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Home , Ardi, Australopithecine, Australopithecus, Berhane Asfaw, Bipedalism, Chororapithecus

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The Partial Skeleton of brief mention of a recovered partial skeleton Ramidus (White, Suwa, and Asfaw 1995). It could only be assumed that this taxonomic Arthur Klages revision implied that the new material must have been morphologically and adaptively Introduction distinct from . Paleoanthro- In 1994 a new species of pologists eagerly awaited the description of Australopithecus was described that was the partial skeleton, especially with the purported to be the new stem hominin1 and implications arising from the new , but the ancestor of the then widely recognized the wait for the publication would be a stem hominin, Australopithecus afarensis protracted one. (White, Suwa, and Asfaw 1994). Mostly a It was not until 2009 that the collection of dental and mandibular aforementioned partial skeleton was fragments, accompanied by a portion of the described in a publication. Bestowed with basicranium, the newly minted the appellation “”, in perhaps a con- Australopithecus ramidus was dated to 4.4 scious attempt to capture the notoriety of the Ma, and, thus, became the oldest famous skeleton, the partial skeleton on record (White, Suwa, ARA-VP-6/500 contained more skeletal and Asfaw 1994). The hominin status of Au. elements than Lucy, but was plagued by ramidus, or rather its bipedality, was extreme fragmentation and horrendous inferred from its morphological similarity to presservation (White et al. 2009). This was Au. afarensis and the inferior orientation of given as the main reason for the delay in the (White, Suwa, and publication (White et al. 2009). A series of Asfaw 1994). As Au. afarensis was already eleven articles described the new Ar. established as an obligate biped2, it was ramidus material and these descriptions suggested that Au. ramidus was likely a relied heavily on virtual reconstructions of bipedal hominin (White, Suwa, and Asfaw the fragmentary using micro-CT 1994). However, it was clear that the new . A critical analysis of the virtual species not only preceded Au. afarensis reconstructions is necessary, as the morpho- chronologically, but that it also displayed a logical inferences found among the more primitive morphology. Au. ramidus descriptions are only as solid as the quality lacked the postcanine megadontia3 of of the reconstructions and one’s confidence Australopithecus, had thinner enamel, in them. and had more -like canines, which From the virtual reconstructions, it was inferred by the authors that Ar. ramidus resulted in the naming of a new specific 4 taxon to distinguish this material from was a hominin, a facultative biped , and was known (White, Suwa, and most likely ancestral to Australopithecus Asfaw 1994). (White et al. 2009). These conclusions were Shortly thereafter, a corrigendum identical to those already postulated in the reassigned this material to a new genus, and original 1994 descriptions based on only a the “root” australopithecine became the few skeletal elements, and perhaps that was “root ground ape” Ardipithecus ramidus to be expected (White, Suwa, and Asfaw (White, Suwa, and Asfaw 1994, 1995). 1994). What was surprising was the There was little justification of this taxo- extremely primitive morphology of Ar. nomic revision in the corrigendum, and no ramidus relative to Australopithecus (White new material was described, other than a et al. 2009). In spite of claims to facultative

TOTEM vol 19 2010-2011 TOTEM 138 terrestrial , Ar. ramidus retained a behaviourally, Ar. ramidus was unlike the completely divergent hallux5 (White et al. extant hominids, and consequently, the 2009). Its primitive foot and discovery of Ar. ramidus questions their morphology suggested that it retained a value as referential models for the LCA substantial arboreal component in its (White et al. 2009). The analysis of Ar. locomotive repertoire (White et al. 2009). ramidus suggested instead that the LCA There were no for knuckle- was, in fact, a generalized arboreal , vertical climbing, or suspension to clambering ape (White et al. 2009). be found in the skeleton, and this suggested In summary, Ar. ramidus was seen to Ar. ramidus was a palmigrade clamberer, a be far more primitive than Au. afarensis and generalized form of arboreal yet derived enough relative to the hominids locomotion (White et al. 2009). for it to be a considered a hominin (White et Their claims for a hominin status al. 2009). It supposedly resembled the rested most notably on the derived morphology of the LCA more than any other characteristics of the and foot that taxa with known postcranial elements supposedly enabled Ar. ramidus to walk (White et al. 2009). Yet, it was strongly bipedally, and the derived characteristics of inferred that Ar. ramidus was both ancestral a vestigial canine/ complex (White to Australopithecus and morphologically et al. 2009). The honing complex6 was similar to , and that it could significantly reduced from that of extant and be easily accommodated into a linear extinct hominids, yet was primitive relative account of hominin (Lovejoy et al. to Australopithecus (White et al. 2009). As a 2009c; White et al. 2009). However, as corollary to the canine reduction, it was many of these claims question the suggested that Ar. ramidus lacked significant conventional wisdom of hominin evolution body-size dimorphism, with Ardi herself and in light of proposed alternative weighing in at a hefty 50 kg, compared to theoretical scenarios of hominin evolution, the diminutive Lucy’s 30 kg estimate (White the Ar. ramidus material requires a detailed et al. 2009). The lack of examination. The level of confidence in the was a surprising claim, as Au. afarensis as virtual reconstructions is crucial to any well as all other extant and extinct hominids interpretations of Ar. ramidus. The in- are, to varying degrees, sexually dimorphic ferences made about Ar. ramidus are largely in both canines and body size (Plavcan based on these reconstructions, and these 2001). interpretations are only valid if the It was also suggested that Ar. reconstructions are sound. Finally, any ramidus differs from the extant hominines in interpretations of Ar. ramidus need to be exhibiting primitive hominid characteristics. placed within the context of what is known This implies that the extant hominines are, about other penecontemporaneous hominin in fact, highly derived in the morphology of and hominine taxa, as well as within their pelvis and limbs, and have acquired competing theoretical approaches to their unique adaptations to vertical climbing hominin evolution. In an attempt to and knuckle-walking convergently and only minimize bias and avoid presupposing any after the and the ancestors particular hypothesis in regards to Ar. diverged from the LCA (White et al. 2009). ramidus, the morphology of this taxon will The lack of a honing complex combined be compared to all relevant hominin and with body-size monomorphism and the hominine taxa. Only then will conclusions novel locomotor repertoire suggested that,

