Philosophers as diverse as Plato, Immanuel Kant and John Stuart Mill tried hard to argue that there is a rational basis for fair and just behavior. However, the best philosophy in the world is only worth so much when there is the chance to make bucket-loads of cash.
And when Darwinism got twisted out of shape by nonscientists, it seemed, wrongly, that there was a biological imperative to selfish behavior. After all, wasn't life all about the "survival of the fittest?" (Herbert Spencer's phrase, not Darwin's).
It is a view that has been hard to kill, and the Gordon Gecko mentality of the 1980s didn't help. But evolutionary psychologists have long recognized that humans have a sense of fairness, even an inbuilt sense of fairness.