Right, standardizing on 25 fps in Europe came from PAL's 25/50Hz rate.

I think 24 fps for sound film originally arose from the requirements of the optical soundtrack: slower running 35mm film wouldn't have enough resolution for decent sound quality. That's my guess anyway.

Silent films were often shot on slower rates around 16-18 fps (which explains the widespread comic "speed-up" look for video copies of silent movies, as the transfer was done the easy way by playing it back at 24).