What contributes to human skin color, and why is there so much variation in skin color among different populations?

Science redefines the meaning of racial categories
DRIVING QUESTIONS
1. What contributes to human skin color, and why is there so much variation in skin color among different populations?
2. Where did the earliest humans evolve, and how do we know?
3. What can genetics and the fossil record tell us about human evolution? 

1-2 pages