How are people formed?

Your problem can be analyzed from two aspects. First, as far as human beings are concerned, if the former is well explained according to the level of contemporary scientific development, it is simply that sperm and eggs are combined in the mother. What puzzles human beings is that they haven't fully figured out where the theory of creating human beings, such as Shen Nu Wa, was destroyed by the spark of human wisdom and replaced by a more rational scientific explanation, which involves the height of human origin. The following is an aspect of human research results.

origin of life

At the beginning of the formation of the earth, due to various physical and chemical reactions, the earth was in an extremely hot state, and there could be no life there. Later, with the slow cooling of the earth and its unique astronomical position and structure, life quietly evolved. People have been paying attention to the origin of life since ancient times, and various hypotheses have emerged. Today, except for problems like genetic machines, the general process outline has gradually become clear.

Creationism holds that life is created by supernatural forces, or is determined by a transcendent beyond matter. This is a primitive concept, which came into being when people's ability to understand nature is very low. Later, it was used intentionally or unintentionally by socialized ideology, which led people who believed in the absolute supremacy of spirit to believe in creationism.

In ancient times, people's ability to understand nature was low, but they were able to carry out abstract thinking activities. According to this phenomenon, they came to the conclusion that life happens naturally. Their representative ideas are China's ancient theory that "maggots are born when meat decays, and midges are born when fish wither" and Aristotle's theory that "some fish develop from silt and gravel".

With the deepening of understanding, people know that maggots come from flies. After Pasteur, people thought that life was produced by parents and spores, that is, life could not be produced naturally. However, the theory of origin does not answer how the original life was formed.

With the great development of astronomy in the century, it is proposed that life on earth originated from the "embryo" of other planets or the universe. This understanding was popular in the19th century, and there are still very few people who insist on this view. The basis is that all living things on the earth have a unified genetic code, and molybdenum, a rare element, plays a particularly important role in the enzyme system.

The theory of chemical evolution advocates studying the origin of life from the law of motion and change of matter. It is believed that under the condition of primitive earth, inorganic substances can be transformed into organic substances, and organic substances can develop into biomacromolecules and multimolecular systems until primitive life is formed. 1924 Soviet scholar a.n. Oparin first put forward this view; Five years later, the British scholar J.B.S Haldane put forward a similar view. They all believe that life on earth is evolved from inanimate matter over a long period of time; This process is called chemical evolution, which is different from biological evolution after the appearance of organisms. Oparin's The Origin of Life on Earth was published in 1936, which is the first monograph on the origin of life in the world. He believes that there is no reducing atmosphere of free oxygen on the primitive earth. Under the action of short-wave ultraviolet and other energy sources, simple organic matter (biological micromolecules) can be generated, and simple organic matter can generate complex organic matter (biological macromolecules) in the primitive ocean, forming an aggregate of multi-molecular systems. After long-term evolution and "natural selection", the latter finally appeared primitive life, namely protozoa. More and more experimental evidence of chemical evolution has been accepted by most scientists.

Research methods ① Simulate primitive earth conditions in the laboratory, and explore the possible process of transforming from unorganized to living matter. ② Looking for chemical fossils and microfossils in paleostrata. Determine their geological ages to study the time and evolution of the origin of life. ③ Using radio telescopes to study interstellar molecules, and using space technology to explore the existence and evolution of life on other planets, so as to study the possible ways of the origin of life on earth. (4) Disassembling and reassembling some structures of existing organisms, and studying the mechanism of biological macromolecules "changing from dead to alive". ⑤ Using modern methods to synthesize protein, nucleic acid and other information molecules artificially, so as to explore the law of the origin of life.

