Western classical music begins with people writing music down, and the people doing the writing were monks. Gregorian chant is a single unaccompanied vocal line, sung in unison, and for a long stretch that was the whole of the notated repertoire. It is worth remembering that the earliest music in this tradition was choral, sacred and unaccompanied, because the assumption now runs the other way.
Polyphony, more than one independent line sounding at once, arrived on top of that chant. The Notre-Dame school in Paris, Léonin and Pérotin in the twelfth and thirteenth centuries, built elaborate upper voices over held chant notes. That is a medieval development rather than a Renaissance one, though it often gets filed forward by a couple of hundred years.