Middle ages music originally had no rhythmic structure, but as the music became more complex, a need for rhythmic unity emerged. With this complexity came rhythmic notation. In the early middle ages, music was monophonic, meaning a single voice or melody line. As time passed, polyphony developed (multiple melodies).
Polyphony is really interesting and led to the highly complex polyphony of the Renaissance, and eventually to the fugues of the Baroque period.
