Ever wondered why dolphins don't build cities? Or why we only have two arms and two legs? Why our feet are the shape they are, and not more ape-like? I certainly have, and the more I read about the way we and every other living creature adapts to fit a niche, or to live within the constraints of its environment, the more I wonder at the way certain basic 'design criteria' crop up again and again.
Take a look at an x-ray photo of a cat's or a dog's fore paw, and you could be looking at a baby's hand. It really is that striking. Certain other features also repeat. the spinal structure, rib cages, even pelvic structures. Looking at bird skeletons soon idetifies that while they also share certain structural similarities, their bones are lighter and possibly stronger than ours, but the wing structures are essentially similar to a 'hand' with 'fingers' and all the bones you'd find in one. Obviously, they are different in size, but the shapes are similar.
When writing scifi, this is something you have to consider - if you 'create' an alien species, you want them to be able to interact with your human characters, so certain similarities are essential. Such as being roughly hominid in shape, perhaps being abole to speak in a way we can interpret. Certainly they need to be able to use tools, perhaps operate complex machines. Now come the constraints. Ever consider why we don't have four arms? The biggest problem is how they would be attached in an operable manner. You would need four shoulder joints, with four shoulder blades for starters. Unless we had the sort of carapace of a spider or a beetle - in which case we'd have a completely different appearance. A human with wings doesn't work either, even with a bird-like bone structure, largely because you need a different shape to the rib cage and a massive 'keel' bone at the sternum to attach the muscles to.
Dolphins and the other members of the whale family are very intelligent, and may well be at least as 'sentient' as we are. There is a lot of evidence to suggest they have a highly developed 'language', impressive cognitive abilities and excellent memories and learning ability. We also know they were once land dwelling - and their skeletons still have the bones structres of 'hands' and vestigial 'feet'. But they are also perfectly adapted to their marine lifestyle and - as far as we know - don't use tools. Besides, bringing them back on land, 'anthropomorphisng' them and making the into spacefarers - or not, depending on the story - doesn't really produce a creature that is believable.
Maybe it's just 'me', but I have wrestled with this problem for years. Even before I started writing seriously, I had trouble with some of the 'alien' creatures created in scifi stories. Tentacles work as limbs in a limited environment. Sure a creature with tentacles can and do move about on dry land, but not terribly efficiently. Add to this the problem of interaction between species, and humans don't function too well under water, or under great pressure even in a 'gas' world.
So I tend to envisage my characters interacting with 'hominid' type creatures - four limbed, with hands, feet designed for upright walking and bodies adapted to the environment that created them. Sure, one can get a bit exotic, like my 'Canids' essentially ape-like creatures more dog-like in their physiognomy, behaviours and appearance - but upright. Or my Lacertians, upright Saurians. Or perhaps even more 'way out' with the Sidhiche, creatures who exist as a form of pure energy, and the Niburu and their symbiotic 'servant' species. Sometimes this is where the 'Laws of Physics' get in the way, since the bigger the creature one envisages, the greater the forces it will be subjected to in any realistic universe. I note this is a problem many scifi authors struggle with, though some simply stick to 'humans in space' or on any other world they create.
Perhaps the answer is to suspend 'science', but I find that difficult ...