Although protein and nucleic acid are important biological macromolecules, they cannot be said to be alive. Only protein Group can call it life. Proteome is a multi-molecular system composed of protein and nucleic acid, which has the ability to form a "phase" and separate it from the original ocean water with its own surface. But it interacts with the environment in the form of an open system. Protein Formation has two characteristics: ① Contrary to the general trend of increasing entropy in inorganic field, it has the characteristic of decreasing entropy. The random thermal motion of molecules in protein is different from the inorganic world, and it has regularity and order. Protein Group's entropy reduction or order is maintained by the continuous supplement of matter, energy and information from the outside world, or protein Group eats negative entropy from the environment. This process is actually expressed in biological language as the metabolism of protein Group, that is, the exchange of material and energy between protein Group and the environment. If this exchange stops, the multi-molecular system of protein Group will change from order to disorder, the entropy value will increase, and the protein Group will disintegrate. Therefore, metabolism is the first important feature of protein group. ② The ability of self-preservation, self-regeneration or self-reproduction of protein group on the basis of metabolism.
From protein, nucleic acid to protein group, it seems that there is only one step away, but this step is difficult in the process of natural occurrence. Although the primitive ocean has accumulated rich organic matter for a long time. Such as amino acids, nucleotides, porphyrins, nucleic acids and protein, the content of which may reach 65,438 0%, but even at such a high concentration, a multi-molecular system of protein and nucleic acids cannot be formed. So, how did the protein Group come into being? At present, there are still different explanations about this issue. One is to concentrate protein and nucleic acid by evaporation, freezing or clay adsorption. In the process of concentration, protein interacts with nucleic acid to establish a certain relationship, and then is surrounded by the formed membrane to form a multi-molecular system. This explanation lacks experimental basis and is unbelievable. Of course, it is desirable to accelerate the interaction between protein and nucleic acid by concentration.
Another explanation is the protein-like microsphere theory, which was put forward by American scholar Fox and others. He believes that after successful aggregation in hot spots, when it is washed away by rain, it will converge and integrate microspheres of the same size and enter the original waters. The process of transforming protein-like proteins into microspheres is as simple as making glutinous rice balls. The microspheres obtained from the simulation experiment look like cells. They are the same size; The outer membrane of double-layer structure is separated from water. They also have metabolism and can germinate and reproduce like yeast, but microspheres contain no nucleic acid. Therefore, Fox and others believe that the origin of life first forms microspheres, and then microspheres provide a unique environment for the later development of nucleic acids.
Another explanation is the famous polymer theory. According to the phenomenon that colloid condenses into aggregates in water, Soviet scholar Obalin proposed that aggregates are the original model of the origin of life. In the experiment, protein, nucleic acid, polypeptide and polynucleotide solutions were used to separate aggregates at a certain temperature and acidity. This aggregate also has metabolism phenomenon, and the addition of chlorophyll also has weak photosynthesis ability, and it will also grow and reproduce. According to this, Obalin and others think that the formation process of aggregates is the reasonable process of the earliest multi-molecular system formation. When someone was studying seawater hundreds of meters to thousands of meters away, he observed the structure similar to aggregates with an electron microscope.
The initially formed polymer may vary. These multi-molecular systems, Dachuan, now have different life types, and some only contain protein molecules, just like protein-like microspheres prepared by Fox; Some are multi-molecular systems composed of protein and lipid or protein, polysaccharide and lipid. Of course, there is a multi-molecular system composed of protein and nucleic acid. At first, these life types all had the ability to develop, but only in the later development did they show high and low.
The system composed of protein and nucleic acid is very similar to today's virus, and perhaps their molecular weight is even smaller than today's virus. They should be cell-free, with protein outside and nucleic acid in the middle, much like Yuanxiao, so we might as well call it protovirus. They live in a "primitive ocean", and primitive cells evolved from this primitive, free-living protovirus. Today's virus is only the offspring of the original virus that has not changed. After the appearance of cells, they have developed into a type that adapts to parasitic life. According to this hypothesis, phage should be the most primitive type of virus; Other viruses are gradually transferred and evolved with the appearance of higher parasites. But many people think that virus is a degenerate organism, which is gradually degraded by bacteria; Others believe that a virus is an escaped organism, a chromosomal substance or a nucleic acid fragment that escapes from a cell. According to these two hypotheses, the types of life in the non-cell stage seem to have become extinct, leaving no offspring.
