When did the aeolian mode change to the harmonic minor?

Question

Looking for explaining the "transformation" of the minor 7th to a major seventh I found this question by Bart Brush in a Choral net forum.

He asks about the modality of folk songs in the oral tradition, and to melodies:

When, or under what circumstances were these altered by sharping the seventh?

Was it only when later arrangements were made using newer harmonizations? Was it done by scholars intent on “improving” or “correcting” the music from earlier times? We can see how this process occurred during the evolution of American hymnals over the last two centuries, and during the transcription and harmonization of folk songs in the last century. Or did there come a time when the harmonic milieu had developed to the point that there was a shift in what the general public liked and valued? What was going on–melodically and modally–in the 16th century and earlier, in the world of simple songs?

https://www.choralnet.org/forums/topic/historically-when-did-melodic-minor-replace-natural-minor/

I remember that I've played at school with my students recorder music (probably in aeolian, dorian or mixolydian mode with a facultative sharpened 7th as leading tone to the final note. Also in older for harpsichord and virginal there occured (#) notated in brackets. So my question is:

when came the augmented 7th up?

1. Was there first the melody that asked for a leading tone?

2. Did it start with the upcoming of the vertical harmonic thinking and composing?

3. What role played the development of the well-tempered pitch?

Answer 1

Two problems with the question that make answering tricky:

1. Very often it can't be said exactly when some artistic thing developed. In the case of the evolution of scales, keys, harmony, etc. - general aspects of music - it is especially difficult to give a precise answer.

2. The terms aeolian mode and harmonic minor are loaded with historic context. I think the term harmonic minor didn't come into use until long after it's actual evolution.

I don't mean to sound glib, but no one in the past said: "let's change the aeolian mode to harmonic minor."

The oldest written record of this evolution should be Musica Ficta. This was the Medieval practice of adding a sharp to the seventh scale degree of the minor modes to create a leading tone.

...When did the aeolian mode change to the harmonic minor?

Roughly, the late Medieval era.

...Was there first the melody that asked for a leading tone? Did it start with the upcoming of the vertical harmonic thinking and composing?

Adding a sharp was a contrapuntal device where the goal was to create a cadence, a clausula vera. The cadence ended on an octave approached by contrary motion where one voice moved by a whole step and the other voice moved by half step.

That exact contrapuntal movement was desired for cadences. In Ionian mode (the major scale) this movement is diatonic so the ^7 scale degree doesn't need alteration. In Aeolian and Dorian modes the diatonic movement would involve whole steps in both voices. In those modes the clausula vera is formed by raising the ^7 degree.

Note that Phrygian mode doesn't require an accidental to make a clausula vera. The movement ^2 to ^1 provides the half step and the ^7 to ^1 is the whole step. It's all diatonic, no accidental required.

Sorry for the long winded technical explanation of the why the ^7 degree is raised. I just wanted to make it clear the reason was contrapuntal.

Keep in mind this is the recorded history of church music. It seems reasonable to speculate church musicians codified musical practices already in use for a long time and perhaps in use outside church music. The origin could be much older than what we know about church music.