The earliest fossils are microfossils, including stromatolites, which are 3.5 billion years ago, but these are all life with cell morphology. From these fossils, we can only get information about primitive cell size, cell wall morphology, cell division and so on. It is impossible to know the source of nucleic acid, protein and information transcription and translation system, and these are the questions that must be answered to study the origin of life. In Darwin's time, a credible conclusion of biological evolution can be drawn simply by description and comparison, but this can't solve the problem of the origin of life. Now, after decades of efforts, people have carried out various complex experiments and gained some understanding of the history of life from the inorganic world.
(A) Several hypotheses about the origin of life
There are many speculations and hypotheses about the origin of life in history, and there are also many controversies. One hypothesis is creationism, which puts the scientific proposition of the origin of life into the field of theology, so it is unscientific.
The second hypothesis is spontaneous generation. This is a widely circulated theory before19th century, which holds that life naturally arises from inanimate matter. Ancient people in China thought that "rotten grass turned into fireflies" (that is, fireflies were produced by rotten grass piles), and carrion gave birth to maggots. Egyptians believe that frogs and eels in the Nile valley are produced by sunlight shining on the soil. In the west, Aristotle (384-322 BC) was a naturalist, and he even compiled a list of species that can naturally appear from inanimate matter. For example, he thinks that decaying corpses and excrement can produce tapeworms, and mucus can produce crabs, fish, frogs and salamanders. Although creationism was dominant in the West in the Middle Ages, spontaneous generation still developed greatly. For example, the "Goose Tree Theory" holds that the combination of conifer resin and seawater salt can produce geese and ducks, so geese and ducks were once classified as vegetarian. /kloc-in the 7th century, the Dutchman J van Helmont made some contributions to the study of photosynthesis, but he advocated spontaneous production in the origin of life. He also used the "experiment" to prove that after 265,438+0 days, a mouse will be produced by stuffing grain and worn-out shirts into a bottle and putting them in the dark. To his great surprise, this "born" mouse is exactly the same as an ordinary mouse. J. van Nelmont's experiment did not rule out the possibility of rats entering from the outside, and their results were obviously wrong.
/kloc-Italian doctor Francesco Redi proved for the first time that carrion can't produce maggots, but maggots are hatched from eggs laid by flies on meat (see figure).
The experiment of hot land was rigorous and convincing, and then people gradually believed that larger animals such as flies, mice and elephants could not happen naturally. However, because Ravenhoek found that there are small animals everywhere, such as ciliates and bacteria, people think that small animals can occur naturally. Lamarck, a pioneer who advocated biological evolution, also believed that trichomonas (such as flagellates and ciliates) could occur naturally, while other organisms evolved from these naturally occurring small organisms. The experiment of Italian biologist L Spallanzani (1729—1799) proves that small organisms are not naturally produced. He boiled the broth in an unsealed bottle, and after standing for a few days, microorganisms appeared in the broth; If the bottle mouth is sealed, then boiled and left standing, microorganisms will not appear in the broth (see figure).
He concluded that the little creatures in the broth came from the air, not naturally. However, naturalists believe that he "tortured" the broth, making it lose its "vitality", and the air in the sealed bottle has also changed, which is not suitable for life.
The experiment of French microbiologist Pasteur finally denied the spontaneous generation theory. According to his fermentation research, Pasteur believed that organisms could not occur naturally in broth or other organic substances, otherwise sterilization and strain breeding would be meaningless. Pasteur did a series of experiments to prove that microorganisms can only come from microorganisms, not from inanimate substances. One of the most convincing but very simple experiments he did was the Goose Neck Bottle Experiment (see photo).
