Given the relatively short existence of rap music, tracing its lineage appears fairly simple. The advances in the genre are well-documented: Kool Herc started turntablism, Kurtis Blow first brought rap to the mainstream, Prince Paul popularized skits, Afrika Bambaataa rocked the sample, and so forth and so on. Then there is Dr Dre's classic 1992 album The Chronic, which represents perhaps the most perplexing turn in hip hop history. The album essentially brought West Coast......