The evolution of the hominin thumb and the influence exerted by the non-dominant hand during stone tool production

Modern humans possess a highly derived thumb that is substantially stronger and more robust than the fingers. Previous hypotheses concerning the evolution of such traits have focused upon the manipulation of hammerstones during stone tool production and of stone tools during their use. To date there has been no research on the manipulative pressures exerted by the non-dominant (core-holding) hand during stone tool production and its potential influence on the evolutionary history of the thumb. Here we provide the first investigation into the frequencies of digit recruitment and the relative manipulative forces experienced in the non-dominant hand during stone tool production. Eight experienced knappers produced flake cutting tools under four distinct conditions while pressure sensors, secured to the volar pads of the thumb, index and middle fingers of the non-dominant hand, recorded manipulative forces. Results indicate that relative to the fingers, the thumb was recruited significantly more frequently and experienced significantly greater manipulative forces during core repositioning events and the securing of the core during flake detachments. Our results support the hypothesis that the robust thumb anatomy observed in the hominin lineage was selected for, at least in part, as a result of more frequent and greater manipulative pressures acting upon the thumb relative to the fingers on the non-dominant hand during stone tool production.