Occam's razor guides us to 'LIMIT' assumption not discard it totally. Without assumption no unproven theory can exist. And no theory on the source of our existence is proven to date.
Granted, without assumption no unproven theory can exist, if you're using a vaguely scientific model. But when you posit creationism and/or ancient astronauts with foot-long pedigrees developing mankind from hominids you've left the rigours of science for the shimmering lands of pseudo-science, where you could also assume my basset hound Sebastian is the lord and ruler of the universe. I mean, he's never /denied/ it and it explains several trciky things that conventionial science can't.
It's the same reason intelligent design oughtn't ever to be taught side by side with real science: when you stop explaining things with strict logic and fact and start positing what you want to be true, it stops being science.