Abstract
Heterochrony, or the evolution of ontogeny, has been well studied in embryology and skeletal development, providing insight into morphological and genetic mechanisms of evolution.1-5 However, heterochronic studies of behavior and cognition lag behind in comparison. In a recent study we investigated the ontogeny of social behavior and cognition in humans' closest living relatives, chimpanzees (Pan troglodytes) and bonobos (Pan paniscus). These two species are estimated to have had a chimpanzeelike common ancestor between 0.86 and 1.8 mya.6,7 Bonobos have been argued to exhibit morphological indications of paedomorphism relative to chimpanzees, especially in the cranium, and to exhibit paedomorphic behavior as adults.6-11 We found that bonobos exhibit developmental delays relative to chimpanzees in several aspects of their social behavior and cognition. Here, we describe how placing these results in the framework of heterochrony contributes to understanding behavioral and cognitive differences between adults of these two species and to our knowledge of hominid evolution in general.