Biomechanics of the human thumb and the evolution of dexterity

Systematic tool production and use is one of humanity’s defining
characteristics, possibly originating as early as >3 million years
ago.​1–3​ 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 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 musculus opponens
pollicis and its interaction with three-dimensional bone shape form.
Results indicate that a fundamental aspect of efficient thumb opposition
appeared approximately 2 million years ago, possibly 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, Australopithecus sediba, previously found to exhibit human-like
thumb proportions. 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.