Welcome to the “Light and Health” series. This series explores how light affects our physical and mental health, sometimes in surprising ways.
For most of our evolutionary history, human activity has been linked to sunlight. Technology has liberated us from ancient sleep-wake cycles, but there is evidence that sunlight has left and continues to leave its mark.
Not only do we still tend to be awake during the day and sleep at night, but we can also appreciate light in many other aspects of our biology.
Light may be the reason our ancestors learned to walk upright and on two legs. Light helps explain the evolution of our skin color, why some people have curly hair, and even the size of our eyes.
As we’ll discuss in future articles in this series, light helps shape our mood, immune system, gut function, and more. Light can make us sick, but please tell us why we are sick and treat us.
If we consider millions of years of evolutionary history, we can see that humans are still creatures of light.
we rose up and left africa
The first modern humans evolved in the warm African climate. One explanation for why humans started walking upright and bipedally is to reduce exposure to harsh sunlight. When we stand up and the sun is directly overhead, much less sunlight hits our bodies.
Curly hair may also have protected us from the hot sun. The idea is to protect your scalp by providing a thicker layer of insulation than straight hair.
Early Homo sapiens had special sun protection in the form of highly pigmented skin. Sunlight breaks down folic acid (vitamin B9), which accelerates aging and damages DNA. In the bright climate of our ancestors, dark skin protected us from this. However, this dark skin still receives enough UV radiation to stimulate the important production of vitamin D.
However, when people colonized temperate regions with low light, each group repeatedly evolved lighter skin colors through different genes. This probably happened rapidly within the last 40,000 years.
As we approached the poles, ultraviolet light decreased, so less pigmentation was needed to prevent sunlight from breaking down the folic acid. Lighter skin also allows your body to produce more vitamin D because it lets in more of the light it lacks. However, there was one major drawback. Less pigmentation means less protection against sun damage.
How our skin pigmentation adapted to movement patterns and changes in light.
This evolutionary background has resulted in Australia having one of the highest rates of skin cancer in the world.
Due to our colonial history, more than 50% of Australians are of Anglo-Celtic descent, with white skin and transplanted to an environment with strong UV rays. It’s no wonder we’re called the “sunburned nation.”
Sunlight also contributes to changes in the human eye. Humans at high latitudes have less protective pigment in their iris. They also have larger eye sockets (and possibly eyeballs), presumably to admit more precious light.
Again, these characteristics make European Australians particularly vulnerable to our harsh light. It’s no wonder, then, that Australia has an unusually high incidence of eye cancer.
we can’t shake our biological clock
Our circadian rhythm, a wake-sleep cycle driven by the brain and hormones, is another heavy evolutionary baggage triggered by light.
Humans are adapted to sunlight. Humans see better in bright light and have sophisticated color vision. However, we have poor vision in dim light, and we lack complementary senses such as acute hearing and a keen sense of smell.
Our closest relatives (chimpanzees, gorillas, orangutans) are also active during the day and sleep at night. This supports the idea that early humans had similar diurnal behavior.
This lifestyle may extend further back in human evolutionary history, from before the great apes to the dawn of primates.
Early mammals were generally nocturnal, using their small bodies and darkness to hide from dinosaurs. However, the meteorite impact that wiped out these fearsome reptiles allowed some surviving mammals, especially primates, to evolve a primarily diurnal lifestyle.
If we inherited our diurnal activity patterns directly from these early primates, this rhythm would have been part of our lineage’s evolutionary history spanning about 66 million years. .
This explains why our 24-hour clocks are so hard to shake. It is deeply rooted in our evolutionary history.
Continuous improvements in lighting technology have increasingly freed us from dependence on daylight, such as fire, candles, oil and gas lamps, and eventually electric lighting. So, in theory, you can work or play whenever you want.
However, when our natural daily cycles are disrupted, such as by lack of sleep, shift work, or jet lag, our cognitive and physical abilities decline.
Futurists are already considering the circadian rhythms necessary for life on Mars. Luckily, a day on Mars is about 24.7 hours long, about the same as ours. This slight difference should be the least of the worries for the first intrepid Mars colonists.
How will humans cope on Mars? At least you won’t have to worry too much about your body clock. Niconomad/NASA/Shutterstock
Light is still changing us
Over the past 200 years or so, artificial light has helped to (partially) disconnect us from our ancestral circadian rhythms. But in recent decades, this has taken a toll on our eyesight.
Many genes associated with myopia (myopia) have become more common in just 25 years. This is a striking example of rapid evolutionary change in the human gene pool.
Also, if you are genetically predisposed to myopia, reducing your exposure to natural light (spending more time in artificial light) increases your chances of developing myopia. These remarkable changes occurred during the lifetimes of many people.
Light will undoubtedly continue to shape our biology for thousands of years to come, but its long-term effects may be difficult to predict.
