From wikipedia

"In his series How Music Works, Howard Goodall presents theories that rhythm recalls how we walk and the heartbeat we heard in the womb. However neither would seem to have any survival value in Man's evolution. More likely is that a simple pulse or di-dah beat recalls the footsteps of another person. Our sympathetic urge to dance is designed to boost our energy levels in order to cope with someone, or some animal chasing us -- a fight or flight response. It is possibly also rooted in courtship ritual."

cited http://www.epjournal.net/filestore/ep03375380.pdf