Chicken & egg thing innit?
The theory I've read into was that when people first began colonising more northern latitudes, they were very likely still predominantly black, with lighter skin an occasional mutation.
The reduced sunlight strength led to a higher incidence of rickets in people with darker skin who were less able to absorb sunlight leading to vit D deficiency. A man with rickets was generally less proficient at running and hunting, whilst those with lighter skin mutations were less affected and became more sought-after mates for women (better providers). With women, rickets can lead to deformities of the pelvic girdle leading to both increased miscarriage chance and birth difficulties. This both lowers the chance of a darker-skinned women successfully reproducing, whilst also in time making them less attractive mates.
So we had both men and women with the lighter skin mutation being more attractive mates, and a higher chance of them being born successfully. Over generations, the lighter-skin gene became the predominant one through natural selection.
Of course, this is just a theory (although I first heard of from Dr Alice Roberts, and she is never wrong ) and there will obviously be more contributing factors - like the lactose breaking-down gene you mention - but it makes logical sense to me as an explanation.
Originally Posted by: Saint Snow