Talking about the magical symmetry day

Now let's compare our right and left hands. There is no doubt that the left hand and the right hand are very different. No matter how the left hand moves or rotates, it cannot coincide with the right hand. As long as you see a hand, you can tell whether it is left-handed or right-handed without seeing its owner. However, the left hand and the right hand are so similar that if we put the photo of the left hand and the mirror image of the right hand together, we can't tell them apart at all. Not only our hands, but also our feet, ears and even the left side of our whole body are mirror images of our right side. Biology says that our body structure is symmetrical. It should be noted that the mirror symmetry of an object about a certain plane is not enough to make the object symmetrical left and right. If an object is also symmetrical about other points, lines or planes, then it cannot be symmetrical left and right. For example, a typical steel cauldron with two ears can be divided into two halves in mirror symmetry, but the half can overlap with the other half after rotation, which is obviously very different from the relationship between the left hand and the right hand. It is not difficult to find that the steel kettle actually has two symmetrical planes and a symmetrical axis. In biology, this body structure with a symmetry axis and several (more or less) symmetry planes is called radiation symmetry. Unlike steel pots, single-handle woks or spoons are symmetrical, and they have only one symmetrical surface.

Looking at our bodies, we will find that besides the left and right, we can also point out four directions: up, down, front and back. Because if we want to discuss biological problems, if we are professional, we should say four directions: front end, back end, belly surface and back surface (after all, we upright animals are not very representative). Together with the left and right sides, they define three mutually perpendicular axes, just like the xyz axis in the three-dimensional coordinate system, which gives our body a clear plane. Obviously, there is no symmetrical relationship between the front and back of the body or the back of the abdomen. The belly in the mirror is still a belly, and there is no danger of confusion with the back. We call the axis of asymmetry between the two poles the polar axis or the heteropolar axis. An axis with a symmetrical relationship between two poles is called a polar axis or a homopolar axis. That is to say, the front and rear axes and the abdominal dorsal axis are polar axes, and the left and right axes are non-polar axes. These two polar axes and a non-polar axis give us a symmetrical figure.

Paying attention to all kinds of creatures in the world, we will find that most of the animal kingdom is symmetrical, and we can see a few members with radiation symmetry, and there are even fewer other symmetrical ways. Outside the animal kingdom, the situation is quite different-there are no symmetrical members at all (asymmetrical creatures can also have symmetrical parts, such as the tentacles of starfish or the leaves of trees). This shows that symmetrical graphics are of great benefit to the lifestyle of animals, so they have been preserved and carried forward. Then let's look at the lifestyle of animals. As the name implies, an important feature of animals is movement, and it is active movement. The typical way of nutrition in the animal kingdom is to get nutrition by "eating" other creatures, and active exercise undoubtedly provides great convenience for eating. Although some animals adopt a fixed lifestyle and a few plants will "eat" other creatures, active exercise is essential for large and complex animals.

Due to the influence of gravity, the near and far ends of the body are usually different, which constitutes the polar axis-generally the ventral dorsal axis in animals. We usually refer to the side facing the ground as the ventral side and the opposite side as the dorsal side. Of course, for a specific species, the division of ventral surface and dorsal surface has biological significance, not just a geometric concept. For example, we don't call it a sloth, and we face down to the abdomen.

For active animals, it is better to have another polar axis-the front and rear axis. In other words, we'd better move on. For animals without front and rear axes, all directions in the horizontal direction are the same. Although this seems convenient, it is actually inconvenient in any direction. Whether fleeing from the enemy or chasing prey, being able to move quickly and flexibly in one direction is obviously more advantageous than not being able to move quickly and flexibly in all directions. The feeding and sensory center of animals is usually located in the front of the body, and this trend is more obvious in animals with higher evolution. For example, our vertebrates' visual, olfactory and auditory receptors are all at the front end. You may think that animals are born to move in the direction of their heads, but in fact, animals' heads are given by movement. A guy who is as fixed as a plant won't evolve a head.

Because the most common movement direction of animals is horizontal, the typical anteroposterior axis direction is also perpendicular to the ventral dorsal axis. Like the division of abdomen and back, the division of front and back is also of biological significance. For animals with left and right symmetry, we usually call the end with a mouth the front end. The front end and the back end are sometimes referred to as the mouth end and the reverse mouth end. The other opening of the digestive tract, the anus, usually grows at the opposite end, but some animals have no anus, and some animals have a U-shaped digestive tract with an anus near their mouths. However, we are only discussing the general situation here. Although coelenterates have a mouth-to-mouth axis, they have no front and back axes. Most of the mouth-to-mouth axes are used by them to cope with the gravity of the earth.

Supported by two mutually perpendicular polar axes, namely, the ventral dorsal axis and the anterior and posterior axis, our animals look in excellent condition. But if the mass or shape on both sides of our bodies are asymmetrical, we still can't become athletes. In order to go straight ahead and turn left and right flexibly, we also need a non-polar axis perpendicular to the plane formed by the front and rear axis and the abdominal dorsal axis-the left and right axis. With these three axes, our body shape will be perfect. Nowadays, animals that are good at flexible and fast movements on the earth all have symmetrical bodies.

