I have got two questions.
- Why do majority of Asian people have black hair?
- Why do majority of people from the northern countries (Sweeden, Norway, Finland) have light (golden) hair?
Thanks for an answer.
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All people don't have those hair colors. All people from those Northern countries aren't blond, and there are blond people outside of Northern Europe; I don't know if you count Melanesians as "Asian", but they have blond people among them:
Still; there are two, really three, different kinds of answers to this type of question.
1) Human populations differ through random drift and assortative mating: populations that are separated will develop different mutations over time that make them look different, and different populations may find different traits attractive (including traits that mark them as part of that population), which would work to amplify those differences even when the populations interact. There is no deep reason for those traits other than "that mutation happened to occur and spread through this population".
2) Certain traits are more or less useful in different environments, and thus will spread through natural selection. The issue here is that this is the kind of explanation people intuitively love (every difference must have a reason for its existence) but it's actually very hard to separate it from the first explanation.
3) Certain traits can be considered "human-default", as in the common ancestor of humans had it, most humans still have it, so the question of "why does a human population have this trait" is less relevant than "why do some human populations not have this trait".
So having said that, black hair seems like the clearly ancestral state, and so Asian populations probably have black hair for the same reason almost all human populations have black hair. And black hair seems common in great apes in general - Orang Utans are the only exception I can think of.
As for Northern Europeans, blond hair and white skin are actually one of those traits where it's hard to separate random variation from natural selection. It is quite likely there are advantages to being fair in high latitudes where there is little sunlight; you need less protection from the sun in the first place, and you need to be able to get enough vitamin D from sunlight on the other. However this applies much more obviously to skin than to hair. Blond hair is the result of very few mutations, which are thought to have appeared around 11,000 years ago. So it could be that those mutations happened to occur and to spread through those populations and not others and that's why they're blond and most other human populations (Melanesians excepted) are not. Or it could be that once those mutations happened they spread particularly well in those populations because it was adaptive in their environment. The question isn't scientifically settled.