The Homology of the Human Pisiform Revealed by Comparative Ossification Timing in Hominoids

Typical mammalian pisiforms, including non-human primates, are elongated and develop from two ossification centers with an associated growth plate. The human pisiform is unusually short and forms from a single ossification center, raising the question of whether it is homologous to the primary or secondary center of other primates. Orangutans also possess a relatively short pisiform compared to other primates; however, it develop from two ossification centers. The pisiform and calcaneus are paralogous structures in the wrist and ankle, and human calcanei retain two ossification centers with a growth plate. We use a comparative analysis of pisiform and calcaneus ossification timing with respect to dental eruption in juvenile Old World monkeys, apes, and humans (n=922) to better understand the developmental processes underlying the identity of the unique human pisiform. Analyses indicate that human pisiforms develop at approximately the same dental stage as the pisiform epiphyses of other primates, and also developed at the same time as the human calcaneal epiphysis. This evidence supports homology between the sole ossification center of the human pisiform and the pisiform epiphysis of other species, indicating that humans have experienced a major change in skeletal patterning of the wrist through the loss of a primary ossification center and the associated growth plate. Furthermore, since the reduced orangutan pisiform retains two ossification centers and presumably a growth plate, pisiform reduction in these two species are the result of different developmental mechanisms.