Inferring hand use in Australopithecus sediba: Analysis of the external and internal morphology of hominin proximal and intermediate phalanges.

The evolution of hominin hand use is characterized by a transition from locomotion to primarily dexterous hand use. However, our understanding of this transition remains unclear with discoveries of hominin hand fossils increasingly pointing towards a diverse range of manual behaviors. The almost complete right hand of Australopithecus sediba (1.98 Ma) shows a unique mix of primitive and derived morphology; an exceptionally long thumb and broad distal phalanx suggest human-like manipulative abilities, while curved phalanges and well-developed flexor sheath ridges (FSRs) on both the proximal and, unusually, intermediate phalanges, suggest the continued importance of climbing and suspension [1]. We analyzed the external and internal morphology of the A. sediba proximal and intermediate phalanges in comparison to extant apes to draw inferences about locomotor and manipulative capabilities. The study sample included microCT scans of manual proximal and intermediate phalanges of rays II-V of Gorilla gorilla (N =12), Homo sapiens (N =14), Pongo pygmaeus (N =17), Pan sp. (N = 18), and A. sediba (MH2). We quantified the degree of curvature via included angle, and variation in FSR morphology via linear measurements. We also assessed the cross-sectional cortical bone properties throughout the shaft using BoneJ (including cross-sectional area (CSA), polar section modulus (Zpol), and polar second moment of area (J)) and mapped variation in cortical thickness using Morphomap [2].

Results reveal curvature in the proximal and intermediate phalanges is lowest in Homo followed by, in an increasing order, Gorilla, Pan, and Pongo. The proximal and intermediate phalanges of A. sediba have intermediate values of curvature, falling within the African ape range, exceeding the mean values of Gorilla and falling below the mean values of Pan. After controlling for size, Gorilla has the shortest (in length) but the most prominent (deepest) and widest FSRs and humans FSRs are short, lacking depth and width, while Pongo and Pan share FSR morphology that is longer but intermediate in width and depth, for both proximal and intermediate phalanges. In A. sediba, the proximal phalanx FSRs are most similar to Gorilla, but longer, while the intermediate phalanges are distinct in having proximal phalanx-like morphology, such that they lack a median bar, and the FSRs are exceptionally deep, exceeding the mean values of each extant species.

The cortical bone analysis indicates that Gorilla has the highest values across all cross-sectional properties (CSA, Zpol, J) while humans have the lowest, and Pongo and Pan are intermediate, for both the proximal and intermediate phalanges. A. sediba is characterized by low cross-sectional values similar to those observed in humans. Initial bone distribution analysis within the extant sample demonstrates that cortical bone is thicker palmarly in great apes and dorsally in humans, in both intermediate and proximal phalanges. Consistent with the human-like CSG properties, A. sediba also has a thicker cortex dorsally. The low CSG properties in the fingers of A. sediba indicate limited strength compared to great apes, which may suggest its hands were not adapted for high loads that are incurred on the hand during climbing or suspension. However, this internal structure is combined with African ape-like curvature, which provides better distribution of the stress experienced during suspension [3], and prominent FSRs on the phalanges, which help reduce strains on the shaft of the bone [3]. Thus, the external morphology of the A. sediba phalanges indicate a hand uniquely adapted for arboreal locomotion but one that lacked the cortical robusticity of extant great apes. This suggests that either arboreal behaviors may not have constituted a large a part of the A. sediba locomotor repertoire, or that the mechanical loads of this behavior were resisted in a different manner than in great apes, or some combination of both.