Frequency and timing of scaphoid-centrale fusion in hominoids

Fusion between the os centrale and the scaphoid has played a central role in many functional and phylogenetic interpretations of hominoid evolution. In particular, scaphoid-centrale fusion shared among African apes and humans has been interpreted as an adaptation in knuckle-walkers, an exaptation in hominins, and has been offered as evidence for a knuckle-walking origin of bipedalism. However, discrepancies in the literature concerning the taxa in which this scaphoid-centrale fusion occurs, as well as the timing and/or frequency of this fusion, have confounded the significance of this trait. This study provides an historical review of the literature on scaphoid-centrale fusion in primates and the first formal investigation into the timing and frequency of this character among primates, with a focus on extant hominoids. Results indicate that there is a significant difference in the timing and frequency of scaphoid-centrale fusion in African apes and humans compared to Asian apes, suggesting that prenatal or early postnatal fusion among hominines is a synapomorphy. Scaphoid-centrale fusion does not occur randomly within primates. Instead, only Homininae and some members of Lemuroidea show consistent and ontogenetically early fusion of these carpals. The consistent occurrence of this trait within only two primate clades and a clear heterochronic trend in timing and frequency of scaphoid-centrale fusion among hominines suggest that this character is primarily phylogenetically controlled. We could not falsify the hypothesis that scaphoid-centrale fusion in African apes is indeed related to midcarpal stability in knuckle-walking, but neither were we able to find direct biomechanical or kinematic evidence to support this hypothesis. A more definitive answer to the question of the functional significance of scaphoid-centrale fusion will have to await more detailed analyses of great ape wrist kinematics.