Watch the unpaired pilot, which featured "Number One" - Pike's first officer, a strong, smart, tall dark haired woman who was technically brilliant and even wore pants. (She made a cameo in this 2nd season, BTW.)

When they aired the pilot, the audience - specifically the women in the test audience - rejected her as unrealistic. So they demoted her to a blonde nurse wearing a skirt for the main series. Society wasn't ready.

Understood in the context of its time, it was very progressive. As Whoopi Goldberg said to her mom when seeing Uhura, "Mom, there's a colored lady on TV and she isn't a maid!" MLK praised the show, bringing Roddenberry to tears.