The Biology Book: From the Origin of Life to Epigenetics, 250 Milestones in the History of Biology (2015)
Skin is the body’s largest organ, weighing about 6 pounds (13 kilograms), and it’s the primary site of interaction with the outside world. Its color has been the source of cultural divisions. Although we tend to focus upon its outward appearance, the skin plays a number of important functions in addition to protecting against mechanical injuries, chemicals, and microbes. It also helps regulate water balance and body temperature, stores fats, and produces hormones and vitamin D3.
Melanin, primary determinant of skin color in humans and also present in the hair and iris of the eyes, is produced by melanocytes, which are found in the bottom layer of the epidermis. Upon exposure to ultraviolet radiation (UVR), melanin production is increased, causing the skin to tan, and it has been long believed that dark skin pigmentation protects the body against the sun’s harmful UVR.
In 2000, anthropologist Nina Jablonski, then at the California Academy of Sciences in San Francisco, and her husband George Chaplin proposed that skin color was an evolutionary adaptation to different levels of UVR to which humans were exposed as they migrated over millennia. They formulated a theory based on their analysis of data generated by NASA’s Total Ozone Spectrometer, which, in 1978, measured the UVR reaching the Earth’s surface in more than fifty countries around the globe—a level that was progressively weaker farther from the equator. They observed a correlation showing the weaker the UVR, the lighter the skin.
The earliest humans had dark hair covering lightly pigmented skin. By the time they moved to East Africa some 1.2 million years ago and lived closer to the equator, they had become functionally hairless and had acquired dark skin pigmentation. Jablonski and Chaplin hypothesized that as they migrated, skin color changes were required to balance the harmful effects of excessive radiation against the competing need to have sufficient UVR for vitamin D3 synthesis; this vitamin is required to maintain sufficient blood levels of calcium and phosphorus, which promote bone growth, and is needed for healthy reproduction.
SEE ALSO: Anatomically Modern Humans (c. 200,000 BCE), Depletion of the Ozone Layer (1987).
Upon exposure to ultraviolet radiation (UVR) from sunlight, the skin tans. Over the generations, society has had mixed feelings about the desirability of tanned skin. For light-skinned people, deeply tanned skin is often in vogue, but is also considered an invitation to skin cancer and prematurely aged skin.