The Everly Brothers (Phil on the left, Don on the right) singing on
The Ed Sullivan Show in 1957.

Harmony singing was a part of rock and roll right from the beginning, but the
three- and four-part harmonies of doo-wop, derived from black gospel and
blues traditions, would never have given us Simon and Garfunkel, the Beatles
or the Byrds. To get those groups, you first had to have the Everly Brothers,
whose ringing, close-harmony style introduced a whole new sound into the
rock-and-roll vocabulary: the sound of Appalachia set to hard-driving
acoustic guitars and a subtle backbeat rhythm. One of the most important
and influential groups in the history of rock and roll, the Everly Brothers
burst onto the music scene in 1957 with their first big hit, "Bye Bye Love,"
which was quickly followed with their first #1 song, "Wake Up Little Susie,"
which topped the Billboard pop chart on this day in 1957.  (History.com)