Research Article

Australopithecus sediba: A New Species of Homo-Like Australopith from South Africa

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Science  09 Apr 2010:
Vol. 328, Issue 5975, pp. 195-204
DOI: 10.1126/science.1184944

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Abstract

Despite a rich African Plio-Pleistocene hominin fossil record, the ancestry of Homo and its relation to earlier australopithecines remain unresolved. Here we report on two partial skeletons with an age of 1.95 to 1.78 million years. The fossils were encased in cave deposits at the Malapa site in South Africa. The skeletons were found close together and are directly associated with craniodental remains. Together they represent a new species of Australopithecus that is probably descended from Australopithecus africanus. Combined craniodental and postcranial evidence demonstrates that this new species shares more derived features with early Homo than any other australopith species and thus might help reveal the ancestor of that genus.

The origin of the genus Homo is widely debated, with several candidate ancestors being proposed in the genus Australopithecus (13) or perhaps Kenyanthropus (4). The earliest occurrence of fossils attributed to Homo (H. aff. H. habilis) at 2.33 million years ago (Ma) in Ethiopia (5) makes it temporally antecedent to all other known species of the genus Homo. Within early Homo, the hypodigms and phylogenetic relationships between H. habilis and another early species, H. rudolfensis, remain unresolved (68), and the placement of these species within Homo has been challenged (9). H. habilis is generally thought to be the ancestor of H. erectus (1013), although this might be questioned on the basis of the considerable temporal overlap that existed between them (14). The identity of the direct ancestor of the genus Homo, and thus its link to earlier Australopithecus, remains controversial. Here we describe two recently discovered, directly associated, partially articulated Australopithecus skeletons from the Malapa site in South Africa, which allow us to investigate several competing hypotheses regarding the ancestry of Homo. These skeletons cannot be accommodated within any existing fossil taxon; thus, we establish a new species, Australopithecus sediba, on the basis of a combination of primitive and derived characters of the cranium and postcranium.

The following is a description of Au. sediba: Order Primates Linnaeus 1758; suborder Anthropoidea Mivart 1864; superfamily Hominoidea Gray 1825; family Hominidae Gray 1825; genus Australopithecus DART 1925; species Australopithecus sediba sp. nov.

Etymology. The word sediba means “fountain” or “wellspring” in the seSotho language.

Holotype and paratype. Malapa Hominin 1 (MH1) is a juvenile individual represented by a partial cranium, fragmented mandible, and partial postcranial skeleton that we designate as the species holotype [Figs. 1 and 2, supporting online material (SOM) text S1, figs. S1 and S2, and table S1]. The first hominin specimen recovered from Malapa was the right clavicle of MH1 (UW88-1), discovered by Matthew Berger on 15 August 2008. MH2 is an adult individual represented by isolated maxillary teeth, a partial mandible, and partial postcranial skeleton that we designate as the species paratype. Although MH1 is a juvenile, the second molars are already erupted and in occlusion. Using either a human or an ape model, this indicates that MH1 had probably attained at least 95% of adult brain size (15). Although additional growth would have occurred in the skull and skeleton of this individual, we judge that it would not have appreciably altered the morphology on which this diagnosis is based.

Fig. 1

Craniodental elements of Au. sediba. UW88-50 (MH1) juvenile cranium in (A) superior, (B) frontal, and (C) left lateral views. (D) UW88-8 (MH1) juvenile mandible in right lateral view, (E) UW88-54 (MH2) adult mandible in right lateral view, (F) UW88-8 mandible in occlusal view, (G) UW 88-54 mandible in occlusal view, and (H) UW 88-50 right maxilla in occlusal view (scale bars are in centimeters).

Fig. 2

Associated skeletal elements of MH1 (left) and MH2 (right), in approximate anatomical position, superimposed over an illustration of an idealized Au. africanus skeleton (with some adjustment for differences in body proportions). The proximal right tibia of MH1 has been reconstructed from a natural cast of the proximal metaphysis.

Locality. The two Au. sediba type skeletons were recovered from the Malapa site (meaning “homestead” in seSotho), situated roughly 15 km NNE of the well-known sites of Sterkfontein, Swartkrans, and Kromdraai in Gauteng Province, South Africa. Detailed information regarding geology and dating of the site is in (16).

