Some good posts above.
Fear of "the different" is an inbuilt and evolutionary useful mechanism that has served the survival of the species very well for many millions of years. It's also a fact: look at the many, many studies that prove even the most liberal and accepting types have an inbuilt, primal, adverse reaction when shown pictures of people that "look different" to them.
It is with us when we are born, it's controlled by the same part of the brain that causes children to fear spiders and snakes before they have ever encountered them. It's not going anywhere in the next few million years, either.
The suggestion above that "the tribe" needs to be seen as humanity as a whole is a good theoretical solution, but unfortunately it runs contrary to every fibre of base human instinct. At the core, it would only work if everybody looked the same.
Fortunately we have developed control mechanisms for these instincts (broadly describable as "society") that subject instinct to external control. For the most part this works incredibly well to prevent murder, rape, theft etc, all of which are actions resulting from a deep-seeded psychological survival mechanism.
The problem is that this external control can be used for good or bad purposes. The holocaust being the prime example. The psych theory of "external locus of control" deals with this in depth and is a great way to understand that people will basically behave in the most reprehensible of ways if they are convinced they must by outside factors.
Humans are flawed and fascinating.