It's generally credited to a monk called Guido of Arezzo - he came up with the idea of standardising notation by inventing the stave (the lines that notes appear on).
Before that, words were given accents to denote the pitch at which they should be sung, but it was all a bit hit and miss as to the tune ever sounding the same twice in a row, and people had to rely on learning songs from memory - the musical version of Chinese Whispers!