Shedding light on skin color

At the beginning of anthropologist Nina Jablonski's lecture yesterday at the Wagner Free Institute of Science, it appeared her audience of about 100 was composed of several different races.

By the end of the free lecture, titled "The Evolution of Human Skin Color," the Pennsylvania State University professor had made a case that we are all just people with varying levels of melanin.

As author of the book

Skin: A Natural History

, Jablonski has studied all aspects of skin, perhaps none more important than why it appears in such a puzzling array of hues. It all comes down to the planet's uneven distribution of sunlight and the universal human need for two vitamins, she explained.

This knowledge was very recently acquired. "Only in the last decade or so has our data allowed us to crack open the mystery," Jablonski said as she began her lecture at the 152-year-old science museum near Temple University.

Nature has painted human skin using one major brown pigment, melanin, which evolved in many species. "It's a natural sunscreen," she said, which is important because humans have a troubled relationship with the sun.

Since we are relatively hairless creatures, our skin gets bombarded by ultraviolet light, which can burn us, destroy the DNA in skin cells, and lead to cancer. Hence an advantage of dark skin.

But there is more to melanin than protection from skin cancer and sunburn. Scientists recently realized that ultraviolet rays penetrating skin destroy the B-vitamin folate. With too little folate, or folic acid, men cannot make adequate sperm and women cannot start healthy pregnancies. So in very sunny places, any genetic mutations that created light skin would likely die out with their owners.

But with melanin offering so many advantages, the question was why anyone would evolve light skin.

Lighter shades came about because humans need some sunlight to penetrate skin and trigger a chemical reaction that produces vitamin D.

To illustrate the devastating effects of vitamin D deficiency, Jablonski showed slides of children with badly bowed legs and softened bones. In women, a lesser deficiency can lead to a narrowed pelvis, making childbirth impossible.

The original skin color was almost certainly very dark, since scientific evidence points to sunny Africa as the cradle of humanity. But once some branches of the human family starting moving north to Asia and Europe, the need for vitamin D gave those with lighter skin an advantage in absorbing the meager sunlight in winter.

Because vitamins lie at the heart of our color differences, locally consumed foods also play a role. Whales and fatty fish can give people some vitamin D, Jablonski said, so diet may explain why the Inuit, who live in Alaska and Greenland, are much darker than people from Northern Europe.

Recent findings from genetics labs show that there are many roads to what we think of as white and black skin - both of which, or course, are really shades of brown. In 2005, for example, scientists found that Europeans became light-skinned through a different combination of mutations than did Northern Asians.

Last year, scientists scraped enough DNA from the bones of a Neanderthal man to show that this extinct branch of humanity carried genes associated with fair skin and red hair.

Currently, Jablonski said, researchers are seeking genetic variants that led to dark skin in far-flung peoples - those from Australia, New Guinea and southern India as well as Africa.

While Jablonski hopes that examining skin through science can help defuse racism and racial tension, she said, she is also concerned with what she calls colorism. Colorism has more to do with perception of beauty, she said. Its primary victims are women.

With a slide of people frying on the beach and an advertisement for bronzer, she explained that colorism has white women thinking they look sickly without a tan. More dangerous still, dark-complexioned women in some countries are driven to use dangerous skin-lightening products, many containing arsenic, mercury and other poisons.

"Why are we always trying to change the way we appear?" Jablonski asked. "Skin color is a beautiful product of evolution. . . . We should revel in it."