The Nature of Human Skin Pigmentation
On Sunday morning at #AAASmtg in Washington DC, Nina Jablonski talked to use about human skin pigmentation as an example of natural selection.
“Human skin is colorful, it’s mostly naked, it’s sweaty, and it’s tough yet sensitive,” Jablonski said. The gradient of human skin pigmentation is very clear in the old world, as it’s lighter in the northern countries and darker in Africa.
But why did human skin pigmentation evolve as it did? When you look at other apes and humans, our relatives have lightly pigmented skin covered by dark hair — this was the ancestral pigmentation of our lineage.
“When you think of Lucy’s species, you can think of lightly pigmented skin covered by dark hair,” she said, noting that eventually as the hominins became more naked they developed more melanin.
When Homo ergaster 1.6 mya was foraging in the savannah, the species would have needed more naked and sweaty skin for keeping cool. In addition, Jablonski’s research has found that permanent dark pigmentation evolved 1.2 mya, at the same time as these other developments — an interesting development!.
Exposed skin could lead to disease states, so by increasing the amount of pigmentation, H. ergaster protected the species and the pigmentation was selected on.
Ultraviolet radiation (UVR) had a lot to do with the advent of darker skin pigmentation. On a map (by George Chaplin based on NASA TOMS7 satellite data) of annual average UVR, Jablonski was able to clearly show the strength of UVR.
“We evolved initially in equatorial Africa and then we had two waves of dispersal from Africa,” Jablonski said. “This really changed the selective genes for human skin fundamentally.”
The key thing about ultraviolet radiation is that wavelengths do different things to the human body. In general, UVR does a lot of damage, such as DNA damage leading to skin cancer.
Skin cancer generally affects people as they’re older beyond their reproductive years. However, the sun has a definitive effect on folate metabolism. One of the key things we see as important, is the effect on folate metabolism on birth defects. Because folate is needed for making DNA and the competition for folate is intense in the presence of UVR, “why not increase the amount of melanin — a superb natural sunscreen — the evolution of permanent melanin was extremely important in high equatorial radiation levels.”
However, melanin comes with a downside. It slows production of vitamin D precursor in the skin, which is essential for calcium metabolism and bone health, as well as incredibly important to the maintenance of a strong immune system.
If we look at our hairy timeline, 6 mya we had hair and light skin, then with dispersal from Africa into India and Asia, then eventually Indonesia and Europe, what did UVR have as an influence?
To Jablonski, it is the combination of needing to protect against DNA damage and folate metabolism as much as possible, but while naturally selecting for less skin pigmentation for keeping the skin light enough to absorb enough UVB rays to create vitamin D.
Indeed, we have an independent evolution of depigmentation of humans in Asia and in Europe. And we also know that the Neandertals experienced the same selection of depigmentation. There are many genes involved in pigmentation and Jablonski said she has even found in some populations, where the amount of UVR changes by season, that people have adapted to change the amount of melanin in their skin throughout the season
Nowadays, accelerated rates of movement around the world, has put humans in environments that are poorly matched, Jablonski said, which has created serious health problems such as with rickets in children, birth defects, and even metabolic syndrome.
For the purposes of education on health, we need to teach that skin color is an adaptation. It is the most visible product of evolution by natural selection on the human body. This is why we need to use it to teach evolution, Jablonski said.
Additional note posted Tuesday, Feb 22. I discovered afterward that Jablonski has a wonderful TED talk that I think everyone should watch, so I posted it below.