People are the way they are because of the benefits of cooperation. People are both selfish and groupish. Humans aren't the only kind of animal to get these benefits. E. O. Wilson observes that social insects account for just 2% of all insect species, but more than half the total insect biomass. Humans and social insects also show altruism, sometimes sacrificing themselves for the good of the group.
Watch E. O. Wilson on “The Social Conquest of the Earth”: