Light skinned people lack melanin because they arose where clothing wasn't optional and sunshine was lacking.

The theory is the lighter skinned people survived in the colder northern climates expressly because their lighter skin allowed more sunlight in, and thus produced adequate amounts of Vitamin D even when clothed for the cold climate.

Those with more melanin and thus dark skin did not produce enough Vitamin D, and died out in the northern darker environments. They did well and produced even darker ancestors in the equatorial areas where the sun was intense and clothing was optional.

And skin cancer is a result of over exposure to the sun.