Around a millennium ago, monks and priests started taking record of the immense catalog of chants they were composing. They came up with a notation system to help others learn the music faster: Sheet music. This system allows for keeping track of more elaborate ideas, that would have been possible relying on the human brain alone. Consequently, these sheets gave way to the growth of a much more extensive and complex music all the way to Classical music.
The Middle Ages
Composers like Perotin and Machaut explored ways of writing multiple vocal lines that went together. Choral music was the art music of the times. Instruments were common, of course, but since they generally weren’t allowed in houses of worship, there is scarcely any notated music including them from the time.