There’s no denying that skin comes in a wide variety of colors and shades. Our favorite description of this myriad of skin tones comes from Skin: Living Armor, Evolving Identity. The touring exhibition’s creators describe skin tone as a sepia rainbow. The story of why humans have this rainbow of skin tones is a tale of trade-offs.
In biology, a trade-off is when two traits are linked, but both cannot exist at high levels at the same time. Increasing one trait decreases the other. The environment and role of the species in its environment selects where on the scale between the two traits is best for survival.
For humans, one trade-off is between protection from the sun and synthesizing vitamin D, and this trade off influences skin tone. Skin tone is determined by how much melanin pigment (specifically one type called eumelanin), is made by melanocyte cells in the outer skin layer called the epidermis. The more melanin produced, the darker the skin tone.
The sun emits UV energy. DNA is prone to breaking when exposed to UV light. When DNA breaks down, it is more likely to make mistakes and lead to cancerous cells. Sunburn is a result of your immune system killing skin cells that have UV light-damaged DNA. Too much UV can also interfere with the body’s supply of folate (vitamin B9) which is also essential for healthy growth.
Melanin is a protective pigment–a natural sunscreen–that helps absorb UV light before it can cause damage to cells below the epidermis. Darker skin, which contains more melanin, provides better DNA protection from the sun and helps protect levels of folate in the body.
UV energy also activates a chemical in our skin that our liver turns into vitamin D. Humans need vitamin D to absorb calcium. Healthy vitamin D levels increase bone density and the immune system. Low levels of vitamin D are linked to issues with bone density and prostate cancer.
Here’s the trade-off: more melanin prevents your body from making vitamin D whereas too little melanin won’t protect you from exposure to high amounts of UV light. This has an impact over evolutionary time.
In parts of the world where sun exposure is higher, such as at the equator, human populations evolved to have darker skin to avoid sun damage. In these locations of the world, damage from UV light is a bigger threat to survival than the ability to synthesize vitamin D.
As ancient human populations migrated away from the equator, they moved to areas where the exposure to UV light was greatly reduced. In these areas, protection from UV light wasn't the challenge to survival; instead, it was getting enough UV light to produce vitamin D. In these populations, humans evolved to produce less melanin, resulting in lighter skin tones.
The amazing array of skin tones we see in humans today is a reflection of the incredible evolutionary story and success of our species. This variation arose through adaptation to the intensity of light where our ancestors lived, a process that happened thousands of years before modern travel. And just as a meteorological rainbow is something people delight in seeing after a rainstorm, we delight in seeing the sepia rainbow all around us.
To learn more about skin, visit the Skin: Living Armor, Evolving Identity exhibition. It’s on display at the Science Museum through January 15, 2023.
