The tendency to perceive others as us versus them isn't exclusively human but appears to be shared by our primate cousins

In a series of experiments, researchers showed that monkeys treat individuals from outside their groups with the same suspicion and dislike as their human cousins tend to treat outsiders, suggesting that the roots of human inter-group conflict may be evolutionarily quite ancient. One of the aspects of human nature is that we evaluate people differently depending on whether they're a member of our ingroup or outgroup. Pretty much every conflict in human history has involved people making distinctions on the basis of who is a member of their own race, religion, social class, and so on. The question the scientists were interested in is: Where do these types of group distinctions come from? The answer is that such biases have apparently been shaped by 25 million years of evolution and not just by human culture.