Humans exhibit what appears to be a unique vocal property: octave equivalence whereby adult male voices are, on average, an octave lower in pitch than those of adult females and children. The evolutionary significance of this seems largely to have escaped notice. While sexual selection might explain why male voices are generally lower, it cannot explain why they should be so much lower than what would be expected for body size, nor why the average difference should be exactly one octave. Nor does a generalised dimorphism convey why precisely tuned octaves feature so commonly in human vocal interaction. The octave features strongly in the organisation of music. A consequence of this characteristic of human pitch perception and production is the capacity to share and respond to vocal pitches (and their instrumental equivalents) as if they are ‘the same’ irrespective of the difference in range, a phenomenon known as octave equivalence. We investigate the nature of octave equivalence from an adaptive perspective and propose a hypothesis for its evolution based on the importance of chorusing for social bonding and pitch-matching in inter-generational exchange.