Short answers, pulled from the story.
The original 1974 Dungeons & Dragons boxed set contained only three main classes: the Cleric, the Fighting Man, and the Magic-User. These three archetypes formed the tactical backbone of early fantasy gaming.
The first supplement Greyhawk added the Thief as a fourth main class in 1975. This addition introduced cunning and stealth to the party dynamic alongside existing roles like the Paladin which appeared as a subclass of the Fighting Man within that same supplement.
Fifth Edition Player's Handbook released in 2014 included twelve base classes with multiple subclasses per class. Players chose archetypes like Berserker Barbarian, Evoker Wizard, Wild Magic Sorcerer, or Beastmaster Ranger at third level or earlier.
Humans enjoyed unrestricted access to any single class without level limits while Dwarves could be Fighters or Fighter-Thieves and Halflings were restricted to the Fighting Man and Thief classes. Elves had limited options including Fighter-Magic-User combinations before Advanced Dungeons & Dragons released its Player's Handbook in 1978 loosening restrictions on race and class combinations.
Monks needed fifteen strength, fifteen wisdom, fifteen dexterity, and eleven constitution according to early ability score requirements. High ability scores granted experience bonuses for progression while unusual stat distributions could prohibit certain roles like fighters requiring at least nine strength points.