This isn't really a linguistics nor biological question as much as a philosophical one. Man is genetically programmed to wonder, and the first thing that early man must have wondered about was his own existence. Therefore, we came to think of ourselves as "beings", ones who are clearly defined and clearly exist. After all, our bodies are the only things that we can truly be sure of (the rest of the world is an illusion anyway, as scientifically proven) and so its existence is tantamount to our thoughts.