For most of our evolutionary history, human activity has been linked to sunlight. Technology has liberated us from these ancient sleep-wake cycles, but there’s evidence that sunlight has left and continues to go away its traces.
Not only are we still awake during the day, but we sleep at night, we will thank light for a lot of other facets of our biology.
Light could have made our ancestors walk upright on two legs. Light 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 learn in future articles on this series, light helps shape our mood, our immune system, our gut function, and way more. Light could make us sick, tell us why we’re sick, and then heal us.
Millions of years of evolutionary history mean that humans are still largely creatures of light.
We got up and left Africa
The first modern humans evolved in the warm African climate. And reducing exposure to brilliant sunlight Is one explanation why people began walking upright, on two legs. When we stand up and the Sun is directly above us, much less sunlight reaches our body.
They also can have curly hair he protected us from the hot sun. The idea is that it provides a thicker layer of insulation than straight hair to guard the scalp.
Early had additional sun protection in the form of highly pigmented skin. Sunlight breaks down folic acid (vitamin B9), accelerates aging and damages DNA. In our light ancestral climates, dark skin was protected against this. But this dark skin still confessed enough UV light to stimulate the obligatory production of vitamin D.
However, as humans colonized temperate zones with lower light, this occurred lighter skin has evolved again and againthrough different genes in several populations. This happened quickly, probably inside the last 40,000 years.
With reduced UV radiation closer to the poles, less pigmentation was needed to guard sunlight from breaking down folic acid. Lighter skin also let in additional thin light, allowing the body to provide vitamin D. But there was one big drawback: less pigmentation meant less protection against sun damage.
This evolutionary background is why Australia has one of the highest rates of skin cancer in the world.
Our colonial history signifies that over 50% of Australians are of Anglo-Celtic descent, have fair skin and are transplanted to an environment with high levels of UV radiation. No wonder we’re described as “a rustic scorched by the sun“.
Sunlight has also contributed to changes in human eyes. People living in high latitudes have less protective pigment of their irises. They have too larger eye sockets (and probably eyeballs), perhaps let in the more precious light.
Again, these characteristics make Australians of European descent particularly liable to our harsh light. So it’s no surprise that Australia has this unusual thing high rate of eye cancer.
We cannot move our biological clock
Our circadian rhythm – the wake-sleep cycle driven by our brains and hormones – is one other piece of heavy evolutionary baggage triggered by light.
Humans are adapted to sunlight. In the brilliant light people can see well and improved it color vision. But we see poorly in dim light, and we lack senses like acute hearing or acute smell to make up for it.
Our closest relatives (chimpanzees, gorillas, and orangutans) are also lively during the day and sleep at night, supporting the concept that early humans exhibited similar daytime behaviors.
This lifestyle probably goes back further in our evolutionary history, before the appearance of great apes, to the origins of primates.
The earliest mammals were generally nocturnal, using their small size and cover of darkness to cover from dinosaurs. However, the meteorite impact that worn out these fearsome reptiles allowed some surviving mammals, especially primates, to largely evolve each day lifestyle.
If we inherited our daylight activity pattern directly from these early primates, then this rhythm would have been part of the evolutionary history of our lineage for nearly 66 million years.
This explains why it is vitally difficult to maneuver our 24-hour clock; it’s so deeply rooted in our evolutionary history.
Next improvements in lighting technology they freed us more and more from our dependence on daylight: fire, candles, kerosene and gas lamps, and finally electric lighting. So theoretically we will work and play at any time.
However, our cognitive and physical performance deteriorates when our internal circadian cycles are disruptedfor instance, because of lack of sleep, shift work or jet lag.
Futurists have already considered the required circadian rhythms life on Mars. Fortunately, a day on Mars lasts about 24.7 hours, which is analogous to ours. This slight difference must have been the least of the worries of the first intrepid Martian colonists.
NikoNomada/NASA/Shutterstock
The light continues to alter us
Over the last 200 years, artificial lighting has helped (partially) disconnect us from the circadian rhythms of our ancestors. However, in recent many years this has come at the expense of our eyesight.
Many genes are related to myopia (nearsightedness). develop into more common in only 25 years, a striking example of rapid evolutionary change in the human gene pool.
And if you’ve a genetic predisposition to myopia, reduced exposure to natural light (and spending more time in artificial light) increases your likelihood of developing myopia. These noticeable changes have occurred over the course of many individuals’s lives.
Light will undoubtedly proceed to shape our biology for millennia to return, but the long-term effects could also be difficult to predict.