Harmonic practice in Western music evolved gradually over hundreds of years. At first, voices joined in unison to sing a melody. In the early Middle Ages, monophonic chant became parallel organum, where voices followed identical contours but with small improvisations.
Over time, the organum became more elaborate, with each voice becoming more independent. In this excerpt from Leonin, one voice moves slowly while the other more floridly decorates the melody.
When the added voices broke free from the original melody, polyphony --multiple parts played together--was born.
Once music was notated, as opposed to improvised, composers became interested in studying and controlling the vertical possibilities. Still, music was primarily viewed “horizontally”—that is, as the sum of melodies. Just as social institutions arose to enable people to live together, harmony began as a way of enabling melodies to “coexist.”
Whereas “harmony” is not a well-defined and separate category in the music of many cultures, it became a central pre-occupation of European music. Chords were classified; progressions were created that were shared from piece to piece. Whereas harmony originated as the result of melodies being combined, now the reverse could happen: Harmony could generate melodies.
For instance, a harmonic progression nicknamed the "Gregory Walker" underlies many American folk songs, including Boil Them Cabbages and Home on the Range.
When harmonic progressions started to get nicknames, a lot had changed since the 14th century! Just as social institutions can develop their own identity and legacy, harmonic practice took on a life of its own. Thinking harmonically has now become common-place: Pop and jazz artists frequently perform from lead sheets that indicate the chord changes. Throughout the world, melodies that were originally performed monophonically are now harmonized. What began as a resultant property of melodies became a foundation of music.