Cold and Clothes led to lighter skin in humans who lived further north

When the human race started in Africa and began migrating from there sometimes northward into europe and further north into Russia, Sandinavia, Sibera etc. eventually our noses changed from the type of noses that shed heat and became more knife edged which held more heat in and yet still shed the rain. Also, other parts of the body sort of modified so they would be less likely to freeze and to fall of like reproductive organs on men and arms and fingers became less elongated and more stubby and thick to hold heat. Also, diet had a lot to do with these changes as well.

But, the biggest factor in lighter skin came from not getting enough Vitamin D from the sun from wearing more clothes. Skin had to get lighter in order to absorb more sunlight to get more vitamin D so people didn't die so much from not enough Vitamin D. However, eventually when they moved back to warmer climates they got skin cancer with the whiter skin unless they slowly got sun and adapted to it.