There is some debate as to the source of the whack which pushed the jazz pendulum back towards a more rhythmic sound. Some say that cool jazz was too scientific, too complex, or even too boring for young musicians. Just as valid, though, seems the argument that the move was caused by a new sound in America: rock ‘n roll.
Rock ‘n roll – a combination of country & western honky-tonk sounds, rhythm & blues (known first as Jump blues), and gospel – had catchy lyrics and melodies, simple blues based chords, and, perhaps most importantly, a solid danceable beat, thanks to jump blues artists like Louis Jordan.
Cool jazz, in contrast, was most often lyricless – floating melodies over highly instrumented sustained complex chords. Intellectual it was. Intense it was. Richly sonorous it was. Danceable it wasn’t!
Whatever the force was, the pendulum back towards a solid consistent beat with simpler chord progressions and more predictable structure was the result.

The starting place for the bop renaissance, which was called the hard bop movement in jazz, was not the West Coast, where cool was king; nor New York City, where bop first was heard. It was in the industrial towns of Detroit and Chicago, and in East Coast cities like Philadelphia that the rejuvenators of bop developed their chops.
Sax players like Dexter Gordon, Sonny Rollins, and Cannonball Adderley; bass player Charlie Mingus; pianists Horace Silver, Joe Zawinul (later founder of Weather Report), Herbie Hancock, Chick Corea, and Keith Jarrett all rode the wave of bluesy hard bop in the late ’50s, and into the ’60s, and ’70s.

Two individuals stand out from the large group of fine musicians who played in this style. Drummer Art Blakey was the source point for the solid drumming patterns which formed the rhythmic foundation of the hard bop sound. Moving away from the erratic drumming of the first bop movement, Blakey set a tight groove for the rest of his band, the Jazz Messengers, to fit around. Blakey led the Messengers which, over the course of thirty years, included dozens of the finest bop soloists of the hard bop style. Just as solid as his beat, Art Blakey was rhythmic mentor for many who would evolve hard bop into the next waves of jazz.
The second player of importance was Miles Davis, who, more than any other player in jazz, redefined himself over and over again. With each redefinition, he succeeded in finding a role on the cutting edge of a new style. From his roots in big band to bop to cool, back to hard bop, and then eventually as a pioneer in the rock-jazz hybrid called “fusion,” Davis was the ultimate musical innovator.
In cool, Davis orchestrated jazz with a classical complexity. In hard bop, Davis once again demonstrated himself to be the master of restraint. With a focused gaze, Davis gave the impression that he was carefully editing the musical ideas in his head, using silence as much as sound to paint the aural picture. His lean trumpet solos were often played in the middle register of the instrument with no vibrato and through a metal Harmon mute. In the late 1960s, Davis would redefine his sound once more.
Art Blakey’s Jazz Messengers: “I Remember Clifford”, Belgium, 1958