Diagnosis. Au. sediba can be distinguished from other species of Australopithecus by a combination of characters presented in Table 1; comparative cranial measures are presented in Table 2. A number of derived characters separate Au. sediba from the older chronospecies Au. anamensis and Au. afarensis. Au. sediba exhibits neither the extreme megadontia, extensive cranial cresting, nor facial prognathism of Au. garhi. The suite of derived features characterizing Au. aethiopicus, Au. boisei, and Au. robustus, in particular the pronounced cranial muscle markings, derived facial morphology, mandibular corpus robusticity, and postcanine megadontia, are absent in Au. sediba. The closest morphological comparison for Au. sediba is Au. africanus, as these taxa share numerous similarities in the cranial vault, facial skeleton, mandible, and teeth (Table 1). Nevertheless, Au. sediba can be readily differentiated from Au. africanus on both craniodental and postcranial evidence. Among the more notable differences, we observe that although the cranium is small, the vault is relatively transversely expanded with vertically oriented parietal walls and widely spaced temporal lines; the face lacks the pronounced, flaring zygomatics of Au. africanus; the arrangement of the supraorbital torus, nasoalveolar region, infraorbital region, and zygomatics result in a derived facial mask; the mandibular symphysis is vertically oriented with a slight bony chin and a weak post-incisive planum; and the teeth are differentiated by the weakly defined buccal grooves of the maxillary premolars, the weakly developed median lingual ridge of the mandibular canine, and the small absolute size of the postcanine dentition. These exact differences also align Au. sediba with the genus Homo (see SOM text S2 for hypodigms used in this study). However, we consider Au. sediba to be more appropriately positioned within Australopithecus, based on the following craniodental features: small cranial capacity, pronounced glabelar region, patent premaxillary suture, moderate canine jugum with canine fossa, small anterior nasal spine, steeply inclined zygomaticoalveolar crest, high masseter origin, moderate development of the mesial marginal ridge of the maxillary central incisor, and relatively closely spaced premolar and molar cusps.

Table 1

List of characters used to diagnose Au. sediba. These characters are commonly used in hominin phylogenetic studies (11, 3840) or have been recorded as diagnostic for various hominin taxa in the past (3, 10, 36). Recognizing the potential pitfalls of performing a cladistic analysis on possibly interdependent characters of uncertain valence, we produced a cladogram from the data in this table as a test of the phylogenetic position of Au. sediba (fig. S3). Our most parsimonious cladogram places Au. sediba at the stem of the Homo clade. Numbers in parentheses in the first column refer to measures presented in Table 2; descriptions of these character states are provided in SOM text S3. Abbreviations are as follows: A-M, anteromedial; costa supr., costa supraorbitalis; intermed., intermediate; lat., lateral; med., medial; mesognath., mesognathic; mod., moderately; MMR, mesial marginal ridge; orthogn., orthognathic; procumb., procumbent; proj., projecting; TMJ, temperomandibular joint.

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Table 2

Craniodental measurements for early hominins in Africa. Au. sediba is represented by MH1. Unless otherwise defined, measurements are based on (6). Some measures were unavailable for specimens of Au. afarensis and Au. garhi, in which case the character states in Table 1 were estimated. Several character states in Table 1 are recorded as variable, although only species average values are presented here. Measurements are in millimeters unless otherwise indicated. Descriptions of character states presented in Table 1 that are based on measurements from this table are provided in SOM text S3. Abbreviations are as follows: br, bregma; ek, ectoconchion; ekm, ectomolare; fmt, frontomolare temporale; ft, frontotemporale; g, glabella; mf, maxillofrontale; n, nasion; ns, nasospinale; or, orbitale; po, porion; pr, prosthion; rhi, rhinion; zm, zygomaxillare; zy, zygion; zyo, zygoorbitale.

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Postcranially, Au. sediba is similar to other australopiths in its small body size, its relatively long upper limbs with large joint surfaces, and the retention of apparently primitive characteristics in the upper and lower limbs (table S2). Au. sediba differs from other australopiths, but shares with Homo a number of derived features of the os coxa, including increased buttressing of the ilium and expansion of its posterior portion, relative reduction in the distance between the sacroiliac and hip joints, and reduction of distance from the acetabulum to the ischial tuberosity. These synapomorphies with Homo anticipate the reorganization of the pelvis and lower limb in H. erectus and possibly the emergence of more energetically efficient walking and running in that taxon (17). As with the associated cranial remains, the postcranium of Au. sediba is defined not by the presence of autapomorphic features but by a unique combination of primitive and derived traits.

