Social origin of Roman Centurions

[Mount Suribachi]

I've always been under the impression that Roman Centurions were men who had been raised from the ranks due to their ability and courage and soldiers, and that this was one of the great strengths of the Roman Army (as Sergeants who have come through the ranks are the backbone of modern western armies).

Furthermore I always thought these guys were "working class", farmers in Republican times, headcount after Gaius Marius. However I read a book recently that claimed centurions were more equivalent of a modern junior officer, and had their social origins in the Roman middle classes, merchants and the like, and an in-between of the Patrician generals, legates and tribunes, and plebian Legionaries.

I'd never heard this theory before, wondered what the fine patrons of the Org thought.