I've always been a great believer that you have to keep producing new things in order to keep life interesting - not only for ourselves, but for the audience as well. That's really always been our principle and way of working.
Look how far the human race has come in terms of air and space travel in the last hundred years. So in the next couple of thousand years, you've got to believe that we're going to be able to do all kinds of amazing things.
I really believe that the aliens are us from the future. It seems to me a very plausible reason that explains a lot of phenomena as opposed to green men with one eye from outer space.