Why don't humans have green hair?

Every kind of life has its own color, and the evolutionary logic behind it is very mind-boggling.

Every life in nature has its own color. ｜From the Internet

Why can we see a colorful world?

Why don't humans have green hair? Polar bears are not white for hiding? African zebras have black and white stripes to regulate body temperature? In fact, every life has its own color, but because we are so accustomed to it, we rarely realize that behind the colorful colors, there is actually a profound evolutionary logic hidden.

Why don't humans have green hair?

In life, we will notice an interesting phenomenon: everyone's hair color is different, black, gray, yellow, brown, silver, but no green. For early humans living in the jungle, green hair is simply a natural camouflage, which can play a perfect protective role. So why do humans give up such an excellent self-protection device?

In fact, human hair is by no means an optional decoration, but a parasol on the head, and its most basic function is to shield against ultraviolet rays. Melanin in the skin is an important line of defense against ultraviolet rays, which extends all the way to the hair, preventing strong ultraviolet rays from reaching the brain epidermis and protecting genes from damage.

Calculations show that black hair can reduce the body's ultraviolet radiation by at least 80%, while green hair has almost no shielding effect on ultraviolet rays. Therefore, it is absolutely wise to give up green hair. For mammals, the thermal regulation ability of green hair is not outstanding, far inferior to black or white hair.

Because humans have a huge brain capacity, the computing process generates a lot of heat energy, and the sun above their heads constantly heats the brain. To complete such a complex heat regulation task, black hair is the best choice, which can prevent excessive heat from reaching the skin and prevent scalp burns.

In Europe, where the climate is damp and cold, people's hair gradually turned light in color to receive more sunlight, allowing sunlight to pass through easily and directly heat the brain. The most fascinating thing about green hair is its concealment function. For animals like humans with tricolor vision, green hair can indeed help hide in the jungle. But the objects of concealment are mainly natural enemies. The natural enemies of humans are large carnivores such as leopards, lions, and tigers. They all have typical two-color vision. Whether green, red or brown, they all look the same to them. Green hair cannot provide effective protection. As for why human hair was originally black, it is because humans are close relatives of chimpanzees, and their hair color must be similar to that of chimpanzees.

Chimpanzees are typical tropical jungle animals. Scientists have long discovered that the warmer and more humid the environment, the darker the animal's body color becomes, until it turns black. The melanin and keratin in black hair are difficult to degrade, making it not only strong but also resistant to fungal erosion.

African green monkeys have olive green fur, which helps them hide in the dim light of the jungle and avoid predators with four-color vision, such as eagles and snakes. But green fur does not protect humans. | Pinterest

Why are polar bears white?

The body color of animals is not the result of random choice, but often has practical considerations; moreover, its main function may not be the hiding and communication functions that we usually think of, but the heat regulation function. Whether it is lower animals or higher animals, they must obtain heat from the environment in an appropriate way. In addition to food, there are also physical ways to obtain heat, such as receiving solar radiation or heat convection in the environment. Different body colors will significantly affect the balance of heat income and expenditure, so choosing the correct body color is particularly important for maintaining body temperature.

Polar bears' skin is originally black, but their hair is almost transparent, and after being superimposed, it appears white. Therefore, many people regard it as a protective color for survival in the ice and snow. However, polar bears mainly hunt seals under the ice, and white hair does not provide much cover, and polar bears and seals mainly recognize each other by smell. Therefore, white hair actually has another value for polar bears, which is to reduce heat loss. On the one hand, white hair does not require synthetic pigments and is a relatively cheap hair; on the other hand, since there is not enough sunlight available in the Arctic, polar bears' dependence on sunlight is greatly reduced. In this case, although white hair absorbs heat the slowest, the rate of heat loss is also correspondingly slower, which can minimize heat loss.

The main value of white hair to polar bears is that it reduces heat loss. | Michael Henry

Why do African zebras have black and white stripes?

Many animals on the African savannah have stripes, but none are as ostentatious as the zebra's stripes. Why do African zebras have black and white stripes? Darwin, who was well-informed, was probably the first person to seriously think about the zebra's stripes. He believed that the zebra's stripes were too ostentatious and were not for camouflage, but more like a "barcode" in nature. Each zebra's stripes are unique, so that it can quickly find its relatives.

However, this conjecture obviously cannot explain one problem: Do animals without barcodes not need to identify themselves? Another theory is that the black and white stripes of zebras may be used to repel mosquitoes. When zebras run fast, the black and white stripes will produce a clear wheel effect, presenting a strong visual camouflage effect, just like the rotating colored lights at the door of a barber shop, creating a strong visual illusion, confusing mosquitoes and making them unable to figure out which direction the zebra is running, thereby reducing the risk of zebras being bitten. However, this cannot explain many problems, such as why wildebeests living in the same environment as zebras did not adopt the same evolutionary strategy.

A study in 2015 may provide the most convincing answer. Scientists have found that the number of black and white stripes varies with different zebra species, with some having more than 20 and some having more than 80. The closer to the equator, the clearer and denser the stripes; in the relatively cool south, the stripes are significantly reduced; when the stripes are thin and weak to a certain extent, the zebra will become a gray horse. This shows that the zebra's stripes may be for regulating body temperature! Black and white colors react differently to sunlight, with black stripes absorbing more heat and white stripes absorbing less heat, so that the air above the stripes will alternate between hot and cold, forming tiny air vortices that can not only repel mosquitoes, but also quickly lower the skin temperature. Under the scorching sun of the African savannah, the skin temperature of zebras is two to three degrees Celsius lower than that of other mammals, which is absolutely of extraordinary significance!

The zebra's black and white stripes may be mainly for regulating body temperature. | Paul Maritz

This article is authorized to be reproduced from the public account "Ten Points Science". The original text is excerpted from the book "The Color of Life: Why Don't We Have Green Hair", written by Shi Jun.