Things that fit together according to the scientific paradigm are what I use to speculate on the design of intelligent aliens.
Living things need an atmosphere. A planet needs a certain minimal amount of gravity to maintain an atmosphere. Intelligent creatures need a large brain. The carrying capacity of a skeleton increases with the square of the linear size of an animal. The weight it must carry increases with the cube of the linear distance. A brain needs to be fairly large so between the gravity and the necessity for a large brain, alien size needs to be near where we are -- too small and not enough brain. Too big and too much weight for bones. They likely have lungs due to the same sort of thing. The necessity for oxygen increases with the cube, the area to transport oxygen increases with the square. They need to gather oxygen from a surface area larger than their ordinary surface area allows. Evolution conserves across different time scales. Animals like us have coverings that can re-evolve on short evolutionary time scales so that when the environment’s ambient temperatures change, the species can evolve on relatively short notice to accommodate. Too little insulation allows too much heat to escape; too much does not allow enough. So, although it may not be ‘hair’ or 'feathers' as we know it, aliens are likely ‘hairy’ or 'feathery' in some sense.
I suspect that intelligent aliens use base 10 because bilateral symmetry is
something favored by evolution (two hands) and five digits seems to be derived
from an amusing characteristic of knotted sheets:
Note: This was in response to yet another 'troll' question on Quora. We keep seeing troll questions about ‘evolutionists’, and they should usually be ignored, but I see why people take the bait from time to time.
A stumbling block for creationists is their apprehension that to accept evolution is to repudiate things like a Biblical narrative. I was not there 6,000 years ago, so I can’t say what actually happened and I don’t really care for the purposes of using the Theory of Evolution as an organizing principle to understand how the living world works.
As my old Biology professor said apropos of Creationists coming to our campus to make their pitch, “It’s not a matter of belief. It is a matter of knowledge and understanding.” Understanding evolution, knowing the parts of the puzzle that fit together so nicely, being able to organize, and predict using an evolutionary paradigm does not require you to ‘believe’ anything. It only requires that you understand.