The organic matter in the primitive ocean is very rich, and the "residents" are extremely rare. So primitive life is like living in a water park, not worrying about "eating" and "drinking" and being free. However, with the formation and reproduction of various primitive life types, the contradiction in the primitive ocean has become increasingly acute. First of all, the multi-molecular system consumes a lot of ready-made small organic molecules. With the decrease of organic matter, multi-molecular systems also begin to compete. In the process of long-term natural selection, when there is a cryptographic relationship between protein and the life type of nucleic acid, and between polynucleotide and polypeptide in it, this life type can acquire the ability of perfectly preserving information. Therefore, the life form composed of protein and nucleic acid has overwhelmingly defeated other types of multi-molecular systems.
virus
Primitive life was perfected and developed in the struggle. The initial cryptographic relationship between the nucleic acid in primitive life and protein must be very simple. With the evolution of life, nucleic acid molecules are getting longer and longer, which can be explained from viruses to bacteria to cells of higher animals. DNA is getting longer and longer, and there is more and more genetic information. In addition, in primitive life, some viruses were similar to today's viruses. They either contain DNA and protein or only RNA and protein; Some primitive life is different from today's virus, which contains both DNA and RNA nucleic acid and protein. Initially, it is likely that both DNA and RNA can establish a cryptographic relationship with protein, thus guiding the synthesis of protein. It's just that DNA molecules have a stable double helix structure, and it is the most advantageous to act as genetic material in natural selection, so in today's organisms, except viruses, the genetic power is monopolized by DNA.
Primitive life is non-cellular, they can't produce organic matter by themselves, and there is no oxygen in the atmosphere, so they live a heterotrophic (ready-made) and anaerobic life. After a long evolution, about 3.5 billion years ago, the internal structure of primitive life gradually became complicated, forming a layer of cell membrane and building a layer of boundary membrane around itself. Because of this special structure, it effectively controls the entry and exit of substances, allows nutrients to flow in and waste to be discharged, and transforms primitive life into primitive cells. With the birth of the first cell, the foundation for the formation of the whole organic world came into being. All modern creatures we see exist in the form of cells. Even non-cellular viruses must invade cells to reproduce. Cell is the structural unit, functional unit and reproductive unit of life, and it is also a great innovation in the history of life.
There was no free oxygen on the primitive earth and no ozone layer in the atmosphere. Ultraviolet rays can come in directly. This condition is conducive to primitive geochemistry, the accumulation of organic matter and the origin of life. Once life is formed, ultraviolet rays will kill life, and the organic matter in the primitive ocean will be exhausted. In the long run, the development of anaerobic and heterotrophic primitive life will be limited. But there is no way to "live" in heaven. The core porphyrin ring of chlorophyll is synthesized in chemical evolution. Life has unlimited variation potential, and a kind of cyanobacteria containing chlorophyll is developed in anaerobic organisms. They use light energy for photosynthesis and directly synthesize inorganic substances into organic substances. Since then, life has solved the problem of "eating" by itself.
Oxygen, one of the products of photosynthesis, gradually produced the ozone layer in the atmosphere. Ozone barrier prevents ultraviolet radiation and protects life. The oxygen produced by photosynthesis also promotes the transformation from reducing atmosphere to oxidizing atmosphere, thus making organisms develop from anaerobic glycolysis to aerobic oxidation, greatly improving the efficiency of biological energy metabolism.
Autotrophic organisms use light energy to synthesize sugar and protein from water, carbon dioxide and ammonia salts. Heterotrophs decomposed organic matter into water, carbon dioxide and ammonia salts by eating autotrophs, and since then, there has been a contradiction between autotrophic and heterotrophic, synthesis and decomposition. Because the unity of opposites of these two pairs of contradictions formed a complete ecosystem and opened up a brand-new road for the great development of future life, cells appeared.
Later, with the further development of the cell, a nucleus appeared in the cell itself. The main component of the nucleus is chromosome, which is a nuclear protein and a combination of nucleic acid and protein. Chromosomes are surrounded by nuclear membranes, forming nuclei. A nucleated cell is called a eukaryotic cell. At present, the body of most living things is composed of eukaryotic cells.
A cell has a basic feature, which can be divided into two parts. 1 cell can divide into two daughter cells under certain conditions. When each daughter cell grows up, it can be divided into two. If this division continues, there will be more and more cells.
The earliest animals were single-celled animals, and the daughter cells produced by division still lived alone. Multicellular animals were developed later. That is to say, in the process of evolution, the heritability of some single-celled animals has changed, and the heritability of their offspring has also changed. The daughter cells they produce are no longer separated from each other, but combined into cell groups.
unicellular animals
The earliest cell groups were also very simple. Although many cells are United, they still manage their own lives separately. Slowly, some simple cell groups have undergone great changes, and the combined cells gradually differentiate into various organs to share all kinds of work in life. In this way, cells begin to work together. Some cells develop into tubes, and the opening of the tube is the mouth. This tube specializes in digesting food and providing nutrition for all cells that live together. Some cells develop into nerves. Nerves can transmit information from one part to another, just like telephone lines. Later, when animals grow up, some cells develop into vascular system, and nutrients can be transported to all cells in the body through blood vessels. Because some cells are far away from the digestive tract, they can't get nutrition directly from the digestive tract.
I don't know how many billions of years these complex changes have gone through. Because those ancient animals are small and soft, it is difficult to leave fossils. However, we already know that all the most important invertebrates developed 500-600 million years ago, and their fossils are displayed in the Natural History Museum.