Comparative metacarpal volume and relative thumb muscle mass in humans and non‐human primates (LB35)

Abstract

Fine precision (pad‐to‐pad) grasping is facilitated by having a relative long thumb and short fingers. Power grasping is aided by having large thumb muscle forces generated by large thumb muscles (e.g., adductor pollicis). Reconstructing precision grasping in fossils is easily done by examining bone lengths. Reconstructing muscle force is more difficult because soft tissue seldom preserves in fossils. The goal of this study was to find a metric that can estimate thumb muscle mass from skeletal elements. Accordingly, we examined the relationship between thumb muscle mass and metacarpal 1 (Mc1) volume in a sample (>220) of human and non‐human primates (>20 species). Relative thumb muscle masses (all thumb muscles, thenar muscles only, adductor pollicis only) were calculated as a percentage of total hand muscles mass. Relative Mc1 volume was calculated as a percentage of Mc2‐5 volume. Metacarpal volumes were obtain from 3D surfaces models derived from CT scanning. Regression analyses demonstrated a significant relationship (p<0.01) between relative Mc1 volume and each measure of relative thumb muscle mass. Relative muscle volumes calculated for the fossil hominin Ardipithecus ramidus are lower than modern humans, further supporting the claims that its hands were more ape‐like in function.Grant Funding Source: Supported by the National Science Foundation (BCS 1317047) and USC URAP