There are two questions that may be answered through the study of chimpanzee cognitive development: First, ‘what are the common cognitive traits shared by humans and chimpanzees?’ and second, ‘what traits are uniquely human?’ Comparative studies have demonstrated similarities between humans and chimpanzees, and also illuminated the fundamental differences. Nonhuman primates climb trees with all four limbs and are well adapted to arboreal life. Nonhuman primate infants cling actively to their mothers; in return, they receive frequent embraces. Mother–infant relationship in these primates, especially simians, is clearly characterized by this clinging–embracing. However, human mothers and their infants are often physically separated from each other. Human infants cannot cling to their mothers. The stability of infants in the supine posture (lying on one’s back on the ground or other surface) is an important but neglected issue related to human posture. Chimpanzee and other nonhuman primate infants are not stable when they are laid on their backs. Human infants are exceptional among primates in this respect; they can assume a stable supine posture and will lie quietly on their back. The stable supine posture gives a firm foundation for face-to-face communication, vocal exchange, and also tool use. This article provides the evolutionary scenario of sociocognitive development that is unique to humans.