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For most of our evolutionary history, human activity was tied to daylight. Technology has freed us from these ancient sleep-wake cycles, but there is evidence that sunlight has left and continues to leave its mark.
Not only are we still awake during the day and asleep at night, but we have light to thank for many other aspects of our biology.
Light may have helped our ancestors start walking upright on two legs. It helps explain the evolution of our skin color, why some of us have curly hair, and even the size of our eyes.
As we'll explore in future articles in this series, light affects our mood, immune system, gut health, and more. Light can cause illness, explain why we're sick, and then help us get better.
Millions of years of evolutionary development show that humans are still, in many ways, beings of light.
We got up and left Africa.
The first modern humans evolved in the warm conditions of Africa. And reduced exposure to intense sunlight is one explanation for why humans began to walk upright on two legs. When we stand with the sun directly overhead, our bodies are exposed to significantly less sunlight.
Curly hair may also have protected us from the scorching sun. The idea is that it provides a thicker layer of insulation than straight hair, protecting the scalp.
Early Homo sapiens had extra protection from the sun in the form of highly pigmented skin. Sunlight destroys folate (vitamin B9), accelerates the aging process, and damages DNA. In the light climates of our ancestors, dark skin provided protection against this. But that dark skin still let in enough ultraviolet light to stimulate vital vitamin D production.
However, as humans colonized temperate zones with less intense light, they repeatedly evolved lighter skin, through different genes in different populations. This happened rapidly, probably in the last 40,000 years.
As UV radiation decreased closer to the poles, less pigmentation was needed to protect against the sun's folate-damaging rays. Lighter skin also let in more of the scarce light our bodies need to make vitamin D. But there was one big downside: Less pigmentation meant less protection against sun damage.
How our skin pigmentation has adapted to migration patterns and changing light levels.
This evolutionary context explains why Australia has one of the highest rates of skin cancer in the world.
Our colonial history means that over 50% of Australians are of Anglo-Celtic descent, with fair skin transplanted into a high UV environment. No wonder we are known as the 'sunburnt nation'.
Sunlight has also changed human eyes. Those living in high latitudes have lower concentrations of protective pigment in their irises. They also have larger eye sockets (and presumably eyeballs), perhaps to let in more valuable light.
These characteristics make Australians of European descent particularly vulnerable to our bright light. It is therefore not surprising that Australia has unusually high rates of eye cancer.
We can't change our biological clock.
Our circadian rhythm—the sleep-wake cycle regulated by our brain and hormones—is another heavy evolutionary legacy caused by light.
Humans are adapted to daylight. In bright light, we see well and have fine color perception. However, in dim light, we see poorly and we don’t have senses like keen hearing or smell to compensate.
Our closest relatives (chimpanzees, gorillas and orangutans) are also active during the day and sleep at night, which supports the idea
Sourse: www.livescience.com
