ERIN MARIE SHEPARD WILLIAMS, then a student at George Washington University, Washington, DC, was awarded funding in April 2009, to aid research on 'Influences of Material Properties and Biomechanics on Stone Tool Production,' supervised by Dr. Alison S. Brooks. Later Homo possesses a derived thumb that is robust and long relative to the other digits, with enhanced musculature compared to extant apes and early hominins. Researchers have hypothesized that this anatomy was selected in part to withstand high forces acting on the thumb during stone tool production. Previous studies indirectly support this hypothesis; however, direct data on loads experienced during stone tool production and their distribution across the hand are lacking. Using a dynamic pressure sensor system and 3-D motion capture technology, manual forces and pressures were collected from six experienced knappers replicating Oldowan tools. Knappers used hammerstones requiring a 3-jaw chuck grip. Peak and strike forces and pressures and impulse and pressure-time integrals were consistently significantly greater on the 2nd and/or 3rd digits compared to the 1st across all subjects. Kinematics data revealed that this distribution pattern was not consistently present during up-swing, however it was established during the down-swing pre-strike phase and continued through swing termination. These results do not support the hypothesis that loads experienced during stone tool production are significantly higher on the thumb compared to the other digit, calling into question hypotheses linking modern human thumb anatomy specifically to stone tool production load resistance.
Williams, Erin Marie, Adam D. Gordon, and Brian G. Richmond. 2012. Hand Pressure Distribution during Oldowan Stone Tool Production. Journal of Human Evolution 62(4):520-532.