A full account of human speech evolution must consider its multisensory, rhythmic, and cooperative characteristics. Humans, apes and monkeys recognize the correspondence between vocalizations and the associated facial postures and gain behavioral benefits from them. Some monkey vocalizations even have a speech-like acoustic rhythmicity, yet they lack the concomitant rhythmic facial motion that speech exhibits. We review data showing that facial expressions like lip-smacking may be an ancestral expression that was later linked to vocal output in order to produce rhythmic audiovisual speech. Finally, we argue that human vocal cooperation (turn-taking) may have arisen through a combination of volubility and prosociality, and provide comparative evidence from one species to support this hypothesis.
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