The basic process of chemical evolution gave birth to the primitive earth of life. The primary earth's crust is weak, the temperature inside the earth is very high, and volcanic activity is frequent. Many gases emitted by volcanoes constitute the primitive atmosphere. One school thinks that the primitive atmosphere includes CH4, NH3, H2, HCN, H2S, C0, CO2 and water vapor, and it is a reducing atmosphere without free oxygen. The main reasons are as follows: ① The radio spectrum analysis of radio telescope shows that the atmospheres of planets such as Jupiter and Saturn, which are far away from the sun and have little change, are all reducing atmospheres composed of H2, he, CH4 and NH3; ② The iron contained in the paleosedimentary rocks is magnetite (Fe3O4) with low oxidation degree, while the iron contained in the later "red bed" is hematite (Fe2O3) with high oxidation degree, which reflects the transition from reduction to oxidation in the original atmosphere. Now the oxidizing atmosphere of the earth is gradually formed through long-term photosynthesis after the emergence of cyanobacteria and plants.

Because there is no free oxygen in the primitive atmosphere and no oxygen layer is formed to block and absorb most of the ultraviolet rays radiated by the sun, all ultraviolet rays can reach the surface of the earth and become the energy source for synthesizing organic matter. In addition, the discharge in the sky, the energy released by volcanic eruption, the radiation in the depth of the earth and cosmic rays in space, and the shock wave generated when meteorites pass through the atmosphere also contribute to the synthesis of organic matter. Among the above energy sources, lightning seems to be more important, because it provides more energy and is released near the ocean surface, so the products synthesized there are easily soluble in water.

The synthesis of small biological molecules, such as amino acids, nucleotides and fatty acids. 1952, S.L. Miller, a graduate student of the University of Chicago in the United States, conducted an experiment to simulate lightning in the primitive atmosphere under the guidance of H.C. Yuri, and * * * got 20 kinds of organic compounds, among which four amino acids (glycine, alanine, aspartic acid and glutamic acid) in1all contained biological proteins. Later, other scholars carried out a large number of simulation experiments, or switched to outside lines, B-rays, high temperature, strong sunlight and so on. As an energy source, or changed to reduce the individual components of mixed gas (such as replacing H2O with H2S, replacing CH4 and H2 with HCN, or increasing CO2 and CO, etc.). ), producing amino acids. However, amino acids cannot be produced by replacing reducing mixed gas with oxidizing mixed gas. At present, 20 kinds of amino acids that make up natural protein, except arginine, lysine and histidine, can be produced by simulation experiments. Most of the small biological molecules that make up nucleic acids can also be formed by simulation experiments; If someone irradiates diluted formaldehyde (HCHO) solution with ultraviolet rays or gamma rays to obtain ribose and deoxyribose; Ultraviolet radiation of HCN produces adenine and guanine. Heating propionitrile (N≡C-C≡CH), KCN and H2O at 100℃ for one day to obtain cytosine. Heating NH3, CH4, H2O, malic acid and polyphosphoric acid to 100 ~ 140℃ to obtain uracil; Adenosine can be produced by mixing dilute solutions of adenine and ribose with phosphoric acid or ethyl metaphosphate and irradiating with ultraviolet rays. Adenosine and ethyl metaphosphate are sealed in a glass tube and irradiated with ultraviolet rays, so that adenosine (AMP) can be produced. In addition, fatty acids can also be obtained by irradiating hydrocarbons and carbon dioxide with high-energy electrons.

Chemical evolution is obviously not limited to the primitive earth, but also occurs in the universe and other celestial bodies. The discovery of organic matter in interstellar molecules and meteorites confirms this. According to L.E. Snyder, by 1978, 37 kinds of interstellar molecules have been discovered, 80% of which are organic compounds. There are a lot of formaldehyde and hydrogen cyanide in interstellar molecules, which are the same as the initial intermediate products in Miller's discharge experiment. When they react with ammonia, they can be hydrolyzed to produce amino acids. 1969 The meteorite that fell in murchison Town, southeast Australia in September was found to contain many kinds of amino acids, the species and contents of which were quite consistent with those produced by Miller's discharge experiment. This shows that it is not only possible for the primitive atmosphere to generate biological small molecules from inorganic substances, but also this process is still happening in the universe.

