From the appearance, the human body is symmetrical left and right, in fact, not only people, more than 99% of the world's animals are left and right symmetrical, especially for terrestrial organisms, almost all of them.
So why does the animal's body have to be symmetrical? The most important thing for any animal is to be "alive", and life activities require energy. Regardless of the animal, obtaining energy is not a simple thing, even if it is a relatively intelligent primitive human, hunger is a common occurrence. Since it is so difficult to obtain energy, it is necessary to minimize unnecessary energy consumption, so the body must be symmetrical. Imagine what would happen if a creature's body wasn't symmetrical? It will be difficult to maintain balance, especially for terrestrial creatures.
If an animal is not able to maintain its balance naturally, it needs to spend more energy to maintain the balance of the body, for example, we consume more energy when standing on one foot than when our feet land on the ground.
So in the evolutionary path of living things, if there is a genetic mutation that causes the body to be asymmetrical, then the individual will soon be eliminated by nature because his energy consumption is too large. Interestingly, while on the surface it appears that humans are symmetrical left and right, the corresponding human internal organs are completely asymmetrical. Doesn't the asymmetry of internal organs cause the human body to lose its balance? In fact, although the placement of human internal organs does not seem to be regular, they are roughly equal in weight on the left and right sides, so it will not affect the balance of the body.
In addition to the brain, the most important internal organs in the human body are the heart, and we all know that the heart grows on the left, so why does the heart grow on the left? Can it grow to the right?
The condition in which the heart grows on the right side exists, which is called "organ ectopicism", and organ ectopy also has two different conditions. The first situation is that the position of the left and right internal organs in the body is reversed, that is to say, the internal organs of a person's body and the internal organs of the body of a normal person show a mirror relationship, just like looking in the mirror, this situation is called "total internal organs reversed". According to the current understanding, the total visceral reversal does not affect health, at least it will not affect the normal survival of a person, and even the longest-lived woman in Europe is the total visceral reversal, she lived to be 126 years old.
Another situation is that only the heart grows on the right side, and the other organs in the body do not or only partially appear "reversed", which also means that a person has a "left and right body axis defect", which is a congenital developmental disease.
There are no people around us who have "left and right axle defects", because this congenital developmental disease will make it impossible for patients to survive, in fact, they cannot even be called "patients", because they usually cannot be born smoothly. Why can't the heart survive without a long pair? To answer this question, you must first know how the heart develops and takes shape. When an embryo is formed in the mother's body, the cells in the embryo that direct development form a temporary organ on their own, which is called the "left and right tissue organs."
Left and right tissues are present in all animal embryos, not only in humans and other mammals, but also in birds and fish.
In the middle region of the left and right histips, there are very long cilia on the cells, the length of these cilia can reach 8 microns, scientists believe that the role of these cilia is to promote the flow of embryonic fluid from the right side to the left side, because the flow of fetal fluid has direction, so it activates some genes that are only expressed on the left side of the cell, which is the beginning of the asymmetric development of the left and right sides of the embryo, and this asymmetrical development is exactly right. At present, researchers have found 5 genes in humans that are involved in the encode of about mytistic cell proteins, and 3 of them have been confirmed to be related to visceral ectopicity, in addition to a gene CIROP is also closely related to visceral ectopic.
After the researchers removed the CIROP gene in the zebrafish, the zebrafish did indeed have organ ectopics in the genes, because after losing the CIROP gene, the heart blood circulation tube developed by the zebrafish embryo would become messy and random.
Later, the researchers followed up 21 people with CIROP gene mutations and found that they all had different degrees of organ ectopia, and 8 of them were total visceral retrolocations, and they all had one thing in common, that is, coronary heart disease. It can be seen that visceral ectopia is a developmental defect, and this defect stems from the deletion or mutation of genes, so there will be some kind of health risks. Of course, causing visceral ectopia is by no means just a problem with the CIRPO gene, there are many factors in it that we are not aware of for the time being. Total visceral reversion is not a "cool" thing, in addition to unknown developmental problems, it will also bring trouble to medical treatment, for a patient with total visceral reversal surgery, the difficulty is obviously higher than normal people.