From vocal homophily to vocal repertoire flexibility: Unravelling the socioecological drivers of language evolution

Sabine Stoll, Carel van Schaik, Judith M Burkart, Konatsu Ono, Nikhil Phaniraj, Zitan Song, Balthasar Bickel

Since diverging from the last common ancestor with bonobos and chimpanzees, the communication system of the hominin lineage underwent a radical transformation. Vocal production learning - the ability to produce novel vocalizations based on experience - is the necessary pre-condition for changing an ape-like communication system into one that is infinitely flexible, extensively learned, and culturally transmitted. However, the evolutionary conditions favoring this shift remain unclear. Drawing on archeological evidence of intensified cooperation for social defense, hunting, and shared child-care among savanna-dwelling hominins, we suggest that the move to open savanna environments around two million years ago created pressures toward communicative capacities beyond the ancestral vocal repertoire. Specifically, we propose that vocal production learning arose from the increased use of preexisting vocal accommodation, where individuals align vocal features as a sign of homophily. We therefore predicted that the function of vocal homophily more closely aligns with the functions of human language than with those of vocal production learning in other species. A systematic comparison of the functions of vocal production learning and vocal accommodation across birds and mammals confirmed this prediction. It also revealed that vocal homophily often cooccurs with key features of human language, such as babbling, tutoring, turn-taking, and high vocal activity, especially in species with shared offspring care. We conclude that increased vocal homophily facilitated the evolution of vocal production learning in the hominin lineage and ultimately language.