He put nutrient solution (such as broth) into a bottle with a curved thin tube. The elbow is open, and air can enter the bottle unimpeded (which makes those who think that Spallanzani's experiment has made the air worse speechless), while microorganisms in the air are prevented from depositing at the bottom of the elbow and cannot enter the bottle. Pasteur boiled the liquid in the bottle to kill all the microorganisms in the liquid, and then let it cool. As a result, no microorganisms appeared in the bottle. At this time, if the curved neck tube is interrupted, the outside air will directly enter the nutrient solution without "precipitation treatment", and microorganisms will appear in the nutrient solution soon. It can be seen that microorganisms are not naturally produced from nutrient solution, but from microorganisms (spores) that already exist in the air. 1864, Pasteur reported his work at the French National Academy of Sciences. The famous naturalist F.A. Puchet, who was scheduled to debate with him, withdrew from the debate. "Life comes from life", that is, biological occurrence wins.
The third hypothesis is the cosmic worm theory. This theory holds that life on earth comes from other planets in the universe, and some microbial spores can attach to interstellar dust particles and fall into the earth, thus giving the earth its initial life. But the physical conditions of the universe (such as ultraviolet rays and temperature) are fatal to life. How does life enter the earth through the universe? It seems impossible for microbial spore-level life forms to fly from outer space, but is it possible that some organic substances that make up life come from outer space? Some people think it is entirely possible. 1959 A carbonaceous meteorite fell from Australia in September, containing a variety of organic acids and amino acids. These amino acids are different from the amino acids that make up protein. They are not L-shaped, but exist in the form of racemic mixture of D-shaped and L-shaped. Some amino acids are not found in life on earth, so they are not from pollution on the ground, but are contained in meteorites themselves. In addition, research on space shows that interstellar matter contains dust particles. The diameter of dust is 0.6 micron, and the diameter of dust is only 0.04 micron ... The temperature of dust is about 10K, so many gases in space are frozen on the surface of dust, and they can complete the process of organic synthesis through the impact of light, electricity and ultraviolet rays, so some organic molecules such as amino acids, purines and pyrimidines can be produced on the surface of dust, which is proved by spectral analysis. Some people think that dust containing these organic molecules was brought to the earth by comets. Comets are made up of interstellar matter. In the early days of the earth's formation, the comet's tail scattered a large number of organic molecules on the earth, thus giving life to the earth. Organic matter does exist outside the earth, and of course there is the possibility of life, but this does not prove that life on earth comes from outer space. But whether life comes from outer space or the earth itself, life always comes from inanimate matter through the stage of chemical evolution, and the conditions for the formation of the earth can meet the requirements of chemical evolution.
In addition, some people think that life, like matter and energy, is eternal, with no occurrence and origin, only spread and change. This view is obviously useless for studying the origin of life.
(b) Life comes from inanimate matter-a new "spontaneous phenomenon"
Although Pasteur's work proves that life can't happen naturally under the present conditions, it can't answer such a question: Since life comes from life, where did the earliest life come from?
1924 A·I· Oparin, a biologist in the former Soviet Union, published a monograph on the origin of life in Russian. After 5α, the British geneticist T.B.S Haldane also published a paper, and put forward the same view as Obalin. 1936, Obalin rewrote The Origin of Life, added content and translated it into many languages. Since then, the issue of the origin of life has once again aroused widespread concern, and many people have studied it. Since 1950s, people have conducted more in-depth experimental research with more advanced experimental techniques and achieved good results.
These studies show that there is no insurmountable gap between living and inanimate objects, which seems to be very similar to the theory of natural occurrence, but in fact it is fundamentally different and can be called the new theory of natural occurrence. According to this theory, life is produced in the long-term evolution of the universe, and it is the evolution process of inanimate matter at a certain stage of the evolution of the universe, rather than being suddenly produced by inanimate organic matter under the present conditions. This theory is recognized by most scientists because of its sufficient foundation and experimental proof, and many researchers continue to study it in depth based on this theory.
(3) the evolution of the universe and the formation of the earth
The origin of life is a part of the evolution of the universe, so it is necessary to understand the evolution of the universe, especially the formation of the earth.
