How is it that you become a PhD expert in nonverbal communication?What would you have to major or minor?

in to do that?

Im interested in body language and humans interactions but Im a Biology major.

But what major would take me on the path of learning more about nonverbal communication and humans' (or animals) interacting dynamics? Or minor in my case maybe.How is it that you become a PhD expert in nonverbal communication?What would you have to major or minor?
Study sign language or music.