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Humans thrive on Earth only when they absorb just the right intensity of just the right types of solar radiation. Low-frequency, infrared sunlight keeps us warm. Moderate-frequency visible sunlight allows us to see and powers photosynthesis, causing plants to grow. But too much high-frequency ultraviolet sunlight and neither animals nor plants flourish. Life on Earth exists only because of a very delicate Goldilocks balance in the frequencies and intensities of solar radiation reaching Earth’s surface.
Earth’s atmosphere filters out the highest energy components of sunlight, including gamma rays, X-rays, and extreme ultraviolet radiation. These high frequencies are completely absorbed in the uppermost atmosphere where they cause ionization of nitrogen and oxygen—forming and heating the ionosphere, 620 to 62 miles (1000 to 100 kilometers) above Earth.
Ultraviolet-C solar radiation is completely absorbed primarily by molecules of oxygen, forming and heating the mesosphere and stratosphere 62 to 6 miles (100 to 10 kilometers) above Earth.
Ultraviolet-B solar radiation is absorbed by ozone molecules, forming and heating the ozone layer 22 to 9 miles (35 to 14 kilometers) above Earth. Ultraviolet-B is the highest energy solar radiation reaching Earth’s surface. But normally, 97 to 99 percent of all ultraviolet-B is absorbed in the ozone layer. This is fortunate because ultraviolet-B has high-enough energy to cause sunburn, skin cancer, cataracts, and disruption of DNA, ultimately causing mutations in plants and animals. Thus, too much ultraviolet-B radiation is damaging and even fatal to life on Earth.
On the other hand, solar ultraviolet-B radiation, when absorbed by human skin, forms vitamin D, which is crucial for healthy bone growth and healthy immune systems. Furthermore, ultraviolet-B is required for healthy plant development. Thus, not enough ultraviolet-B is also damaging to life on Earth.
The crux of this delicate Goldilocks balance is how much solar ultraviolet-B radiation reaches Earth’s surface and how much is absorbed by the cells making up the skin of animals and the surface layer of plants.
The intensity and duration of sunlight is greatest in the tropics, decreasing toward the poles. The primary way our bodies adapt to these changes over long periods of time is by adjusting skin color, eye color, and hair texture.
Our ancestors evolved from apes in the jungles of eastern Africa 6 to 7 million years ago. Around 1.2 million years ago, as the world descended deeper into the last ice age, a mega-drought drove early humans into open savannah grasslands. They began to walk upright in open sunlight. They lost their body hair, evolved more efficient perspiration, and evolved short, curly, Afro-textured hair to protect their brain from the heat of direct sunshine.
But the primary evolutionary adaptation to bright sunlight is skin color produced by melanin. Melanin is a dark brown to black pigment that occurs naturally in the hair, skin, and the iris of the eyes in people and animals. Melanin is generated by exposure to ultraviolet radiation causing tanning of skin. Melanin absorbs up to 99.9 percent of ultraviolet-B radiation before it can penetrate deeper into the body, where is can damage tissue and DNA. Too much ultraviolet-B radiation causes depletion of folate that is required for the body to make DNA and RNA and to metabolize amino acids.
Thus, humans living in intense sunlight in equatorial regions, long before clothes were invented, developed dark skin, short, curly, dark Afro-textured hair, and brown eyes. Homo sapiens, members of our species, evolved around 200,000 years ago in the Great Rift Valley of east Africa located today in the countries of Ethiopia, Kenya, and Tanzania and in South Africa. Beginning 70,000 to 50,000 years ago, some Homo sapiens began migrating out of the tropics into mid-latitudes where sunlight was less intense. They were in search of better food sources and living conditions. At higher latitudes, they evolved, over thousands of years, lower levels of melanin, leading to lighter skin, lighter eye color, and lighter hair color so that they could absorb adequate sunlight. Plus, they evolved straighter hair that blankets the head, providing increased warmth. These lower levels of melanin appear to have evolved in a heritable way in as little as 100 generations—approximately 2500 years.
There is an exceedingly high correlation between skin color and the latitude at which a population lived for thousands of years. The less intense the sunlight, the lighter the indigenous people’s skin. That is why fair skin, blonde hair, and blue eyes are so prevalent in Scandinavia.
As humans moved to higher latitudes, they developed clothing which meant less skin was exposed to sunlight, hastening decreases in melanin. They also developed diets rich in vitamin D, such as seafood, fatty meats, egg yolks, mushrooms, and, more recently, fortified foods such as milk, orange juice, and cereals.
During the last 12,000 years, after the last ice age, humans began living in the far north. Innuit developed a diet rich in vitamin D, emphasizing fatty fish and sea mammal blubber. Eurasian populations emphasized reindeer meat, organs, and fat.
Light-skinned people can produce vitamin D in their skin at rates 5 to 10 times faster than dark-skinned people. With the rapid dispersal and mixing of populations worldwide in recent centuries, up to 50 percent of world population today may not get enough sun and 40 percent of U.S. residents are deficient in vitamin D. How much sunshine is needed for a healthy body is a function of skin color—the darker the skin, the more sunshine is required.
The intensity of sunlight in the tropics has also influenced social customs. For example, the modern siesta, a nap after a midday meal, is a common way for those working outside to avoid the most intense sunshine in the early afternoon.
Thus, the existence and health of humans and plants living on Earth is determined by having just the right Goldilocks balance between how much solar ultraviolet-B radiation is absorbed by the ozone layer versus how much reaches Earth’s surface. Human bodies adapt over thousands of years to solar intensity at the latitude where their ancestors lived. Dark skin, brown eyes, and Afro-textured hair made it possible for early humans to thrive in the bright sunlight of tropical East Africa. Lighter skin, lighter eyes, and straighter, more insulating hair made it possible for humans to survive at higher latitudes as they roamed the world in search of food.
About the Author
Dr. Peter Langdon Ward earned a BA at Dartmouth College and a PhD at Columbia University in geophysics. He worked 27 years at the United States Geological Survey, leading a group of more than 140 scientists and staff and playing a lead role in establishing a major national research program.
He earned two national awards for explaining science to the public. He and his work were featured on Good Morning America. Ward has worked full time in retirement, at his own expense, since 2006, carefully reexamining all the evidence and theories for why climate has changed throughout Earth history.