"the well-tempered clavichord" is early in the transition to well temperaments, previously organs (and harpsichords) couldn't play in all keys which was limiting. It may well be the oldest thing we have written (I don't know) however there were a lot of different experiments.

You can tell that this wasn't equally tempered because composers talked about the "color" of each key, a concept that doesn't make sense unless the keys sound different.