For fun's sake!

I was thinking about the TOK earlier today. About what defines a human being. And after giving it some thought I felt that what makes us human is not our tendency to classify things in right and wrong, the fact that we build houses or play music. It's our feeling of guilt. Guilt fill no purpose in surviving or carry on your genes to a next generation. Is it simply a feeling we've created fro ourselves? Or does it actually fill some kind of purpose. If anyone knows plez tell me.