I don't need to know exactly how they did it, there is strong evidence to suggest how they did it, even stronger evidence as to when they did it. We have no evidence of aliens or divine intervention doing it. That's the point. As Heis mentioned, if aliens did it, that would be the coolest shit I've ever heard of, not only that, it would completely prove that we're not alone in the universe, one of the biggest questions we can ask as humans. I would welcome the idea. There is just nothing at all that says they did. The absence of evidence is not evidence. Just because we don't know with 100% certainty how it was done does not mean we can justifiably introduce unverifiable conclusions.
How do we know a different species all together didn't do it? Why is asking how aliens did it any more valid than some ancient unidentified Earthbound species? Both are equally as valid because there is no evidence for either. Meanwhile, we know humans have been around for hundreds of thousands of years, we know ancient humans built structures to last through time, and we know it is possible to accomplish such feats with only human innovation, ingenuity and a lot of time to do it. This suggests it was ancient man that did it. We even have writings from ancient Egypt detailing how they did it.
I had a really tough time following the movie, I also didn't like the language the post apocalyptic tribe used, I could barely understand it, or the facial FX, they seemed a little off and not very believable, especially the last few scenes here the Chinese girl was supposed to be an English girl, I didn't buy that for a second.. And I thought the ending would tie everything together a little better than it did..
Titties though.. so that was cool.