“Rap” frequently is considered as having its roots in a rhyming practice of the antebellum South called ‘The Dozens’. In The Dozens what happens is two people face off and make rhymes at each other as some sort of challenge. Rap contests of the latter day frequently play on the model of The Dozens with rappers hurling rhyming insults at each other.
http://en.wikipedia.org/wiki/The_Dozens
George Carlin joked about it in his album “Occupation Fool”