Mr. Spock was not the original second in command of the USS Enterprise: in the pilot of the show the role was filled by a character called Number One, a cold, logical, and efficient woman played by Gene Roddenberry’s second wife Majel Barrett-Roddenberry. Except at that point, she was only the lover that he’d taken outside of a failing marraige, and NBC’s executives were... concerned that she had such a vital role, and that the second main character of the show would be a woman. They also didn’t much like the cold, logical alien known as Mr. Spock, and according to Barrett herself, Roddenberry "kept the Vulcan and married the woman, 'cause he didn't think Leonard [Nimoy] would have it the other way around." So, Mr. Spock was installed as the logical second officer of the Enterprise, and Barrett was given the more secondary (and emotional) role of Nurse Chapel. However, you can still see a bit of Number One in the episode The Menagerie which re-purposes much of the footage from the unaired pilot for Star Trek: The Original Series.