Many have noted through the years that it appears that Mister Spock is the only non-human aboard a other wise all human ship, but is he? It's a little to easy to say that there are plenty of non-humans in Star Fleet, oh but they're just off camera, but what if they're right in front of us. There were several non-human races who had the appearance of humans. Who then is the alien?
First possibility, Sulu. He looks to be of Japanese descent, but Sulu isn't a Japanese name. Like so many non-humans during TOS he only had a single name. Even the later addition of a Japanese first name doesn't preclude mixed species ancestry. His home town being San Fransisco increases the chances that at least one of Sulu's parents were non-human. San Fransisco is Star Fleet's headquarters, many non-humans live there. Star Fleet personel, civilian employees, government employees (Federation and Eath) and family members. If both of Sulu's parents were non-human, their ancestors may have immigrated to Earth generations before Sulu's birth, they choose to give their son a name from their new home land.
And have you considered that there very well might be three different Sulu's on the Enterprise? There is the Lt. Sulu we usual see at the helm and who enjoys fencing. There is a second Lt. Sulu who works in the botany lab, he is a close friend of Janice Rand and has a plant named Gertrude (The Man Trap) . A third Sulu is assigned to astro sciences, a physicist perhaps, of unknown rank, maybe a petty officer (WNMHGB).
Yeoman Janice Rand is very obviously another non-human. You see, that isn't a elaborate hair style, that is in fact the actual shape of Rand's head! The enlarged size of the cranial cavity of Rand's race (Randizoids) accommodates a brain of superior abilities, important for Rand's many complex duties.
Also transporter chief Kyle, he has this truly weird off world accent.