Basically, believers of Ancient Astronaut theory believe that some time ago (possibly as long as twenty thousand years ago) aliens (from space, not Mexico) came to Earth and gave knowledge to primitive humans and (here's the sexy part!) possibly interbred with them.
The basis of this theory is simple. There are things that are, even in our scientifically advanced state, beyond our explanation. For example: How did stone age (or was it bronze age?) humans build the Great Pyramids with their limited technology? Why are pyramids found oceans apart (Egypt and South America)? Why do supposedly separate peoples have similar ideas about religion and architecture? Why so many eerie examples of what appear to be spacemen in cave art? Etcetera, etcetera.
The simple answer: Aliens set all this shit in motion. Aliens, so the argument goes, are quite literally the parents of us all.
My threefold argument with Ancient Astronaut theory is this.
1. It completely discounts human ingenuity and willpower, and does not take into account the all-encompassing devotion of early humanity to their gods - their rulers.
Now, AA theorists get around this by claiming that Pharaohs, Mayan kings and the like WERE the gods - aliens. Of course, there's no physical evidence that proves aliens and humans interbred, and, one can only assume that creatures from other planets would have different numbers of chromosomes... making interbreeding impossible. But AA theory isn't about hard evidence.
2. The theory not only underestimates humanity, it grossly underestimates the alien element. My questions about aliens always start with our depiction of them. Why are they shaped like us (arms, legs, etc.)? Why do they travel in spaceships? Why the obsession with sex? The probing and the breeding experiments?
If aliens were so far advanced, wouldn't they be over sex? Why would they mate with hairy, stinky, possibly diseased humans? If they were so smart, wouldn't they have known about syphilis? Gonorrhea?
Put simply, an advanced alien boning a human would be akin to the Dalai Lama fucking a spider monkey.
If the aliens were so advanced, why the hell would they build pyramids out of stone? Wouldn't they create some wild, indestructible alloy? After all: They just traveled an untold amount of light years to get here.
And my guess is they weren't flying in a stone ship.
And finally, 3. The Ancient Astronaut theory does for today what the gods of yesteryear did for early civilizations - It gives a simple explanation for the difficult to explain.
Why does it rain? The gods do it. Why does the sun rise? The gods do it.
How did the Egyptians build the Pyramids?
Aliens helped them (or just built the Pyramids themselves as "landmarks" for future landings).
Ancient Astronaut theory is like that rusty bucket I used to use to fetch water in New Hampshire: It just doesn't hold. It underestimates everything, all for a simple "new gods" answer: Nothing more than a write-off embarrassment for our supposedly "technologically advanced" race.