Reconstructing the evolution of laughter in great apes and humans

Background. Human emotional expressions, such as laughter, are argued to have their origins in the displays of ancestral primate species. In order to test this hypothesis of phylogenetic continuity most directly, the current work examined the acoustics of tickle-induced vocalizations from infants and juveniles of all four great ape species, as well as tickle-induced laughter produced by human infants, coded the obtained acoustic data as character states, and submitted them to phylogenetic analyses. Results. Acoustic outcomes revealed both important similarities and differences among the five species, and phylogenetic trees reconstructed from those data matched the well-known genetic relationships among the species. Conclusions. Taken together, the results provide strong evidence that tickling induced laughter is homologous in great apes and humans, and supports the more postulation of phylogenetic continuity from nonhuman displays to human emotional expressions. Findings also indicate that distinctively human characteristics in laughter such as predominantly regular, stable voicing and consistently egressive airflow are nonetheless shared with our closest biological relatives, the great apes.