Ah, right, so the question of the starting point is a good one. I don't know the exact historical answer, but it's worth noting that if your are in minor scale (or Aeolian mode) then the tonic of the scale (for A minor) is indeed A.

Quick check with Claude hints that various modes were in common use in church music prior to major scale (Ionian mode) taking more central place in (western) music theory.