Did you know that in the original Star Trek, Spock was supposed to wear red makeup, to make his skin look red? Yes, I know with the whole "green-blooded" thing, his skin would look green... but we are talking about the original Trek. Roddenberry wanted Spock to have red skin. Unfortunately, many viewers back then still had black & white televisions, and red skin would have made Spock look black... and there was no way Trek would have gotten to stay on television at that time in history, with a black second in command. Having a black woman as an officer on the bridge was progressive enough.

The red hue was abandoned, as when viewed on television sets it made Nimoy look dark skinned. 

https://en.wikipedia.org/wiki/Development_of_Spock

Thus the politics of discrimination decided how this "alien" would appear.

Roddenberry also felt that other alien designs should not require extensive makeup or prosthetics around the mouth and eyes, as this would interfere with the actor's ability to show emotion during their performance. So his tradeoff on making aliens look human, was that the actors could do their jobs easier. Odd considering his costume choices over the years, but whatever.