If humans "adapt" to their environment, then the adaptation would encompass more than just their skin tone. People who follow this logic will also state that Asian and Eskimo people groups have squinty eyes as an adaptation to blowing sand and snow, yet Arabs, who have dwelt in sand for hundreds of years, don't have squinty eyes. Adaptation also isn't something that starts and stops, it is a system. If people had adapted to their climate in the past, they would continue to adapt, and still be adapting today. Asians, who no longer live in a sandy environment, would be undergoing adaptaion to their current environment. With the passing of time their eyes would become more open.
If you look around you'll notice that the human body doesn't adapt well to the natural environment. That is why we've gone to such great lengths to adapt the environment to us.
If you look around you'll notice that the human body doesn't adapt well to the natural environment. That is why we've gone to such great lengths to adapt the environment to us.