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of its phylogenetic positioning and related primitive relative to Australopithecus, in inferences be offered. which only the largest specimens show Dentition traces of distal edge wear (Kimbel and The dental elements are by far the Delezene 2009; Suwa et al. 2009b). Thus, best preserved and most numerous elements Ar. ramidus is depicted as evidence of a recovered for Ar. ramidus and were linear transition in the reduction of the examined in detail (Suwa et al. 2009b). The honing complex from Ar. kadabba through most significant hominin characteristic of to Australopithecus (Suwa et al. 2009b). the Ar. ramidus dentition is the reduction of Molar enamel thickness in Ar. the canines, particularly in crown height, ramidus was found to be intermediate relative to extant and extinct hominines between the thin enamel of both (Suwa et al. 2009b). There were no and extant hominines, and the statistically significant differences between thick enamel of Australopithecus (Suwa et the inferred male and female canines in size al. 2009b). Again, it was implied that Ar. or shape in the recovered sample, or in other ramidus fits within a linear progression in words, the canines were monomorphic and the increase of molar enamel thickness from could not be confidently split into two the hominines to Australopithecus sexually dimorphic clusters (Suwa et al. (Suwa et al. 2009b). The postcanine 2009b). In this regard Ar. ramidus was said dentition is primitive in size, however, and to fit the hominin pattern (Suwa et al. does not display any sign of the megadontia 2009b). The sample size of 21 canines was inherent in Australopithecus (Suwa et al. claimed to be statistically large enough so 2009b). The postcanine dentition was also that it assured that males are represented in found to be monomorphic in size and the Ar. ramidus sample, which is not the supported the claim that there was limited case for either Sahelanthropus or sexual dimorphism present in Ar. ramidus (Brunet et al. 2002a; Senut et al. 2001; (Suwa et al. 2009b). In general, however, Suwa et al. 2009b). there seems to be only a limited amount of The overall size of the canines in Ar. sexual dimorphism in hominin and hominine ramidus are similar to that of female postcanine dentition (Plavcan 2001). Molars , but they differ significantly in are less dimorphic in hominins and their shape—they are diamond-like, rather hominines than they are in the other than dagger-like in Pan or other hominines anthropoids, and it is harder to infer the sex (Suwa et al. 2009b). There is no evidence of of a specimen based on the postcanine a functional canine/premolar honing dentition in these taxa (Plavcan 2001). complex in Ar. ramidus, but there are It was, rather, the canines that played vestiges of a diastema7 (Suwa et al. 2009b). a major role in the assessment of the Ardi The canines are, thus, further derived in the partial skeleton as that of a female and the hominin direction from those of determination of the degree of sexual Ardipithecus kadabba, which are dimorphism present in Ar. ramidus (Suwa et significantly more dagger-like in shape and al. 2009b; White et al. 2009). The canines of still show evidence of a diastema and distal Ardi were the second smallest out of thirteen edge honing (Haile-Selassie, Suwa, and known canines, and this was interpreted to White 2004; Suwa et al. 2009b). The suggest that Ardi was likely a female (Suwa canines of Ar. ramidus do express some et al. 2009b; White et al. 2009). However, as distal edge wear, as the crowns are not fully previously mentioned, Ardi had a large body reduced, and therefore, they are still mass for an early hominin. Based on a

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sample of recovered humeri, Ardi’s was intact computer generated images, little either the second or third largest of the eight resemblance to the original fossils. known elements (White et al. 2009). Thus, To complete the cranial Ardi was a large and yet a female, reconstruction, the temporo-occipital region and this resulted in the claim that Ar. from the original published Ar. ramidus ramidus was a monomorphic species (White material (White, Suwa, and Asfaw 1994) et al. 2009). However, as mentioned above, was rescaled to fit the newly described Ar. the canines could not be separated into two ramidus skull (Suwa et al. 2009a). The statistically distinctive clusters, and it is resultant virtual reconstruction depicts Ar. possible that Ardi was a male with small ramidus with a very hominin-like degree of canines. It has to be determined whether it is reduction in facial prognathism, a foramen more parsimonious that Ar. ramidus was a magnum clearly inferior on the cranial base departure from the Miocene hominine and accompanied by a short basicranial length, Australopithecus pattern of sexual dimorph- and a hominine-like cranial capacity (Suwa ism, or that Ardi was, in fact, a male with et al. 2009a). These morphological simil- small canine teeth, keeping in that arities to Sahelanthropus tchadensis are small is only a relative term. Hominine and repeatedly stressed, and it is ultimately con- especially hominin canines are also less cluded that the skull validates the hominin sexually dimorphic than in other anthropoids status of S. tchadensis as they share a similar (Begun 2009; Kimbel and Delezene 2009). constellation of derived characteristics (Suwa et al. 2009a). Skull The skull of Ar. ramidus exceeded Pelvis the pelvis in terms of its incompleteness, The importance of the pelvic and distortion and fragmentation (Suwa et al. femoral morphology in interpreting the 2009a). The cranial vault was compressed to hominin status of Ar. ramidus cannot be such an extent that the cranial height of the overstated. Lovejoy and colleagues (2009d) recovered elements measured only 35 see derived features in these elements that mm (Suwa et al. 2009a). Like the pelvis, the indicate Ar. ramidus was a facultative biped, skull was also the focus of a major virtual capable of effective bipedal walking. Like reconstruction; luckily, the recovered facial other skeletal elements, the pelvis was said elements and the supraorbital torus did not to display a mosaic constellation of traits suffer extensive post deposition distortion (Lovejoy et al. 2009d). The ischium was (Suwa et al. 2009a). Interpreting the morph- large, robust and ape-like, while the ilium ology of Ar. ramidus is handicapped by the was short inferiorly-superiorly, like those in limited number of images of the original later hominins adapted to obligate bi- fossil elements accompanying the published pedalism (Lovejoy et al. 2009d). The ilium descriptions. The authors of these papers also featured an anterior inferior iliac spine, rely on the images from the virtual recon- a bipedal hominin trait that is not present in structions, which are based on heavily extant hominines (Lovejoy et al. 2009d). manipulated micro-CT scans, for the basis However, these claims are open to for their morphological analyses (Suwa et interpretation due to the poor preservation al. 2009a). This fact should be kept in mind and fragmentary of the recovered throughout discussions of Ar. ramidus as the pelvic elements. The pelvis of Ar. ramidus is skeletal elements, rendered as complete and represented by two extremely fragmentary and badly distorted os coxae, and

