Evolution and development of the primate limb skeleton

The order Primates is composed of many closely related lineages, each having a relatively well established phylogeny supported by both the fossil record and molecular data.1 Primate evolution is characterized by a series of adaptive radiations beginning early in the Cenozoic era. Studies of these radiations have uncovered two major trends. One is that substantial amounts of morphological diversity have been produced over short periods of evolutionary time.2 The other is that consistent and repeated patterns (variational tendencies3) are detected. Taxa within clades, such as the strepsirrhines of Madagascar and the platyrrhines of the Neotropics, have diversified in body size, substrate preference, and diet.2, 4–6 The diversification of adaptive strategies within such clades is accompanied by repeated patterns of change in cheiridial proportions7, 8 (Fig. 1) and tooth‐cusp morphology.9 There are obvious adaptive, natural‐selection based explanations for these patterns. The hands and feet are in direct contact with a substrate, so their form would be expected to reflect substrate preference, whereas tooth shape is related directly to the functional demands of masticating foods having different mechanical properties. What remains unclear, however, is the role of developmental and genetic processes that underlie the evolutionary diversity of the primate body plan. Are variational tendencies a signature of constraints in developmental pathways? What is the genetic basis for similar morphological transformations among closely related species? These are a sampling of the types of questions we believe can be addressed by future research integrating evidence from paleontology, comparative morphology, and developmental genetics.