The synthesis of biomacromolecules can be inferred. Biological macromolecules (monomers) washed into the primitive ocean by rainwater can form biological macromolecules (polymers) such as protein and nucleic acid through interaction. However, monomer must undergo dehydration and condensation when it becomes polymer. It is obviously a big problem to dehydrate and condense in the primitive ocean. At present, there are three credible views on synthetic peptides of amino acids: ① American scholar S.W. Fox thinks that amino acids in primitive oceans may be washed to the hot zone near volcanoes, and then evaporated, dried and condensed to form protein-like substances. If protein-like substances are washed back into the ocean, other reactions may occur further. Their basis is that 20 kinds of natural amino acids are neutralized and mixed by acid and alkali, and heated at 170℃ for several hours, so that an amino acid polymer with some characteristics of natural protein-protein-like can be obtained. (2) Other scientists, such as Israeli A. Carter Carsky, believe that amino acids in primitive oceans are peptides synthesized on some special clay. They first let amino acids react with adenylate in the laboratory to produce aminoacyladenylate, which contains free energy. When it is adsorbed on the surface of special clay such as montmorillonite, it can condense to form polypeptide. As early as 195 1, British scholar J.D. Bernard put forward the theory that some clay lamellae can adsorb charged molecules and become the original catalytic centers because they contain a lot of positive and negative charges. In 1960s, British scholar A.G. Keith-Smith further put forward the view that life originated from clay. He believes that the chemical evolution that led to the emergence of life was carried out in clay. In 1980s, NASA scientists discovered that some clays have the function of storing and transporting energy, which supported Keith Smith's view. (3) Japanese scholar Shiro Akashi put forward the theory of polyglycine, which can avoid the difficulty of dehydration condensation and explain the formation of peptides. He thinks that HCHO produced in primitive atmosphere can react with NH3 and HCN to generate aminoacetonitrile, which is polymerized first and then hydrolyzed to generate polyglycine, and finally reacts with aldehydes and hydrocarbons to generate different side groups, forming protein composed of various amino acids.

Scientists' experiments of synthesizing nucleic acids by simulating primitive earth conditions have also been successfully reported. For example, some people heat nucleoside and polyphosphoric acid to 50 ~ 60℃ to obtain polynucleotides; Someone heated uridylic acid and polyphosphoric acid to obtain polyphosphoric acid, but it was not connected by 3' and 5' phosphodiester bonds. Later, some people used cytidine and polyphosphoric acid to synthesize a short-chain nucleic acid consisting of about 5 nucleotides at 65℃, containing 3' and 5' phosphodiester bonds, which is the same as biological nucleic acid. However, in the absence of enzymatic reaction, it is difficult to synthesize longer polynucleotides linked by 3' and 5' phosphodiesters or polymers composed of several mononucleotides.

With the appearance of multi-molecular system, biological macromolecules must form a system and a boundary film to be obviously isolated from the surrounding environment and further evolve. Therefore, people think that the formation of multi-molecular system may be an essential stage in the process of chemical evolution before the emergence of life. At present, there are two main experimental models for studying multi-molecular systems: aggregates and microspheres. ① Coalescent layer model. Oparin glued the aqueous solution of gelatin and the aqueous solution of Arabic gum together. Under the microscope, he saw countless droplets, that is, aggregates. Later, it was found that protein mixed with sugar, protein mixed with protein, and protein mixed with nucleic acid, all of which may form aggregates. Oparin added phosphorylase to the solution containing histone and Arabic gum, and the enzyme was concentrated in the aggregated body drops; When glucose-1- phosphoric acid is added to the solution, the latter will diffuse into the droplets and be polymerized into starch by phosphorylase. The energy required for polymerization is provided by the phosphate bond of glucose-1- phosphoric acid, and the released phosphoric acid diffuses out of the droplets as "waste". Because gum Arabic is a sugar, synthetic starch can increase the volume of aggregates. When the aggregates become too large, they will spontaneously split into several droplets. If phosphorylase and amylase are added to the aggregate, both enzymes will be concentrated in the droplet, and then two-step reaction will occur: glucose-1- phosphoric acid enters the droplet and is polymerized into starch by phosphorylase; Amylase breaks down starch into maltose, which diffuses back into the surrounding solution together with phosphate. This droplet can be kept as an open system for a long time. It can be imagined that if the aggregate itself can produce phosphorylase and amylase (containing gene nucleic acid system that can "guide" the synthesis of these two enzymes), and there is enough glucose-1- phosphate as "food" in its surrounding environment, it can be synthesized and decomposed; If the synthesis speed is greater than the decomposition speed, the aggregate can "grow" and "reproduce" through division. Because aggregate model can show these simplest biological characteristics, it attracts people's attention.

