings indicate that there were other, Outside those at Makapansgat, some of the oldest likely more arboreal hominin taxa living at the remains of South African Australopithecus are those same time and in the same part of East Africa as belonging to the skeleton Stw 573 (“Little Foot”), Au. afarensis. The hominin foot from Burtele, from Member 2, (Clark, 1998). The Ethiopia (Haile-Selassie et al., 2012), dated to dating of this skeleton has been the subject of 3.3 Ma, is argued to have had an opposable hal- considerable disagreement, and although a review lux, thus indicating pedal arboreal grasping. It of that research in conjunction with new dating has long been argued that hallux opposability methods place it between 2.6 and 2.2. Ma, the is not evident in Au. afarensis. Such a morpho- most recent findings indicate that it is more likely logical condition would also be incompatible to be aged to 3.2 Ma (Granger et al., 2015). If as with the 3.6 Ma Au. afarensis set of hominin old as 3.2 Ma, the Stw 573 skeleton has a combina- footprints at Laetoli, , which are mark- tion of features in the foot entirely different to the edly human-like (e.g. Raichlen et al., 2010). contemporary Au. afarensis remains from Hadar Although the consensus view remains that the (Harcourt-Smith & Aiello, 2004). If Stw 573 is prints were made by that taxon (e.g. White et al., younger, it still represents a distinct pattern from 1984), they have been argued to be incompat- Au. afarensis, and given that it has a more primi- ible with the pedal morphology of Au. afarensis tive talo-crural joint, it either indicates a reversal (e.g. Tuttle et al., 1990, 1991; Harcourt-Smith in morphology from Au. afarensis to Au. africanus, & Hilton, 2005). The Au. afarensis foot, as rep- or two distinct lineages with different types of resented by remains from Hadar, has an inflated mosaicism in the foot (Harcourt-Smith, 2002). navicular tuberosity and a low orientation of the Elsewhere in the skeleton the relative size of the sustentaculm tali on the calcaneus (Sarmiento & long bone joints in Au. africanus and Au. afaren- Marcus, 2000; Harcourt-Smith & Aiello, 2004; sis are argued to be broadly alike (e.g. McHenry, Harcourt-Smith et al., 2015). The former feature 1986; Dobson, 2005), although there are also is indicative of a weight-supporting middle part argued to be distinctly different muscle attachment of the medial foot (Elftman & Manter, 1935) configurations in the pelvis (Haeusler, 2001). while the latter feature suggests a low medial Haeusler (2001) in fact asserts that the older Au. longitudinal arch (Morton, 1935). Architectural afarensis was more human-like in its mode of bipe- reconstructions of the medial column of the dalism that the more recent Au. africanus.

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The recently described Au. sediba remains and the taxonomic affinities of much of the point to a late species of South African australo- Olduvai postcranial material have been challenged pith completely distinct in its postcranial skele- (Constantino & Wood, 2007). New discover- ton and gait from other contemporary hominins ies have also expanded the locomotor diversity from the region (Au. africanus and Paranthropus within the genus. While the postcranium of H. robustus) in having a pelvis that shares a number ergaster and H. erectus is predominately human- of important features with those from the genus like, analyses of the upper limb indicates that the Homo, such as shorted ischia and more verti- position of the scapula and the degree of humeral cally orientated ilia (Kibii et al., 2011). Given torsion in these taxa falls outside the modern the relatively small of Au. sediba, this human range (Larson, 2009, but see Roach & is a strong indication that selection for obligate Richmond, 2015). This would have had implica- bipedalism in this taxon was a more likely rea- tions for arm and trunk movement during bipedal son for a more Homo-like pelvis than anatomical walking, perhaps leading to a gait different from reconfiguration to accommodate larger-brained that of modern . More drastically, the neonates during parturition. However, the leg diminutive remains of H. floresiensis has a shoul- and foot of Au. sediba are mosaic and are argued der configuration unlike that of modern humans to indicate a hyperpronating form of bipedalism (Larson et al., 2007), a long foot relative to the leg, distinct from that of both early Homo and other a hallux shorter than in humans and a foot lacking species of Australopithecus (DeSilva et al., 2013). a medial longitudinal arch (Jungers et al., 2009). Little can confidently be said about the post- In combination these features would have resulted cranial morphology of the genus Paranthropus, in a gait definitively different that of modern although Robinson (1972) argued that the humans, and would likely have impaired com- extensive remains from , South plex bipedal behaviours such as running. Most Africa, indicated a locomotor repertoire that was recently the H. naledi remains from South Africa more terrestrial than that of the contemporary (Berger et al. 2015), although lacking a geologic Au. africanus. In east Africa the recent discovery age, indicate further experimentation with bipe- of new Paranthropus boisei postcrania at Olduvai dalism within Homo. The foot is largely human- Gorge, Tanzania (Domínguez-Rodrigo et al., like (although it has phalanges more curved than 2013) indicates that that taxon may have had in modern humans) (Harcourt-Smith et al., 2015) powerful and large forelimbs, perhaps indicating whereas the shoulder joint has a cranially orien- some degree of arboreality. This is distinct from tated glenoid, and highly curved manual phalan- the more gracile upper limb remains attributed to ges, both adaptations to arboreal climbing (Berger contemporary early Homo from northern Kenya et al., 2015). Given that the H. naledi talus is far (Leakey et al., 1978; Aiello & Dean, 1990). more derived than that of OH 8 (if that belongs to H. habilis), and the cranial morphology is closer to basal Homo than not, this indicates a different Locomotor diversity in the genus suite of locomotor adaptations in H. naledi com- Homo pared to its likely closest relatives, H. habilis, H. ergaster and H. erectus. Recent descriptions of fem- Until the last ten years or so the earliest mem- postcranial remains from northern Kenya (Ward ber of the genus Homo with associated postcra- et al., 2015) support this assertion. The remains nial elements, H. habilis, was argued by most to are argued to be Homo-like, but possess femoral have been an obligate biped (e.g. Day & Naiper, midshaft and pelvic inlet morphologies distinct 1964; Susman & Stern, 1982). Recent finds from those found in African H. erectus, indicat- have challenged that assertion. Ruff (2009) has ing that at 1.9 Ma there were at least two distinct shown that the OH 62 partial skeleton retained postcranial morphotypes in east Africa attributed ape-like structural properties in its long bones, to the genus Homo. How these differences relate W.E.H. Harcourt-Smith 23

