Abstract:
The human hand, a tool of utmost importance in our everyday activities, is markedly distinct from other extant and fossil primates. The evolution of hand bone morphology, how it was used in manipulative and locomotor activities, and the emergence of human-like hand use are therefore of great interest to Paleoanthropologists. This dissertation aims to contribute to our growing knowledge about isolated and associated hand bones of hominids and hominins. It contains three case studies applying virtual methods to analyze the morphology of hand bones with a particular focus on functional morphology. The first case study focuses on an intermediate phalanx associated with the Miocene ape Ouranopithecus macedoniensis. The specimen (RPl-87) was previously identified as a hand bone based on a comparative analysis of linear measurements. To reassess this anatomical allocation, a morphological investigation of the phalanx was conducted using three-dimensional geometric morphometrics, thereby allowing for a more detailed comparison of shape differences. The results contradict the previous assessment and instead suggest that RPl-87 was likely a foot bone from the second or fifth ray. The comparison with phalanges of extant primates also highlights morphological similarities with the terrestrial primates Gorilla and Papio. This study, therefore, tentatively supports a primarily terrestrial locomotion for O. macedoniensis, agreeing with evidence from other disciplines. The second and third studies aim to reconstruct habitual manual activity in early hominins. Study II combines the multivariate analysis of muscle attachment site (entheseal) patterns of the first metacarpal with a multivariate analysis of overall bone shape, thereby addressing habitual activity and mechanical efficiency within the same framework. We investigated a comparative sample of later Homo, including Homo sapiens and Homo neanderthalensis, and different recent great ape species. The early fossil hominin sample includes Homo naledi, Australopithecus sediba, Australopithecus afarensis, Australopithecus africanus, and SK84, an early hominin from Swartkrans with disputed species attribution. According to the results, later Homo and great apes differ clearly in their attachment site proportions of the first dorsal interosseus, a muscle heavily involved in tool-related behaviors. All early hominins, except for A. africanus, present entheseal patterns characterized by a relatively large attachment site of this muscle, supporting human-like hand use and, potentially, tool use in these species. Importantly, A. sediba and A. afarensis combine human-like entheseal patterns with an ape-like morphology of the first metacarpal. This study highlights that, despite their limited manual dexterity (as shown by metacarpal shape), some hominins as early as A. afarensis habitually used their thumb similarly to humans and potentially engaged in tool-related behaviors. Consequently, our results suggest that biomechanical adaptations for increased manual dexterity on the hand skeleton occurred long after the emergence of human- like hand use. The third study builds upon these results and expands the analysis of entheseal patterns to more hand muscles, focusing on the Australopithecus sample. The analysis shows that later Homo and the extant great apes differ in entheseal patterns. Like in Study II, the later Homo group shows a proportionally large attachment site of the first dorsal interosseus. Additionally, individuals of this group are characterized by large attachment sites on the fifth finger. The muscles in this pattern are involved in behaviors that play important roles in everyday human-like hand use, such as power grasping, in-hand stabilization and manipulation of objects. Among the australopiths, A. sediba and A. afarensis show similar, although not identical, attachment site proportions to later Homo. Our results thereby support a human-like hand use in these species that heavily relies on the muscles on the margins of the hand and, notably, the fifth finger. The fifth ray of A. afarensis, in particular, had previously been assessed as too inefficient and limited in movement to allow for the production of Lomekwian stone tools. These contradicting results once again highlight the need to differentiate between bone morphology that is behaviorally plastic and morphology that represents biomechanical adaptations. In contrast to the other australopiths, the entheseal pattern of A. africanus is unlike both that of later Homo and the extant great apes, suggesting that it engaged in a unique range of manual activities. In summary, this study produces evidence for human-like hand use in some Australopithecus specimens and emphasizes the value of activity reconstruction for human-evolutionary studies. Altogether, this dissertation uses virtual methods to investigate hand (and foot) bone morphology to address manual and locomotor behaviors in the hominid and hominin fossil records. It also adapts these methods to allow a meaningful examination of isolated bones frequently occurring in the fossil record.
geometric morphometrics