I myself laughed off the whole "ancient alien" hypothesis a few years back, but I'm slowly coming around to the theory of an ancient colonization by an extraterrestrial species. There are things like Rh negative people that can't quite be explained in terms of a Darwinian narrative of human evolution. A couple accounts (of dubious scientific integrity, I admit) state that early hominids, i.e. the Neanderthals, were a result of genetic engineering by an alien race who came to Earth; Neanderthals (and everyone alive today who possesses Neanderthal genetics) were alien hybrids, who became further hybridized by interbreeding with Homo Sapiens = Cro-magnon man. I'm not sure how I feel about this, but it is pretty interesting. Scientists say the universe is expanding every year, so it must have been smaller in the past, and that (along with other factors, I'm sure) may have made ancient travel to other planets/galaxies, and alien settlements, entirely plausible and much more doable than such a feat would be today. Especially if this putative alien race had the advanced technologies required to do this... long before "modern man" entered the picture and we put a man on the moon (and thought that was such an intrepid accomplishment).
I get goosebumps when I think about the very real possibility of a great-grandchild of mine being a Martian.