[woliveirajr]

If we compete for resources, sometimes my tribe and yours aren't exactly friendly. Perhaps seeing you (alone or in groups) wasn't exactly a pleasant experience, so I might not like your hair, skin color, eyes format, and so on... Not liking you and not being liked by you might have happened in the past.

And if my group was more aggressive, there are some probability that yours didn't even survived and mine did. Or if I was more "racist", perhaps my genes would spread better. So, some genetic that makes me don't like "different" people might have a role.

If our genes didn't evolve (in that aspect) in some few thousand years, simply because there wasn't enough ambiental/social/whatever pressure in that direction... here we are.