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unfortunately, no sacrum was recovered breadth of the sacrum was estimated based (Lovejoy et al. 2009d). To say the pelvis on the angle of the articulation of the pubic was heavily reconstructed would be an symphysis; assuming they can articulate the understatement. Claims for the presence of a two halves of the pubic symphysis properly, greater sciatic notch and a forward curvature the resulting alignment of the os coxae or flaring of the iliac blade, both bipedal should reveal the sacral width. But the pubic characteristic, are not at all discernable in symphysis is also fragmentary and distorted, the images of the original left os coxae and the reconstructed pubis symphysis also (Lovejoy et al. 2009d). The pelvic elements differs in morphology from that of the are not presented in normal anatomical original fossil (Lovejoy et al. 2009d). positions, but rather are photographed at Thus, the validity of the unusual angles which obscure the reader’s reconstructed sacrum can be called into interpretations of certain pelvic features, question, and as a result, the shape of the such as the reported greater sciatic notch entire pelvic girdle. Where the recon- (Lovejoy et al. 2009d). It would have been struction differs from the fossils, it differs helpful to have a completely lateral view towards the hominin direction. It should also included to allow for study of the be noted that this reconstruction is still far acetabulum, the projection of the anterior less hominin-like than that of Au. afarensis inferior iliac spine, and the aforementioned (Lovejoy et al. 2009d). The ilium is most greater sciatic notch. likely shorter in inferior-superior length than Major portions of the iliac crest that of a hominine but the ischium and pubis appear to be missing from the of the are more akin to those of hominines. The ilium, and this makes it hard to determine morphology of the sacrum and the presence the vertical height of the ilium; the ilium of either a greater sciatic notch or any may be higher inferiorly-superiorly than the anterior curvature of the iliac blades are fossil would indicate. However, the debatable based on the fossil evidence reconstruction seems to imply that the presented at this point. The pelvis is, majority of the pelvis is there, as the therefore, primitive relative to Au. afarensis reconstruction is equal in vertical height to and appears to be only partially derived in the fossil pelvis (Lovejoy et al. 2009d). the hominin direction. Also, the blade of the fossil ilium is It is interesting to note that, while a extremely flat and lacking in any hominin- complete tibia was recovered for Ar. like curvature and is very hominine-like in ramidus, as it is clearly visible in the this regard. Lovejoy and colleagues (2009d) photograph of Ardi’s skeletal elements, no conclude that this is due to taphonomic description of the tibia was included in the processes, and their reconstructed os coxae analysis of the skeleton (David Begun, turned out to be very different morpho- personal communication, 2010; White et al. logically than the recovered fossil, as it is far 2009a). This is surprising as the morphology more hominin-like in appearance. of the tibia is highly informative in Also presented as hominin-like is the discerning bipedality, as was the case in the sacrum that is said to be wider description of the proximal tibia of mediolaterally than the narrow sacra of Australopithecus anamensis (Leakey et al. extant hominids (Lovejoy et al. 2009d). 1995). In Au. anamensis the medial and However, no sacrum was ever recovered, lateral condyles of the tibia were and their reconstructed sacrum is purely approximately equal in size, lengthened hypothetical (Lovejoy et al. 2009d). The anteriorly-posteriorly, and concave in shape,

TOTEM vol 19 2010-2011 TOTEM 142 similar to those found in bipedal hominins locomotion and grasping, and this might be and unlike the smaller convex condyles its greatest adaptive advantage (Tocheri et found in extant quadrupedal hominids al. 2008). The overall postcranial (Leakey et al. 1995). As the move towards morphology of Ar. ramidus and the hand, in obligate bipedalism required the re- particular, is said to rule out the knuckle- distribution of body-mass through two walking hypothesis as the form of rather than four, the remodelling of the knee locomotion in the LCA (Lovejoy et al. joint is thought to be highly diagnostic of 2009b, 2009c; White et al. 2009). It is bipedalism, and the Au. anamensis tibia is highly doubtful that Ar. ramidus was a regarded as the earliest uncontested knuckle-walker, but it needs to be evidence of bipedalism in the fossil record established that Ar. ramidus is indeed a (Harrison 2010; Leakey et al. 1995; hominin for this information to invalidate Richmond, Begun, and Strait 2001). One the knuckle-walking hypothesis. Also, it is can only speculate why the tibia of Ar. possible that all vestiges of knuckle-walking ramidus was not similarly subjected to such might have been lost by this time in hominin an analysis. evolution, although I find this to be unlikely given the amount of time available (perhaps and feet less than one million years based on the Where as the skull and pelvis of Ar. youngest molecular divergence dates) ramidus were both highly fragmented and (Harrison 2010; Patterson et al. 2006; distorted, a major boon was the recovery of Richmond, Begun, and Strait 2001). intact hand and foot elements, bones that are Complicating the knuckle-walking hypo- not often recovered but are valuable assets thesis is the suggestion that the appearance in assessing the phylogeny and locomotion of knuckle-walking adaptations in the hand of Ar. ramidus (White et al. 2009). The are a result of plastic remodelling of the hand morphology is superficially that of a bone, and are not genetically derived generalized arboreal hominid and does not adaptations (Tocheri et al. 2008). As such, show any specific adaptations to knuckle- the degree of remodelling depends upon the walking or suspension (Lovejoy et al. weight of the animal, and the degree to 2009b). Superficially, the hand is long and which knuckle-walking was employed as a features curved phalanges, but the pollex8 is form of locomotion (Tocheri et al. 2008). larger and more robust than that of Pan; the Pan paniscus hand bones can show no signs metacarpals are shorter and lack the ridges of diagnostic knuckle-walking traits in their and grooves that provide stability in the bone , and, thus, knuckle-walking knuckle-walking Pan and Gorilla (Lovejoy may have been employed by early hominins et al. 2009b). The hand is primitive relative without evidence showing up in the fossil to Australopithecus, which had shorter record (Begun 1993; Shea and Inouye phalanges relative to the hallux, but it must 1993). This is only mentioned to show the be noted that Au. afarensis also retained difficulties in trying to infer behaviour from arboreal characteristics in the hand; it was fossilized morphology. not until the emergence of that the Enough major foot elements were hand became significantly restructured also recovered to clearly demonstrate that (Tocheri et al. 2008). Ar. ramidus had a divergent hallux, another It should also be noted that extant primitive characteristic that appears to be hominid hand morphology is generalized lacking in the more derived Au. afarensis and highly adaptable to various forms of (Lovejoy et al. 2009a). The divergent hallux,

