How come there are different races while today’s humans are descended from about 2000 individuals from Africa?

Answer

The variation known among the human races is a phenomenon that is geographically bound. For example, with regard to skin color, it has been seen that there is a close relationship between the average amount of sunlight that falls in a certain area and the skin color of the local population. The connection here is: the stronger the incidence of sunlight (and harmful UV rays), the darker the skin is (as protection against those UV rays).

The origins of modern humans are in Africa, where indeed genetic data shows that a limited number of individuals gave rise to the human population spread all over the world. The fossil evidence suggests that modern humans originated in Africa about 200,000 years ago. Now, races only came into existence once humans started migrating. Roughly about 100,000 years ago, some individuals of this African modern man left the African continent, and moved to eastern Asia, among other places (modern man would have reached Australia about 60,000 years ago). These areas had a different climate to UV radiation, which means that a change in skin color must have occurred (a skin color should not be too pale, but also not too dark so as not to disrupt the production of certain vital substances). About 40,000 years ago, other individuals moved from Africa to Europe via the Middle East, where there was still a different light climate. This then gave rise to the skin color of the Caucasian race.

These migrations, which involved very great distances, were, of course, not completely covered by the same individuals who had departed from Africa. A limited group has probably left, with several generations having passed over before they ended up in China and Australia, for example. In the meantime, of course, they had multiplied, with not all individuals migrating further in one and the same direction. Thus, the entire South East Asia was colonized, and in a later phase also Europe (and later than the American continent).

In this way you get a world population that originates from a limited number of individuals, but which have become locally adapted to, among other things, the prevailing light climate.