Relation between muscle architecture and first metacarpal morphology, and its implications for human hand evolution.

Previous studies have proposed that our ability to produce and use stone tools was the primary selective pressure explaining the evolution of the human hand. Derived traits in humans include a robust first metacarpal and longer thumbs relative to the other fingers. Along with other anatomical peculiarities, humans can exert forceful precision and have powerful grips, and can resist loads during tool production and use. Despite this biomechanical explanation for the morphology of the human hand, limited work has been done on the soft tissue and, therefore, the relationship between the hand bones and the muscles most heavily relied upon during tool-related behaviours still requires thorough investigation. For this purpose, we have dissected 23 forearms and hands of fresh human cadavers of known sex and age at death, and dissected all the muscles attached at the first metacarpal (the first dorsal interosseous, opponens pollicis, and abductor pollicis longus muscles). Variations in physiological cross-sectional area, muscle mass, and fibre length were compared with metacarpal anatomy. In no case bone traits were a significant predictor of muscle features. In contrast, sex and age predicted muscle architecture in several cases, thus substantially affecting the functional analysis based on linear measurements of this bone. The data, therefore, failed to provide a deductive framework for predicting muscle recruitment based on measurements of bone from the fossil record.