Cranium. The cranium is fragmented and slightly distorted. The minimum cranial capacity of MH1 is estimated at 420 cm3 (SOM text S4). The vault is ovoid, with transversely expanded, vertically oriented parietal walls. The widely spaced temporal lines do not approach the midline. Postorbital constriction is slight. The weakly arched supraorbital torus is moderately developed and laterally extended, with sharply angled lateral corners and a weakly defined supratoral sulcus. A robust glabelar region is evident, with only a faint depression of the supraorbital torus at the midline. The frontal process of the zygomatic faces primarily laterally and is expanded medially but not laterally. The zygomatic prominence does not show anterolateral expansion. The zygomatics are weakly flared laterally, resulting in an uninterrupted frontal profile of the facial mask that is squared superiorly and tapered inferiorly. The zygomaticoalveolar crests are long, straight, and steeply inclined, resulting in a high masseter origin. The root of the zygomatic begins at the anterior margin of M1. The nasal bones are widened superiorly, become narrowest about one-third of the way down, and flare to their widest extent at their inferior margin. The nasal bones are elevated as a prominent ridge at the internasal suture, with an increasingly anterior projection inferiorly. The bone surface of the maxilla retreats gently away from the nasal aperture laterally, resulting in an everted margin of the superolateral portion of the aperture relative to the infraorbital region. The inferolateral portion of the nasal aperture becomes bluntly rounded. The infraorbital region is slightly convex (18) and is oriented at an approximately right angle to the alveolar plane. There is a trace of a premaxillary suture near the superolateral margin of the nasal aperture. Prominent canine juga delineate moderately developed canine fossae. Anterior pillars are absent. The inferior margin of the nasal aperture is marked by a stepped nasal sill and a small but distinct anterior nasal spine. The subnasal region is straight in the coronal plane and only weakly projecting relative to the facial plane. The face is mesognathic. The palate is consistently deep along its entire extent, with a parabolic dental arcade.

Mandible. Descriptions apply to the more complete juvenile (MH1) mandible unless otherwise stated. The nearly vertical mandibular symphysis presents a weak lateral tubercle, resulting in a slight mental trigone, and a weak mandibular incurvation results in a slight mentum osseum. The post-incisive planum is weakly developed and almost vertical. Both mandibular corpora are relatively gracile, with a low height along the alveolar margin. The extramolar sulcus is relatively narrow in both mandibles. In MH1, a moderate lateral prominence displays its greatest protrusion at the mesial extent of M2, with a marked decrease in robusticity to P4; in MH2 the moderate lateral prominence shows its greatest protrusion at M3, with a marked decrease in robusticity to M2. The alveolar prominence is moderately deep with a notable medial projection posteriorly. The anterior and posterior subalveolar fossae are continuous. The ramus of MH1 is tall and narrow, with nearly parallel, vertically oriented anterior and posterior borders; the ramus of MH2 is relatively broader, with nonparallel anterior and posterior borders (fig. S2). The mandibular notch is relatively deep and narrow in MH1 and more open in MH2. The coronoid extends farther superiorly than the condyle. The condyle is mediolaterally broad and anteroposteriorly narrow. The endocondyloid buttress is absent in MH1, whereas in MH2 a weak endocondyloid buttress approaches the condyle without reaching it.

Dental size and proportions. The dentition of the juvenile (MH1) is relatively small, whereas preserved molars of the adult (MH2) are even smaller (Fig. 3 and fig. S4). For MH1, the maxillary central incisor is distinguishable only from the reduced incisors of Au. robustus. The maxillary canine is narrower than all canines of Au. africanus except TM 1512, whereas the mandibular canine falls well below the range of Au. africanus. Premolars and molars are at the lower end of the Au. africanus range and within that of H. habilis–H. rudolfensis and H. erectus. Molar dimensions of the adult individual (MH2) are smaller than those of Au. africanus, are at or below the range of those of H. habilisH. rudolfensis, and are within the range of those of H. erectus. Au. sediba mirrors the Au. africanus pattern of maxillary molars that increase slightly in size posteriorly, though it differs in that the molars tend to be considerably larger in the latter taxon. Conversely, the Au. sediba pattern varies slightly from that seen in specimens KNM-ER 1813, OH 13, and OH 65 and H. erectus, wherein the molars increase from M1 to M2 but then decrease to M3. In broad terms, the teeth of Au. sediba are similar in size to teeth of specimens assigned to Homo but share the closely spaced cusp apices seen in Australopithecus.

