What makes you think that humans have never adapted to cold climates? Most of Europe is cold, and humans have lived there for tens of thousands of years. People living in Eurooe slaughtered animals for their fur, something that Africans did not need to do. A lot of European adaptions, including their appearance was for survival in cold, forested climates. People who have blue eyes, for instance, actually do see better in the dark.
The brown skin of most Africans is 'camouflage' in African environments. The lighter skin of Europeans is 'camouflage' in colder environments. The darker skin of most Africans is an adaption to hair loss and heat. The skin color offers some protection against the equatorial sun. The lighter skin of European makes it easier for them to absorb vitamin D from sunlight. Europe has less sunlight than Africa. Studies show that it take about 10,000 years, plus the right amounts of vitamin D and folic acid to go from dark skin to light skin.
Although I know that racism is alive and well (I especially think that biological theories are often off), I am not willing to throw the baby out with the bathwater. Many of my fellow African Americans sound like Trump, although Trump is more mean-spirited than just ignorant.