One of the most significant results of recent Liver cell research is the mapping of human DNA. In doing so, researchers have been able to trace the entire human race back to the beginning, back to one female born in Africa, more than likely a woman with very darkly pigmented skin. They're saying that the human population has at least one common ancestor! This knowledge is important when trying to find a liver transplant donor, because the donor doesn't have to be of the same race or "skin tone" as the receipiant.
Recesssive genes for fairer skin and straight hair were somehow introduced into the gene pool. Genetic studies indicate that inbreading or incest can produce genetic anomalies where the recessive genes for lighter skin tone and straighter hair are manifested. Kinky hair-dark skin parents have a curly-hair dark skin child and their society elevates that single child to a position of honor, if they are the only one. But when a curly-hair dark skin child mates with another curly-hair dark skin child and they produce one curly-hair dark skin child, a kinky-hair dark skin child, a curly-hair lighter skin child, and a straight-hair lighter skin child a cast system and/or racism is born. some people choose to call this human nature!
In a cast society where skin color and hair texture are determining factors for acceptance, the minority is usually ostracized, enslaved, and/or forced out of the encampment. Once the recessives got away from the dominant group, they would sire children with dominant genes of kinky hair-dark skin, but also children with curly or straight hair and fair or fairer skin tones. Innbreading would begin all over again. Other differences developed, including eye coloring and eye-shape.
Some groups would go farther North to get away from sun exposer and heat, while others went East or West of what we now call "darkest Africa". When they went as far East or West as they could on the continent, they took to the seas and populated islands and other continents. They went farher North and crossed to other continents. Survival of the fittest in terms of wars and disease also have a great deal to do with which descendants survived and which ones disappeared off of the planet. This process of developing three different races and many skin tones took a long, long time.
All of this helps to explain why there are skin tone differences on both the African continent and other continents.