Biomechanics of the Thumb and the Evolution of Human Dexterity

Abstract

Tool use is one of humanity’s defining characteristics, possibly originating as early as > 3 million years ago. Although heightened manual dexterity is considered to be intrinsically intertwined with tool use and manufacture and critical for human evolution, its role in the emergence of early culture remains unclear. Most previous research on this question exclusively relied on direct morphological comparisons between early hominin and modern human skeletal elements, assuming that the degree of a species’ dexterity depends on its similarity with the modern human form. Here we develop a new approach to investigate the biomechanical efficiency of thumb opposition, a fundamental component of manual dexterity, in several species of fossil hominins. Our work for the first time takes into account soft tissue as well as bone anatomy, integrating virtual modeling of the muscle opponens pollicis and its interaction with three-dimensional bone shape form. Results indicate that efficient thumb opposition appeared approximately 2 million years ago, likely associated with our own genus Homo, and did not characterize Australopithecus, the earliest proposed stone tool-maker. This was true also of the late Australopithecus species, A. sediba, previously found to exhibit human-like thumb morphology. In contrast, later Homo species, including the small-brained H. naledi, show high levels of thumb opposition dexterity, highlighting the increasing importance of cultural processes and manual dexterity in later human evolution.