One of the great articles I found in my internet perusal was a round-table in Spin in 1989 with a bunch of the hip hop old guard discussing the evolution of the form. I've picked out a few key quotations from the article and will illustrate below with representative clips. This quotation comes from Jazzy Jay, a DJ, who worked with Afrika Bambaataa, and record producer who help found the iconic Def Jam Records with Rick Rubin and Russell Simmons:
Today you got younger kids growing up with idols like L.L. Cool J, KRS-One or Big Daddy Kane, and they don’t say, “Let me go rehearse my stuff. Let me go buy me some equipment and try to better myself.” They say, “Let me sit down, write this five-minute rap, so I can make a record.” Whereas when we started out, you had idols like Kool Herc, Afrika Bambaataa, maybe Grandmaster Flash, DJ Breakout, Cold Crush Brothers, and you wanted to be more than just a rapper talking on a record … The kids would look at that and they would see the unity and strength in a group situation, whereas right now it’s like, “I could be a soloist. I’ll go all for myself and just call up RCA records.” Where rap wasn’t accepted before, now it is being accepted because big business is involved.
("Dropping Science," John Leland and Robin Reinhardt, Spin, August, 1989)
L.L. Cool J is from Long Island, KRS-One is from the Bronx, and Big Daddy Kane is from Bedford-Stuyvesant in Brooklyn.
L.L. Cool J, "Mama Said Knock You Out," 1990