Humans to me can be rotten than trash and lower than animals & insects or pure and beautiful, although i haven't seen anyone like that - Beautiful and pure. Anyways back to the question. To me, what defines a human is how they live their life and their purpose of life. From what i've seen, majority or almost all my life that humans are very selfish. (Yes, i know everyone is selfish, but i meant it as a negative selfish. For your own gain or to satisfy your own goal and desire while not caring about what happens to other people/friends/family. Not the positive selfish where you do it for another person gain while not caring about their feeling and just doing what's best for them.)
Anyways... Back to your question, what defines a human being? My answer is a human being cannot be define.. They are literally all split up in different category by nature. You can only decide what's a human being by yourself. I'm a neutral person, so it doesn't really bother me if humans are selfish disgusting people or kind beautiful noble people.