Rap grew from exactly the place you would expect, The Bronx. Originally rap was about expressing yourself verbally, talking about parties and good times over a beat, hence the first rap record to chart in August 1979 by the Sugarhill Gang. But it wasn't supposed to be the first rap record.
Sylvia, the lady who owned Sugarhill records saw the rap scene growing behind the scenes and herded a few producers into the studio, gave them some lyric sheets to learn and made them sound like they'd been buddies since school on 'Rappers Delight'.
The intended first rap record was being prepared by thrash metal guitarist Russell Simmons with Kurtis Blow on vocals in time for December 1979, Christmas Rapping. But Sylvia got there first, that's business! Incidentally Russell Simmonds went on to meet entrepreneur Rick Rubin, together they formed Def Jam records that became the most successful rap label, the rest is history.
Rap and Hip Hop have never been mainstream pop music but they're featured in lots of song breaks, influenced sampling (James Brown being the most sampled artist) and still appear regularly today in many songs.
'Toasting' is another of the Rap scenes early building blocks, my knowledge of this was broadened by the autobiography of Neville Staple (Origional Rude Boy) from The Specials, himself a 'toaster'. I forget now but I'll do some research and explain this method in detail, if you're a Toaster then mail us and tell me all about it!