Humans thrive on Earth only when they absorb just the right intensity of just the right frequencies of solar radiation. Low-frequency, infrared sunlight helps us keep warm. Moderate-frequency visible sunlight makes it possible for us to see and powers photosynthesis, causing plants to grow. Much higher frequency ultraviolet-B sunlight is needed for our skin to produce vitamin D necessary for healthy bone growth and immune systems. Ultraviolet-B also helps in the formation of serotonin, a chemical that contributes to our feelings of wellbeing and happiness. But too much ultraviolet-B solar radiation causes sunburn, skin cancer, cataracts, mutations, and other damage to DNA. Too much ultraviolet-B inhibits plant growth.
Life on Earth exists in a very delicate Goldilocks balance in the frequencies and intensities of solar radiation reaching Earth’s surface. Over hundreds of generations, our bodies are able to adapt to the levels of sunlight common where we live by changing skin, hair, and eye color, hair style, eye style, and even the length of our limbs.
Starting around eight million years ago, the land in east Africa, in regions known today as Ethiopia, Kenya, and Tanzania, began rising, ultimately reaching nearly one mile above sea level today. This uplift occurred as Africa began rifting apart, forming the East African Rift Valley. This uplift gradually changed weather patterns, causing tropical jungles to become open savannah grasslands.
It was this uplift and change in climate that led to the evolution of humans. Chimpanzees in the east Africa jungle began evolving into humans starting around six million years ago as the jungles thinned. Our ancestors, in order to survive, had to climb down from their trees and learn how to walk and run in order to find scarce vegetable food sources and to hunt abundant wild animals. This evolution became especially rapid around one million years ago as savannah grasslands became dominant.
As our ancestors began to live in bright sunlight, they lost most of their body hair, exposing more skin. The skin of all animals contains a dark brown to black pigment called melanin. Melanin is enhanced by exposure to ultraviolet radiation. It is an increase of melanin that causes the tanning of your 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.
In this way, the skin of all of our distant ancestors became dark brown to black. Melanin in their eyes and hair was also darkened by intense sunlight. They evolved more efficient perspiration, and evolved short, curly, Afro-textured hair to allow more air circulation close to the skin, thus protecting their brain from the heat of direct sunshine.
Today, we can control how much sunlight we absorb by choosing sunglasses, a hat, and other appropriate clothing and by using suntan lotion that is typically labeled to provide “broad spectrum UVA/UVB protection,” which means it absorbs sun’s most damaging ultraviolet rays. But various types of sunscreen lotions have only been around for 5,000 to 10,000 years and clothing for maybe only as much as 170,000 years. Before that time there was little protection from sunshine except to stay in the shade. But humans needed to keep moving around in search for food.
As climate changed into and out of ice ages all during this time, modern humans migrated north, out of Africa, in search of food, beginning 70,000 to 50,000 years ago. They populated southern Asia and even reached Australia. Others migrated into Europe less than 55,000 years ago.
As humans migrated to higher latitudes, their skin and eyes became lighter, their hair became flatter, limiting air circulation near the skin to provide more insulation. The furthest north these people could go during the ice ages was Scandinavia where today we find the lightest skin, the blondest hair, and the bluest eyes. There is a remarkably close correlation between skin color and the latitude where a given population lived for thousands of years. Scientists think that skin color becomes heritable over roughly 100 generations or approximately 2500 years.
As humans moved north, they adapted to less sunshine by adding foods rich in vitamin D to their diets. Fatty foods rich in vitamin D include salmon, herring, sardines, cod liver oil, tuna, egg yolks, and mushrooms, all of which became popular in Europe among people with whiter skin. During the last 10,000 years, after the last ice age, humans began living in the far north where sunshine is much less intense and is almost non-existent during the winter. The Inuit of northern Canada developed a diet rich in vitamin D, emphasizing fatty fish and sea mammal blubber. Northern 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 required or the more vitamin D must be absorbed from diet.
Many of the physical differences among people from different regions on Earth are the result of adapting to climate. The legs and arms of people living in colder climates tend to be shorter, reducing body surface area. This helps conserve heat. Noses for people living in very dry and cold areas tend to be longer and thinner. People living in Siberia and northern Asia have developed an epicanthic fold of the upper eye lid that provides greater insulation for the eye and sinuses from the effects of cold, especially from freezing winds. It might even provide some protection from snow blindness.
Humans are remarkably similar worldwide. Our different looks are determined primarily by the environment where our ancestors lived for thousands of years. Skin color, eye color, hair color, and hair style are determined primarily by the intensity of sunlight. Length of limbs, nose shape, and eye shape are determined primarily by the temperatures common in those regions.
Most humans were born, lived, and died in the same region. In the last 700 years, people have been able to migrate large distances by boat within weeks. In the last hundred years people have moved large distances by airplane within hours. With the rapid mixing of populations, many of the differences are becoming homogenized, but they will still last for thousands of years. Let’s enjoy our physical differences rather than dividing them into social classes.
