First, let's try to answer an auxiliary question: why does a person have some parts of the body symmetrical, while others are not?
Symmetry is a basic property of most living things. It is very convenient to be symmetrical. Think for yourself: if you have eyes, ears, noses, mouths and limbs on all sides, then you will have time to feel something suspicious in time, no matter which side it sneaks up on, and, depending on which side it is, it is suspicious, - eat it or, conversely, run away from it.
The most flawless, "most symmetrical" of all symmetries is spherical, when the body does not differ in the upper, lower, right, left, front and back parts, and it coincides with itself when turning around the center of symmetry at any angle. However, this is possible only in an environment that itself is ideally symmetrical in all directions and in which the same forces act on the body from all sides. But on our land there is no such environment. There is at least one force - the force of gravity - which acts only along one axis (up-down) and does not affect the others (front-back, right-left). She pulls everything down. And living beings have to adapt to this.
So the next type of symmetry arises - radial. Radially symmetrical creatures have a top and bottom, but no right and left, or front and back. They coincide with themselves when rotating around one axis only. These include, for example, starfish and hydras. These creatures are inactive and are engaged in "quiet hunt" for living creatures passing by.
Anemones (sea anemones) are an example of radially symmetrical organisms. Drawing from Ernst Haeckel's book The Beauty of Forms in Nature.
Image from the site en.wikipedia.org
But if some creature is going to lead an active lifestyle, chasing prey and escaping from predators, another direction becomes important for him - anteroposterior. The part of the body that is in front, when the animal moves, becomes more significant. All the senses "crawl" here, and at the same time the nerve nodes that analyze the information received from the senses (for some lucky ones, these nodes will later turn into the brain). In addition, there should be a mouth in front in order to catch the prey that has been overtaken. All this is usually located on a separate part of the body - the head (in principle, radially symmetrical animals have no head). This is how bilateral (or bilateral) symmetry arises. In a bilaterally symmetrical creature, the upper and lower parts, front and back parts differ,and only the right and left are identical and mirror images of each other. This type of symmetry is typical for most animals, including humans.
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Leonardo da Vinci's Vitruvian Man shows an example of bilateral symmetry.
Image from the site en.wikipedia.org
In some animals, for example, annelids, in addition to the bilateral one, there is one more symmetry - metameric. Their body (with the exception of the most front part) consists of identical metameric segments, and if you move along the body, the worm "coincides" with itself. In more advanced animals, including humans, a weak "echo" of this symmetry remains: in a sense, our vertebrae and ribs can also be called metameres.
Human ribs have some features of metameric symmetry. First thoracic - the first thoracic vertebra, first lumbar - the first lumbar.
Image from ru.wikipedia.org
So, why a person has paired organs, we figured out. Now let's discuss where the unpaired came from.
First, let's try to understand: what is the axis of symmetry for the simplest, radially symmetric, primitive multicellular organisms? The answer is simple: it's the digestive system. The whole organism is built around it, and it is organized so that every cell of the body is close to the "feeding trough" and receives a sufficient amount of nutrients. Imagine a hydra: its mouth is symmetrically surrounded by tentacles that drive prey there, and the intestinal cavity is in the very middle of the body and is the axis around which the rest of the body is formed. The digestive system of such creatures is one by definition, because the whole organism is built "for it".
Gradually, the animals became more complex, and their digestive system also became more and more perfect. The intestines lengthened to digest food more efficiently, and therefore had to collapse several times to fit in the abdominal cavity. Additional organs appeared - liver, gallbladder, pancreas - which were located in the body asymmetrically and "moved" some other organs (for example, due to the fact that the liver is located on the right, the right kidney and the right ovary / testicle are shifted downward relative to the left) … In humans, from the entire digestive system, only the mouth, pharynx, esophagus and anus have retained their position on the plane of symmetry of the body. But the digestive system and all its organs remained with us in a single copy.
Now let's look at the circulatory system
If the animal is small, it has no problem with the nutrients reaching every cell, because all cells are close enough to the digestive system. But the larger the living being, the more acute the problem of supplying food to the “distant provinces” located at a great distance from the intestine, on the periphery of the body, arises for it. There is a need for something that would “feed” these areas, and besides, connect the whole body together and allow distant regions to “communicate” with each other (and in some animals, it would also carry oxygen from the respiratory system throughout the body). This is how the circulatory system appears.
The circulatory system is built along the digestive system, and therefore it consists, in the most primitive cases, of only two main vessels - the abdominal and dorsal - and several additional connecting them. If the creature is small and weakly mobile (such as a lancelet), then in order for the blood to move through the vessels, it is enough to contract these vessels themselves. But for relatively large creatures leading a more active lifestyle (for example, fish), this is not enough. Therefore, a part of the abdominal vessel in them turns into a special muscular organ that pushes the blood forward with force - the heart. Since it arose on an unpaired vessel, then it itself is "lonely" and unpaired. In fish, the heart is symmetrical in itself and in the body is located on the plane of symmetry. But in land animals, due to the appearance of the second circle of blood circulation,the left side of the heart muscle becomes larger than the right one, and the heart shifts to the left side, losing both the symmetry of its position and its own symmetry.
