One cannot really compare music of the 40s and the 50's but to listen to Roy Brown (he wrote it) and then Elvis singing the same song is a revelation. Similarly I consider Reconsider Baby to be the finest track Elvis made for RCA and if you listen to Lowell Fulson's original (he wrote it) you realise that Elvis could transform an OK song into something sublime.
The roots of modern music are indeed interesting and also intertwined, especially when you expand beyond 'rock 'n' roll' into other genres e.g the late Curtis Mayfield's influence on reggae music