There is a popular view that the universe began with a sudden big bang 150 3 billion years ago. After that, a huge nebula composed of hydrogen and helium appeared in the universe, which split into many smaller nebulae. For some unknown reasons, the nebula began to shrink and rotate slowly. First shrink into a flat disk, and at the same time gradually increase the speed. When contracting, the internal contraction is faster and the external contraction is slower. To a certain extent, a dense entity is gradually formed inside. This is a forming star, also known as a primitive star. Some substances continue to fall on its surface, increasing its size and mass. Before shrinking, the temperature of the nebula is very low. Due to gravity contraction and density increase, the heat generated by intermolecular friction cannot be dissipated quickly, so the temperature rises. This process continues, and the temperature continues to rise, until there is extremely high pressure in the center, and hydrogen atoms undergo thermonuclear reaction at high temperature, releasing huge energy. At this time, a star was formed. This is how the sun is formed.
There is still a lot of gas and dust around primitive stars. One part of it falls on the protostar, and the other part is thrown out due to the acceleration of rotation. The thrown material, first, will continue to revolve around the protostar; Second, they will attract and collide with each other and gather into small pieces. These small lumps will also attract foreign substances and gradually increase in the process of rotation. This process has led to the formation of many planets, and the earth is such a planet. The formed planet can also attract smaller nearby celestial bodies to become its satellites.
Our solar system was formed in the above way (this is a generally accepted theory at present), the earth is a part of the object that fell out when the sun was formed, and the moon is a small sphere captured by the earth.
When the earth was first formed, it was completely different from the present earth environment. This is very important, because life will only appear under the conditions at that time. So let's take a look at the physical conditions when the earth was just formed.
When the earth was first formed, its composition was mainly hydrogen and helium and some solid dust. At first, its temperature was relatively low. The earth originally formed had a core, which was formed by the polymerization of solid dust and surrounded by a layer of gas, forming the first atmosphere, that is, the primary atmosphere. As the earth shrinks, the temperature rises gradually. When the temperature reaches a certain level, the outside atmosphere disappears completely. This is because the gas with relatively small molecular weight is separated from the gravity of the earth and scattered by the strong solar wind.
Then the surface temperature of the earth gradually decreased, and the internal temperature was still high, indicating frequent volcanic activity. The decomposition of materials in the earth produces a large amount of gas, which breaks through the surface and forms the second atmosphere, that is, the secondary atmosphere. This atmosphere is also different from the atmosphere of the earth now. It is reduced and contains no oxygen and nitrogen. It is generally believed that it contains all hydrogen compounds, such as water vapor (H2O) synthesized by hydrogen and oxygen, ammonia (NH3) synthesized by hydrogen and nitrogen, methane (CH4) synthesized by hydrogen and carbon, and hydrogen sulfide (H2S) synthesized by hydrogen and sulfur. The atmosphere formed by these newly generated gases is stable, because their temperature is not enough to make gas molecules move too fast and escape from the earth's gravity. Life is produced under such atmospheric conditions.
When the earth was first formed, there were no rivers and oceans, but there was a certain amount of water vapor in the atmosphere. When the earth's surface temperature drops again, because the internal temperature is still high, frequent volcanic activity spews more water vapor. The water vapor in the atmosphere is saturated and cooled, and the rain falls to the ground, and the rain gathers in the subsidence and depression of the earth's crust, forming rivers and oceans. When the surface temperature of the earth's crust drops below 100℃, they become water instead of water vapor.
When the water vapor in the atmosphere condenses into rain, some other gases in the atmosphere dissolve into water. Some soluble compounds on the surface of the earth's crust are dissolved in water, so many compounds, including the most primitive organic compound (methane), accumulate in the primitive ocean, laying a material foundation for producing more complex compounds. The primitive ocean has thus become the birthplace of primitive life. As for the energy needed for life, according to the situation of the earth at that time, in addition to the decay of radioactive isotopes in the crust and the heat released by volcanoes and hot springs, it may also come from ultraviolet rays and lightning (table 1). Although light is the biggest source of energy, the energy of each quantum is low, and there was no photosynthesis at that time, so it was useless. Each quantum of ultraviolet light contains high energy, which can break the valence bond of organic molecules, thus promoting a variety of chemical reactions leading to the formation of new molecules.
