First there was the Fighting Man and the Magic User.
Fighting Man was anyone who wasn't a Magic User.
Elves could adventure as either Fighting Men or Magic Users, and had to pick which on each adventure.
Cleric is a newer class than a player who played a Vampire "Sir Fang" -- the Cleric was invented as the anti-Vampire.
Thief was pretty old. The first Thief I used a spell-like system of abilities. They learned an ability, and could use it all day long. So a Thief with "lockpicking" skill could just pick locks, automatically. As they gained levels they gained more skills.
The Gygaxian thief was next, possibly inspired by tales of the above thief, but used a pile of tables. Prior to it, when a Fighting Man or a Magic User tried to do thief-like things, you'd describe what you are doing and the DM would assign a chance of success/failure (including automatic). It was one of the early cases of "another class gets features, suddenly you get worse at those features" in D&D.
Demihumans evolved into their own fixed class; Elves where Fighter-Mages, Dwarves where variant Fighters, and Halflings where variant Thieves.
Paladins/Rangers where "super Fighting Men" with extra abilities tacked on. You had to get lucky with your stats roll to play them, and they where human-only (with race-as-class, that isn't surprising).