ONCE UPON A TIME ALL HUMANS HAD DARK SKIN

Skin color, eye color, and hair texture are the primary ways human bodies adapt to intense sunlight in the tropics.

- Dr.Peter Langdon ward

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. [#1- on next spread]

Pictured above: Temperature in Earth’s atmosphere decreases with increasing altitude except in the stratosphere, which is heated by absorbing solar ultraviolet-C and ultraviolet-B radiation, and in the thermosphere, which is heated by absorbing extreme ultraviolet, x-ray and gamma-ray radiation; right: All solar ultraviolet-C radiation is absorbed in the stratosphere. Nearly all solar ultraviolet-B radiation is absorbed in the stratosphere and especially in the ozone layer. Most ultraviolet-A radiation reaches the Earth’s surface.

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. [#2]