Research Article

The Upper Limb of Australopithecus sediba

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Science  12 Apr 2013:
Vol. 340, Issue 6129, 1233477
DOI: 10.1126/science.1233477


The evolution of the human upper limb involved a change in function from its use for both locomotion and prehension (as in apes) to a predominantly prehensile and manipulative role. Well-preserved forelimb remains of 1.98-million-year-old Australopithecus sediba from Malapa, South Africa, contribute to our understanding of this evolutionary transition. Whereas other aspects of their postcranial anatomy evince mosaic combinations of primitive (australopith-like) and derived (Homo-like) features, the upper limbs (excluding the hand and wrist) of the Malapa hominins are predominantly primitive and suggest the retention of substantial climbing and suspensory ability. The use of the forelimb primarily for prehension and manipulation appears to arise later, likely with the emergence of Homo erectus.

Paleoanthropological fieldwork in Africa and western Asia continues to enhance the fossil representation of Plio-Pleistocene hominin upper limb remains (17). Improvement in this record has fostered a new appreciation for variability in hominin forelimb morphology and limb proportions but has at the same time raised new issues about the locomotor and manipulative behavior of various species of Australopithecus and early Homo. One area of current uncertainty involves the functional and phylogenetic importance of variation within the genus Australopithecus. Compared with the earlier species Au. anamensis and Au. afarensis, limb and joint proportions in Au. africanus reflect a more ape-like morphology and possibly a greater degree of arboreality (8, 9). The same may be true of postcranial material attributed to H. habilis (10, 11), although the fragmentary condition of the representative specimens prevents definitive conclusions (1214), as does the possibility that these two partial skeletons do not represent H. habilis (7). In addition, a 2.5-million-year-old partial skeleton that may represent Au. garhi is said to evince derived, Homo-like interlimb proportions (i.e., relative elongation of the lower limb) coupled with ape-like brachial (forearm to arm) proportions that are more primitive than seen in earlier Au. afarensis (15). There thus appears to have been considerable evolvability in hominin limb proportions (16), likely involving substantial parallelism as well as evolutionary reversals, making it difficult to identify the immediate morphological precursors to the human-like proportions seen in early H. erectus (4, 17, 18).

A second open question concerns the functional meaning of morphological variation within Au. afarensis. Two relatively complete scapulae of this species have recently been discovered in Ethiopia, one representing a 3.3-million-year-old juvenile from Dikika (DIK-1-1) (3, 19), the other a 3.6-million-year-old adult from Woranso-Mille (KSD-VP-1/1 g) (6, 20). Multivariate analysis of linear and angular measurements of DIK-1-1 highlights its similarities to gorilla and orangutan scapulae in the orientations of both its spine and glenoid fossa and in the proportions of its supraspinous and infraspinous fossae, suggesting a high degree of arboreality in Au. afarensis (3, 19). Conversely, in a multivariate analysis of angular measurements, the adult Woranso-Mille scapula clusters with modern humans and not African apes, suggesting a loss of arboreal function in Au. afarensis (6). It is not clear whether this disparity in results is the consequence of differences in the developmental ages of the two specimens [but see (19)] or is the result of using different variable sets and comparative samples in the analyses of the two specimens [see (6)].

Upper limb remains of Au. sediba (7), from the 1.98-million-year-old site of Malapa, South Africa (21, 22), enhance our understanding of variation in australopith morphology. The forelimb is particularly well represented in the Malapa hominins (Fig. 1). The subadult male, Malapa Hominin 1 (MH1), preserves the lateral half of the right clavicle, a right humerus that is complete save for the proximal epiphysis and apophysis of the medial epicondyle, a proximal shaft fragment of the left humerus, the proximal half of the right ulna (lacking the olecranon apophysis), and the distal epiphysis of the right radius. The adult female (MH2) preserves a complete clavicle, humerus, radius, and ulna and a relatively complete scapula from the right side, as well as portions of the left clavicle, scapula, and humerus. Both specimens also preserve manual remains, which have been treated elsewhere (23).

Fig. 1 Malapa upper limb elements in anterior view (unless otherwise noted).

