Twice times two: Dual mechanism for double rhythmic meter in orangutans and the evolution of human song.

Rhythmic pulse, the division of a beat into subordinate patterns, is the backbone of music. Across the world's musical traditions, the division of the primary beat into two equal parts - "double meter" - represents a prototypical pulse, also found in singing nonhuman primates. The last great ape common ancestor was, however, a non-singing species. How rhythmic pulse evolved in human song and music is, thus, enigmatic. Here, we analyze wild male orangutan long calls, which are structurally isochronous (i.e., with a steady of 1:1 rhythm). Males divided the primary rhythm into 1:2 and 2:1 subordinate patterns and did so by two distinct mechanisms: tempo changes as used by other primates and voiced in-exhale alternations as still used today by some human song traditions. Findings confirm double-meter in a non-singing great ape and suggest the two-phase cycle of the phonatory-respiratory system may have been leveraged for the evolution of human song and music.