Fig. 3

Dental size of a selection of Au. sediba teeth compared to other early hominin taxa; see fig. S4 for additional teeth. Dental measurements were taken as described by Wood (6). Owing to small sample sizes, H. habilis and H. rudolfensis were combined. (A) Upper central incisor mesiodistal (MD) length. (B) Upper canine MD length. (C) Lower canine MD length. (D) Square root of calculated [MD × BL (BL, buccolingual)] upper third premolar area. (E) Square root of calculated (MD × BL) upper second molar area. (F) Square root of calculated (MD × BL) lower second molar area. Measures were taken on original specimens by D.J.D. for Au. africanus, Au. robustus, and Au. sediba. Measurements for Au. afarensis, H. habilis, H. rudolfensis, and H. erectus are from (6). P4 is not fully erupted on the right side of MH1, therefore measures of the maxillary postcanine dentition are presented for the left side only. Dental metrics for Au. sediba are as follows (MD, BL, in millimeters): Maxillary: MH1: RI1 10.1, 6.9; LI2 7.7 (damaged), 5.1; RC 9.0, 8.8; LP3 9.0, 11.2; LP4 9.2, 12.1; LM1 12.9, 12.0; LM2 12.9, 13.7; LM3 13.3, 14.1; MH2: RM3 11.3, 12.9. Mandibular: MH1: LC 8.0, 8.5; RM1 12.5, 11.6; RM2 14.4, 12.9; RM3 14.9, 13.8; MH2: RM1 11.8, 11.1; RM2 14.1, 12.2; RM3 14.2, 12.7; LM3 14.1, 12.5.

Postcranium. Preserved postcranial remains of Au. sediba (table S1) denote small-bodied hominins that retain an australopith pattern of long upper limbs, a high brachial index, and relatively large upper limb joint surfaces (table S2). In addition to these aspects of limb and joint proportions, numerous other features in the upper limb are shared with sibling species of Australopithecus (to the exclusion of later Homo), including a scapula with a cranially oriented glenoid fossa and a strongly developed axillary border; a prominent conoid tubercle on the clavicle, with a pronounced angular margin; low proximal-to-distal humeral articular proportions; a distal humerus with a marked crest for the brachioradialis muscle, a large and deep olecranon fossa with a septal aperture, and a marked trochlear/capitular keel (19); an ulna with a pronounced flexor carpi ulnaris tubercle; and long, robust, and curved manual phalanges that preserve strong attachment sites for the flexor digitorum superficialis muscle.

Numerous features of the hip, knee, and ankle indicate that Au. sediba was a habitual biped. In terms of size and morphology, the proximal and distal articular ends of the femur and tibia fall within the range of variation of specimens attributed to Au. africanus. However, several derived features in the pelvis link the Malapa specimens with later Homo. In the os coxa (Fig. 4), Au. sediba shares with Homo a pronounced acetabulocristal buttress; a more posterior position of the cristal tubercle; a superoinferiorly extended posterior iliac blade, with an expanded retroauricular area; a sigmoid-shaped anterior inferior iliac spine; a reduced lever arm for weight transfer between the auricular surface and the acetabulum; an enlarged and rugose iliofemoral ligament attachment area; a tall and thin pubic symphyseal face; and a relatively short ischium with a deep and narrow tuberoacetabular sulcus. These features are present in taxonomically unassigned postcranial remains from Koobi Fora (KNM-ER 3228) and Olduvai Gorge (OH 28), which have been argued to represent early Homo (20), as well as in early Homo erectus (21). An os coxa from Swartkrans (SK 3155) has been considered by some to also represent early Homo (22) but can be seen to possess the australopith pattern in most of these features. In addition, Au. sediba shares with later Homo the human-like pattern of low humeral-to-femoral diaphyseal strength ratios, in contrast to the ape-like pattern seen in the H. habilis specimen OH 62 (table S2).

