god got all constipated, and eventually he was so backed up, that everything combined was collecting in his ass.

when he finally passed that disgusting mass, the result was Earth.

god felt so sorry for the inhabitants, to be a mutilated hybrid of reality, and the worst part is we clogged his toilet, so now we're dead center between heven and hell, hence the whole yin yang thing, the existance of both good and evil.