I think it is interesting that people automatically point out to the garden of eden or biblical stories as to why these monuments were here.
Hmm, they always leave out those little bits of information. Abstract thinking paved the way for civilization, thinking outside the box lead to building things that they wouldn't have otherwise. I believe the reason so many point to religion, is because it was one of the first stitchings that held a people together and gave a semblance of order. I'm sure if we turned back the clock and observed, we would find all our speculations wrong.