Fig. 4

Representative ossa coxae, in lateral view, from left to right, of Au. afarensis (AL 288-1), Au. africanus (Sts 14), Au. sediba (MH1), and H. erectus (KNM-WT 15000). The specimens are oriented so that the iliac blades all lie in the plane of the photograph (which thus leads to differences between specimens in the orientation of the acetabula and ischial tuberosities). MH1 possesses derived, Homo-like morphology compared to other australopithecines, including a relative reduction in the weight transfer distance from the sacroiliac (yellow) to hip (circle) joints; expansion of the retroauricular surface of the ilium (blue arrows) (determined by striking a line from the center of the sphere representing the femoral head to the most distant point on the posterior ilium; the superior arrow marks the terminus of this line, and the inferior arrow marks the intersection of this line with the most anterior point on the auricular face); narrowing of the tuberoacetabular sulcus (delimited by yellow arrows); and pronouncement of the acetabulocristal (green arrows) and acetabulosacral buttresses.

Although aspects of the pelvis are derived, the foot skeleton is more primitive overall, sharing with other australopiths a flat talar trochlea articular surface with medial and lateral margins with equal radii of curvature, and a short, stout, and medially twisted talar neck with a high horizontal angle and a low neck torsion angle (table S2 and fig. S5). The calcaneus is markedly primitive in its overall morphology: the bone is strongly angled along the proximodistal axis, with the point of maximum inflexion occurring at an enlarged peroneal trochlea; the lateral plantar tubercle is lacking; the calcaneal axis is set about 45° to the transverse plane; and the calcaneocuboid facet is vertically set and lacks an expanded posterior projection for the beak of the cuboid (23).

Discussion. The age and overall morphology of Au. sediba imply that it is most likely descended from Au. africanus, and appears more derived toward Homo than do Au. afarensis, Au. garhi, and Au. africanus. Elsewhere in South Africa, the Sterkfontein cranium Stw 53, dated to 2.0 to 1.5 Ma, is generally considered to represent either H. habilis (10, 24, 25) or perhaps an undiagnosed form of early Homo (26). It played an important role in the assignment of OH 62 to H. habilis (27). However, the derived craniodental morphology of Au. sediba casts doubt on the attribution of Stw 53 to early Homo [see also (28)]: Stw 53 appears to be more primitive than MH1 in retaining closely spaced temporal lines; marked postorbital constriction; a weakly developed supraorbital torus; narrow, nonprojecting nasal bones; anterior pillars; marked nasoalveolar prognathism; medial and lateral expansion of the frontal process of the zygomatic bone; and laterally flared zygomatics. If Stw 53 instead represents Au. africanus, the assignment of OH 62 to H. habilis becomes tenuous. Attribution of the partial skeleton KNM-ER 3735 to H. habilis was tentatively based, in part, on a favorable comparison with OH 62 and on the hypothesis that there were no other contemporaneous nonrobust australopith species to which it could be assigned in East Africa (29). As a result, the interpretation of KNM-ER 3735 as H. habilis also becomes uncertain.

The phylogenetic significance of the co-occurrence of derived postcranial features in Au. sediba, H. erectus, and a sample of isolated fossils generally referred to Homo sp. indet. (table S2) is not clear: The latter might represent early H. erectus, it might sample the postcranium of H. rudolfensis (which would then imply an evolutionary pathway from Au. sediba to H. rudolfensis to H. erectus), or it might represent the postcranium of H. habilis [which would suggest that OH 62 and KNM-ER 3735 (two specimens with ostensibly more primitive postcranial skeletons) do not belong in this taxon]. If the latter possibility holds, it could suggest a phylogenetic sequence from Au. sediba to H. habilis to H. erectus. Conversely, although the overall postcranial morphology of Au. sediba is similar to that of other australopiths, a number of derived features of the os coxa align the Malapa hominins with later Homo (H. erectus) to the exclusion of other australopiths. Additionally, Au. sediba shares a small number of cranial traits with H. erectus that are not exhibited in the H. habilisH. rudolfensis hypodigm, including slight postorbital constriction and convexity of the infraorbital region (18). Following on this, MH1 compares favorably with SK 847 (H. erectus) in the development of the supraorbital torus, nasal bones, infraorbital region, frontal process of the zygomatic, and subnasal projection. However, MH1 differs from SK 847 in its relatively smaller size, the robust glabelar region, the weakly developed supratoral sulcus, the steeply inclined zygomaticoalveolar crests with a high masseter origin, and the moderate canine juga, all features aligning MH1 with Australopithecus. It is thus not possible to establish the precise phylogenetic position of Au. sediba in relation to the various species assigned to early Homo. We can conclude that combined craniodental and postcranial evidence demonstrates that this new species shares more derived features with early Homo than does any other known australopith species (Table 1 and table S2) and thus represents a candidate ancestor for the genus, or a sister group to a close ancestor that persisted for some time after the first appearance of Homo.

