The hand of Australopithecus sediba

Here we describe the functional morphology of the Australopithecus sediba hand, including the almost complete hand of the presumed female Malapa Hominin (MH) 2 skeleton and a single, juvenile metacarpal from the presumed male MH1 skeleton. Qualitative and quantitative comparisons with extant hominids and fossil hominins, ranging from Ardipithecus to early Homo sapiens, reveal that Au. sediba presents a unique suite of morphological features that have not been found in any other known hominin. Analyses of intrinsic hand proportions show that the MH2 hand has a thumb that is longer relative to its fingers than recent humans and any other known hominin. Furthermore, the morphology of the hamatometacarpal articulation suggests that the robust fifth metacarpal was positioned in a slightly more flexed and adducted posture than is typical of Neandertals and humans. Together, this morphology would have facilitated opposition of the thumb to the fingers and pad-to-pad precision gripping that is typical of later Homo. However, the remarkably gracile morphology of the first ray and the morphology of the lateral carpometacarpal region suggest limited force production by the thumb. The distinct scaphoid-lunatecapitate morphology in MH2 suggests a greater range of abduction at the radiocarpal joint and perhaps less central-axis loading of the radiocarpal and midcarpal joints than is interpreted for other fossil hominins, while the morphology of the hamatotriquetrum articulation suggests enhanced stability of the medial midcarpal joint in extended and/or adducted wrist postures. The MH2 proximal phalanges show moderate curvature and, unusually, both the proximal and intermediate phalanges have well-developed flexor sheath ridges that, in combination with a palmarly-projecting hamate hamulus, suggest powerful flexion and that some degree of arboreality may have been a functionally important part of the Au. sediba locomotor repertoire. Finally, the MH1 and MH2 third metacarpals differ remarkably in their size and degree of robusticity, but this variation fits comfortably within the sexual dimorphism documented in recent humans and other fossil hominins, and does not necessarily reflect differences in function or hand use. Overall, the morphology of the current Au. sediba hand bones suggests the capability for use of the hands both for powerful gripping during locomotion and precision manipulation that is required for tool-related behaviors, but likely with more limited force production by the thumb than is inferred in humans, Neandertals, and potentially Homo naledi.