The Qur'an is just the rule book and it was supposed to be revealed to Muhammad in a series of revelations, a bit like Moses getting the Ten Commandments up the mountain.
It's hard not to see the major religions as different interpretations of similar word-of-mouth stories and legends - all bound up into a fabric that could support the priests as a profession and satisfy people's need for a belief in a higher being. Sort of supply-demand market dynamics with vested interests and entrepreneurs really.