The discovery of a <1.95-million-year-old (16) australopith that is potentially ancestral to Homo is seemingly at odds with the recovery of older fossils attributed to the latter genus (5) or of approximately contemporaneous fossils attributable to H. erectus (6, 30). However, it is unlikely that Malapa represents either the earliest or the latest temporal appearance of Au. sediba, nor does it encompass the geographical expanse that the species once occupied. We hypothesize that Au. sediba was derived via cladogenesis from Au. africanus (≈3.0 to 2.4 Ma), a taxon whose first and last appearance dates are also uncertain (31). The possibility that Au. sediba split from Au. africanus before the earliest appearance of Homo cannot be discounted.

Although the skull and skeleton of Au. sediba do evince derived features shared with early Homo, the overall body plan is that of a hominin at an australopith adaptive grade. This supports the argument, based on endocranial volume and craniodental morphology, that this species is most parsimoniously attributed to the genus Australopithecus. The Malapa specimens demonstrate that the evolutionary transition from a small-bodied and perhaps more arboreal-adapted hominin (such as Au. africanus) to a larger-bodied, possibly full-striding terrestrial biped (such as H. erectus) occurred in a mosaic fashion. Changes in functionally important aspects of pelvic morphology, including a reduction of the sacroacetabular weight-bearing load arm and enhanced acetabulosacral buttressing (reflecting enhancement of the hip extensor mechanism), enlargement of the iliofemoral ligament attachment (reflecting a shift in position of the line of transfer of weight to behind the center of rotation of the hip joint), enlargement of the acetabulocristal buttress (denoting enhancement of an alternating pelvic tilt mechanism), and reduction of the distance from the acetabulum to the ischial tuberosity (reflecting a reduction in the moment arm of the hamstring muscles) (20, 32) occurred within the context of an otherwise australopith body plan, and seemingly before an increase in hominin encephalization [in contrast to the argument in (33)]. Relative humeral and femoral diaphyseal strength measures (table S2) also suggest that habitual locomotor patterns in Au. sediba involved a more modern human-like mechanical load-sharing than that seen in the H. habilis specimen OH 62 (34, 35). Mosaic evolutionary changes are mirrored in craniodental morphology, because the increasingly wide spacing of the temporal lines and reduction in postorbital constriction that characterize Homo first appeared in an australopith and before significant cranial expansion. Moreover, dental reduction, particularly in the postcanine dentition, preceded the cuspal rearrangement (wide spacing of postcanine tooth cusps) that marks early Homo.

The pattern of dental eruption and epiphyseal fusion exhibited by MH1 indicates that its age at death was 12 to 13 years by human standards, whereas in MH2 the advanced degree of occlusal attrition and epiphyseal closure indicates that it had reached full adulthood (SOM text S1). Although juvenile, MH1 exhibits pronounced development of the supraorbital region and canine juga, eversion of the gonial angle of the mandible, and large rugose muscle scars in the skeleton, all indicating that this was a male individual. And, although fully adult, the mandible and skeleton of MH2 are smaller than in MH1, which, combined with the less rugose muscle scars and the shape of the pubic body of the os coxa, suggests that MH2 was a female. In terms of dental dimensions, MH1 has mandibular molar occlusal surface areas that are 10.7% (M1) and 8.1% (M2) larger than those of MH2. Dimorphism in the postcranial skeleton likewise is not great, though the juvenile status of MH1 tends to confound efforts to assess adult body size. The diameter of the proximal epiphysis for the femoral head of MH1 (29.8 mm) is approximately 9.1% smaller than the superoinferior diameter of MH2’s femoral head (32.7 mm). It is likely that MH1 would have experienced some appositional increase in joint size before maturity, thus this disparity would probably have decreased somewhat. The distal humeral epiphysis of MH1 is fully fused and its articular breadth (35.3 mm) is only marginally larger than that of MH2 (35.2 mm). Thus, although the dentition and postcranial skeleton are at odds in the degree of apparent size differences, the overall level of dimorphism, if these sex attributions are correct, appears slight in the Malapa hominins and was probably similar to that evinced by modern humans.

