From a production point of view, the cost of creating elaborate new alien makeup every week would be prohibitive. From a Star Trek perspective, Gene Roddenberry felt that the human-like species featured on the show would have evolved under similar circumstances (i.e. a Class-M environment). It was later revealed that a certain humanoid species "seeded" different planets in the galaxy with their DNA, thus solving the mystery of similar species. (Star Trek: The Next Generation "The Chase").

Advances in computer-generated imagery (CGI) have made it possible for some alien creatures to be less humanoid in their appearance. Star Trek: Voyager had Species 8472, and Star Trek: Enterprise features two Xindi variants that are created by artists using computer technology.