if you are doing behaving exactly like everyone else, it shows you are well adjusted and your genes and brains function normally, thus your babies are more likely to survive. It's arbitrary, but so are words. There is no reason why the word "word" has to be "word", but by showing you understand and speak language you show your fitness. Just think of non-verbal communication as a language that everyone speaks and biting your lip the right way is the same as saying "word" the right way. You'd think I'm having a stroke if I say "gooyallodsiu" instead of "word" and it's the same for body language.

As an avid autism enjoyer myself, some of us are just mute and deaf in this language.