Not if they live in similar environments and face similar environmental pressures, in which case alien species will have similar adaptions to Earth species. It is called convergent evolution, and since alien bodies obey the same laws of physics as ours, there will be similarities. An alien that must move quickly in water will be torpedo shaped, like dolphins and sharks. A brain will probably be folded, even if it is an alien brain, since that increases its surface area without increasing volume. A creature that flies will be aerodynamically shaped. There might be alien creatures who have adaptions that no Earth creature presently has, but there will be many similarities where the same adaption works well.
I tried to combine the systems of multiple earth creatures to generate a relatively new anatomy. I do agree that the brain structure is very human. I was unable to think of a way to increase the brain surface area without increasing its volume without folding.
Incerase brain structure without filding? Easy - Sponge like brain structure with holes inside - Multiplying number of brain lobes (like my Velorans) - Developing whole cortex brain (like second brain of my Asymmetrans) - Developing additional brains (like my Asymmetrans) All these ideas are almost certianly impossible in earth vertebrate anatomy, but may work well in aliens.
So then imagine something what not resembles any type of earth organ. It is EXTREMELY difficult. Human brain allow to think about objects what resembles things what were earlier seen. Designing something totally diferent from all the things known, is not a problem, human brain was designed to solve.
When designing believably looking aliens uou can make them three way - Fusing features of many strange and very different earth creatures (like above). - Base your aliens on animal mutants and teratomas what are kept in formalin jars in some science laboratories (like aliens of Barlowe "Expedition"). - Base your aliens on non organic, human made artifacts (like engines), what will give them bimechanical apperance (like Giger's Xenomorph from "Alien").
Abiogenesis aliens are some of the most original of the all created ones i have ever seen. Look how earthy compared to them look aliens from "Avatar", "Star wars" and lots more.
And there is also phenomenon of convergence. When mollusks and chordates become separated phyllums, they had only light sensitive cells and no true eyes. Now both cephalopods and vertebrates have very advanced eyes what looks suprisingly simmilar. Of course there are some differences in eye anatomy, but resemblance is true amazing for totally separated evolution. Why? It is because eyes is one of the most useful organs and both cephalopods and vertebrates evolved it to most perfect level possible.
Beside some kind of eyes, other organs what would also very probably evolve on heterotrophic multicellular (animal like) alien organisms are: - One way digestive tract - Some kind of jaws on mouth - Front and rear of the body with sensory organs and mouth probably on the front (some kind of head) - Probably some kind of limbs especially on land creatures - Probably ome kind of fins in swimming species
But of course these things may look totally different from earth animals with so disturbing apperance, human can no imagine.
True enough, on the other hand there are only so many ways to do things like increase surface area (brain), manipulate an endo-skeleton (muscle, or some similar hydrolic system, perhaps), etc.
I keep Tarantulas... (I know, they are earth critters - I just like to go off-topic) and they have the most amazing anatomy and physiology. They have a donut-shaped brain, and their esophagus goes right through the middle of it. They have only flexor muscles... to extend their limbs they use blood pressure. Cool stuff.
True. Pity that there are no freshwater species and they are so hard to breed in captivity. Odontodactylus scyllarus are especially beautiful. I always wanted to have them, but marine aquarias are so difficult comparing to freshwater... The same situation is with cephalopods... But for now I breed 24 species of both freshwater and land arthropods (+ 12 species of vertebrates), so even without mantis schrimps its not so bad