Ahem.
(Put's on serious hat)
Belly buttons come from where the umbilical cord was cut and tied off following birth.
Adam and Eve were (allegedly) not born, so why did they have umbilical cords?
Or, if no umbilical cords, then why belly buttons?
(Normal service will now be resumed).
Edit: I forgot the biggest theological question:
If God made Adam in his own image, and Adam had a belly button, then why does God have a belly button?