Clearly before my time. I suspect that sometime in the late 1940 is when it happened for the first time. The Guardian investigates: Will the creator of modern music please stand up?
Instead of Ike Turner, writer of Rocket 88 in 1951, they should look at Big Joe Turner and Shake, Rattle and Roll.
Though, like any movement, music or otherwise, it's often impossible to pinpoint the exact date that a trend started or first began.