Your question would be much better applied to height discrimination, which is something that’s almost never mentioned, but is a lot more indicative of the nature of discrimination itself.
It is instinctual, as others have said, but it has nothing to do with tribalism or war, its about resources. Discrimination is almost always about resources (the notable exception being gender/orientation based discrimination, which I guess is religious?).
The discrimination against small people (and obesity and age as well), is more basic, and likely older (in evolutionary terms), and is oriented towards hunting and fighting. We think less of smaller, fatter, and older people because they’re assumed to be less capable of gathering (and fighting for or defending) basic resources.
Discrimination against races is more recent, and more societal, and is more about monetary resources, and isn’t even entirely a matter of race. Poor white people can be discriminated against in the exact same way for the exact same reasons. Racism is more classist than discrimination against height, weight age, etc. but is essentially still a matter of these classes being seen as less capable of getting resources.
You can see it more easily if you look objectively at the discriminatory tendencies of women (and I mean that in a very generalized way). They tend to be far more discriminatory towards resource based biases… Height, weight, physical condition… They’re often inexplicably attracted to overly aggressive partners, occasionally to their own detriment. The more instinctual a woman is, the more likely to pursue the overly aggressive men. Race isn’t anywhere near as much a factor, and there are notable exceptions in all factors for women if a man obviously has a lot of resources already (no indictment intended ladies, just is what it is, and generally)
And of course it’s more obvious among women for the same reason… The disparity (again, in a very general sense) between male and female in ability to gather and defend resources affects women’s choices of partners more so than men.
Being black isn’t a disqualifier if you can access resources…
How much discrimination do you think obviously wealthy black people face? A much as a poor black person?
It’s not a matter of skin color in reality, that shit’s nothing more than some extra melanin. The difference is the perception that black people are poor… People see a young black guy in a high end Audi and assume he’s a drug dealer, because they have this societally developed perception that a person of his melanin content is incapable of acquiring that kind of money legitimately. The discrimination is centric to the resources, always has been. There are white people who get just as discriminated against as any other poor person, it’s a huge factor in why Trump’s stupid ass has the following he does… He plays that shit up and nobody else will touch it with a ten foot pole…