Human skin color a compromise by nature
The singer belts out one clear note, and holds it; the wine glass absorbs sound energy from the air, vibrates and shatters.
In a somewhat similar manner, soup in a microwave oven absorbs energy and boils, even as the bowl stays cool. Soup is mostly water, and water absorbs microwaves.
Leaves are green because chlorophyll absorbs every color but.
The world is made of molecules, clusters of atoms. Like a well-designed antenna, every molecule absorbs specific “colors” of the electromagnetic spectrum. The “colors” may be visible, like green, or invisible, like microwaves.
DNA, which holds the blueprints that keep us alive, is a good antenna for ultraviolet light – so good, in fact, that it can absorb enough energy to be damaged. Damaged DNA can lead to mutations, which in turn can lead to cancer and other diseases.
Evolving under the strong African sun, our ancestors’ DNA needed protection from ultraviolet (UV). A brownish pigment, melanin, made their skins dark and blocked the sun.
For millions of years, melanin saved our skin. But when groups of ancestors left Africa for lands to the north, a serious problem arose.
Our bones and teeth are made of calcium phosphate (CaPO4), a molecule that doesn’t dissolve in water (if it did, our limbs would be rubbery, our teeth soft).
The insolubility of CaPO4, however, makes it hard to move from the gut (where it’s present in food) into the blood, which then takes it to our bones and teeth. CaPO4 dissolves in the stomach but, in the alkaline conditions of the small intestine, tends to bond together again – to re-solidify. It must be absorbed before that happens, lest it be lost.
In the lining of our gut lie tiny molecular machines made of protein that latch onto calcium atoms and send them into the blood. Once the calcium’s been hauled through, phosphate tends to follow.
The calcium-binding protein is not a permanent part of the intestinal wall, however. It must be continually re-manufactured. This requires a signal, telling the body to make it. The signal is a hormone: calciferol, also known as vitamin D.
Vitamins, by definition, are substances we get from food, needed in tiny amounts. Vitamin D is available in some foods but, if conditions are right, we don’t have to eat it … we can make it ourselves … almost.
Our bodies can make a molecule that is very, very close to vitamin D, but, at almost the last step, stop short. Almost the only task remaining is to break the bond between two atoms sticking together. Normally, we separate atoms with molecular machines called enzymes (such as those that digest our food); but in this case, we don’t have an enzyme that can do the job.
The atoms to be separated, however, compose an excellent antenna for ultraviolet. After our bodies manufacture the molecule to become vitamin D, we secrete it into the blood, which carries it to the skin. Exposed to the sun’s UV, the bond is broken, manufacture of the vitamin completed.
Carried back into the body, vitamin D triggers the formation of the calcium-binding protein, and a host of other bone- and tooth-building reactions.
When some of our ancestors moved north out of Africa to settle in higher latitudes, the sun drew lower, and its light lost its strength. Less likely to cause cancer, weaker UV was also less able to form vitamin D. Evolution favored those whose skin produced less melanin, and the skin of those who moved north grew light.
Alan Stahler is an amateur astronomer and amateur naturalist who trained as a biologist, gives private lessons, and teaches biology and geology at Bitney Springs Charter High School. His radio science programs can be heard at noon on alternate Tuesdays on KVMR, 89.5 FM. You can write him in care of The Union, 464 Sutton Way, Grass Valley, 95945.
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