A Geometric Morphometric Examination of Hominoid Third Metacarpal Shape and Its Implications for Inferring the Precursor to Terrestrial Bipedalism

The third metacarpal has been a focus of study when examining questions surrounding early hominin locomotion since this bone is adapted to the diverse range of positional behaviors performed by extant hominoids. The shape of this bone is potentially under strong selective pressure related to the biomechanical demands of terrestrial knuckle-walking, arboreal clambering, and brachiation performed by extant hominoids since the hand directly interacts with the substrate during the performance of these movements. The objective of the present study was to explore shape variation of the third metacarpal and examine how different parts of the bone discriminated between hominoid genera that perform these different locomotor behaviors. In addition to examining general interspecies variation, shape analysis was applied to testing the knuckle-walking hypothesis for human evolution. Fourteen 3D landmark coordinates were collected on hominoid third metacarpals, and principal component analysis and Procrustes distances were used to examine metacarpal shape. Comparable measurements were collected on fossilized third metacarpals of Australopithecus afarensis as an early hominin test case for examining the knuckle-walking hypothesis. Analyses that included landmarks collected on both ends of the bone distinguished humans from great apes and presented a strong functional signal related to suspensory locomotion among nonhuman hominoids, whereas the distal articular surface provided the strongest knuckle-walking signal. The shapes of Australopithecus afarensis metacarpals examined in the current study did not provide evidence for a trajectory of shape change in early hominin evolution that started from a metacarpal adapted for terrestrial knuckle-walking. Anat Rec, 302:983-998, 2019. © 2018 Wiley Periodicals, Inc.