It not very often I disagree with Chris but I think it's unlikely he's right about us all being white originally.
If the current scientific theory about the origins of man is to be believed then every one of us came out of central Africa. Assuming this is the case, it's unlikely that Homo Sapiens had light skin. It stands to reason that a creature such as ourselves with precious little body hair would have survived long as a species living in the central African sun without factor 50 sunscreen.
Again, assuming it's true that we all came out of Africa, it seems far more likely to me that Homo Sapiens started out having very dark skin and as they began to migrate and populate the less sun-drenched parts of the world, over a period of hundreds of thousands of years (anywhere from 400,000 and 250,000 years ago) and tens of thousands of generations, certain 'tribes' of humans gradually found that their skin pigmentation became lighter. This would make evolutionary sense too as organisms that receive no light are often very light skinned (sometime even transparent) as they have no biological need to repel sunlight.