Something new happened in sacred music sometime between 700 and 900 to take the first steps in the development of polyphony. Give its proper name and briefly describe it.

It was called Organum

Explanation:

Organum is in a way a precursor to the harmony and it is the first example of polyphony in the medieval chant music wherein one or in some cases until the fourth or fifth a parallel voice was added to the main voice, on the fifth note or sometimes on a parallel note in the same octave.

This parallel voice gave a deeper sense  of music and was in a lot of ways the precursor of what would become polyphonic harmony.