Hard Bop was only an evolution of existing styles. If you listen to Ellington & the Armstrong all stars from the 50s there are certain hard bop elements in play. This seems to come from from the percussive elements from the bass, piano & drums.
Seems appropriate to post this interesting quote from an interview with saxophonist Dave Liebman attempting a historical perspective on the evolution of modern jazz:
“Dave Liebman: Placing Free Jazz And The Avant Garde In Musical And Historical Perspective
Dave Liebman: Let me have a shot at setting the history. This is just running around in my head and I'm not sure how accurate it will be. But basically, I want to talk about how a case could be made that more or less every five years in the 1950s and 1960s something happened to make the music change. For one thing, Charlie Parker's passing in 1955 signaled the end of the bebop era. It was codified by that time and a lot of guys could do it. Hard bop was raising its head and there was an accumulated repertoire that was being laid out which everybody could latch onto. Then, when you get to 1959 and 1960, two recordings strike me the most about that year: Coltrane's Giant Steps (Atlantic, 1960), in particular the title tune, and Miles Davis' Kind of Blue (Columbia, 1959), especially the tune "So What."
All About Jazz: What was it that struck you so much about "So What?"
DL: It was the official beginning of modal playing in jazz. For simplicity's sake, modal playing is using a scale rather than a chord cycle as the basis for improvising. It is first heard on the Milestones (Columbia, 1958) recording with Miles on the title track, but "So What" became more famous than "Milestones." What's really interesting is that John Coltrane, who of course always comes up when we talk about that era, was involved in both of those records: his own Giant Steps and Miles Davis' Kind of Blue within a month of each other. "Giant Steps" was the epitome of playing chord changes at a very fast speed, (in some ways akin to Bird playing "Cherokee"). The album Kind of Blue and particularly "So What," were by contrast to "Giant Steps" basically no chord changes. So Coltrane was playing the least and the most chord changes on two record dates within a month of each other!”
Full interview here:
www.allaboutjazz.com/dave-liebman-placing-free-jazz-and-the-avant-garde-in-musical-and-historical-perspective-dave-liebman?width=768