When I was twelve or so my Grandmother gave me a book by a guy named Erich von Daniken called Chariots of the Gods. It is a psuedo-scientific fantasy (presented as fact) explaining many of these, and many other unknowns of the ancient world as gifts from space travellers. I believe one of his theories was that the space travellers were actually human time travellers. I'll tell you, that book really got my twelve year old imagination going. I'd really like to read it again. I'm sure it hasn't aged well (especially to my forty year old sceptical mind), but it had all of the pictures in this thread and many others too (Stone Henge, Easter Island, even the origin of religion from the contact with aliens).
Thanks for reminding me of this. I'm going to pick up a copy of it.