First off, in the Northern (and Southern) latitudes, the amount of solar radiation received is far less than in the mid latitudes, therefor dark skin isn't needed. In fact it can interfere with Vitamin D production. Secondly, Our ancestors lived through several extreme ice ages in Eurasia. Cansidering that for most of that time Mankind was somewhere near the bottom of the food chain, it just didn't pay to be dark meat in such an environment. Easy pickings for dire wolfs, big cats, cave bears, and a whole host of other large nasty predators. We had to "lighten up" to better blend in. Kinda like the story of the light/dark moths living around London during the early industrial revolution.

I personally think that such an extreme and unforgiving climate, coupled with small fairly isolated populations helped to concentrate "intelligence" genes within the populations of the Eurasian peoples. Stupid people, on average, simply wouldn't live long enough to reproduce in such a climate. This would help to explain, for the most part, why civilization never took hold in "the crib" of Mankind.