Jon I think what we think of as musical is closely tied to us being used to our scales. Why we chose eight notes, I don't know. Maybe it's because it just "felt" right, or maybe it feels right because we are so used to hearing it.

In other cultures, they have other scales, based on different intervals, not just larger intervals (like a pentatonic scale)but also with intervals so small that we have problems differentiating between the different notes.

Magpie