Pérotin (c. 1200), aka Pérotin the Great or Pérotin the Master, was the successor to Léonin at the Notre Dame school of polyphony.
Pérotin, as well as Léonin, were written about by a source known only as Anonymous IV, and If it weren’t for him we wouldn’t be able to attribute either of these men to the works that they composed.
These two composers really are the very beginnings of the history of western music.
Check out his Monophony: Beata viscera and his Polyphony: Viderunt omnes and Sederunt principes