Well, it has an interesting subject to the movie: Crystal skulls.
From what I read on them, some people say that they were produced by an anciant culture that has vanished since. Others say that this ancient culture was extraterrestial one, and that they are the ones who made us, humans, by genetic manipulation of early ape like creatures. That's why the missing link was never found, the development was not natural...
Some books said that we were made as slave labor for the UFO people, and then we rebeled and escaped their compounds. the story of the rebellion is in the bible: It's called being thrown out of the garden of Eden.
I read in a spiritual forum that the Crystal Skulls can be used in meditation to access other dimensions, so it will be interesting to see what story Lucas has developed for us...
He's a very good story teller.