[link to original on tumblr]
---&---
I'd always thought that the Isley Brothers' version was the original, but apparently not. For what it's worth, the Isley Brothers' version is in F major (which, based on the other songs I know by them, seems to be their favorite key); the Tremeloes' is in C major; and the Beatles' is in D major, which makes the guitar part in the middle (it's a horn part in the Isleys' version) easy to play.
In my recording, I skipt the first verse entirely because it would have been pretty much the same as the second.