MH1: UW88-1 right clavicle (superoanterior view); UW88-34, 42, and 88 right humerus; UW88-36 left humerus; UW88-3 right ulna; and UW88-12 right radial epiphysis (distal view). MH2: UW88-172 manubrium, UW88-38 right clavicle (superoanterior view), UW88-94 left clavicle (superoanterior view), UW88-56 right scapula, UW88-103 left scapular acromion, UW88-104 left scapular glenoid fossa (lateral view), UW88-197 left scapular body fragment, UW88-57 right humerus, UW88-101 left humerus, UW88-62 right ulna, and UW88-85 right radius.

The Pectoral Girdle

The overall morphology of the upper limbs of Au. sediba broadly resembles that of other australopiths. The right clavicle of MH2 is absolutely and relatively short and is relatively gracile compared with the Woranso-Mille (Au. afarensis) specimen KSD-VP-1/1f (6) and early and modern Homo (table S1), all of which are larger bodied. When the acromial ends of the clavicles of both MH1 and MH2 are held horizontally, their shafts inflect inferomedially (Fig. 2), indicating a shoulder position higher than that of living humans (24). The morphology of the sternoclavicular joint surfaces of the clavicle and manubrium of MH2 also suggest an oblique orientation to the clavicle (Fig. 2) and imply a high scapular position. Like other australopiths and Homo, the deltoid scar is on the anterior surface of the lateral curve of the shaft, not on the superoanterior surface as in chimpanzees, indicating a human-like lack of clavicular rotation (24). However, unlike humans, MH2 and, to a lesser extent, MH1, have prominent conoid tubercles that form distinct posterior flanges (although not as prominent as those seen in chimpanzees) (Fig. 2). Both specimens have a pronounced angular margin continuous with the tubercle medially and separating the inferior from the posterior surfaces, as also seen in Au. afarensis specimens A.L. 288-1 (25), A.L. 333x-6/9 (26), A.L. 438-1 (27), and StW 606 (28). Similar but perhaps less-developed angular margins are present in the Au. africanus specimens StW 431 and StW 582.

Fig. 2 Clavicular morphology in Au. sediba, Pan troglodytes, and Homo sapiens.

(A) Specimens in anterior view, from top: MH1 right side, MH2 left side, MH2 right side, chimpanzee, and human (comparative specimens from the A. H. Schultz collection). All specimens are oriented such that the acromial ends are in the horizontal plane, showing the inferomedial inclination of the clavicular shafts in Au. sediba and Pan. Note the enlarged conoid tubercle in MH2 (arrow). The enlarged conoid tubercle of MH1 extends more dorsally than inferiorly. (B) The MH2 clavicle in inferior perspective. The inferomedial aspect of the sternoclavicular joint surface (white arrow) forms a 124° angle with the medial surface of the joint, such that when the clavicle is articulated with the manubrium, the shaft is oriented at an angle of ~120° to the vertical. An elevated margin (black arrow) bounds the articular surface, such that when the clavicular shaft is lowered beyond 115°, contact between the clavicular and sternal articular surfaces is no longer possible. All specimens to same scale.

The scapula (Fig. 3) possesses a cranially oriented glenoid fossa (as indicated by its high glenoid-medial angle), a markedly cranially directed spine (reflected in its low spinal-medial angle), and a large attachment area for supraspinatus (relative to the size of the glenoid fossa and the infraspinous fossa) (tables S3 and S4). Supraspinous to infraspinous fossae proportions are very similar to those of the juvenile Au. afarensis specimen DIK-1-1 (3) and are intermediate between those of Homo and the African apes (table S4). The glenoid fossa of MH2 is narrow relative to its height, with proportions most similar to orangutans and gorillas (table S4). The vertebral border is convex, the acromion process is long and curved (as in apes), and the inferior axillary border has a prominent flange at the origin of teres major, indicating hypertrophy of this adductor and medial rotator of the humerus (Fig. 3).

Fig. 3 Three-dimensional rendering of the reconstructed MH2 right scapula in ventral (left), lateral (middle), and dorsal (right) views.

In silico reconstruction involved rejoining the superior fragment (preserving much of the supraspinous fossa, the lateral root of the acromion, and the superior half of the glenoid fossa) with the body inferiorly. When possible, aligning separate fragments was performed at the level of individual trabecular struts. The right scapula lacks the acromion process; thus, the left side acromion process was mirrored and aligned to obtain the best fit with overlapping preserved portions of the lateral spine.

