Light helps explain the evolution of our skin color, why some of us have curly hair and eye size. And light shapes us to this day.
For most of our evolutionary history, human activity has been associated with 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 do we still tend to be awake during the day and asleep at night, but we can thank light for many other aspects of our biology.
Light may 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.
Related: Which Animals Are Evolving the Fastest?
As we'll explore in other articles in this series, light helps shape our mood, our immune system, how our gut works, and so much more. Light can make us sick, tell us why we are sick, and then heal us.
A million years of evolutionary history means that humans are still very much creatures of light.
We got up and left Africa
The first modern humans evolved in warm African climates. And reducing exposure to harsh sunlight is one explanation why humans started walking upright on two legs. When we stand up and the Sun is directly overhead, much less sunlight hits our bodies.
The curly hair may also have protected us from the hot sun. The idea is that it provides a thicker layer of insulation than straight hair to protect the scalp.
Early Homo sapiens had extra protection from the sun in the form of heavily pigmented skin. Sunlight breaks down folate (vitamin B9), accelerates aging and damages DNA. In our light ancestral climates, dark skin protected against this. But this dark skin still admitted enough UV light to stimulate vital vitamin D production.
However, when humans colonized temperate zones with lower light, they repeatedly evolved lighter skin through different genes in different populations. It happened quickly, probably within the last 40,000 years.
With reduced UV radiation closer to the poles, less pigmentation was needed to protect sunlight from breaking down our folate. Lighter skin also let in more rare light, so the body could make vitamin D. But it had one big drawback: less pigmentation meant less protection from sun damage.
How our skin pigmentation has adapted to migration patterns and changing light.
This evolutionary background contributes to Australia having one of the highest rates of skin cancer in the world.
Our colonial history means that over 50% of Australians are Anglo-Celtic, light skinned, transplanted to a high UV environment. No wonder we are described as a "scorched earth".
Sunlight has also contributed to the variability of human eyes. People from high latitudes have less protective pigment in their irises. They also have larger eye sockets (and probably eyeballs), perhaps to admit rarer light.
Again, these characteristics make Australians of European descent particularly vulnerable to our harsh light. So it's no surprise that Australia has an unusually high rate of eye cancer.
We can't shake our body clocks
Our circadian rhythm—the wake-sleep cycle driven by our brains and hormones—is another heavy evolutionary burden triggered by light.
Humans are adapted to daylight. In bright light, people see well and have refined color vision. But we see poorly in the dark and lack senses such as keen hearing or a keen sense of smell to compensate.
Our closest relatives (chimpanzees, gorillas, and orangutans) are also active during daylight hours and sleep at night, reinforcing the idea that the earliest humans had similar diurnal behaviors.
This lifestyle probably goes back further into our evolutionary history, before the great apes, to the very dawn of primates.
The earliest mammals were generally nocturnal and used their small size and cover of darkness to hide from the dinosaurs. However, the meteorite impact that wiped out these fearsome reptiles allowed some surviving mammals, especially primates, to develop a largely diurnal lifestyle.
If we inherited our daylight activity pattern directly from these early primates, then this rhythm would have been part of our lineage's evolutionary history for nearly 66 million years.
This explains why it is very difficult to shake our 24-hour clock; it is so deeply rooted in our evolutionary history.
Gradual improvements in lighting technology freed us more and more from dependence on daylight: fire, candles, oil and gas lamps, and finally electric lighting. So we can theoretically work and play at any time.
However, our cognitive and physical performance deteriorates when our internal circadian cycles are disrupted, such as by lack of sleep, shift work, or jet lag.
Futurists have already considered the circadian rhythms needed for life on Mars. Fortunately, a day on Mars is around 24.7 hours long, so it's similar to ours. This slight difference should be the least of worries for the first intrepid Martian colonists.
Light keeps changing us
In the last 200 years or so, artificial lighting has helped (partially) uncouple us from our ancestral circadian rhythms. But in recent decades it has cost us our sight.
Many genes associated with myopia (myopia) have become more common in just 25 years, a remarkable example of rapid evolutionary change in the human gene pool.
And if you have some genetic predisposition to myopia, reduced exposure to natural light (and spending more time in artificial light) makes it more likely. These noticeable changes have occurred during the lifetime of many people.
Light will undoubtedly continue to shape our biology for millennia to come, but these longer-term effects may be difficult to predict.
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