Researchers speculate about how prehuman species developed the capacity for complex language. One theory suggests that humans' apelike ancestors first communicated through gestures. Once the neural circuits for gesture-based language had evolved, those same brain areas could have switched over to verbal communication. Indeed, research has shown that modern apes use the same area of the brain to interpret hand signals as humans use to process spoken language.

Working at the Yerkes National Primate Research Center in Atlanta, Frans B.M. de Waal and Amy S. Pollick observed communications among 34 captive chimpanzees and among 13 captive bonobos, also known as pygmy chimpanzees. The researchers logged every hand gesture, facial expression, and vocal cry that one animal directed at another. They also noted the social context—playing, grooming, fighting, having sex, eating, and so on—in which each signal occurred.