Table 1 Early Earth Energy
About the age of the earth: Because the sun, planets and meteorites are all formed by the same cosmic nebula, we can roughly know the age of the earth according to the age of meteorites. The age of meteorites can be calculated according to the decay of isotopes in meteorites, or directly according to the decay of isotopes in rocks of the earth itself. Both calculations show that the age of the earth should be 4.6 billion years.
chemical evolution
The earliest stage of life is chemical evolution, that is, from small inorganic molecules to primitive life (see figure).
Primitive life is the beginning of cells. The continuous evolution of cells, from prokaryotic cells to eukaryotic cells, from single cells to multicellular cells, is the stage of biological evolution.
The whole process of chemical evolution can be divided into four successive stages:
1. Generating organic molecules from inorganic molecules
In the laboratory, people have successfully synthesized inorganic substances into organic substances. In 1828, Weller first synthesized urea from lead oxide and ammonium. In the future, a large number of organic compounds will be continuously synthesized from inorganic substances. But is there a process of synthesizing organic matter from inorganic matter in nature? As early as the 1920s, Oberlin and Haldane speculated that such a process might occur in the reducing atmosphere of the early earth. The primitive atmosphere contains a lot of hydrogen compounds, such as methane, ammonia, hydrogen sulfide, hydrogen cyanide and water vapor. These gases may synthesize some simple organic compounds, such as amino acids, nucleotides and monosaccharides. Under the action of external high energy (such as ultraviolet rays, cosmic rays, lightning and local high temperature, etc.). According to this hypothesis, people simulated the original environmental conditions when the earth was formed in the laboratory and carried out experiments.
The first person who proved by experiments that inorganic substances can be converted into organic molecules in the primitive earth environment was S Miller of the University of Chicago. He installed a closed circulating device (see figure), which was filled with CH4, NH3, H2 and steam to simulate the primitive atmosphere.
Water is put in a flask with a closed device to simulate the primitive ocean. Then he heated the flask to make water turn into steam to circulate in the tube, and at the same time introduced an electric spark into the tube to simulate the lightning discharge in the primitive sky, so that the gas in the tube reacted. The condensation device on the tube makes the reactant dissolve in water vapor and condense at the bottom of the tube. A week later, he checked the condensed water in the pipeline and found that it contained a variety of amino acids, organic acids (such as acetic acid and lactic acid) and organic molecules such as urea (Table 2). Some amino acids, such as glycine, glutamic acid, aspartic acid and alanine, are the same as those that make up natural protein.
Table 2 Organic matter obtained from Miller's simulation experiment
Since then, many people have carried out similar experiments, and some people have switched to other gases and obtained roughly the same results. For example, some people use methylcyclohexane, water vapor and ammonia to synthesize propionitrile, hydrogen cyanide and some amino acids by radiation. In terms of various energy sources, in addition to spark discharge, ultraviolet rays, shock waves, gamma rays, electron beams and high temperatures (heated to 1000℃) are also used, and these experiments can also be successful. Particularly remarkable is the role of ultraviolet rays, because ultraviolet rays are the largest source of energy in the primitive atmosphere of the early earth. It is conceivable that with the composition of primitive atmosphere, small molecules such as amino acids can be formed under the action of ultraviolet rays.
In addition to amino acids, other small organic molecules, such as purine, pyrimidine and other bases, ribose, deoxyribonucleic acid and fatty acids can also be formed under the same conditions. It is even reported that nucleotides, porphyrins, nicotinamide and other compounds have been found in these experimental compounds. For example, adenine is synthesized by electron beam shooting with a mixture of methane, ammonia, water vapor and hydrogen; Ribose and deoxyribose are synthesized by irradiating diluted formaldehyde solution with ultraviolet rays or gamma rays. C2 ~ C 12 monocarboxylic acids (including acetic acid, propionic acid, butyric acid, isobutyric acid, isovaleric acid and isohexanoic acid) were synthesized from methane and water by spark discharge method.