And the relationship between left-right symmetry and movement is not limited to the biological world. The appearance of man-made fast and flexible machines is almost always in the form of high symmetry from left to right. Note that speed and flexibility are both important here. Although the satellite is fast, it is in a fixed orbit and does not need to change its motion mode anytime and anywhere. Such a machine doesn't have to be symmetrical. It should also be noted that the movement here is the movement of the machine, not the movement of the internal parts of the machine. Just as we can't treat the movement of our hands as the movement of our bodies. Of course, we can make some immobile things, such as houses, symmetrical to the left and right for aesthetic needs, but we should make most things used for flexible movement symmetrical to the right and left. Seeing this, you will know how some science fiction writers lack common sense: advanced aliens always travel and fight in symmetrical radiation flying saucers! If one day I meet flying saucers and symmetrical fighters driven by science fiction writers, I will definitely give them a vivid lesson.

Of course, some people like to give some counterexamples, trying to explain that the symmetrical way of body structure has nothing to do with movement. They will say that jellyfish with symmetrical radiation live a free-swimming life, starfish are active hunters, and barnacles with symmetrical left and right live a fixed life. However, these examples only show that radiation symmetry is not completely unsuitable for sports, and symmetrical creatures can also choose a fixed lifestyle, and the correlation between left-right symmetry and active sports ability cannot be denied. Moreover, animals with symmetrical radiation are aquatic. The buoyancy of water counteracts part or all of gravity, making the disadvantage of radial symmetry less obvious. Moreover, jellyfish, starfish and so on. Are not very athletic animals.

From the evolutionary point of view, animals with symmetrical left and right appeared later and had more complicated structures than animals with symmetrical radiation. Left-right symmetry has one more polar axis than radial symmetry. The increase of the number of polar axes and the decrease of symmetry contain more information. Due to the uneven distribution of cell contents, a typical fertilized egg has animal poles and plant poles. That is to say, the fertilized egg already has a polar axis, which is not spherically symmetric but radially symmetric. Of course, some researchers don't like to call this structure with symmetry axis and "countless" symmetry planes radial symmetry, but cylindrical symmetry. There is only one polar axis from fertilized egg to two germ layers (endoderm and ectoderm), which is the later oral axis. So coelenterates, these guys with two germ layers, naturally grow into radial symmetry. The appearance of mesoderm gives the embryo another polar axis-ventral dorsal axis. Animals with three germ layers can be divided into two categories: protozoa and metazoa. There are many kinds of protozoa, such as arthropods, annelids, mollusks, nematodes and so on. There are not many kinds of animals, mainly including chordates and echinoderms. However, the status of metazoa should not be underestimated. Most of the behemoths (animals) on the earth and us elves are metazoans. The appearance of mesoderm in protozoa and metazoa is different, but they both give their owners symmetrical figures.

Of course, the reality is much more complicated. There are also a few coelenterates with left-right symmetrical structures, while echinoderms with mesoderm such as starfish and sea urchins have radially symmetrical bodies. /kloc-At the beginning of the 9th century, some scientists put echinoderms and coelenterates in the same phylum. Later, it was known that echinoderms were much more complicated than coelenterates, and their larvae were symmetrical left and right, and "returned" to radial symmetry in the later development. This seems to be a retrogression, but we know that evolution has no direction.

As mentioned above, we will make the appearance of a fast and flexible machine like a car highly symmetrical. But if you open the hood of the car, you will find that the interior of the car is very asymmetrical (only the two sides have roughly the same mass). Although the uniform distribution of body mass makes the interior symmetry more favorable, it is almost impossible to make such a complicated internal structure symmetrical in a limited space. Similar to cars, our bodies are not symmetrical inside. The heart of our vertebrates is on the left side of the body, and the liver and gallbladder are on the right side. In fact, the asymmetry of the internal structure of the body is a very common phenomenon in the animal kingdom, but the more complex the structure, the more obvious the asymmetry of the internal structure of the animal. In other words, the left and right axes are also the polar axes inside our bodies.

There are also some animals whose shapes are asymmetrical, such as insects and nematodes. Of course, the most striking thing is gastropods. Many members of Gastropoda in mollusks have spiral shells, such as snails and snails, and their shells are not symmetrical about any plane. As we know, snail shells can be left-handed or right-handed. But most of the shells we usually see are right-handed, and the shells in the left hand are not easy to see. Of the 70,000 known species of snails, only about 10% is left-handed. In addition, the proportion of left-handed snail shells varies greatly in different regions. According to statistics, 27% of 446 species of snails in Turkey are left-handed, 16% of 330 species of snails in Central and Western Europe are left-handed, and 345 species of snails east of the American Rocky Mountains are right-handed. So, if you live in the eastern United States, you don't have to bother looking for left-handed snails.

