Sean Carroll in Preposterous Universe:
If extraterrestrial life is out there — not just microbial slime, but big, complex, macroscopic organisms — what will they be like? Movies have trained us to think that they won’t be that different at all; they’ll even drink and play music at the same cafes that humans frequent. A bit of imagination, however, makes us wonder whether they won’t be completely alien — we have zero data about what extraterrestrial biology could be like, so it makes sense to keep an open mind. Arik Kershenbaum argues for a judicious middle ground. He points to constraints from physics and chemistry, as well as the tendency of evolution to converge toward successful designs, as reasons to think that biologically complex aliens won’t be utterly different from us after all.