Evolution is a random process. Humans do not have as keen a sense of smell or hearing as dogs because we don’t need it – our eyesight compensates. Vertebrates, cephalopods and insects all have only two eyes because that’s all you strictly need for a wide field of vision or depth perception. Evolution in a natural enviroment rules out certain possibilities. As Dr. Ostrow in Forbidden Planet said on examining the footprint left by the invisible Id Creature, “On any planet, Captain, this thing’s a monster!” Meaning, it did not appear to reflect any plausible combination of evolutionary adaptations.
Can’t conscious beings do better?
In Bruce Sterling’s first novel, Involution Ocean (1977), the backstory involves a long-extinct race of beings who had the technology to design new bodies for themselves, and occasionally switched one form for another. The various forms described had nothing in common and a person encountering the race for the first time would assume he was seeing an assembly of sentients from different planets. (The only one I remember is a centaur form with a ring of eyes around its head.) Suppose you could do that. Invent something it would be cool to be – and that could live on Earth’s surface, with Earth’s gravity and air pressure and air content and temperature range. What shape? How many limbs? What senses? Multiple eyes for a complete spherical field of vision? Would you be adapted to live in air or in water, or both? Would your brain be in a head out in front/top as in vertebrates, or would you locate it at the center of your body for better protection? What manipulative organs? A hand with an extra thumb on the other side might be very dexterous. Do you want four hands, or is that overdoing it? Want a prehensile tail? A prehensile nose? A prehensile . . . never mind.
But it has to be plausible: E.g., if you want to see radio waves, you need eyes as big as radio dishes; if you want to see x-rays . . . I don’t know how flesh and bone could manage it at all. If you want to fly – and remain sentient – you need to account for the need to carry the three-pound weight of a human-sized brain in a body small enough for flight (cube-square law, you know – a roc could never fly on this planet; the largest known flying birds are condors and they don’t fly very well). If you want to be a telepath – a thing unknown in nature – you’d better provide a pretty convincing mechanism for telepathic communication, and specify whether and how it enables you to read the minds of all beings, or only those with the same equipment.