Based on multivariate analyses of size-standardized shape variables, the MH2 scapula is morphologically most similar to scapulae of Pongo (29). In one analysis, MH2 also was not significantly different from Gorilla, but in no cases did it cluster with Homo, Pan, or Hylobates. MH2 is most similar to Pongo in measures of glenoid and spinal angle (table S3), including measures of glenoid fossa or spine orientation relative to the superoinferior long axis of the scapular body (the glenoid-medial and spinal-medial angles, respectively). The long axis of the scapula is defined by the superior and inferior angles of the body (30) and lies parallel to the vertebral column in humans, providing an indication of the anatomical orientation of the bone (31). Thus, angles based on this axis are likely to be better indicators of glenoid fossa and spinal orientation than are angles based on the more commonly used axillary border. In its axillary-vertebral angle, the MH2 scapula falls closest to the mean value observed in the Gorilla sample (table S3), denoting a scapula that is mediolaterally long relative to its superoinferior breadth, but not quite to the extent seen in humans. MH2 is also similar to Pongo and Gorilla in having a glenoid fossa that is dorsoventrally narrow relative to its superoinferior height (as reflected by its low glenoid fossa index) (table S4). The shape of the body of the MH2 scapula, as measured by the scapular index, is most similar to that seen in Homo and Pan (table S4). The relative size of the supraspinous fossa (relative to the size of both the infraspinous fossa and the glenoid fossa) is greater than the mean values seen in Pongo and Homo but smaller than the means of the African apes and gibbons (table S4).

To explore the claim that purported differences in morphological affinities of the Dikika [gorilla-like (3)] and Woranso-Mille [human-like (6)] australopith scapulae are a function of the choice of measurement variables (6), we conducted three separate principal component analyses (PCA): one using only the 10 linear and angular variables employed by (3), one using only four of the five angles employed by (6), and one using the combined variable set from both studies (29). Our analyses also included samples of the Asian apes, which were not represented in either of the previous studies. We found that, regardless of which variable set we employed, MH2’s position in multivariate space fell closest to the centroid of the Pongo sample (i.e., in no case was the Euclidean distance of MH2 from the Pongo centroid significantly larger than the average distance of each individual specimen of Pongo from its own group centroid). When only the angles employed by (6) were used, MH2’s position also did not significantly differ from the Gorilla centroid. In all three analyses, MH2 was significantly distant from the Homo, Pan, and Hylobates centroids. These results suggest that, when Asian apes are included in the analysis, the analyses are fairly robust with respect to choice of variables; the morphological affinities of the MH2 scapula with those of Pongo was a consistent finding. This further suggests that the differing, previously reached conclusions concerning australopith scapular morphology [human-like (6) versus ape-like (3)] are mainly a function of composition of the comparative samples, perhaps combined with the ontogenetic status of the specimen from Dikika (32). This conclusion is supported by more recent studies of both the Woranso-Mille (20) and Dikika (19) scapulae that report morphological affinities with Pongo.

The importance of the Pongo-like scapular morphology of MH2 is unclear. Orangutans are the only large-bodied ape to retain a predominantly arboreal lifestyle, and they engage in the greatest amount of forelimb suspension during locomotion of any of the great apes (33). However, orangutans also display the greatest degree of variability in locomotor and positional behaviors of any of the apes, including vertical climbing, quadrumanous climbing and suspension, quadrupedalism, and a small amount of pronogrady (33). Pongo-like scapular morphology, related to highly variable arboreal positional and locomotor behaviors, has been argued to represent the primitive condition for hominids, with the scapulae of Pan and Gorilla reflecting derived specializations for greater suspension and quadrupedalism (20). By this model, the Pongo-like morphology of australopith scapulae reflects their descent from a generalized arboreal ancestor that was not specialized for arboreal suspension (20, 34). The Pongo-like morphology of MH2 is consistent with this hypothesis. However, it is important to note that Pongo engages in substantially more forelimb suspensory locomotion than do African hominids (33, 35), making it difficult to argue that the African apes are somehow more specialized for suspensory behavior. In most aspects of scapular shape (tables S3 and S4) MH2 is intermediate between Pan and Homo, and thus also consistent with the hypothesis that the chimp-human last common ancestor was largely chimplike in its postcranial morphology (36). By this model, the morphological and possibly behavioral intermediacy of the australopiths between a Pan-like arboreal ancestor and fully terrestrial modern humans resulted in scapular morphology homoplastically similar to that of Pongo. Regardless of which of these models is correct, the comparative evidence shows that MH2 possessed a scapula that would have been well suited to the arm positions associated with the kinds of antipronograde postures and forelimb suspensory behaviors commonly employed by highly arboreal orangutans.