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in concert with the robust pollex, would ancestral to known fossil hominins (Bernor suggest that Ar. ramidus operated as an 2007). As a result, Pan is most often used as arboreal clamberer, using the hands and feet a referent to hypothesize the morphologies to grasp branches securely (Lovejoy et al. and behaviours of the LCA, especially those 2009a; Lovejoy et al. 2009b). However, synapomorphies9 that are shared by Pan and despite the clearly divergent and primitive Gorilla, as the principle of parsimony hallux, it was claimed that the foot of Ar. suggests that these traits would be present in ramidus was unlike that found in any the LCA (Begun 2007; Richmond, Begun, hominine in that the second pedal was and Strait 2001). However, it must be adapted to provide stability when travelling remembered that both of these lineages have bipedally and it lacked the flexibility of the been evolving since they diverged from the Pan foot that enables the to LCA, and traits that are assumed to be excel at vertical climbing (Lovejoy et al. homologous may in fact represent 2009a). Thus, the foot of Ar. ramidus is homoplasies (White et al. 2009). Thus, the claimed to be both primitive in its ability to discovery of fossils of panins and those climb, yet derived enough to allow it to walk hominines ancestral to the LCA are of bipedally (Lovejoy et al. 2009a). It is utmost importance to constructing difficult to make comparisons with the foot evolutionary relationships and in addressing of Ar. ramidus to other hominins or whether traits are in fact homologies or hominines as this combination of adapt- homoplasies. ations is unlike any described previously. The specific morphology of the LCA The feet of both Pan and Homo would be needs to be indentified if questions of considered derived relative to Ar. ramidus hominin evolution are to be resolved but in different directions (Lovejoy et al. satisfactorily. The extant hominines may be 2009a, 2009c). There have yet to be any highly derived morphologically, thus publications critiquing the interpretations of obfuscating the morphology of the LCA. the Ar. ramidus foot. However, the foot Most Middle or Late Miocene hominids appears to be superficially very primitive in display one or more hominin characteristics, morphology as does the hand previously such as thick molar enamel, reduction of described. In combination, the hands and canines and reduced facial prognathism – feet suggest that an interpretation of the foot characteristics that are absent from extant as an to arboreal clambering hominines. This complicates the picture of seems to be on a more solid footing than what are primitive characteristics for the does the additional inference that the foot LCA (Begun 2002, 2007). Furthermore, if was equally capable of facultative questions of hominin evolution are to be bipedalism. resolved, future research needs to be directed at the discovery of these hominines Ardipithecus and the hominines as few fossils currently exist. In the One issue of concern regarding the meantime, Ar. ramidus can only be com- descriptions of Ar. ramidus is a lack of pared to the few hominine taxa presently comparison of this material to known fossil known, and it should be kept in mind that hominines and hominids. This is partly any tentative conclusions may be open to understandable as one obstacle in obtaining major revisions following future discoveries. a consensus in hominin evolutionary studies There is a large gap in the fossil has always been the dearth of known record between the last known African and hominine fossils, both contemporaneous and European hominids at roughly 10 Ma and

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the emergence of the first purported hominin hominines, including the recently discovered at 7 Ma (Bernor, 2007). There is still a (Begun 2007; Senut 2007). question as to whether or not these Middle However, differs from Miocene African hominids gave rise to the Dryopithecus and extant hominines in LCA, or whether a Middle Miocene having derived characteristics shared with European hominid returned to Africa to hominins, including molar enamel that spawn the LCA (Begun 2009; Harrison approaches Australopithecus in thickness 2010). Until very recently, no African along with a similar adaptation to heavy hominids were known from this window, mastication (Begun 2007; Senut 2007). The which formed the basis of the European similarities between Ouranopithecus and origin hypothesis (Begun 2002, 2009). If Nakalipithecus, both chronologically and they were not found in Africa, it implied that morphologically may, in fact, support the they must have come from elsewhere hypothesis that a European hominid moved (Begun 1994). into Africa to give rise to the hominines. Dryopithecus had often been There have been major mammalian dispersal postulated as a generalized hominid that events throughout the Tertiary Period, and may have given rise to the hominines, as it just because extant forms are found in shares primitive characteristics with extant certain locales today, it does not necessarily hominids, such as a suspensory form of mean that their ancestors must have locomotion. It does lack the derived charac- originated in the same place (Begun 2009). teristics of the Asian hominids (the The Eurasian origin hypothesis may pongines), traits which would have excluded be challenged by newly described African Dryopithecus as being a possible ancestor to hominids. Note that I have been referring to the African hominines (Begun 1994, 2009). both the African and European forms as While molecular genetics have solidified the hominids, as only those hominids of the Homo-Pan (Patterson et al. 2006), lineage that gave rise to the LCA can fossil analyses of Dryopithecus, compared to properly be described as hominines, and the both Pan and fossil hominins, suggest that issue to which lineage represents the Dryopithecus cannot be ruled out as a hominines remains unresolved (Harrison possible ancestor to this clade (Begun 1992, 2010). The first of these African fossil 1994). Dryopithecus may instead be a sister hominids, , was described taxon of an unknown Late Miocene in 2007 and is represented by a number of European hominid that returned to Africa to individual teeth dating to 10 Ma from East spawn the hominines (Begun 2002). Other Africa (Suwa et al. 2007). The molars of Late Miocene European forms include the Chororapithecus were said to be similar in possibly facultative biped, size and morphology to those of Gorilla, and (Begun 2002, 2007). However, evidence for the new taxa was forwarded as a possible bipedality in Oreopithecus is even less gorilla ancestor (Suwa et al. 2007). Later in certain than it is for Ar. ramidus, and it is the same year, Nakalipithecus was also more likely that Oreopithecus was a published represented by a right partial suspensory ape, with an autapomorphic10 and individual tooth fragments; dentition (Begun 2002, 2007). this new taxa was also dated to 10 Ma Ouranopithecus has also been suggested as a (Kunimatsu et al. 2007). Nakalipithecus was possible LCA for the hominins, as it is the interpreted as being ancestral to extant best known late Miocene form, with hominines, the molars supposedly being dentition morphologically similar to extant similar in morphology to extant hominines

