What Causes Skin Tanning?

Summertime, Credit; canva.com

What causes the skin to tan?

After a fun day at the beach, you come back home burning red. You should’ve listened to your mother when she told you for the hundredth time: don’t forget to wear sunscreen.

The largest organ of the body

Have you ever really thought about your skin? Our entire body is covered by it and its derivatives (hair, nails, and sweat glands), making the skin the largest organ in the body. The skin, or as scientists call it the integumentary system, creates a barrier between the external and internal worlds. It serves important functions like protection (injury, microbes, and environment), thermoregulation (regulation of body temperature), energy storage (increase in weight), and sensation (touch and temperature).

The layers of the skin

The skin is not just an elastic sheet, but a complicated structure made up of three basic layers.

1. Epidermis: the outermost layer with keratinocytes associated with protection and skin color.

2. Dermis: middle layer with hair follicles and sweat glands associated with sensation and regulation of body temperature.

3. Hypodermis: deepest layer made up of fat cells associated with insulation against cold weather, energy storage, and cushioning.

The components of the skin
The components of the skin, Credit: WIkimedia/BruceBlaus

Skin deep, literally. But despite the multiple functions and intricate details of each layer, our main focus in this article will be on the epidermis to better understand how it protects our skin from spending too much time under the sun.

Melanin to the rescue - Skin tones: how much melanin are you producing?

Human skin tone is remarkable for its beautiful diversity. Our skin color is determined by our genetics, as in what genes we inherit from each of our parents. But what sort of instructions do these genes provide for the skin cells?

The color of our skin is produced by specialized cells called melanocytes located in the lowest layer of the epidermis. These are the cells that the genes regulate to produce the skin pigment known as melanin. The amount of melanin we produce determines the color of our skin.

Melanocyte, Credit: Wikimedia/BruceBlaus

It’s not just about the color, though. Melanin plays an important role in protecting your skin against the sun’s harmful ultraviolet (UV) rays, which can burn or kill skin cells and even damage the DNA enough to cause skin cancer.

Early human migration and vitamin D

Species adapt to their environment to maximize survival. Our skin tone underwent such a process, but this was the result of years of evolution and natural selection.

According to the evidence we have, modern humans (homo sapiens) originated in Africa. Groups of humans started to gradually move throughout the rest of the continents, away from the equator with the blazing sun. Although the sun’s UV light can be damaging to our skin cells, that same light plays an important role in producing vitamin D required for healthy bones and strong immunity. In regions like Africa where the sun is very strong, high melanin and dark skin were vital to protect our ancestors from UV but allow enough of it to go through the skin and stimulate vitamin D production. However, as our ancestors migrated north, sunlight became weaker and was unable to dodge the protective melanin to induce our skin to make vitamins. This put our ancestors at risk and natural selection began to select individuals with lighter skin; only humans with lower levels of melanin were able to produce vitamin D. This variation in skin tone was actually an evolutionary response that took years to manifest!

Out of Africa: early expansions of our ancestors
Out of Africa: early expansions of our ancestors, Credit: WIkimedia/NordNordWest

Why does your skin tan?

In addition to our genes, some of our traits are affected by the outside world to some extent. This allows our body to react to the outside world to improve survival. Tanning is one such example.