Interrogating Carpus Functional Morphology in Haplorhine Primates: Implications for the Evolution of Hominin Locomotion and Dexterity

Hominin forelimbs have evolved from primarily locomotive to manipulative appendages over ~6 million years. As such, hand functions in fossil hominins and the Pan-Homo last common ancestor (LCA) are intensely debated, with carpal morphology central to this debate. However, owing to their irregular and challenging shapes, only few studies have comprehensively quantified carpal morphology. Here, I characterize the overall carpal morphology of a broad sample of anthropoid primates and several fossil hominin species (Australopithecus afarensis, Au. africanus, Au. sediba, H. floresiensis, H. naledi, H. neanderthalensis) using a spherical harmonics (SPHARM) analysis of shape quantification. With these data, I employ various quantitative approaches — including principal components analysis, supervised machine learning approaches, hierarchical clustering analysis, and phylogenetic comparative methods — to investigate the evolution of forelimb locomotion and manual dexterity in anthropoid primates, with a particular focus on the evolution of knuckle walking in African apes. Results across all analyses show that carpal specializations in African apes are shared with taxa that do not knuckle walk, suggesting that knuckle walking does not require many morphological changes to a generalized hominoid carpus (Chapter 1). A small number of traits in the scaphoid, lunate, trapezium, and capitate were found to be shared between hominins and African apes, likely associated with fusion of the scaphoid and centrale that may have evolved to dissipate compressive loads on the hand or to limit wrist extension (Chapters 1 and 2). Although further exploration of alternative functional explanations is warranted, knuckle walking remains the most robust hypothesis for these traits and functional demands. Notably, results show that later Homo (i.e., H. neanderthalensis, H. sapiens) possess distinct scaphoid and capitate morphology likely associated with sophisticated tool manufacture/use and that the scaphoid and capitate morphology of African apes are intermediate between humans and all other sampled taxa (Chapter 2). The carpals of fossil hominins show a general temporal trend from African ape-like to human-like, with a piecemeal loss of ape-like traits and recruitment of human-like traits causing some Australopithecus capitates to resemble those of palmigrade monkeys (Chapter 2). Although hominin carpals typically equally resemble those of chimpanzees and gorillas, the triquetrum and hamate of hominins more closely resemble those of gorillas (Chapters 2 and 3). This trend appears driven by the overall proportions of these bones, potentially corroborating existing research on the digits that indicates that gorillas and hominins possess more ancestral hominine manual proportions than those of chimpanzees. Paired with other evidence from the hamate and the hominoid fossil record, these results suggest that suspensory behavior may have evolved repeatedly in hominoids (Chapter 3). The research outlined here places the functional morphology of hominin carpals within a broader phylogenetic context and thus provides new insights concerning the evolution of several anthropoid manual behaviors, including below-branch suspension, knuckle walking, and stone tool manufacture/use.