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in both size and enamel thickness was a surprise, not only for its early date of (Kunimatsu et al. 2007). The only other 7 Ma and interesting mosaic of primitive African hominid known from this period, and derived characteristics, but more so for and again only from dental remains, is where it was discovered—in Chad, more , but this taxa presents than 2,500 km west of the East African morphologies that are distinct from extant fossil hotspots (Brunet et al. 2002a). Some , and Samburupithecus may be too have argued that S. tchadensis must have derived to be ancestral to extant hominids predated the divergence time of the LCA, (Kunimatsu et al. 2007). Unfortunately, in and as such, it is unlikely to be a hominin the case of Nakalipithecus, the occlusal (Wolpoff et al. 2002). However, according surfaces of the molar teeth are badly to molecular studies, there may have been a damaged, thus rendering any phylogenetic11 period of hybridization occurring between comparisons problematic (Kunimatsu et al. these soon to be separate lineages, which 2007). would suggest that fossils presenting While these discoveries show that different constellations of mosaic traits there were indeed a few hominids present in should be expected (Patterson et al. 2006). Africa between 10 Ma to 7 Ma, the small Both S. tchadensis and Ar. ramidus are hypodigms12 and the poor quality of the compatible with this prediction. specimens makes any phylogenetic com- Despite predating Ar. ramidus by parisons to earlier hominids and later over two million years, S. tchadensis is the hominins contentious. Here, it should be only other skull available for comparison noted that the dearth of hominine fossils in until Au. afarensis made an appearance, and Africa may amount to an absence of a detailed comparison is necessary. The evidence, rather than an evidence of braincase of S. tchadensis is primitive and absence, as known fossil hominid and hominine-like in its cranial capacity, while hominin sites represent an area equal to only the face is highly derived and hominin-like 0.1% of the African continent (Bernor (Brunet et al. 2002a). It lacks the facial 2007). Since searches for fossil hominins prognathism of extant hominines, and all have historically received more attention known canines are reduced in crown size from palaeontologists than those for fossil and present with apical wear (Brunet et al. hominids, and since hominins and hominids 2002a, 2005). There is no functional likely occupied different ecological niches, canine/premolar honing complex in known it is highly likely that fossil African specimens, and the canines only show a hominids are out there waiting to be found. slight degree of distal edge wear (Brunet et The recently discovered hominins of Chad al. 2002a, 2005). Both skulls feature short (Brunet et al. 1995, 2001) lend credence to basicranial lengths and inferiorly located this alternative hypothesis of hominine positions of the foramen magnum (Suwa et origins, and thus far, researchers may have al. 2009a). Taken together, they are been looking in the wrong places. indicative of bipedalism to the degree that this trait can be inferred from cranial Ardipithecus and Sahelanthropus material alone (Suwa et al. 2009a; tchadensis Zollikofer et al. 2005). S. tchadensis shares After a gap of roughly three million all these derived hominin-like characteristics years in the African hominid fossil record with the virtual reconstruction of Ar. comes the purported hominin ramidus. S. tchadensis was also the subject Sahelanthropus tchadensis. S. tchadensis of a CT-based virtual reconstruction

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(Zollikofer et al. 2005), but the fossil skull explanation (Brunet et al. 2002b; Wolpoff et of S. tchadensis was 95% complete to begin al. 2002). with and did not suffer major distortion; In the description of the skull of Ar. thus, the confidence in its reconstruction ramidus, the hominin characteristics it greatly exceeds that of the Ar. ramidus shared with S. tchadensis were trumpeted to reconstruction. the degree that it was claimed that Ar. There are significant differences in ramidus validated the hominin status of S. the two skulls, as should be expected in tchadensis (Suwa et al. 2009a). It had been fossils that are greatly separated geograph- suggested previously that the plethora of ically and temporally. The robust supra- early hominin taxa may not belong in orbital torus of S. tchadensis is unique separate genera, and that when further among extant hominines and early hominins evidence is discovered, these genera may and clearly separates it from Ar. ramidus have to be revised; perhaps they might all be (Brunet et al. 2002a; Suwa et al. 2009a). S. sunk into Ardipithecus (White et al. 2006). It tchadensis also lacks the supraorbital sulcus is unsurprising in hindsight that the present in Ar. ramidus (Suwa et al. 2009a; similarities to hominins and Ar. ramidus Zollikofer et al. 2005). While both crania would be stressed in a comparison of these have similar cranial capacities, as mentioned two taxa. Of course, since 2.5 million-years above, Ar. ramidus has a braincase that is and 2,500 km separate the two fossils, it superficially similar to that of Au. afarensis, stretches credulity that a simple case of while S. tchadensis has a braincase that is anagenetic evolution can be assumed, long and low and distinct from those of early especially in light of the number of morph- hominins and extant hominines (Kimbel and ological differences between the two taxa. Delezene 2009; Suwa et al. 2009a; Zollikofer et al. 2005). The occipital region Ardipithecus and Orrorin tugenensis projects posteriorly, resulting in an oblong Discovered just prior to S. tchadensis or angular occipital region when viewed and dating to 6 Ma, Orrorin tugenensis is laterally. S. tchadensis also features heavy the next oldest of the purported first nuchal cresting, again unlike that of Ar. hominins, and the fossil material described ramidus (Brunet et al. 2002a, 2002b). The S. increases morphological diversity of these tchadensis skull may express more early taxa (Senut et al. 2001). The hypodigm robusticity as it is a male specimen, while for O. tugenensis includes mandibular and the Ar. ramidus skull is supposedly a female maxillary fragments, numerous teeth, and, (Brunet at al. 2002a; Suwa et al. 2009a). most significantly, postcranial elements Like the possibility that Ardi may, in fact, be including the BAR 1002’00 femur (Senut et male, it was suggested that the skull of S. al. 2001). The canines of O. tugenensis are tchadensis is that of a female hominine very similar to those found in female Pan rather than a male hominin (Wolpoff et al. and are the most primitive of any of the 2002). However, if the lack of facial purported first hominins (Senut 2007). They prognathism and canine reduction is are dagger-like in morphology and display attributed to S. tchadensis as being a female vestiges of a canine/premolar honing hominine, one is left to explain the complex and an accompanying diastema robusticity of the occipital torus and nuchal (Senut et al. 2001; Senut 2007). While the cresting. As a result, this interpretation postcanine teeth were smaller in size than appears to be a less parsimonious those of Australopithecus, they displayed thick molar enamel similar to those of later

