The Byrds, historically, are the inventors of folk rock, the genre that was trendy in the 60s, drawing inspiration a bit from electric Dylan and a bit from traditional American music.
A pleasant, carefree music, particularly in line with the pacifist ideas of American youth, who were gearing up for the hippie movement. In a nutshell, when people think of the Byrds, they think of "Mr. Tambourine Man", "Turn Turn Turn (To Everything There's A Season)", and the "Jingle Jangle" of Roger McGuinn's 12-string Rickenbacker.
It was only in 1966, when psychedelia began to gain significant influence in American rock, that Gene Clark left the group, and Chris Hillman on bass, Mike Clark on drums, McGuinn, and the legendary Dave Crosby on guitars decided to record something beyond three simple chords.
"Eight Miles High" is the beginning of the metamorphosis: beyond being a hit single, it's an ode, an ode to psychedelia that marks the maturity of the four both musically and intellectually.
Just a year before, McGuinn & company were topping the charts with some folk ballads and a couple of simple harmonies; with "5 D", all of that finally becomes history, and the Byrds manage to create something magical, surreal, something eight miles high!
"Fifth Dimension", the track that lends its name to the entire album, is the first chapter of this lysergic tour; followed by the traditional "Wild Mountain Thyme" and "John Riley", the spacey atmospheres of "Mr. Spaceman" (which relatively recall "Space Oddity" by David Bowie), the guitars of "I See You" and "Hey Joe", famous thanks to Jimi Hendrix's version.
"5 D" is the first major contribution from the Byrds to lysergic music. The album, as I've already mentioned, marks the evolution of the four's compositional genius, but the compositions are not yet entirely psychedelic: their music is still naive, in some respects uncertain.
The metamorphosis will be completed with the subsequent "So You Want To Be A Rock & Roll Star" from "Younger Than Yesterday" and "Artificial Energy" from "The Notorious Bird Brothers".