Counterpoint, why do dolphins and sharks look so alike when they have a completely different phylogenetic tree? They separately developed traits that were very well suited to the medium of water. Convergent evolution means that it isn't entirely unlikely that whatever we may find has similar traits to beings on earth, because we all exist in a universe of solids, liquids, and gasses.
Counter-counterpoint: both species come from the same tree of life, several of their traits taken for granted come from previous species' adaptation, e.g. bilateral symmetry. When you see, say, a molusk with the same bodyplan as a shark, we can talk about it being a common possibility.
