Scabadus said:
Many races have two legs, two arms, one head and one torso.
I've written a few sci-fi stories, and in each one I attempt to logically decide exactly how an alien race would look. This is one of the most important parts of any science fiction.
Bear with me as I hit you with a wall of text.
The most obvious reason is that as humans we will better identify with humanoid species. If you make every species look like jellyfish, slugs, insects, or some otherworldly thing the consumers of your story won't identify with them and thus they'll never be usable characters.
However I think there is a scientific explanation, or at least a plausible sci-fi explanation.
My basic idea is that evolution is not something where the
process varies wildly, but something where the
results vary wildly.
All the sapient species are terrestrial - they don't fly in the air or swim in the sea (obviously the Hanar are an exception). So we then know that they evolved to live on land. Creatures that live on land have legs (2, 4, 6, doesn't matter at this point). Humans and Asari are mammalian (belly button), Turians and Krogan are reptilian (eye structure, scaly skin), Salarians are amphibious (eye structure, eggs as children). For the sake of argument let's put Quarians and Volus as mammals as well. Vorcha I remember some very odd explanation in the codex, and I would say Batarians look most like amphibians due to the 'slimy' quality of their skin, which implies the ability to live both underwater and on land. So there are no sapient species with 2 legs, 2 arms, and a torso that are insectoid, annelid, echinoderm, etc. They are only mammals, reptiles, or amphibians. To my limited knowledge, it is standard for almost all mammals, reptiles, and amphibians to have four limbs as the first iteration of their land-dwelling ancestors.
So we've determined that it is logical for all of the species mentioned to start their individual evolutionary changes with a torso and four limbs. But why do those species become sapient? What drives the evolution of intelligence, the increased brain size, and eventually organized civilization? The answer, I believe, is the ability to use and
create tools.
To do this, you need to free at least two limbs. Why did humans evolve this ability? we migrated to the plains, where standing on our hind legs and growing tall gave us the ability to see a greater distance and watch for predators. Essentially, we were prey. So each sapient species went through evolution as prey to something else. They all evolved to stand on hind legs and eventually walk upright, which freed the other two limbs for tool making - something they would need to defend against their predators. The process of making and using tools necessitated a larger brain, thus becoming sapient.
EDIT: Forgot the non-crew races like Vorcha, Batarians, and Volus.
Obviously Elcor and Hanar don't fit into the above model, but I personally don't think the Elcor make any sense. There is no way they could create things, they don't have the limbs necessary to use tools. At least the Hanar have tentacles that you can reasonably assume can use tools. An obvious reason for both of these species to be sapient would be Prothean interference.