It is worth mentioning that adenine is the most easily obtained alkali in the simulation experiment. Adenine is nothing more than a pentamer of hydrogen cyanide, so it is easy to synthesize.
The other three bases, guanine, cytosine and thymine, must be produced through complex reactions. With adenine, it is possible to produce ATP high-energy compounds. Adenine, ribose and phosphate solutions can be irradiated by ultraviolet light from 240 nm to 290 nm to generate ADP and ATP. Therefore, it is probably because adenine is easy to produce, so ATP, a molecule that provides energy for life activities, was produced early in life. It is precisely because adenine is easy to produce that ATP has become a widely distributed energy-supplying substance in the process of life evolution.
The simulation experiments made by Miller et al. and the data from meteorites prove that amino acids, adenine and other organic compounds can be synthesized before biogenesis (pre-biosynthesis) and have strong repeatability. A meteorite falling in Australia contains many identical amino acids, and its relative content is roughly the same as that proved by Miller. This coincidence provides strong evidence for Miller's idea of pre-biosynthesis. Miller believes that if these organic substances only exist in cosmic dust, comets, asteroids, meteorites and so on. Their content is too small to form the basis of the origin of life. Only when they come to the earth and accumulate in the primitive ocean can life appear.
2. Generating biomacromolecules from small organic molecules
The two most important cornerstones of living matter are protein and nucleic acid, so a key question of the origin of life is how these small organic molecules form biological macromolecules such as protein and nucleic acid.
There are also some experiments and speculations about protein and nucleic acid synthesis. It is generally believed that amino acids and nucleotides can be accumulated and concentrated in seawater for a long time. Under appropriate conditions (such as adsorption on inorganic mineral clay), amino acids and nucleotides can be polymerized to form original protein and nucleic acid respectively.
According to experiments, this kind of polymerization is realized in two ways: ① solution polymerization, which occurs under the adsorption of clay surface. The fine particles of clay are charged, which can make monomers such as amino acids adsorb on it and gather in large quantities, which is beneficial to polymerization. For example, adding hydrogen cyanide to a diluted amino acid solution (this product was found in Miller's experimental solution) can generate peptides at room temperature. If glycine is dissolved in hydroxide solution and heated to 140℃ and 19h, it can be directly polymerized into polyglycine. Under the adsorption of clay, polyglycine and formaldehyde can form complex polypeptide chains containing serine or threonine. ② concentration polymerization. Some people think that in some small corners of the ocean or small water bodies like lakes, the content of molecules such as amino acids in water can be very high due to long-term evaporation. This solution can directly produce "protein-like" polypeptide at higher temperature. F. Fox simulates primitive earth conditions. After mixing some amino acids, pour them into hot sand or clay at 160℃ ~ 200℃, evaporate water and concentrate amino acids. After 0.5h ~ 3.0h, amber transparent substance, i.e. protein-like substance, is produced. This substance is called protein-like, because it has some characteristics of protein, such as color reaction, peptide chain structure, amino acids produced after hydrolysis, which can be hydrolyzed by protease and its enzyme activity is weak. However, it has some characteristics different from protein, such as no optical rotation, poor order and no immune response.
In addition, other polymerization methods were found in the simulation experiment. If the mixture of hydrogen cyanide and ammonia is heated, the product obtained after hydrolysis will contain peptide-like substances. So some people think that peptides may be formed by hydrolysis of cyanohydrin polymers. All these indicate that the origin of protein may have many ways.
There are similar experiments in the synthesis of nucleic acid macromolecules. For example, a single nucleotide can be polymerized into a polynucleotide by heating at high temperature. As long as polyphosphate exists, a single nucleotide can form a polynucleotide at 50℃ ~ 60℃.
In a word, substances similar to protein and nucleic acid have been made under the condition of artificially simulating the primitive earth. However, compared with protein and nucleic acid in modern life, these products still have a certain distance. Their structure is relatively simple, the degree of order is relatively low, and their functions are not very specific. For example, an enzyme has low activity and specificity, and an enzyme can have several functions; One nucleic acid can serve as the function of several nucleic acids. After a long period of evolution, these molecules have become more orderly and complex protein and nucleic acid molecules.
