Since H. G. Wells's War of the Worlds startled Victorian sensibilities with the outlandish notion of an invasion from Mars, we have become increasingly obsessed with the possibility of extraterrestrial life. From Klingons to Ewoks to giant blobs of goo, we have imagined space aliens in every conceivable form. But if aliens do exist (and they probably do), what do they really look like? Would we recognize alien life if we saw it? Given the rules that science has devised for life on earth, can we predict how evolution might proceed in environments quite different from our comfortable air-and-wa