The Body: A Guide for Occupants

by Bill Bryson

“Melanin is a superb natural sunscreen,” says Jablonski. “It is produced in cells called melanocytes. All of us, whatever our race, have the same number of melanocytes. The difference is in the amount of melanin produced.” Melanin often responds to sunlight in a literally patchy way, resulting in freckles, which are technically known as ephelides. Skin color is a classic example of what is known as convergent evolution—that is, similar outcomes that have evolved in two or more locations. The people of, say, Sri Lanka and Polynesia have light brown skin not because of any direct genetic link but because they independently evolved brown skin to deal with the conditions of where they lived. It used to be thought that depigmentation probably took perhaps ten thousand to twenty thousand years, but now thanks to genomics we know it can happen much more quickly—in probably just two or three thousand years. We also know that it has happened repeatedly. Light-colored skin—“de-pigmented skin,” as Jablonski calls it—has evolved at least three times on Earth. The lovely range of hues humans boast is an ever-changing process. “We are,” as Jablonski puts it, “in the middle of a new experiment in human evolution.” It has been suggested that light skin may be a consequence of human migration and the rise of agriculture. The argument is that hunter-gatherers got a lot of their vitamin D from fish and game and that these inputs fell sharply when people started growing crops, especially as t...