Human design aesthetics: It seems that each feature on the human body is either
Furthermore, each double is paired with a single of comparitive scale. Singles, on the other hand, can exist on their own.
For example: The head is a single. Ears and eyes are both pairs; nose and mouth are both single of the same scale. The philtrum is paired with the nostrils. Testicles, arsehole. Arms as a pair go with the head; legs as a pair with the torso. Nipples, bellybutton. You see, on every size scale the rule is obeyed.
I don't pretend to know why, but I feel that if we are to genetically engineer humans we need to be aware of and guided by these principals so we can create something that looks "right". An extra arm placed off-centre wouldn't do, you see, but Medusa-style hair of snakes would be fine (so long as it was balanced by a single of similar scale -- say, a large beard).
Incidentally, this also explains questions of more theological concerns. Devils have horns to balance the tail. And we can even make predictions: Given that we're only adding wings to the human form to make an angel, and we aren't adding a balancing single, the wings must be enormous so as to be the right scale to use the whole body as the balancer. This means that angels can justly be regarded as superhuman rather than (if they had smaller wings and a single balance) submen, or even (if they had no balance) simply mutants.