Aside from the preponderance of evidence and predictions that have come from the theory of evolution, I can only say that there is no other demonstrable mechanism to explain our existence. The creation explanation can be immediately disgarded, since there are many hundreds of variations of these and none are testable.
But lets get a few conventions out of the way first:
1) We are discussing evolution, not the origin of the universe. These are two different concepts and deserve separate threads. Personally, I'll likely not post in the universe thread since I have limited education in astronomy and cosmology. I make the distinction because in nearly every evolution argument I've been in, within a few posts the opponent to the fact of evolution will bring up the beginning of the universe.
2) The word "theory" does not some how imply a hierarchy that evolution aspires to become, specifically moving on to be a "law." Theories and laws are two different, but related, things. Laws are generalizations, principles or pattersn in nature, while theories are the explanations of those generalizations. Theories in science are very different that the "theories" of colloquial speech in that they are based on one or more hypotheses that have been tested in some way.
But I can't imagine that anyone these days would dare challenge evolution as an explanation for life as we know it. There simply aren't any other viable explanations and the evolution explanation has far too much evidence supporting it. Okay... I'm trolling a little. But just a little