Who did introduce the mediant keys in to music harmony progression?

Question

Which composer(s) introduced as the first time mediant chords and mediant keys in their compositions - or in which period the use of this kind of modulation was coming up?

Answer 1

In my opinion, it would difficult to pinpoint exactly who would have used a mediant chord "first" in the history of Western art music. The late classical and romantic era presented many examples of modulations or harmonic movement in "third" relations.