How did natural selection give caucasians coloured hair/eyes and mongoloids/negroids black?

If climate and selective breeding created vastly different populations in Europe and Asia from essentially the same 'people group', then why do the Chinese look different from the Europeans when they share the same climate? Is it possible there were for example, Caucasian-looking people in China long ago who were then ostracised and prevented from passing on their genes, thus producing an absolutely homogeneous population? It's quite hard to believe that this could be the case and vice versa in Europe. It's simply.. too complete. There was never any record of a Caucasian in China or a Chinese in Europe until exchanges began.

How did natural selection give caucasians coloured hair/eyes and mongoloids/negroids black?

Any isolated population will, over time, develop traits that are both unique to that population and widespread in said population.

By the way, for the record, its is not the caucasians that have colored eyes and hair. Blue eyes are the ones without any coloring pigment. Pale hair are the ones with less melanin. Why do notherners have less melanin? Because it impeded the absorbption of the light needed to sythesize D vitamin under the less bright northern sun, while the population closer to the equator still need to protect themselves from too much light.

In the case of asian people, if you take a world map, you will notice that the bulk of China lies closer to the equator than Scandinavia, where the blue eyes/blond hair probably developped.

Could there have been caucasian looking people in China? Possible. The variation in the traits would have been a very progressive process over several generations, but if someone was a tiny teeny bet more "caucasian" than his parents, his offspring may not have been, depending on the person he/she had kids with; so in the end, it is the whole population that varies slightly over time. The exception would have been isolated tribes, which may have been eliminated because people often fear the different (and that applies also the the possibility of more asian looking people in Europe).

There are at least the Jomon (thought to be ancestors to the Emishi) from the north of Japan who were described as having caucasian physical appearance. If you look at picture of some from Russia, you will see people with blond hair and blue eyes, but the eye shape is oriental in appearance.

How did natural selection give caucasians coloured hair/eyes and mongoloids/negroids black?

the question you pose in my aproximation is very interisting, there those who would argue that the diffrence in people is soley based on external events, delging into uncharted waters, such as space and time, to find a true meaning mabey you should check out quantum phisycs with a twist of extraterrestial life forms.

How did natural selection give caucasians coloured hair/eyes and mongoloids/negroids black?

A little basic research would show you that not only there are records but Mumford bodies deep into area now occupied by the PRC.

The people know as Han are not a homogeneous population.

How did natural selection give caucasians coloured hair/eyes and mongoloids/negroids black?

My guess would be that Europe, being closer to Africa, has been invaded by new races more often than has Asia and there has a more mixed population. In genetic terms, Africans are even more diverse than are Europeans. It could also have to do with Europe being on a releatively northern latitude. Therefore, Europeans don't need dark hair to protect them against the sun.

Darwin thought it was a matter of sexual preferences, dark hair being fashionable in Asia and therefore blond Asians had less chance of sex and less chance of passing on their genes. Personally, and don't think this is very plausible although it could be the explanation for some more perculiar racial characteristics, such as the blue penises in one South-American tribe.