There are two competing forces, then, affecting human skin tone: the need to protect against cancer and the need for vitamin D3. The interesting thing is that humans have evolved to carefully regulate the amount of UV light penetrating the skin, and hence carefully balancing the trade-off between vitamin D3 and cancer.
A recent review reports that skin reflectance is lowest (i.e. melanin is highest) at the equator, then gradually increases, about 8% per 10° of latitude in the Northern Hemisphere and about 4% per 10° of latitude in the Southern Hemisphere. This pattern is inversely correlated with levels of UV irradiation, which are greater in the Southern than in the Northern Hemisphere.
There are, of course, exceptions to the rule, but these are informative in their own right. For example, the Inuit natives of the arctic are dark-skinned. This can be explained by the fact that their traditional animal-based diet provides plenty of vitamin D3. Another example is the difference in skin color between the dark skinned Bantu and light skinned Khoisan inhabiting Southern Africa. It poses a puzzle unless one considers that the Khoisan are derived from one of the earliest migrations into Southern Africa, whereas the Bantu migrated from West Africa more recently, perhaps within the past 1000 years.
Jablonski and Chaplin end their essay by writing, "Our current knowledge of the evolution of human skin indicates that variations in skin color, like most of our physical attributes, can be explained by adaptation to the environment through natural selection. We look ahead to the day when the vestiges of old scientific mistakes will be erased and replaced by a better understanding of human origins and diversity. Our variation in skin color should be celebrated as one of the most visible manifestations of our evolution as a species." Read more about skin color here.