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<jats:p> Since the announcement of the species <jats:italic>Australopithecus sediba</jats:italic> , questions have been raised over whether the Malapa fossils represent a valid taxon or whether inadequate allowance was made for intraspecific variation, in particular with reference to the temporally and geographically proximate species <jats:italic>Au</jats:italic> . <jats:italic>africanus</jats:italic> . The morphology of mandibular remains of <jats:italic>Au. sediba</jats:italic> , including newly recovered material discussed here, shows that it is not merely a late-surviving morph of <jats:italic>Au. africanus</jats:italic> . Rather—as is seen elsewhere in the cranium, dentition, and postcranial skeleton—these mandibular remains share similarities with other australopiths but can be differentiated from the hypodigm of <jats:italic>Au. africanus</jats:italic> in both size and shape as well as in their ontogenetic growth trajectory. </jats:p>

<jats:p> The discovery of a relatively complete <jats:italic>Australopithecus sediba</jats:italic> adult female skeleton permits a detailed locomotor analysis in which joint systems can be integrated to form a comprehensive picture of gait kinematics in this late australopith. Here we describe the lower limb anatomy of <jats:italic>Au. sediba</jats:italic> and hypothesize that this species walked with a fully extended leg and with an inverted foot during the swing phase of bipedal walking. Initial contact of the lateral foot with the ground resulted in a large pronatory torque around the joints of the foot that caused extreme medial weight transfer (hyperpronation) into the toe-off phase of the gait cycle (late pronation). These bipedal mechanics are different from those often reconstructed for other australopiths and suggest that there may have been several forms of bipedalism during the Plio-Pleistocene. </jats:p>

<jats:p> Two partial vertebral columns of <jats:italic>Australopithecus sediba</jats:italic> grant insight into aspects of early hominin spinal mobility, lumbar curvature, vertebral formula, and transitional vertebra position. <jats:italic>Au. sediba</jats:italic> likely possessed five non–rib-bearing lumbar vertebrae and five sacral elements, the same configuration that occurs modally in modern humans. This finding contrasts with other interpretations of early hominin regional vertebral numbers. Importantly, the transitional vertebra is distinct from and above the last rib-bearing vertebra in <jats:italic>Au. sediba</jats:italic> , resulting in a functionally longer lower back. This configuration, along with a strongly wedged last lumbar vertebra and other indicators of lordotic posture, would have contributed to a highly flexible spine that is derived compared with earlier members of the genus <jats:italic>Australopithecus</jats:italic> and similar to that of the Nariokotome <jats:italic>Homo erectus</jats:italic> skeleton. </jats:p>

<jats:p> To characterize further the <jats:italic>Australopithecus sediba</jats:italic> hypodigm, we describe 22 dental traits in specimens MH1 and MH2. Like other skeletal elements, the teeth present a mosaic of primitive and derived features. The new nonmetric data are then qualitatively and phenetically compared with those in eight other African hominin samples, before cladistic analyses using a gorilla outgroup. There is some distinction, largely driven by contrasting molar traits, from East African australopiths. However, <jats:italic>Au. sediba</jats:italic> links with <jats:italic>Au. africanus</jats:italic> to form a South African australopith clade. These species present five apomorphies, including shared expressions of Carabelli’s upper first molar (UM1) and protostylid lower first molar (LM1). Five synapomorphies are also evident between them and monophyletic <jats:italic>Homo habilis/rudolfensis</jats:italic> + <jats:italic>H. erectus</jats:italic> . Finally, a South African australopith + <jats:italic>Homo</jats:italic> clade is supported by four shared derived states, including identical LM1 cusp 7 expression. </jats:p>

<jats:p> 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 <jats:italic>Australopithecus sediba</jats:italic> 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 ( <jats:italic>Homo</jats:italic> -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 <jats:italic>Homo erectus</jats:italic> . </jats:p>

<jats:p> The shape of the thorax of early hominins has been a point of contention for more than 30 years. Owing to the generally fragmentary nature of fossil hominin ribs, few specimens have been recovered that have rib remains complete enough to allow accurate reassembly of thoracic shape, thus leaving open the question of when the cylindrical-shaped chest of humans and their immediate ancestors evolved. The ribs of <jats:italic>Australopithecus sediba</jats:italic> exhibit a mediolaterally narrow, ape-like upper thoracic shape, which is unlike the broad upper thorax of <jats:italic>Homo</jats:italic> that has been related to the locomotor pattern of endurance walking and running. The lower thorax, however, appears less laterally flared than that of apes and more closely approximates the morphology found in humans. </jats:p>