adamah - the melanocyte count being fixed I believe in regards to once a person is born. Hence a black person won't ever become full white unless they have a michael jackson type disorder. However I don't believe it's fixed for generation upon generation over thousands of years.
Why are black/africans darker then arabs? While arabs for thousands of years were wearing full body garments, african blacks as evidenced by recent history would be close to fully naked.
If the amount is fixed over generation upon generation and not just once one is born, then the skin color tone wouldn't match up with sun exposure over thousands of years. Fully naked blacks or fully clothed arabs over thousands of years would have the same skin tone. Or maybe reversed, etc.
If there were different human races, different kinds of humans in their true form from creation, I believe they have been wiped out. The giants, etc.
As for modern day humans, if you are saying that skeletons with larger jawbones were different races, I would disagree with that. Just like skin color can change over thousands of years due to sun exposure over generations, diet changes and eating habits could change our jaw structure. This is even proven in a recent study, I'm just cutting and pasting this part......
"The rise of agriculture allowed for the development of complex societies and technologies that likely wouldn't have been possible otherwise. It also wreaked havoc on human health. And we can add a new downside to the list: our misshapen mouths.
That's the finding of University of Kent researcher Noreen von Cramon-Taubadel, who examined 295 human jaws from various museum specimens around the world. She found that, even adjusting for climate, geographic, and genetic variation, a consistent pattern emerged: people from agricultural societies have significantly smaller mouths that their counterparts in hunter-gatherer societies.
In itself, that wouldn't be such a bad thing, but the problem is that we all have exactly the same number of teeth. Von Cramon-Taubadel speculates that agricultural societies tend to produce people with smaller jaws because the ground grains and processed animal products we eat are softer than the wild plants and animals eaten by hunter-gatherers."