The Arm and Forearm

Humeral torsion angles in the Malapa hominins (29) (table S8 and fig. S12) suggest a more posteriorly directed humeral head than that seen in the only other australopith (A.L. 288-1) for which this morphology can be determined [although estimated torsion in partial proximal humeri attributed to australopiths suggest even more medially directed heads than A.L. 288-1 (37)]. MH1 and MH2 thus support the conclusion that the high torsion angles seen in humans and African apes evolved in parallel (37, 38). The low degree of torsion in the Malapa humeri suggests that the scapula occupied a more lateral position on the mediolaterally narrow, ape-like upper thorax (39) [otherwise the elbow would have a lateral set, which among extant apes is seen only in gibbons as an adaptation to brachiation (38)]. The inference of a laterally positioned scapula is also supported by the absolutely and relatively short clavicle of MH2 (table S1) (40). Other aspects of humeral size and shape fall within the range of variation of other australopiths (tables S9 and S10). Both MH1 and MH2 have humeri with elevated and rugose (MH1) to moderately rugose (MH2) deltopectoral crests [as seen in other australopiths and humans (34)].

The morphology of Au. sediba’s elbow also falls comfortably within the range of variation of Au. afarensis and Au. africanus in most aspects of size and shape (tables S9 to S12). The Malapa hominins share with other australopiths and African apes a strongly projecting humeral medial epicondyle and a pronounced humeral lateral epicondyle with an elongated and distinct brachioradialis crest. They also share with other australopiths a relatively large insertion for biceps brachii on the radius (table S11), enhanced leverage for triceps brachii on the ulna, and an anteriorly oriented ulnar trochlear notch (table S12). Although these and other features (7) indicate retention of the primitive condition of enhanced brachial and antebrachial musculature, the forelimb of MH2 exhibits relatively poor mechanical advantage for the elbow flexors biceps brachii and brachialis compared with other australopiths (tables S11 and S12). The trochlear surfaces of both the MH1 and MH2 ulnae exhibit keeling greater than that seen in humans. The ulnar shaft of MH2 is straight (like a human) in lateral view, whereas her radius is mildly curved in anterior perspective. The distal radii of both MH1 and MH2 are like those of other australopiths in having larger lunate than scaphoid articular surfaces and in possessing weakly developed but distinct dorsal ridges on the radiocarpal articular margin (41).

Limb Proportions

MH2 is the only australopith with complete preservation of the long bones of the upper limb. Early hominin specimens preserving even relatively complete long bones are rare; thus, forelimb proportions tend to involve considerable estimation for most taxa, and their ranges of variation and sample central tendencies are incompletely known. Still, some tentative conclusions can be reached. Humeral length relative to femoral head diameter in Au. sediba (7.86 to 8.15) is intermediate between that of Au. afarensis (9.20) and early H. erectus (6.93 to 7.38) (7). When humeral plus radial length is plotted against femoral head diameter (Fig. 4A), MH2 can be seen to have an arm that is long for her body size (as measured by femoral head diameter) relative to most humans but still within the 95% confidence limits of the human sample. MH2’s brachial index (100 × radial/humeral length) of 84 falls within the estimates for A.L. 288-1 (Au. afarensis) and OH 62 [attributed to H. habilis, but see (7)] and above that of the Nariokotome H. erectus specimen (Fig. 4B). In a regression of radial on humeral length, MH2 falls above the 95% confidence limits about the human sample but within that of the chimpanzee sample (29) (fig. S13). Although it is important to note that some modern humans have brachial indices as or more extreme than that observed in MH2, one would have less than a 1% chance of drawing such an individual at random from a sample of modern humans. The high brachial index observed in the Ardipithecus ramidus specimen ARA-VP-6/500 has been argued to represent the primitive condition for hominins (42), and the high estimated value for Lucy [a value of 92 being the preferred estimate of (42)] has been interpreted as a retention of primitive arm proportions in Au. afarensis. If this is the case, the lower value seen in MH2 would then represent another derived, Homo-like feature in Au. sediba and, importantly, one not observed in OH 62. Nevertheless, given the great uncertainty involved in the estimation of the brachial index in other australopiths and OH 62, there is at present little basis for inferring in Au. sediba a reduction in relative forearm length from the primitive condition. The taxonomically unassigned [but possibly Au. garhi (15)] BOU-VP-12/1 specimen has an estimated brachial index of 97.9 to 102.2 (15), and in bivariate space it straddles the upper 95% confidence limits about the Pan sample (fig. S13). Given the positions of the other fossil hominins in bivariate space, this extreme result (relative elongation of the forearm greater than that of most chimpanzees) is highly improbable. It is therefore likely that the humeral length of this specimen has been underestimated—and, thus, the inference of human-like humeral/femoral proportions in this 2.5-million-year-old fossil (15) is unsupported.