Therefore, at present, we are not sure that these pathways are the pathways of protein and nucleic acid production, but we can say that these pathways are very likely.
3. The formation of multi-molecular system and the appearance of primitive life
Biological macromolecules are not primitive life. All kinds of biological macromolecules do not show the phenomenon of life when they exist alone, and only when they form a multi-molecular system can they show the phenomenon of life. This multi-molecular system is the bud of primitive life.
How did the multi-molecular system come into being? Oberlin and Fox have done a lot of experiments and put forward the theory of aggregates and microspheres respectively.
Oberlin's coacervate theory holds that biological macromolecules, mainly protein solution and nucleic acid solution, can form aggregated droplets, which is a multi-molecular system with certain life phenomena.
Oberlin's first experiment was this: He mixed the aqueous solution of gelatin (protein) with the aqueous solution of gum Arabic (sugar). Before mixing, this solution is transparent, and after mixing, it becomes turbid. Under the microscope, it can be seen that droplets, that is, aggregates, appear in a homogeneous solution. There is a clear boundary between them and water.
Protein, nucleic acids, polysaccharides, phospholipids and peptides can also be used to form such aggregates.
The diameter of aggregated droplets is 1 micron ~ 500 μ m, and the peripheral part of aggregated droplets thickens, forming a membrane-like structure separated from surrounding media. Oberlin has been able to make the aggregated droplets have primitive metabolic characteristics, make them exist stably for hours to weeks, and make them grow and reproduce indefinitely. For example, when phosphorylase is added to the solution of histone and gum Arabic, the enzyme is concentrated in the droplets of the aggregate. If glucose-1- phosphoric acid is subsequently added to the surrounding culture medium, the latter will diffuse into the aggregate and enzymatically polymerize into starch, which will increase the volume of the aggregate. The phosphate bond in grape-1- phosphoric acid can provide the energy needed for polymerization, while the inorganic phosphate released during polymerization is discharged from the aggregate as waste. For example, histone and RNA are made into aggregates, then RNA polymerase is added to the aggregates, and ADP is added to the surrounding culture medium as "food". In the aggregate, ADP interacts with RNA polymerase to generate polyadenylic acid (ADP provides energy), and polyadenylic acid increases the total RNA in the aggregate, thus generating droplets and splitting them into sub-droplets. Oberlin also simulated the experiment of polymer photosynthesis. He added chlorophyll to polymer droplets and methyl red and ascorbic acid as "food" to the culture medium. When the aggregated droplets are irradiated by visible light, the excited electrons in chlorophyll reduce methyl red, while the electrons released by ascorbic acid are used to replace the electrons in chlorophyll.
This process is similar to the photosynthesis of green plants (water molecules reduce NADP to NADPH under the action of light energy).
In addition, the aggregates can absorb different substances from the surrounding media, so that the aggregates can "grow" and "reproduce" after growing to a certain extent (small aggregates are separated by "germination"). It seems that aggregates are selective in absorbing foreign substances. On the whole, it has a certain structure. If the aggregate absorbs the enzyme, the enzyme can work in the aggregate (synthesize or decompose something). There is a clear boundary between aggregate and surrounding environment, which is a way to form the original film.
It can be seen that aggregates can show certain life phenomena.
These characteristics make people imagine that the solution of polynucleotide and polypeptide, or the solution of nucleic acid and protein, after concentration, forms an aggregate system at a certain temperature and other suitable environmental conditions, which is an important stage of the formation of primitive life. Only the aggregates formed by nucleic acid solution and protein solution will evolve into primitive life. The aggregates of other colloidal solutions were eliminated in the process of natural selection. Some people have found substances similar to agglomerates in the deep sea from hundreds of meters to thousands of meters, which is considered as a direct proof: polymer systems similar to agglomerates have indeed occurred; Polymers are indeed similar to primitive creatures. Once, an experienced biologist mistook it for a bacterium.
