The evolution of speech remains an elusive scientific problem. A widespread notion is that vocal learning, underlined by vocal-fold control, is a key prerequisite for speech evolution. Although present in birds and non-primate mammals, vocal learning is ostensibly absent in non-human primates. Here we argue that the main road to speech evolution has been through controlling the supralaryngeal vocal tract, for which we find evidence for evolutionary continuity within the great apes.

Animal communication has long been thought to be subject to pressures and constraints associated with social relationships. However, our understanding of how the nature and quality of social relationships relates to the use and evolution of communication is limited by a lack of directly comparable methods across multiple levels of analysis. Here, we analysed observational data from 111 wild groups belonging to 26 non-human primate species, to test how vocal communication relates to dominance style (the strictness with which a dominance hierarchy is enforced, ranging from 'despotic' to 'tolerant'). At the individual-level, we found that dominant individuals who were more tolerant vocalized at a higher rate than their despotic counterparts. This indicates that tolerance within a relationship may place pressure on the dominant partner to communicate more during social interactions. At the species-level, however, despotic species exhibited a larger repertoire of hierarchy-related vocalizations than their tolerant counterparts. Findings suggest primate signals are used and evolve in tandem with the nature of interactions that characterize individuals' social relationships.