If that "selfishness is survival" and "scarche goods will alwaas lead to fights all against all" is really ture you can easily see if you look at e.g. Favelas.
Yes, there are "wars". There is a lot of fighting out of frustration. There is a lot of theft.
But there is also a deep and wide net of help. There is even pride when the people manage to get their hands (legally) on a bucket of paint so that they can paint their tiny sheds.
"We may not have much", those sheds say "but at least we have a beautiful home that everyone can enjoy looking at".
And even in the darkest corner of the black continent where millions of people die of hunger, theft and murder is not something everyone does.
Human nature is vast and responds to its situation. I talk about framing a lot because framing will dictate how your human nature expresses itself. If you see love and beauty, you will reflect that. So human nature basically just mirrors what it sees.