TOTEM vol 19 2010-2011 TOTEM 147 hominins (Senut et al. 2001). O. tugenensis, also be noted that the critics of Galik et al. thus, presented a dental morphology that (2004) included the describers of Ar. was expected in a transitional form (Senut et ramidus, and as such, they were not likely to al. 2001). The biggest surprise surrounding agree with a conclusion that shunted this O. tugenensis, and what suggested that this lineage to an evolutionary side branch taxa was a hominin, was the morphology of (Ohman, Lovejoy, and White 2005). In a the BAR 1002’00 femur (Senut et al. 2001). similar vein, it was the describers of O. The femur was described as not only being tugenensis that were involved in the most that of a biped, but of a biped with a fully vocal opposition to the hominin status of S. modern bipedal that had lost all of its tchadensis, as it usurped their position as the arboreal adaptations (Senut et al. 2001). discoverers of the oldest hominin (Wolpoff Since O. tugenensis was suggested to be et al. 2002). more modern in femoral morphology than The status of the BAR 1002’00 Australopithecus, which Orrorin preceded femur was revisited in a later analysis that in time, its discoverers concluded that O. would both validate its status as a biped, tugenensis was directly ancestral to the while at the same time concluding that the genus Homo, and that the genus external morphology of the O. tugenensis Australopithecus lay on an evolutionary side femur is indistinguishable from that of branch (Senut et al. 2001). Australopithecus (Richmond and Jungers The highly derived Homo-like 2008). The femur was compared anatomi- morphology of the femur rested on an cally to those of Pan, Australopithecus and analysis of the gross morphology of the Homo. It found that the femur was least bone and an examination of the cortical similar to Pan and did not fall within the bone thickness of the femoral neck (Galik et range of Homo variation (Richmond and al. 2004). In extant hominines, cortical bone Jungers 2008). This study both vindicated O. thickness is symmetrical around the circum- tugenensis as a biped, while simultaneously ference of the femoral neck, a response to invalidating the notion that O. tugenensis the variability in loading stresses placed was a separate lineage from upon the neck due to their arboreal Australopithecus that would have given rise adaptations (Galik et al. 2004). In bipeds, to Homo through the hypothetical taxa the stress pattern is less varied, and, thus, Praeanthropus (Richmond and Jungers cortical bone thickness is asymmetrical; a 2008; Senut 2007). Parsimony would biped should theoretically be identifiable suggest that early bipeds would have an based on the pattern of cortical bone Australopithecus-like femur, and yet would thickness (Galik et al. 2004). O. tugenensis be more primitive dentally. O. tugenensis fit was found to match the pattern of Homo, within this hypothesis. both in internal and external bone However, the description of the new morphology (Galik et al. 2004). Critics were Ar. ramidus material affects the inter- quick to point out that the resolution of the pretation of the O. tugenensis material, CT scans was so poor that no confidence increases the morphological variation could be placed on any conclusions based on present in early hominins and complicates the internal morphology (Ohman, Lovejoy, the constructions of phylogenetic relation- and White 2005). As encountered previously ships. While the postcanine teeth of O. with Ar. ramidus, the use of new tugenensis are more primitive than does not necessarily provide Australopithecus in lacking the megadontia easy solutions for palaeontologists. It should of that genus, they appear to be derived

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relative to the much younger Ar. ramidus became a player in the first hominin sweep- (Begun 2004; Senut 2007). O. tugenensis stakes, and the hesitancy to name a new shows the first signs of molar enamel species may have, in part, been influenced thickening and increase in molar size prior by a desire to return Ar. ramidus to to Australopithecus (Begun 2004; Senut prominence in hominin discussions. Even 2007). Ar. ramidus and S. tchadensis are more influential may be the theoretical more alike in postcanine dentition than approaches of the discoverer, who clearly either is to O. tugenensis (Brunet et al. 2005; sees anagenetic processes operating in early Suwa et al. 2009b). To further complicate hominin evolution (Haile-Selassie, Suwa, matters, while the postcanine teeth in O. and White 2004). tugenensis are more derived in the hominin Just as Au. ramidus was reassigned direction than those of S. tchadensis, the to a new genera, the Ar. ramidus kadabba canines appear to be more primitive; they material was eventually elevated to the display not only vestiges of a honing species level as Ar. kadabba along with the complex and diastema but also retain some description of an expanded hypodigm of the dagger-like morphology of Pan (Haile-Selassie, Suwa, and White 2004). Ar. (Senut et al. 2001; Brunet et al. 2005). kadabba has canine teeth that are more While O. tugenensis also has an primitive and Pan-like than Ar. ramidus, Australopithecus-like femur at 6 Ma, the 4.4 with higher canine crowns and a more Ma Ar. ramidus has the postcranial dagger-like morphology, as well as remnants morphology of an arboreal hominine of a canine/premolar complex and its (Richmond and Jungers 2008). accompanying diastema (Haile-Selassie Unfortunately, while femoral fragments of 2001; Haile-Selassie, Suwa, and White Ar. ramidus have been recovered, they 2004). The Ar. kadabba canines are most lacked the proximal end that would enable a similar to the penecontemporaneous, O. direct comparison to O. tugenensis (Lovejoy tugenensis, although they are relatively more et al. 2009d). incisiform in morphology (Begun 2004; Haile-Selassie, Suwa, and White 2004; Ardipithecus ramidus and Ardipithecus Senut 2007). The postcanine dentition of Ar. kadabba kadabba differs from O. tugenensis in The last of the purported first having thinner molar enamel, even thinner hominins, Ardipithecus kadabba, underwent than that found in Ar. ramidus (Begun 2004; a process of taxonomic revision similar to Haile-Selassie, Suwa, and White 2004; Ar. ramidus. Ardipithecus ramidus kadabba Senut 2007). was originally described as a subspecies of As mentioned previously, O. Ar. ramidus, a rather odd decision in that the tugenensis shows signs of postcanine fossil material predated Ar. ramidus by 1.2 enamel thickening and molar enlargement Ma and featured significant morphological that is not present in Ar. ramidus or the differences to the later specimens (Haile- earlier Ar. kadabba (Haile-Selassie, Suwa, Selassie 2001). A fairly fragmentary and White 2004; Senut 2007). The post- collection of teeth and arm bones, along canine dentition of Ar. kadabba is most with a single phalanx were recovered similar to Ar. ramidus, although Ar. kadabba from multiple locations and spanned a time may be more primitive in having slightly range of 5.8 to 5.2 Ma (Haile-Selassie thinner enamel and slightly more pro- 2001). These finds extended the genus back nounced molar cusps (Haile-Selassie, Suwa, in time to a point where Ardipithecus and White 2004; Suwa et al. 2009b).

