Why the Asians have such an eye cut?
To begin with, it is worth noting that the term "Asians" is inaccurate from the point of view of the anthropologist – will more correctly say "people of the Asian race" or "Mongoloids". There is, of course, the assumption that a narrow eye cut with Epicatus (fold in the inner corner of the eye. – approx. Red.) saves eyes from dust Central Asia. But the assumption is rather incorrect, since, firstly, the Mongoloids arose in the middle course of Juanhe, and not in Central Asia. Secondly, the narrow cut of the eye does not save from dust, since the dust that scored under the epicantus is worse than the dust in a wide eye, where it can wash the tear. Thirdly, the world is full of groups that live in dusty places, but have extensive eyes, such as Taways and Berbers.
Apparently, there is no point in the form of eye. In its creation, it was not classical driving natural selection, but genetic and automatic processes and sexual selection. Genetic and automatic processes – the processes of statistically random changes in the frequencies of neutral signs. For example, a gene drift, when the next generation is randomly increasing, one of the options had passed, and the other becomes less. That is, if in the first generation there were 50 x 50 people with wide and narrow eyes (and assume that they are inherited without any domination), then in the following without any selection, the frequency can be, for example, 45 x 55, and through a number of generations – even 0 x 100. There are other options for such processes – the "Founder’s Effect" and "Bottle Bottle Effect".
Another factor – sexual selection when women like some kind of option is not because it is useful, but because you like. If women – the ancestors of the Mongoloids liked men with narrow eyes, then their descendants were the same.
Most likely, mutations created a variety, genetic and automatic processes shifted frequencies to some level, and sexual selection picked up and finally strengthened one of the options.
By the way, not all the Mongoloids have Epicatus. In Southeast Asia and Indonesia, many groups do not have it at all, and the eye cut has another.