Evolution of speech rhythm: a cross-species perspective

Cognition and communication, at the core of human speech rhythm, do not leave a fossil record. However, if the purpose is to understand the origin and evolution of speech rhythm, alternative methods are available. A powerful tool is comparative approach: studying the presence or absence of cognitive/behavioral traits in other species, drawing conclusions on which traits are shared between species, and which are recent human inventions. Here we apply this approach to traits related to human speech rhythm. Many species exhibit temporal structure in their vocalizations but little is known about the range of rhythmic structures perceived and produced, their biological and developmental bases, and communicative functions. We review the literatures on human and non-human studies of rhythm in speech and animal vocalizations to survey similarities and differences. We report important links between vocal perception and motor coordination, and the differentiation of rhythm based on hierarchical temporal structure. We extend this review to quantitative techniques useful for computing rhythmic structure in acoustic sequences and hence facilitating cross-species research. While still far from a full comparative cross-species perspective of speech rhythm, we are closer to fitting missing pieces of the puzzle.