Darwin proposed that music and dance may be part of courtship display leading to reproduction, and hence preservation of genes. Sexual selection could act on either or both music and dance, but we argue it may act most powerfully on their synergistic rhythmic co-performance. We suggest that motoric and temporal capacities evolved for, and adapted to, essential biological functions were exapted to working drills and to dance, and in combination with auditory capacities, to music. We propose that the recently observed correlation between bodily symmetry and perceived dancing quality in Jamaican dance is in part a reflection of the rhythmic abilities of the dancers: their capacity to produce and to synchronise with an isochronic pulse, and their simultaneous capacity to elaborate polyrhythms of movement in relation to that pulse. We elaborate empirical tests of these ideas. Since an exaptation can become a secondary adaptation, music/dance may have become an adaptation that favours reproductive success through sexual selection via display, as has been argued to apply to the studied Jamaican dancers and their community. The combination of dance with music may amplify such phenotypic success. © 2009 by ESCOM European Society for the Cognitive Sciences of Music.