1970: Deejay u-roy hits Jamaican pop charts with 3 songs in the top ten, featuring a style known as "toasting" one of the foundation for what would soon be called mcing or rapping. (he didn't invent toasting... it goes back centuries, possibly). other individuals in the states that were also already doing this were Jocko Henderson and Cab Calloway, to name a couple.
1975: Around this time herc would get a brotha by the name of coke la rock to say crowd pleaser rhymes on the mic which would then lay the foundation for mc'ing or later known as rap.
1979: (Spring Records) first rap record ever recorded by a non rap group "King Tim Iii"(personality jock) by the fatback band.
1979: First rap hit record "rappers delight" by the sugarhill gang, even though they were not actually a group of rappers at all, and big bank hank used grandmaster caz's rhymes by asking to borrow some (hence the "cassanova" reference in the song)
1980: The punk group blondie released the record "rapture" which featured lead singer debbie harry doing what she considered rap. but by making mention of legit hip hopers as fab 5 freddie and grand master flash, the song is widely excepted in the hip hop community, and helps to fuse the punk and hip hop scene together.
1981: Funky 4 plus one more make the first national tv appearance on nbc's saturday night live.
1980s - current: With breakthrough lyricists continually coming up, from run dmc to rakim to public enemy and many others over the next two decades, mc's took center stage. this is partly due to the fact that they could create a product that would be easier for "any man" to understand (e.g. words, as opposed to understanding what a dj is doing with their hands). rap is close to being the #1 popular music in america, particularly amongst people under the age of 30, and mc's get most of the credit for hip hop (they are the icons, mainly).