Watch: Paul McCartney sings original version of "Yesterday" with Jimmy Fallon

Paul McCartney's most popular song didn't come to him fully formed. The Cute Beatle, like many great songwriters, often thinks up the melody before the lyrics, then tries out various combinations of words and subject matter before arriving at a version that's ready to record. Such was the case with "Yesterday," still believed to be the most-covered tune in popular music.

But even without its final lyrics, McCartney knew he had a blockbuster on his hands:

McCartney's initial concern was that he had subconsciously plagiarised someone else's work... "For about a month I went round to people in the music business and asked them whether they had ever heard it before. Eventually it became like handing something in to the police. I thought if no-one claimed it after a few weeks then I could have it."

Upon being convinced that he had not robbed anyone of his melody, McCartney began writing lyrics to suit it. ...the working opening verse was "Scrambled Eggs/Oh, my baby how I love your legs". [Wikipedia]

The other night, McCartney performed the song on Jimmy Fallon with the host as it was originally written. It's, uh, pretty terrible, to put it mildly. If you're a big fan of either performer, you'll probably enjoy this "for-completists-only version." The rest of us can enjoy the second clip, which is McCartney performing it solo soon after the Beatles version came out.