If I recall correctly, Sir Paul started off with a trumpet because it was a heroic instrument at the time. It's in the bass player book.
Ah,here it is.
"I would have been about 15 or something,and me Dad bought me a trumpet,because it was kind of a heroic instrument at that time. Me dad had been a trumpet player,so he showed me a bit.But I realized I couldn't sing with the trumpet and I wanted to sing as well,so I asked him if he wouldn't mind if I traded it in for a guitar."
He got a guitar yada yada yada, no one wanted to be the bassist, boom, Sir Paul.
Dedication:Staying up all night and then having to search around at 5:30 in the morning to get out a bass quote.
Good night.Erm...morning.