The microsphere theory was put forward by Fox. Fox found that microspheres can be formed by heating and concentrating dried amino acids or "protein-like" obtained in the laboratory (see figure). The microspheres are stable in solution, and the diameter of each microsphere is very uniform, about 1 micron ~ 2 microns, which is equivalent to the size of bacteria. Microspheres show many biological characteristics, such as ① there is a double-layer film on the surface of microspheres, which makes microspheres shrink or expand with the change of osmotic pressure of solution. If sodium chloride and other salts are added to the solution, the microspheres will shrink; ② It can absorb protein-like substances in solution and grow, and can reproduce in a way similar to bacterial growth and division; ③ Electron microscope showed that the ultrastructure of microspheres was similar to that of simple bacteria; ④ The existence of surface film makes microspheres selectively adsorb external molecules, and after ATP adsorption, they show the activity similar to cytoplasmic flow.
No matter what kind of multi-molecular system, the following three points are very important to continue to evolve into primitive life:
First, there must be a certain physical and chemical structure in the multi-molecular system, which is an important condition for the origin of life. With a certain physical and chemical structure, that is, a certain organization, we have the ability to absorb substances and carry out chemical reactions, and these reactions can be carried out in a certain way. With a certain organization, the system will be stable, not easy to be destroyed, and can survive and move towards an independent "life" without being affected by the external environment. The regular spatial arrangement of molecules is the main basis of a certain physical and chemical structure of a multi-molecular system.
Second, the main components of a multimolecular system must be protein and nucleic acid. Only with these two types of macromolecules can a multimolecular system establish a transcription and translation system and realize its genetic function. There were many kinds of aggregates or microspheres in the early ocean of the earth, but the aggregates formed by gelatin solution and Arabic gum solution were eliminated by natural selection because they could not be replicated, and only the polymer system containing nucleic acid and protein was selected to remain. Other macromolecules, such as polysaccharides and lipids, also participate in nucleic acid and protein system, completing their specific functions.
Third, the formation of the original film. The surface of a multi-molecular system must have a film. With the membrane, it is possible for the multi-molecular system to be separated from the external medium (seawater) and become an independent and stable system, and it is also possible to selectively absorb the required molecules from the outside to prevent harmful molecules from entering, so that various molecules in the system have more opportunities to collide with each other and promote the chemical process. So, how did the original film come into being and how did it develop into a double-layer film? Some people think that lipid molecules (phospholipids) are adsorbed on the interface of multi-molecular system, and protein molecules interact with lipid molecules, and lipid molecules are adsorbed on lipid molecules or buried in lipid layers, thus forming lipid protein layers. Continue to develop, this lipid protein layer becomes a bilayer under certain physical effects, and then absorbs some other molecules such as polysaccharides to become the original membrane of the bilayer.
The structure and function of the original membrane have been constantly improved and complicated in the evolution process, and it has become the current biofilm.
Nuclease and Nucleic Acid-protein System There is a problem here, that is, modern biology tells us that nucleic acid can only be synthesized under the action of protein (enzyme), and protein can only be synthesized when its corresponding nucleic acid sequence exists. Therefore, it is hard to imagine that in the early days of the earth, nucleic acids and protein with such complex structures would be naturally produced at the same time and have complex interactions. So, what kind of chemical process can form a multi-molecular system in which nucleic acid and protein are interdependent?
T Cech of the United States studied the RNA of Tetrahymena protozoa, and found that the intron (L 19RNA (see figure) cut from Tetrahymena precursor molecule has strong enzymatic activity, which can polymerize nucleotides into polynucleotides and cut the polynucleotides into fragments of different lengths while keeping itself unchanged.
It can be seen that it is a true enzyme, named ribozyme. It is characterized by information and catalysis. Its discovery makes people tend to regard RNA as the first molecule of protein-nucleic acid problem in the origin of life, and synthesize protein through the enzymatic action of RNA, thus resulting in the genetic system of RNA protein. Because protein has 20 different side chains, molecular conformation.