Supporting Online Material

www.sciencemag.org/cgi/content/full/328/5975/195/DC1

SOM Text 1 to 4

Figs. S1 to S5

Tables S1 and S2

References

References and Notes

  1. The H. erectus hypodigm includes African specimens that are referred to the taxon H. ergaster by some. Unless otherwise stated, we collectively refer to H. habilis, H. rudolfensis, H. erectus, and H. ergaster materials as “early Homo.
  2. Rak (36) describes a feature in the infraorbital region of Au. boisei that he refers to as a nasomaxillary basin: a concave depression that is surrounded by a more elevated topography. We see a similar concavity in the infraorbital region of specimens of H. habilisH. rudolfensis (KNM-ER 1470, KNM-ER 1805, KNM-ER 1813, and OH 24), although it is not clear whether they represent homologous structures. In specimens of Au. africanus, Au. sediba, and H. erectus, we recognize a slight convexity in this area.
  3. Some humeri that are probably best attributed to Australopithecus lack marked development of the trochlear/capitular keel [or “lateral crest”: see (37)], and thus the absence of a marked crest does not reliably differentiate Australopithecus from Homo. However, although some specimens of early Homo (such as KNM-WT 15000) have crests that are more strongly developed than those of modern humans, none exhibit the marked crests of the australopiths. Thus, the marked crest seen in the Malapa humeri can be seen to be shared with Australopithecus rather than Homo.
  4. It is possible that the more Homo-like humeral-to-femoral diaphyseal strength ratios in Au. sediba reflect a relative reinforcement of the femoral diaphysis in the context of femoral elongation (resulting in longer bending-moment arms) without a change in locomotor behavior. At present, we are unable to directly assess the absolute and relative length of the femur in Au. sediba.
  5. We thank the South African Heritage Resources Agency for the permits to work at the Malapa site; the Nash family for granting access to the Malapa site and continued support of research on their reserve; the South African Department of Science and Technology, the South African National Research Foundation, the Institute for Human Evolution, the Palaeontological Scientific Trust, the Andrew W. Mellon Foundation, the AfricaArray Program, the U.S. Diplomatic Mission to South Africa, and Sir Richard Branson for funding; the University of the Witwatersrand’s Schools of Geosciences and Anatomical Sciences and the Bernard Price Institute for Palaeontology for support and facilities; the Gauteng Government, Gauteng Department of Agriculture, Conservation and Environment and the Cradle of Humankind Management Authority; E. Mbua, P. Kiura, V. Iminjili, and the National Museums of Kenya for access to comparative specimens; Optech and Optron; Duke University; the Ray A. Rothrock Fellowship of Texas A&M University; and the University of Zurich 2009 Field School. Numerous individuals have been involved in the ongoing preparation and excavation of these fossils, including C. Dube, B. Eloff, C. Kemp, M. Kgasi, M. Languza, J. Malaza, G. Mokoma, P. Mukanela, T. Nemvhundi, M. Ngcamphalala, S. Jirah, S. Tshabalala, and C. Yates. Other individuals who have given significant support to this project include B. de Klerk, C. Steininger, B. Kuhn, L. Pollarolo, B. Zipfel, J. Kretzen, D. Conforti, J. McCaffery, C. Dlamini, H. Visser, R. McCrae-Samuel, B. Nkosi, B. Louw, L. Backwell, F. Thackeray, and M. Peltier. T. Stidham helped construct the cladogram in fig. S3. J. Smilg facilitated computed tomography scanning of the specimens. R. Clarke and F. Kirera provided valuable discussions on these and other hominin fossils in Africa.
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