I have written about Jablonski and her work on evolution of skin color and roles of Vitamin D before (here). This article published in Scientific America in 2002 is today widely read by undergraduate anthropology students. In this article, Jablonski and Chaplin reject that skin cancer is the major evolutionary factor for dark-colored skin in Equatorial areas. They suggest that
Vitamin D and folic acid are two important factors of biological adaptation responding varying level of UV radiation, and cultural adaptation and recent migration were also important factors understanding the observed geographical pattern of skin color.
The people living in equatorial areas, whether they have dark colored or light colored skin, can get enough sun light to synthesize Vitamin D. However, they have to have dark colored skin to protect them from harmful UV radiation. Overexposure to the UV radiation can cause skin cancer, and skin cancer is deadly to the individuals, but it does not reduce reproductive success of individuals, because people usually have skin cancer after reproductive age. Instead, overexposure to UV ration may lead to reduced reproductive success of individuals, because overexposure to UV radiation may reduce the level of folate, an essential Vitamin B, in their blood. The low level of circulating folate is associated with higher risk of babies with spina bifida and low sperm counts.
For those people living in high latitude areas, where they can get enough sun light to synthesize Vitamin D only in limited time of the year, having light-colored skin is advantageous. In high latitude areas, people with dark colored skin tend to have low level of circulating Vitamin D, and low level of circulating Vitamin D is associated with problems with bone development and maintenance, and immune system, etc (see here).
Probably, her major contribution is to publicize how evolution can explain skin color variation around the world and to demonstrate that the field of anthropology can help people appreciate the genetic and biological variation that exists in the world.