On Earth, human breasts are unique on Earth, why? Because of the more than 5,000 mammals in the world, why are only humans with permanent breasts?
In human women, the role of the breast is to produce and secrete milk to feed the baby, on the earth, all male creatures are born on the female as a template, which is why human males also have breasts, because after the embryo is produced, it is actually a female, from the end of the first month of development, the development of the breast has begun, but in the 6th and even the 8th week. The SRY gene of human males has only begun to be expressed, and the nipples and mammary glands of individual males have not developed further since then, but only grow with the body. Therefore, human males also have the same chest structure as women.
Both females and males develop breasts from the same embryonic tissue. During puberty, estrogen and growth hormone together cause female human breast development, while in other mammals the female animal's breast is only during ovulation or lactation, which is a temporary breast, which is only manifested at a specific time, the purpose is to produce milk, and the breast will gradually disappear after the lactation period. Other times their breasts are flat.
But human women are not, this problem has plagued scientists for hundreds of years, scientists have proposed a lot of hypotheses, the current mainstream hypothesis is that humans have permanent breasts in response to the need for body temperature regulation and energy storage and brain evolution of the individual development. We need to know that as we evolved from apes to humans, the brain began to evolve, and as the brain evolved, the energy required was also increasing. The human brain only accounts for 2% of the total body weight, but the human brain is the organ that consumes the most energy in the whole body, and can consume 20%-25% of the body's calories when resting, which is equivalent to 350-450 calories a day. Children's brains are even more hungry, and the brains of five- and six-year-olds can consume more than 60 percent of the calories a day.
This is because the brain needs to complete the signal transmission through the synaptic cells of neurons all the time, complete the various functions of the body, and triggering the synapse requires a large number of ions to cross the cell membrane, which consumes a lot of calories. At the same time, the brain doesn't really "rest." When we sleep, it still consumes calories to keep signals between cells and maintain body function.
Fat, on the other hand, is the body's form of energy storage, and the increase in subcutaneous adipose tissue triggers an increase in estradiol levels (E2), a natural estrogen secreted mainly by ovarian mature follicles that promote and regulate the normal development of female sexual organs and parasexual characteristics, such as breasts and gluteals.
In addition, the increase in meat in the early human diet allowed for further hormonal changes, such as more dehydroepiandrosterone (DHEA/S) synthesis, which was essential for brain evolution. DHEA/S is also very easy to convert to E2 in E2-sensitive body parts, such as the breast and gluteal area, leading to fat buildup in these areas, resulting in a growing breast.
This also results in a high proportion of fat in the human body among all non-aquatic mammals. Many anthropologists point out that humans during times of food scarcity maintain a strong metabolism in the brain because of their high proportion of body fat — and the hips, abdomen, and chest are the main distribution areas of our fat tissue.
Arguably, the appearance of permanent breasts is the most important sign of the evolution of apes into adults, which provided energy for the evolution of the human brain, thus evolving a brilliant human civilization.
As humans walk upright, changes in breasts also change men's sexual choices, and breasts also have the function of attracting mates, thus gaining the advantage of reproduction, just like the gorgeous feathers of male peacocks.
Male peacock in order to obtain the opportunity to reproduce, the excess energy is used to grow gorgeous feathers, then the female peacock at a glance to know that the male peacock nutrition and health is good, dragging such a bulky tail has not been eaten by predators, indicating that this male peacock has strong survivability and good genetic foundation. Similarly, breasts can reflect a woman's nutritional health and fertility.
After humans had permanent breasts, the size of the chest also changed due to geographical reasons. We need to know that in humans, the EDRA gene is a key factor affecting the development of human skin, hair, nails, teeth and sweat glands.
Europeans and Americans are EDAR 370V allele, while East Asians carry the EDARV370A gene, of which women carrying the 370V allele, the number of cups is generally not small, at least C cups; and women with 370A variant genes, the chest is difficult to grow.
This is due to the fact that the central region of China 35,000 years ago was relatively warm and humid. More sweat glands can help to sweat faster, increase mobility, and access resources, which is a great choice advantage, and it is good for the people who lived there to live on hunting and gathering at that time. The increase in the mammary glands also ensures that women can still breastfeed well in the case of small breasts.
In addition, genetic variation is also related to sexual selection, and coarse hair and small breasts are visible sexual signals to the naked eye, because East Asian men prefer this trait, which can quickly become more common as women who carry this trait have more children.
Of course, scientists have always been puzzled, after the genetic mutation led to the emergence of the first women with "permanent breasts", how did the small number of them magically make the "taste" of men on breasts produce a one hundred and eighty degree change, thus continuing this gene, and finally completely eliminating the number of women with only "temporary breasts" that were absolutely dominant in evolution, so that all women on the earth today have "permanent breasts"?
Since we can't travel back in time to the distant past to see the evolutionary events of that time, this will most likely remain a mystery forever.