Selflessness is Selfish

I was debating an argument with a friend, and it led me down a path of deep thought-processes, and resulted in me coming up with a very interesting conclusion. I think that all humans are selfish, and at their core, do things for the sole benefit of themselves. Let us say, that an average man, who is at danger of being evicted from his apartment due to being behind on rent by say, $500. Today is the deadline. He is walking on the road and finds $500 in a wallet on the sidewalk. He now has two options, turn it in or take it and escape eviction. Would he keep the money?

Now, for the second example, replace the danger of eviction with “need bail for $500 because his brother is in jail and if he doesn’t pay bail, the brother will be executed by death sentence in a month’s time.” Will he keep the money or turn it in? (regardless his moral and ethical sense)

I think that in a dire enough situation, when faced with choices that are either very beneficial to oneself, or not at all beneficial, the person concerned will always put his own survival above anything else.

What do you think?