again, we are talking about conditions for humans to survive, not alien species. without proof there can be no certainty that it isnt possible, just as there is no certainty that it is. think outside the box a bit. there were scientists not too long ago that said anti-matter didnt exist, but here we are. black holes were thought of as non-existant, now wormholes may be real. entertainment(movie)companies the media, and the government, and the scientific community has perpetuated these "false truths" about aliens, ufo's and such for years, and we still hold on to them until something happens, they are proved wrong, and a different way of thinks is produced, and now someone has an explination for the new phenomenom. why not read the writing on the wall(or cave, whatever the case may be) and realize that the possibility is there. if the writers of the bible (i will point to scriptures if needed to prove my point)and other ancient civilizations talk about ufo's and alien's who are we to say they are wrong? maybe we arent as receptive to them as they were, too arrogant to accept them, maybe we live among them, who knows for sure? i have read some books and can point to some websites that would shower you with facts and references to alien presence on earth(including the NASA website to make the connection between plantet x aka nibiru from the cuniform tablets of ancient sumer, and the planet on the approach coming back into our solar system) and i bet there will still be a doubt in someones mind that it is impossible. let's look outside the box a bit. it is possible.