Fig. 4 Limb proportions in Au. sediba, fossil hominins, and extant hominoids.

(A) Scatter plot of upper limb length (humerus maximum length + radius maximum length) regressed on femoral anteroposterior (A-P) head diameter for Pleistocene/Holocene Homo, recent Pan and Gorilla, and fossil hominins. Homo sapiens is indicated by red crosses, Pan troglodytes by blue triangles, and Gorilla gorilla by black triangles. Fossil hominins are indicated in green, with MH2 represented by a star. Potential range of upper limb length values for A.L. 288-1 are indicated by a solid line between its minimum and maximum values. The point for KNM-WT 15000 is based on the specimen’s superoinferior femoral head diameter. The ordinary least squares regression lines for the comparative samples are represented by solid lines, and the reduced major axis (RMA) regression lines for the comparative samples are the dashed lines. H. sapiens RMA formula: Y = 10.643X + 76.078, r = 0.65, n = 865; P. troglodytes RMA formula: Y = 15.235X + 71.368, r = 0.61, n = 59; G. gorilla RMA formula: Y = 15.353X + 35.459, r = 0.81, n = 45. (B) Brachial indices in Au. sediba (MH2), extant hominoids, and fossil hominins. Box-and-whiskers plots show the median (vertical line), upper and lower quartiles (box), range (whiskers), and outliers (circles) for each group. For individual specimens, boxes represent the range of estimated values [see (12)]. Redrawn from (12) using data from that study, except for H. sapiens (n = 1093 samples), P. troglodytes (n = 59 samples), and G. gorilla (n = 46 samples) [see (29)] and ARA-VP-6/500 [data from (42)].

Forelimb Function and Locomotion

Au. sediba shares with other australopiths forelimb features that have been interpreted as indicative of an arboreal habitus (43, 44). Both relatively high shoulders and pronounced clavicular conoid tubercles have been suggested to be adaptations for enhanced elevation of the clavicle and arm in climbing and suspension (24). Likewise, an upturned scapular glenoid fossa is thought to reflect habitual use of the arms in elevated positions (43). The high spinal angle seen in Au. sediba increases the mechanical advantage of the serratus anterior in upward rotation of the scapula, as well as increasing the horizontal component of the force vector of the upper part of the trapezius (the component that contributes to the rotary force couple with the serratus anterior) (45), allowing for more forceful rotation of the scapula when moving the trunk on a fixed arm during suspensory behaviors. The arm is moderately long relative to body size, and the brachial index of Au. sediba is high, reminiscent of the condition in apes for whom arboreal forelimb suspension remains a substantial portion of their behavioral repertoire. Muscle attachment surface areas and enthesis rugosity from the shoulder to the hand in the Malapa hominins indicate moderate hypertrophy (that is, greater than usually seen in humans but less than commonly seen in the large-bodied apes) of the brachial and antebrachial musculature, including supraspinatus, teres major, brachioradialis, flexor carpi ulnaris, and the other extrinsic wrist and digital flexors (7, 23). Two features that have been argued to be developmentally plastic traits associated with climbing and suspension in the apes (46, 47) can be observed in Au. sediba: The ulnar trochlear surface is moderately keeled, and the manual phalanges are moderately curved (23). Although it could be argued that many of these features reflect functionally irrelevant retentions of primitive traits from a more arboreal ancestor, the presence of developmentally plastic features indicative of climbing and suspension during ontogeny supports the inference of the retention of some degree of arboreal behavior in the locomotor repertoire of Au. sediba. In the aggregate, the upper limb of the Malapa hominins was well adapted for arboreal locomotion.

