One of the most familiar of science fiction tropes is that aliens will be repulsively unfamiliar and very dangerous indeed. The first is almost certainly wrong, the latter may well be correct. And why? Simply because if you want to understand what alien biospheres will be like, then there is no better place to start than on Earth. I will argue that be in terms of extremophiles, molecular biology, functional morphology, nervous systems and even intelligence and consciousness terrestrial systems provide an excellent guide. Not only have they reached the absolute limits of what is possible anywhere, but the number of available solutions is an infinitesimally small fraction of all theoretical possibilities. Tool making, intelligent, cultural, vocalizing species are evolutionary inevitabilities. So too should the aliens, but if so where are they?