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However, the question remains as to whether Canine reduction, postcanine megadontia, thin enamel is actually the primitive thick enamel, and unambiguous evidence of condition for the LCA. The recovered toe bipedalism in both the lower limbs and phalanx was said to be evidence in support pelvis are all synapomorphies of the genus of Ar. kadabba being a bipedal hominin, in Australopithecus (Kimbel and Delezene concert with its supposed affiliation with Ar. 2009). All of these undisputed early hominin ramidus (Haile-Selassie 2001). However, characteristics are either absent or while the phalanx was recovered in roughly ambiguous in Ardipithecus. contemporaneous deposits, it was found some distance from other Ar. kadabba Au. anamensis specimens (Haile-Selassie 2001). As such, it The Ar. ramidus material was is possible it might not even belong to this originally compared to Au. afarensis, the taxon, and interpreting bipedalism from a stem hominin of the day (White, Suwa, and single phalanx strains credulity. Asfaw 1994). Since that time, an older and The discovery of Ar. kadabba more primitive species of Australopithecus certainly increases the morphological vari- has been discovered (Leakey et al. 1995). ation present in early hominins, especially Au. anamensis pushed the genus back to 4 when it is recognized as a species in its own Ma, and, as it is either primitive or similar right. The original notion that the hominin morphologically to Au. afarensis, Au. fossil record had reached a level of fidelity anamensis is an excellent candidate to be the that one could begin to identify subspecies ancestor of Au. afarensis (Kimbel and should never have been taken seriously. Ar. Delezene 2009; Leakey et al. 1995; White et kadabba is most likely the species ancestral al. 2006). The morphology, geography and to Ar. ramidus as it appears to be primitive chronology of these two species overlap to in all respects (Haile-Selassie, Suwa, and such a degree that there that is some White 2004; White et al. 2006). The question as to whether Au. anamensis should primitive nature of Ar. ramidus and doubts be treated as a distinct taxon, or whether it as to whether it is the ancestor of represents the early morphology of a Australopithecus or not, has led to a chronospecies of Au. afarensis (Haile- questioning of its hominin status (Harrison Selassie et al. 2010). The original 2010). That being the case, the hominin description of Au. anamensis listed status of the even more primitive Ar. diagnostic characteristics that separated it kadabba must be more uncertain in light of from Au. afarensis (Leakey et al. 1995). the new Ar. ramidus material. We now turn These included smaller and, thus, more to the hypothesized decedents of Ar. ramidus primitive postcanine dentition, larger canine to address these issues. teeth, and a sharp recession of the anterior/inferior region of the mandible, all Ardipithecus and Australopithecus characteristics that distinguished Au. Ar. ramidus was originally assigned anamensis from the specimen of Au. to the genus Australopithecus, and was afarensis (Leakey et al. 1995; Kimbel and thought to be a primitive form of this genus, Delezene 2009). However, comparing the and, ultimately, the stem australopithecine holotypes of species is no longer an (White, Suwa, and Asfaw 1994). Now that adequate methodology to assigning more is known about both Ar. ramidus and specimens to taxa; rather, the entire Australopithecus, the evolutionary relation- hypodigm needs to be taken into ships of these taxa should be re-examined. consideration (Haile-Selassie et al. 2010). It

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is here that the rich hypodigm of Au. traced from 4.0 Ma to 3.0 Ma, in enough afarensis can shed light on hominin detail that there seems to be no evidence of a evolution in a way individual fossils cannot sudden transition from one form to another (Kimbel and Delezene 2009). (Kimbel and Delezene 2009). Contrast this scenario with the claim that Ar. ramidus Au. afarensis evolved into Au. anamensis between 4.4 Ma More is known about the taxa Au. and 4.0 Ma (Lovejoy et al. 2009c; White et afarensis than any other early hominin, and al. 2009). Major evolutionary changes in for the first time in , we almost all skeletal elements of Ar. ramidus can now study the evolution within a would be required in a short period of time, particular lineage (Kimbel and Delezene and there is no known fossil evidence to 2009). As mentioned above, the holotypes support this transition. For example, there is for these two taxa are morphologically no hint of the evolution of the distinctive, but these distinctions become australopithecine megadontia and thick blurred when the two hypodigms are molar enamel in Ar. ramidus, which would compared in full (Haile-Selassie et al. 2010). have been indicative of a major ecological Au. afarensis is now known to be a shift and would likely have required a morphologically variable and sexually di- significant amount of time to evolve. There morphic species, with early specimens should be in situ evidence of this transition displaying morphologies more similar to in if Ar. ramidus was truly that of Au. anamensis than they are to the ancestral to Australopithecus. When you larger and more robust specimens of Au. also consider the slow pace of evolutionary afarensis from later time periods (Kimbel change in the Ardipithecus linage from 5.8 and Delezene 2009). With the expansion of Ma to 4.4 Ma, it is surprising that there the Au. anamensis hypodigm, even more would be an unprecedented shift in morphological and geographical variation morphology from Ardipithecus to has been uncovered, and it is now suggested Australopithecus in a small amount of time. that these two taxa may represent the same This undermines the claim that Ar. ramidus species (Haile-Selassie et al. 2010). There was the ancestor of the genus now appears to be a lack of diagnostically Australopithecus. derived characteristics that would clearly separate Au. afarensis from Au. anamensis Conclusions and phylogenetic (Haile-Selassie et al. 2010). It may be the interpretations case that a speciation event did occur, and it Only when Ar. ramidus is analyzed should be recalled that a daughter species in the context of other early fossil hominins would look very similar to the parent can conclusions be reached in reference to species. As a result, increasing numbers of its phylogenetic placement and the pattern fossils may actually cloud the picture, as a that early hominin evolution has followed. fine-grained analysis could capture so much To reiterate, the fragmentary nature of the small scale evolutionary change that the Ar. ramidus skeleton casts doubt on the speciation event could be lost in the clutter. quality of the virtual reconstructions of the However, the transition from Au. skull and pelvis in particular, and, thus, anamensis to Au. afarensis presents by far interpretations based on these the best evidence of anagenetic evolution in reconstructions may be subject to revision. the hominin fossil record (Haile-Selassie et Still, elements of the skeleton provide al. 2010). Small evolutionary changes are valuable insights into early hominine

