An exploratory assessment of the fourth and fifth carpometacarpal joints of humans, non-human great apes, and fossil hominins using three-dimensional geometric morphometric analysis

The range of motion afforded to the fourth and fifth metacarpals at their respective carpometacarpal joints are fundamental for the effective execution and utilization of uniquely-human grips and hand postures which are considered to have played a pivotal role in the deliberate manufacture and use of stone tools. However, despite their importance, functional morphological variation between humans, non-human primates, and hominin taxa contemporaneous with the earliest lithic tools are under-explored.

The aim of this project was to compare, using landmark-based three-dimensional geometric morphometric analysis, the morphology of the fourth and fifth carpometacarpal joints of modern humans and non-human great apes in order to identify functionally important morphologies that facilitate enhanced movements of these joints in modern humans. The hamate-metacarpal joint surfaces of available extinct hominin taxa were then compared to extant genera in order to identify human-like structures in the hominin fossil record, and to assess human-like movements of the hamate-metacarpal joints in selected hominin taxa.

This study identified several features of the fourth and fifth carpometacarpal joints of humans that are conducive to increased freedom of movement and load transmission compared with non-human taxa. Several of these features can be identified to varying extents in fossil hominin specimens contemporaneous with early Palaeolithic sites. However, the full suite of derived human features does not emerge until relatively late in the palaeontological record. The results and interpretations of this research indicate that the hamate-metacarpal joints of hominin taxa contemporaneous with the earliest-known lithic technology were not as mobile as those of modern humans and suggest that the fourth and fifth digits were utilized differently. Furthermore, hominin species known to make and use lithic tools display non-modern morphologies, suggesting that the full suite of human-like features of these joints developed relatively late in our evolutionary history.