In a nutshell it goes like this:
Approx 200, 000 years ago aliens came to Earth to mine gold which they needed for some reason. To assist them in this operation they took some of the semi-intelligent apes who were knocking about and spliced their own genes in with them to create an intelligent worker who would also, conveniently, be self replicating. This is us, Homo Sapiens Sapiens.
Once the aliens had the gold they required most of them left, leaving us to it. The remainder divided the place up amongst them and ruled as gods until about 10,000 years ago when there was a decision made to wipe us all out ('The Flood', although 'flood' may be a euphemism for some sort of biological attack). A few humans survived this, scattered everywhere,, and a few aliens did their best to help them re-build some civilisation from the ashes. They too then left, after appointing 'kings' from amongst the humans, and passing on the 'divine right to rule.'
I like this theory because it explains so many mysteries quite neatly (including why gold is so valued even today even though it has few practical uses, the concept of 'god' or 'gods', the idea of divine rule, some odd aspects of our own physiology, how we appear to have evolved so quickly, how some of the 'impossible' constructions of antiquity were built (alien technology), and about a hundred other things.
The only problem is that to accept it as a very real possibility you have to agree on two things:
1) intelligent and advanced aliens do/did exist.
2) They were able to get here from wherever they are.
What say you?