Incidentally, Gene Roddenberry did write into his original 1964 Star Trek pitch something of a justification for the aliens to all look very similar.
1. We would only be visiting "Class M" worlds. Which, in the original pitch, meant they were very very Earth-like.
2. He came up with some pseudo science called Parallel Evolution/Parallel development. That most humanoid societies developed along the same lines.
This was his two-pronged attempt to justify using and reusing props from currently produced films. He remarked basically something to the effect "If we just finished shooting an Egyptian or a western movie, we can make clever reuse of set design and wardrobe." Hence why you have episodes where the crew beam down to a planet run by the 1930s Chicago mobs, or why you end up with episodes like "Who Mourns for Adonais" where they meet Apollo. That one, more than many of Trek's episodes really does tie into some of the same ideas of Prometheus and Alien : Covenant. With it's ties to our mythology, but also with the attitude of Apollo himself.