The growing fossil record of australopith and early Homo shoulder and forelimb remains (3, 4, 6, 27, 48, 49) shows that the emergence of the Homo body plan entailed a reorganization of the pectoral girdle, involving relocation of the scapula from high and dorsal to lower and more lateral on the thorax, with a concomitant shift from an obliquely to horizontally oriented clavicle (37). This change was accompanied by a downward rotation of the scapular glenoid fossa (less cranial to more lateral) and changes in the angular relationships between the scapular spine, glenoid fossa, and body from a Pongo-like to a human-like morphology (3, 6, 37). This reorganization of the shoulder, first evident in H. erectus, was accomplished in the context of relative stasis in clavicular length (proportional to body size) and a moderate decrease in humeral torsion angle (37). If the Malapa hominins represent a stem species of the Homo clade (7, 22), they show that relocation of the scapula to the lateral thorax preceded its downward shift and the concomitant reorganization of scapular architecture. Given the evidence for the retention of some climbing abilities in Au. sediba, this suggests that downward relocation of the hominin scapula only occurred later with a full-time commitment to terrestrial environments. The establishment of a shoulder configuration intermediate between that of australopiths and modern humans, as seen in early H. erectus (4, 37), likely signals the initial transition to upper limb function away from locomotion and predominantly toward manipulation and prehension.

Supplementary Materials

Methods and Results

Figs S1 to S13

Tables S1 to S12

References (5167)

References and Notes

  1. Methods and background are available as supplementary materials on Science Online.
  2. Hailie-Sellasie et al. (6), citing Graves (30), argue that the scapular axis is too variable in Homo to be of use in evaluating scapular architectural differences between taxa. The study by Graves (30), however, concerned variation in the form of the vertebral border and says nothing about variation in the position of the superior and inferior angles (that is, the scapular axis). Radiographic analysis of scapular position in humans at rest shows that the two landmarks that define the axis lie equidistant to the spinous processes of their adjacent vertebrae (50). Furthermore, in our samples, although it is the case that the glenoid-medial angle tends to be more variable than other angles, the spinal-medial angle is not (table S3). This suggests that variance in the glenoid-medial angle is reflecting variation in the orientation of the glenoid fossa more so than variation in the anatomical reference axis. These observations suggest that the long axis of the scapula can be used as a reliable indicator of the orientation of the bone with the arm at rest.
  3. Acknowledgments: We thank the South African Heritage Resource 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 Gauteng Provincial Government, the South African Department of Science and Technology, the African Origins Platform (AOP), the South African National Research Foundation (NRF), the Evolutionary Studies Institute (University of the Witwatersrand), the University of the Witwatersrand’s Vice Chancellor’s Discretionary Fund, the National Geographic Society, the Palaeontological Scientific Trust, the Andrew W. Mellon Foundation, the Ford Foundation, the U.S. Diplomatic Mission to South Africa, the French Embassy of South Africa, the Claude Leon Foundation, the Strategic Planning and Resource Allocation Committee of the University of the Witwatersrand, the A.H. Schultz Foundation, Duke University, the Ray A. Rothrock ’77 Fellowship and International Research Travel Assistance Grant of Texas A&M University, and the Oppenheimer and Ackerman families 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; Gauteng Department of Agriculture, Conservation and Environment and the Cradle of Humankind Management Authority; and our respective universities for ongoing support. We thank the Virtual Image Processing Laboratory and the Microfocus X-ray Computed Tomography Facility of the Palaeosciences Centre at the University of the Witwatersrand; for funding these facilities, we thank the University of the Witwatersrand Office of Research and the NRF SRIG (Strategic Research Infrastructure Grant) and AOP funding programs. For access to comparative specimens, we thank E. Mbua, P. Kiura, V. Iminjili, and the National Museums of Kenya; B. Billings, B. Zipfel, and the School of Anatomical Sciences at the University of the Witwatersrand; S. Potze and L.C. Kgasi at the Ditsong Museum; E. Westwig at the American Museum of Natural History; L. Gordon at the National Museum of Natural History; and M. Ponce de León and C. Zollikofer at the Anthropological Institute and Museum. For technical and material support we thank, Duke University, the University of Zurich 2009 and 2010 Field Schools, A.B. Taylor, C.E. Terhune, and C.E. Wall. Numerous individuals have been involved in the ongoing preparation and excavation of these fossils, including C. Dube, 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 major support to this project include B. de Klerk, W. Lawrence, 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. J. Smilg facilitated computed tomography scanning of the specimens. The Au. sediba specimens are archived at the Evolutionary Studies Institute at the University of the Witwatersrand. All data used in this study are available upon request, including access to the original specimens, by bona fide scientists.

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