to potentially different types of bipedal locomo- the context of the earlier date for the emergence of tion will need to be further explored. Homo that coincides with a shift to more open and arid habitats (DiMaggio et al., 2015).

Consideration of locomotor diversity in the genus Homo within light of Conclusions increased taxonomic diversity Based on current evidence from the foot it is Although beyond the scope of this paper, it now clear that there was considerable variation in is worth briefly considering how we define the fossil hominin pedal anatomy (Fig. 1). Given that genus Homo within the context of increased hom- the foot is the only structure to interact with the sub- inin postcranial and associated locomotor diver- strate in bipeds, this likely indicates variation in the sity. Many of the traditional characters used to “types” of bipedalism found in different fossil hom- define Homo have been challenged or reevaluated inin taxa Though the evidence is meagre for early over the last twenty years (e.g. Wood & Collard, hominins, by 4.2 Ma it is apparent there may have 1999), and recently several finds have contributed been two distinct locomotor modes, in Ar. ramidus to this process. We now know that and Au. anamensis respectively. These two patterns production (Harmand et al., 2015) and possible continue throughout the , as evidenced by hominin tool use (McPherron et al., 2010) pre- the Au. afarensis remains and Burtele foot (Haile- date the emergence of the genus Homo. Brain size Selassie et al., 2012). The potential incompatibility increase is also an increasingly problematic crite- of the Au. afarensis foot and the Laetoli footprints rion to use (Spoor et al., 2015) which leaves two indicates possible further variation in that time major usable criteria: the dental apparatus and the period. The early Au. sediba remains locomotor skeleton. New finds that push back point to experimentation with a Homo-like pelvis the earliest evidence of the genus (Villmoare et al., in late South African Australopithecus, in combina- 2015) indicate that a reduced dentition predates tion with a reconstructed gait unique to the genus. other bony adaptations in the jaw of early Homo, In early Homo we now see an increased diversity making dental modification a robust criterion for in postcranial anatomy and associated locomotor membership of the genus, although it is also evi- behavior, which indicates a continuation of loco- dent that there was considerable diversity in dental motor experimentation late into the hominin fossil and gnathic morphology between early Homo taxa record (as indicated by H. florsiensis). New finds (Spoor et al., 2015). Postcranially, there are now suggest at least two distinct postcranial morpho- new species of Homo with locomotor adaptations types at 1.9 Ma (Ward et al., 2015), and H. habilis indicating either a different of bipedalism to and H. naledi are argued to have retained a degree that of modern humans (e.g. H. floresiensis), or of arboreality in their locomotor repertoires, while a retention of some arboreality in the locomotor H. floresiensis would have had a gait markedly dis- repertoire (e.g. H. naledi). tinct from that of modern humans (Jungers et al., Given these problematic and confounding 2009). It follows, then, that it is perhaps necessary factors, obligate and, more importantly, modern to reevaluate obligate, human-like bipedalism as a human-like, bipedalism should perhaps not be meaningful diagnostic criterion for membership of used as a criterion for membership of the genus the genus Homo. Homo. Instead, a combination of postcranial traits It is thus from a broad spectrum of locomo- and archaeological evidence that clearly indicate tor experimentation that modern human biped- a distinct behavioural shift in activity patterns, alism emerged. While this may be confusing to landscape use and resource acquisition (including some, it largely supports what we know for many materials for tool-making) should be considered other aspects of the human condition, that the more closely. This is especially important within story is complex and diverse.

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Fig. 1- A graphical summary of pedal anatomical variation in the hominin fossil record.

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