The appearance asymmetry here is not contradictory to the relationship between left-right symmetry and motion mentioned above, because these appearance asymmetries are only slightly different-just like the antenna of a car is located on a certain side-and will not actually affect the motion. However, gastropods, which are obviously asymmetrical in shape, just move slowly.

The polarization of the left and right axes adds more information to the structure of the body and also creates new problems. In the above article, we talked about the polarization of the ventral dorsal axis. The polarization of the left and right axes seems to have a higher "technical content" because before the appearance of the ventral dorsal axis, the embryo was cylindrically symmetric. Just like a beer bottle with a mouth-to-mouth axis, if we want to put a label on its side and call this side the back, then the label is the same in any direction, that is, it is random. But the polarization of the left and right axes cannot be random, and its direction must be fixed. For example, the heart is always on the left and the liver is always on the right. These traits are all determined by genes. Mutations in some related genes will cause abnormal distribution of internal organs. According to statistics, one in every 8,000 surviving newborns suffers from abnormal distribution of internal organs. Most of these developmental abnormalities are accompanied by serious physiological defects, and only when all internal organs grow into mirror images will there be no defects. This completely mirrored individual has only one in about 20,000 surviving newborns. It is estimated that the actual proportion may be higher, because such individuals have no obvious defects and are easily overlooked.

The left-right asymmetric development of embryos has always been a fascinating puzzle. Today, scientists have discovered dozens of genes related to asymmetric development. Nodal is probably the most striking one in recent years. Nodal belongs to the transforming growth factor-β (TGF-β) family, and its effect on left-right asymmetric development has been verified in vertebrates such as mice, chickens and Xenopus. In the embryos of these animals, Nodal protein is specifically expressed on the left side and plays a role, and knocking out Nodal gene will cause asymmetric development disorder. In addition, the researchers also found the role of Nodal homologous protein in primitive chordates, ascidians and echinoderms. Interestingly, Nodal acts on the right side in sea urchin embryos. Since echinoderms and chordates belong to metazoa, people naturally want to know whether there is a homologous gene of Nodal in protozoa. At the beginning of this year, a research paper in Nature reported the role of Nodal homologous protein in the asymmetric development of snails. Scientists in the laboratory of the University of California, Berkeley chose a right-handed snail and a left-handed snail for research. They found that Nodal was specifically expressed on the right side in right-handed snail embryos, while Nodal was expressed on the left side in left-handed snail embryos. This interesting research promoted Nodal's history at least before animals parted ways.

However, people's research on the development of left-right axis polarization is still very preliminary. Now we know that the difference in gene expression between the left and right sides of embryos must be caused by earlier differences. This is like a domino. Every domino moves because of the collision of the previous domino. And where the first domino is, people still don't know. From the above discussion, we know that the first domino can't fall randomly, otherwise about half of people's hearts will grow on the right. So, how do our embryos tell which side is left and which side is right? You know, left and right are just people's conventions, and there is almost no physical difference between them. Although there is a law of parity non-conservation in physics, there seems to be no evidence to show the contribution of weak interaction in life activities. Perhaps this question is good news for believers of God. They can say that the first domino was knocked down by the hand of God, and only his old man has this ability. However, we know that the causes of all phenomena in nature can only be found in nature.

Although most physical laws are still valid after mirror image transformation, asymmetry is very common in biochemical reactions. If the molecules of matter cannot coincide with their own mirror images like our hands, we call them chiral molecules. Chiral molecules have two mirror isomers, just like the relationship between left hand and right hand, and biochemical reactions usually only "know" one of them. For example, glucose, which supplies us with energy, is dextrorotatory (D form), while amino acids (except glycine without chirality) used to synthesize protein are levorotatory (L form). This seems to provide a way for organisms to distinguish between left and right. Of course, this does not mean that biochemical reactions do not conform to physical laws. In principle, we can indeed rotate all chiral molecules in the organism one by one at the same time, and life activities can be carried out as usual (the current technology can't do it yet). However, there is no need for organisms to care about the symmetry of physical laws, as long as they can distinguish left and right in their own bodies. It's like if you live in China or the United States, you can teach your baby that the steering wheel side of a car is the left side, although this is not "scientific" because in some countries, the steering wheel of a car is on the right side.

We can imagine that if an L- alanine molecule is pressed in the center of the body so that its carboxyl group points to the ventral side of the mouth and its methyl group points to the ventral side of the mouth, then its amino group refers to the left side of the back, and the hydrogen atom refers to the right side of the back. After all, it's a bit strange to decide the left and right by fiddling with an amino acid molecule, but macromolecules in organisms, such as protein, nucleic acids and polysaccharides, are also chiral, and they can also form larger complexes, such as chromosomes and cytoskeleton, which plays an important role in cell morphology and intracellular material transport. If we distinguish between left and right through them, it will be much more "realistic". Of course, this model is only a hypothesis at present, and there are other models, all of which are in the hypothetical stage. In short, there is still a long way to go to solve the mystery of left-right asymmetry.

Do you also think that the phenomenon of left-right symmetry and left-right asymmetry is amazing?

(Author: Intron, Ph.D. in Biochemistry and Molecular Biology)