Rap born in the communities with bad economic and social conditions. The first 'rappers' were forced to cope with several problems. The bad socio-economic conditions of the most African American people in the 80s were a trouble for all the musicians and artist. You couldn't buy the necesary thing to play music, a bass, a trompet, or even a guitar were to expensive for the most of the people. The only real 'free' instrument that they had was the voice. They, with time, started to made some improvisation with the only instrument that they had availabe. The first African American 'rappers' also started to create their own ways and instruments to create their own music.
I really belive that rap is a response of the black power to solve and cope with the diferent kind of problems. It is an example of pass over the problems and perseverance.