The thought has flitted through my mind ever since I was about 12...that UFOs could be time machines from our own future, rather than spaceships carrying aliens from other star systems. But I never really thought long and hard about that alternative explanation.
But apparently Spielberg has put more thought into it than I have. By saying that aliens are really "us half of a million years from now" he is postulating that creatures like the Greys are what we will evolve into. Little bodies, and big heads.
Certain things in UFO lore would make more sense if that were true. Creatures that evolved on planets in other solar systems would not likely be able to interbreed with us (it would worse than trying to cross a human with an oak tree). But if Saucer jockeys are not aliens but are a later model of our own species then all of these tales about breeding and making hybrids with us via abductions would make sense. We likely could interbreed with a later version of our own species. And maybe they like to freshen up their genepool with our more primitive DNA. Just a thought. 