Wednesday, May 27, 2015

The Biology Behind Human Skin Color

This fascinating subject is explained so clearly in the following article. It makes me wonder if fair-skinned people using sun block regularly would change skin color if they stopped using it. And, I am not talking about getting a sunburn which gives a short-lived pink tint to light skin.

I remember the cod liver oil. Mom fed it to me almost religiously. “It’ll make your bones strong,” said she, “so you don’t get rickets.” Even after she described rickets, I could not really envision the condition.
But I’ve seen plenty of photos of it since and learned that it is tied up with climate and the color of one’s hide. So let’s go there.
The first thing to realize is that skin color is a tough subject to study. One can’t just use people’s faces — they’re too subject to color alteration due to sun and weather, even tanning or whitening agents. So researchers have settled on using comparisons from one body area not much exposed to those effects: the color of the inner upper arm. Unless you’re an expert on those, be a little tentative about what you think you know about human skin colors.
The second point is that the genetic basis of color is not simple either. We’ve got a few genes identified which have major effect, but there are others with minor effects and we don’t yet have detailed gene sequencing studies to resolve the issue.
In addition to all this, the subject has been so politically explosive that few researchers want to get involved. One major exception is Nina Jablonski of Pennsylvania State University who has spent more than 20 years on the subject. Others have joined in, but I rely here mostly on an article in the November 21, 2014 issue of Science featuring her work, and her 2012 book "Living Color."
Humans make two major types of skin pigment: Eumelanin gives the browns and blacks of darker-pigmented people. Pheomelanin gives various shades running from yellow to red. All people except albinos have both these pigments, and the relative amounts thereof account for the color differences of peoples worldwide. And these are not reliable indicators of “race” — a term which has no objective value at all. There is more color variation within each of the traditional “races” than there is between them, and color is useless as a categorical term.
But there are geographical patterns, of course. Peoples living near the equator are more darkly pigmented, and among those groups (as elsewhere, e.g., Tibet, Bolivia) those living at high altitudes or near the oceans have the higher levels of eumelanin. In contrast, peoples living farther from the tropics have the highest levels of pheomelanin and are light-skinned, with yellow or red hair common. Similarly, peoples with higher eumelanin ratios have the greatest ability to tan — not always obvious with the already dark pigmentation — and those with higher amounts of pheomelanin are most likely to sunburn. Tanning is a property of eumelanin production.
Humans, according to overwhelming evidence, began in Africa. Our ancestors there probably had dark pigmentation. As peoples moved elsewhere in the world (the great “human diaspora”), nature progressively selected pigment variations (reduction, some re-acquisition) that best fitted their new environments. That takes at least a score or more thousand years. So now let’s go to the biology.
Ultraviolet radiation (UVR) from the sun is of course most intense in equatorial regions.  It is more intense at high elevations. And it reflects much more from water surfaces than from terrestrial environments, so persons living on the ocean coasts are exposed to more than those living inland.
UVR generates Vitamin D in the skin. We need that for bone strength, a good deal of immune system function, heart health, and other basic body functions. Too little Vitamin D and children get rickets -- adults are subject to osteoporosis and other diseases.
But one can get too much sunlight as well. Sunburn is an obvious condition, and that relates to higher rates of skin cancers. But UVR also appears to destroy folate, well-established as critical for neural tube development in developing fetuses. So nature has to balance the UVR level that produces healthy bones while not generating so much that it destroys folate and causes conditions such as spina bifida. Tanning is helpful for this in peoples of moderate skin color. They live primarily in areas of seasonally-variable UVR: intense amounts in the summer, much less in the winter. Tanning thickens the skin and generates eumelanin production in summer and reverses that process in winter.
Human migrations, particularly in the last 400 years, have produced many “mis-matches” — people living in areas and in cultural conditions for which their bodies are not adapted. It was high levels of rickets and tuberculosis among freed African slaves who moved into America’s northeastern states after the Civil War that produced studies to demonstrate that they were not getting enough Vitamin D for healthy bones and immune systems.
Cod liver oil solved the problems, and helped even lighter-pigmented people in temperate zones who may have not gotten enough Vitamin D in winter.

1 comment:

  1. Interesting article on skin color, Barbara. Hope to see you next week.