The human extensor digitorum profundus muscle with comments on the evolution of the primate hand

Forelimb dissections on 14 genera of anthropoids including humans and 17 cases of human aneuploids has revealed a high incidence of “atavistic” musculature (Barash et al., 1970;Aziz, 1981a) in the aneuploids. The phenotypic specificity of this aneuploid musculature clearly manifests developmental retardation and instability (Shapiro, 1983) revealing not only the likely course of embryonic myogenesis in chromosomally normal humans (Cihak, 1972, 1977) but also information relevant to ontogenetic and evolutionary changes. The extensor digitorum profundus proprius complex is particularly illustrative of these characteristics of aneuploid musculature. Our examination of the variation of this muscle complex in human aneuploids and between primate genera reveals how normal ontogeny may proceed, as well as the morphological basis for the evolutionary changes in hand structure and function amongst Primates. We also consider the phylogenetic and functional significance of changes in the extensor digitorum profundus proprius with reference to the divergent locomotory and manipulative capabilities and behavior of Primates.