By Sr. Fidelis
Simple chants of only 2 or 3 notes, that circle around a single pitch, are most likely very ancient. However, over time, the final pitches began to descend, and we see an expansion of range from the Reciting Tone to the Home Tone. Below are 3 brief antiphons—all beginning very similarly on LA and SOL. But look at the endings! The first ends on LA—obviously one of the very early ones.. The second antiphon descends to a MI. We know this as Mode IV; and the final antiphon descends to RE, which gives us the formula for Mode I.
Here is a very simplistic “snap shot” of some of the changes that took place in the Gregorian repertoire over the centuries.