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morphology. As Ar. ramidus is primitive to and White 2004; White et al. 2006). This Australopithecus in all major respects, it anagenetic interpretation of canine evolution cannot be excluded as a potential ancestor is belied by O. tugenensis, and Ar. kadabba, on these grounds (Lovejoy et al. 2009c; which had more prominent canines and White et al. 2009). However, since the two honing complexes than did the earlier S. taxa were only separated by 400 Ka, it is tchadensis (Brunet et al. 2005; Senut et al. unlikely that Ar. ramidus was, in fact, 2001). ancestral to Australopithecus (Harrison Thick molar enamel and an increase 2010). In addition, Ar. ramidus may lack in the size of the postcanine dentition is synapomorphies with Australopithecus - another hominin synapomorphy that has those shared derived traits that are crucial in been used to sort out phylogenetic establishing phylogenetic relationships. The relationships (Kimbel and Delezene 2009; pelvis is described as one major synapo- White et al. 2009). Thick molar enamel is a morphy, but based on the condition of the shared hominin characteristic, and time and fossils and the reliance on the virtual again it has been inferred that intermediate reconstruction, this claim is easily disputed. enamel thickness was the intermediate con- The other major synapomorphy Ar. dition in hominin evolution (Begun 2004). ramidus shared with Australopithecus is the However, many of the often ignored reduction of the canine teeth. Much has been Miocene apes also had thick molar enamel, written on the reduction of the canine in and the thin enamel of extant apes may be hominin evolution, and clearly, later the derived and not the primitive condition hominins have reduced canines and the (Begun 2007). Here too with the postcanine absence of a canine/premolar complex teeth, there does not appear to be evidence (Haile-Selassie, Suwa, and White 2004; of a linear trend over time. It is O. White et al. 2006). However, the reduction tugenensis that again does not fit the pattern, of the canines may be of limited value in but this time by being derived in postcanine sorting out the phylogenies of early anatomy relative to later hominins (Begun hominins. It is true that there appears to be a 2004; Senut 2007). If one is still hoping to trend towards canine reduction from the find evidence of anagenetic evolution in the Late Miocene through to, and continuing early hominin fossil record, disregarding O. within, the genus Australopithecus (Haile- tugenensis, there appear to be similarities Selassie, Suwa, and White 2004; White et between S. tchadensis and Ar. ramidus in the al. 2006). But this evolution of the canine is skull and dentition (Suwa et al. 2009a, most likely akin to the expansion of cranial 2009b). Indeed, this claim has been made, capacities in the Homo lineage, or the and the similarities between the two taxa changing enamel thicknesses of hominids have been stressed, suggesting that these during the Miocene. It is clear encephal- early hominins may belong to the same taxa ization was a trend occurring over time in (White et al. 2006). However, while dentally Homo, but it is of limited value in esta- similar and sharing similarities of the blishing evolutionary relationships between basicranium, the skulls of these two taxa are taxa, or in defining a taxon, just as enamel morphologically and geographically dis- thickness as a trait viewed in isolation is of tinctive (Suwa et al. 2009a, 2009b; little phylogenetic value. Examining the Zollikofer et al. 2005). reduction of canines in isolation biases the If one is looking for similarities and analysis towards a linear interpretation of shared characteristics in early hominin taxa, hominin evolution (Haile-Selassie, Suwa, one will surely find them. All of these early

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hominins, or contemporary hominines for Even if this is the case, rather than being a that matter, will have to be similar in disappointment, Ar. ramidus would be an morphology to the LCA (Harrison 2010; equally valuable discovery, indicating the Patterson et al. 2006). Most relevant to large amounts of morphological variation assessing phylogenetic relationships and present in both early hominin and hominine determining hominin status are the synapo- morphologies. morphies and autapomorphies. Taking this This study of the Ar. ramidus partial into consideration, even based on the limited skeleton concludes that, rather than fitting hominin evidence available, there is a nicely into an anagenetic pattern of early considerable amount of morphological and hominin evolution, the unique morphology geographic variation present in known taxa of Ar. ramidus and indeed the unique (Begun 2004). This variation is most visible morphologies of the limited purported when all hominin taxa are submitted to a hominin taxa known, are best described by a detailed comparative analysis as this paper cladogenetic pattern of evolution. More has attempted to do. hominin taxa are likely to be forthcoming, Ar. ramidus is best interpreted as a and the focus of future research should be unique species in its own right, rather than finding fossil panins and other hominines, as attempting to accommodate it within well as addressing the question of the origin preconceived notions as to how hominin of the African hominines. Searches for evolution should operate. The very primitive fossils should be expanded away from morphology of Ar. ramidus validates the Eastern Africa, as it is very unlikely that all removal of the hypodigm from the major evolutionary events in hominin Australopithecus. It could qualify as a evolution would have occurred there. Rather phylogenetic hominin based on the derived than fill in a missing gap in the hominin nature of the canine teeth, the inferior lineage, Ar. ramidus has most likely location of the foramen magnum and the provided yet another branch in what is most short basicranium. S. tchadensis is likely a bushy family tree. considered to be a hominin based on similar evidence (Zollikofer et al. 2005). That does not imply that either form was truly bipedal. Acknowledgments If hominins are defined phylogenetically as any taxa on the lineage following the I would like to thank Dr. Andrew Nelson for divergence with the LCA, the earliest his supervision and encouragement hominins may not necessarily have been throughout the development of this project bipedal (Harrison 2010). In fact, the as well as Dr. David R. Begun, Dr. Ian postcranial material of Ar. ramidus is most Tattersall and Dr. Matt Tocheri for taking indicative of an arboreal animal, rather than the time to respond to the inquiries of a a terrestrial biped. If it is a hominin, it could lowly undergraduate student. best be described as “a damned odd one” (Ian Tattersall, personal communication, Notes 2010). As such, it is highly likely its assessment as a hominin could be revised 11 This paper follows a that places all the based on future fossil discoveries. It is also great apes, , and their combined ancestors in the family . This is in line with recent likely that some of the ‘missing’ fossil taxonomic practices based on categorizing taxa hominines, perhaps including Ar. ramidus, exclusively on evolutionary relationships (Harrison may have been misidentified as hominins.

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2010). Thus, all of these taxa will be referred to as 11 phylogenetics: The study of evolutionary hominids. The African great apes, humans, and their relationships among a group of organisms. combined ancestors are assigned to the subfamily 12 hypodigm: All of the known material of a taxon. and will be referred to as hominines (Harrison 2010). Humans and their ancestors, or those taxa found on the branch leading to human after the divergence point with the last common ancestor (LCA) are assigned to the tribe and will be referred to as hominins (Harrison 2010). These are the taxa that have been traditionally referred to as hominids. The chimpanzees and their ancestors, or those taxa found on the opposing branch after the divergence point with the LCA are assigned to the tribe Panini and will be referred to as panins (Harrison 2010). While the renaming of hominids as hominins, and simultaneously expanding the definition of hominid to be more inclusive might be confusing, it is becoming more common in paleoanthropology, and has the benefit of recognizing phylogenetic relationships in the nomenclature, and thus should become the standard practice in the discipline. 2 obligate biped: An animal adapted to full-time bipedal locomotion. An obligate biped could still maintain an arboreal component in its behavioural repertoire but would no longer employ a quadrupedal form of locomotion. 3 postcanine megadontia: A significant enlargement of the and molars and typified by the genus Australopithecus. 4 facultative biped: A quadruped capable of sustained bipedal locomotion for a significant period of time as part of its normal behavioural repertoire. 5 hallux: The first pedal digit (the big toe). 6 honing complex: An association between the upper canine and lower first premolar where a projecting upper canine is sharpened or honed against the front of the lower first premolar. Honing complexes are present in extant hominids but missing in known hominins who lack large projecting canines. 7 diastema: A space in the tooth row that accommodates a large projecting . Diastemata are present in extant hominids but missing in known hominins who lack large projecting canines. 8 pollex: The first manual digit (the ). 9 synapomorphy: A shared derived character present in one or more taxa and their last common ancestor but not present in the previous common ancestor. 10 autapomorphy: A unique derived character that evolved for the first time in a particular taxon but is not present in sister taxa or in their shared last common ancestor.

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