12,000 years ago, since humans entered the agricultural society, humans all over the world began to continuously try to domesticate wild animals and plants. We “cheat” them into our settlements and screen out the traits we need to better meet our needs for food and production, living materials and life partners. Among all domesticated animals, cows, pigs, sheep, chickens, dogs, and cats are relatively successful examples. They live with humans all over the world. But why are animals such as rhinos, tigers, and zebras not domesticated?
In fact, to become a domestic animal, wild animals need to meet some requirements. First, they must not be too picky eaters; their food must be found near human settlements. Second, the animal growth cycle cannot be too long. Although elephants are a good labor force, it takes 22 months from pregnancy to birth, and it takes about 15 years to reach adulthood and mating. Therefore, humans are more willing to domesticate cattle with shorter growth cycles. Third, animals must be willing to accept human breeding. Although the ancient Egyptians used cheetahs as pets, the mating process of large cats is very troublesome and requires long-distance running and chasing, so it is not suitable for domestication.
Compared with domesticating animals, the process of humans domesticating plants is more difficult. Wheat, barley, rice, sorghum… The staple food of human beings are almost all grasses. More than 10,000 years ago, the grains of wild wheat after maturity may not be much larger than the seeds of the dogtail grass (also grass family) seen today. So, what is the reason to persuade mankind to try to domesticate gramineous plants? The answer may be an unexpected mutation.
For a long time, wild wheat in Asia Minor was divided into two types: 7 chromosomes and 14 chromosomes. After these two kinds of wild wheat are mature, the seeds will fall on the ground, which causes great inconvenience for humans to collect seeds. But about 10,000 years ago, ancient humans discovered a mutant variety whose seeds will remain on the ears of wheat after maturity. This is a new variety with 21 chromosomes formed after the two wheats are crossed. The seeds left on the wheat ears after maturity may not be conducive to wheat reproduction, but greatly shortens the time for humans to collect mature wheat seeds. Therefore, this variety was continuously domesticated by ancient humans, and eventually wheat became an important staple food for all humans. The domestication process of other grasses may be similar to that of wheat being domesticated.
In daily life, we often use words such as “shameless” and “face thicker than the wall” to describe the contempt for some people. So how thick is the human skin?
Facial skin belongs to human skin, and skin is also the largest organ of the human body. Human skin is composed of epidermis, dermis, and subcutaneous tissue. The thickness of epidermis is 0.1 mm to 0.3 mm, and the thickness of dermis is approximately 0.3 mm to 2 mm. The thickness of subcutaneous tissue depends on the degree of personal obesity. In general, we define the thickness of the skin as the thickness of the epidermis, with an average of 0.1 mm to 0.3 mm. The thickness of the face is of course within this range.
Because the functions of various parts of the body are different, the thickness of the corresponding skin is naturally different. For example, when a person walks, the sole of the foot will rub against the shoe, so the skin on the sole of the foot is the thickest. The facial skin is relatively thin, and biologists found that the thickness of human facial skin is basically no more than 0.1 mm. However, the thickness of a person’s face does not exceed the thickness of the body’s skin. The skin has a defensive function. For women who often use exfoliating skin care products, their facial skin will be slightly thinner, which makes the skin more prone to invasion by bacteria and sunburn.
The thickness of a person’s skin is closely related to the person’s age. The older the person, the thicker the skin. The thickness of a person’s face is about 0.04 mm when he is about 9 months old, 0.07 mm when he is 15 years old, and the thickness is basically fixed when he is 35 years old, about 0.1 mm.