But Michael Morris, a drone changes its pitch as needed when the melody changes so it is still harmony.

I suspect that even when there is only one note, we 'hear' acommpanying harmonies and that is why everyone and most things react to music. I think harmony is an intrinsic part of Earth itself. Think of the sounds of wind. Think of the sounds of an avalanche. Think of Nature's colors. For some reason, mixing green with blue or orange with mauve in Nature is harmonious. Nature can get away with things that we cannot. We still have a lot to learn.