The speech-like properties of nonhuman primate vocalizations

The origins of speech, the most complex form of animal communication, remain a puzzle. Human speech and nonhuman primate vocalizations have traditionally been viewed dichotomously, with several aspects of speech having no clear analogues in the calls of our primate relatives. The putative unique aspects of speech include a diverse array of learned sounds that are rapidly produced in rhythmic strings and continuously recombined in new sequences. However, recent research challenges the idea that these features are indeed unique to humans and suggests more continuity between nonhuman and human primates than was previously appreciated. Here we review recent findings in four areas of this emerging continuity. In light of these studies, we argue that the evolution of human speech abilities most likely originated in a primate ancestor capable of (1) producing a 'speech-ready' range of vowel-like sounds, (2) vocalizing with simultaneous rhythmic mouth movements, (3) combining long strings of varied and structured sounds and (4) exercising some volitional control over calls that were modified based on experience. Taken together, these results suggest that the considerable latent vocal ability that we observe in nonhuman primates is consistent with the hypothesis that a key step towards human speech was the evolution of greater cognitive control of the vocal apparatus (and not the evolution of speech-specific anatomical adaptations). By shifting research emphasis away from the mechanics of how speech is produced to the conditions that favoured more diverse, open-ended and imitative vocal systems, we hope to encourage new avenues for future comparative research.