A guest post adapted from an email by songwriter John Woods of Shine: A Burlesque Musical and The Wet Spots. Noting Led Zeppelin as the first folks who essentially covered without attribution is an interesting distinction. In popular folk & blues music – going back centuries – 'remixing' has been going on in the form of putting new lyrics to familiar chords and melodies, lyrics that are often funny and more immediately relevant than the ones they're replacing. (In that sense, Weird Al is the most traditional songwriter of the late 20th century.) Inevitably, slight changes will occur to the melodies and the chords after years of lyrical mutations and the songs would change.
This starts to get complicated when we bring copyright and royalty into the picture. These laws were designed to work with a Tin Pan Alley professional songwriting culture that was producing neo-classical/jazz hybrid pieces for the hit parade and musical theater. While these folks might lift a piece of melody here and there, it was fairly easy to establish if a piece was overall harmonically and melodically original, and afford it legal copyright.
Rock music came out of folk and blues traditions that had been remixing for centuries. But there hadn't been any money involved in these traditions. Nor was there an idea of a professional class of performers in these genres until the advent of recording. Music was more of a social- rather than performance-based phenomenon. Everyone could sing along and every instrumentalist could take a verse. In such a culture, the idea of a song as a property is irrelevant. No-one invented it, no-one owned it, people could fuck with it, speed it up, put new words in, etc. We still have this phenom in popular jokes that make the rounds (who owns "knock knock, who's there?" or urban myths?).