George Carlin explains it the best. In one of his hilarious rants he goes on about people wanting to "save" everything and says the worst are those who want to "save the planet." He says, "the planet is not going anywhere, we are." "The planet's fine, we're the one's who are f*#ked." And he is so right. The earth will eventually decide that its had enough of us and change the climate or create some kind of disaster in which the planet no longer support human life. "The planet will shake us off like a bad case of fleas."
I doubt said aliens will have anything to do with the end of the human race.