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?
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?
Was there first the melody that asked for a leading tone?
Did it start with the upcoming of the vertical harmonic thinking and composing?
What role played the development of the well-tempered pitch?