It is always perilous to try and suss out the origins of Rock 'N Roll. If you say some aspect was first - first electric guitar or first back-beat - or pivotally influential, someone is sure to immediately find an earlier example, or lengthy interviews with early rockers who all point to a particular artist as an influence. The discussions can deteriorate into vagueness, or one-upmanship, or both. "Well, what about jump music?" "Well, what about Dixieland/jive/boogie/jug band?"
Andrew Hickey is reportedly writing a five-volume history of R&R and I wish him well. It must be a labor of love, because he is going to have to spend the entire time talking with posers who are trying to prove him wrong and showing off the stray facts that they know, claiming them as authoritative proofs. Not me. I know my stray facts are just that and others know much more.
However, being the Assistant Village Idiot, I do have some observations that I think helpful so long as you don't regard them as full explanations of...well, of anything.
Rock descends from many ancestors, including some surprises. I once looked with some disdain at a person who told me that polka music influenced rock-n-roll. Yet he made a clear case quickly. Did I think that Western Swing influenced rock? Well, yes, there's that whole Bill Haley/Buddy Holly strain, sure. Did I hear how polka influenced Western Swing? Okayyyy...sure...but was the influence the part that went on into rock? Yes, he asserted. It's the back-beat, hitting the 2nd and 4th beats of the measure. Very polka. I also thought but didn't want to say aloud because I thought this was already out of hand, also very oompah band. I read once that it was the sound of trains on tracks, the sound of freedom and adventure, that resonated with so many people. Could be. But with a train, how the hell do you know where the beginning of the measure is?
The back beat was in many styles growing up in the early to mid 20th C. The lines between genres were porous, and musicians would play a wide variety of songs. A guy's gotta make a living, and it was the 60s kids who got into being purist and authentic about whatever style they favored. What we perceive in our day as a very different style might only be a difference in instrumentation. The song is very much the same, but this one has a fiddle and that one has horns and that one has an accordion. Rock music doesn't have accordions, and yet...sometimes you can make the switch in your head and hear the lack of difference. Drum/no drum. Electric bass/upright. Steel guitar/accordion. The feel is different, but...
So there are rabbit holes everywhere. I reject few or none, but do think it is worth having perspective. Bob Wills might say what he was playing in 1957 wasn't any different than what he was playing in 1937, and there are some "truth elements" as we say about that, he was mostly just being a prick and it's not true.
The lead-in influences were themselves hybrids, more often based on marketing than on similarity. Country music and Western music had some mutual influence, but it was mostly a product of record labels and the effect of the early 40s ASCAP/BMI conflict. They were different. Country (and folk) did not influence rock all that much. Western Swing a whole lot. Rhythm & Blues became a style of mutual influence, but it got its name because people were no longer comfortable calling it "race music." Record labeling and concert venues again. "Rhythm" was a catch-all for all those jives and jumps and hokum and "blues" was - it's own history, maybe as complicated as rock's. Not the same, but the same people produced their records, because those were the folks who would touch that.
So R&B&Western Swing all together here each of those based on other styles in their turn, performed about four years earlier than this (1946) at minimum. But Rock 'N Roll supposedly didn't come along until 1952, 1954. Sometimes it's not just the musical style solidifying, it is our understanding of the style that lags behind the innovation.
Don't believe me? After you pick up the beat and the tune, start humming "When the clock strikes two, three, and four, If the band slows down we'll yell for more."