The vitamin D theory is popular but some scientists disagree with it. Not all northern populations have light skin and everyone was outdoors so much then that vitamin D deficiency should not have been a major problem. Natural selection requires many people to die before breeding age to choose a preference. A vitamin D deficiency was probably not enough to kill off so many in such short a time. Mothers play an under-appreciated role in cosmetic trait selection, by playing favorites. This is hard to understand in modern times because we have food security, but they didn't have that back then and mothers often had to choose among their many children. Some proof that blond hair, light skin, and less hairy bodies was cultivated by mothers is that children are whiter, blonder, and less hairy than adults. Most European babies have blond hair and blue eyes, then these traits are free to change once the mother has bonded to the child.
I doubt very much that either Neanderthals or their homo contemporaries had to choose among their “many” children. I have seen no literature indicating that either group had large numbers of children. Usually, children would be born 4 to 5 years apart as the mothers would be nursing until around that age, and often nursing prevents ovulation. Also, in current primative cultures, it is often taboo for a male to mate with a woman who is lactating.
Sunken Civ has posted several items about the ginger gene which some scientists belive is Neanderthal in origin and is found in large numbers of north western europeans. Regarding northern peoples who were neither blond or redhead, coastal people like Esquimos and Scandinavians had access to fish and other seafood which has a high Vitamin D component especially in the liver. Furthermore, since it was cold, these peoples were not necessarily getting much sun exposure on their fur clothed skin.
Arctic natives have darker skin, but they live TOO far north for skin-synthesis of Vitamin D from sunlight to work, so light skin would have had no survival value to them. In the winter they get little or no sunlight, and all year they are covered up enough that they get minimal sun exposure. They get all the vitamins they need from fish and meat.
This also tends to disprove the vegan contention that vegetarianism is the most "natural" diet for humans. It's really hard for humans to get all the nutrients we need from just plants.