② Microsphere model. Fox et al. dissolved acidic protein in dilute salt solution, and after cooling, countless microspheres were observed under the microscope. The microspheres have a double-layer membrane and are relatively stable. They contract in hypertonic solution and expand in hypotonic solution. They can "reproduce" through budding and division, and show enzymatic activities such as hydrolysis, decarboxylation, amination, deamination and redox. But the protein-like protein is not protein, it has a certain amount of peptide bonds and a large number of other chemical bonds, which can not be completely hydrolyzed by protease. Its enzyme-like activity is very low, almost mainly decomposable, but what is more significant in evolution is the activity of synthetase. Protein-like substances are produced by using 20 natural amino acids as raw materials to simulate the dry and hot conditions of the primitive earth, which is more convincing than ready-made substances (such as gelatin and Arabic gum). ) is produced by aggregates from organisms, so it is widely valued.

The evolution from a multi-molecular system to the origin of primitive life is the most critical step. There are two important problems to be solved: how biofilm is produced and how genetic devices originated. (1) Formation of biofilm: Only when the boundary membrane becomes a biofilm can the multi-molecular system evolve into a primitive cell. The basic structure of biofilm is that phospholipid molecules are embedded with dynamic functional protein molecules. It is generally believed that liposomes may be the model of primitive biofilm. Liposome is an artificial cell-like structure, which is composed of lipid molecules and surrounded by a water-containing chamber. Liposomes can usually be obtained by ultrasonic treatment of phospholipids in water. It is generally believed that phospholipids must be formed in the primitive ocean, and liposomes can easily form with phospholipids. Liposomes embedded in functional proteins such as glycoprotein may develop into primitive biofilm after long-term evolution. ② Origin of gene device: At present, there is no experimental model, only some indirect data are used to speculate. Many scientists believe that the relatively stable life at first may be a polymer made by Oparin in the laboratory, which is mainly composed of protein and nucleic acid. As you can imagine, at first, it was a multi-molecular system of various components, and later it was crushed into something unsuitable for survival, and what was suitable for survival was preserved. After such "natural selection", the multimolecular system based on protein and nucleic acid finally survived and developed. Among them, nucleic acid can replicate itself and act as a template, while protein plays a structural and catalytic role. It can be inferred that protein and nucleic acid did not appear first, but co-evolved in the multi-molecular system from the beginning, which promoted the development of life together. The key problem here is how to identify the base sequence of nucleic acid with the amino acid sequence, so that the codons on today's messenger ribonucleic acid (mRNA) can match with 20 natural amino acids as accurately. If this problem can be clarified through experiments, it will be easier to solve the problem of evolution from multi-molecular system to primitive cell.

The fossil record of the origin of life is about 4.6 billion years old. At present, the oldest known sedimentary rock on the earth is in Isuva, southwest Greenland, with an age of about 3.8 billion years. Some organic microstructures have been found in this sedimentary rock, which have been proved to be produced when the water surface is discharged. 1978 ~ 1980, Australian scholar D.I. groves and others reported that some filamentous microfossils were found in the strata of Warawoona Group in Northpole, Western Australia, 3.5 billion years ago (left).

The origin of optical rotation All amino acids in protein are L-type, and natural sugars such as ribose and deoxyribose in nucleic acid are D-type. If organisms ingest sugars or amino acids with opposite optical rotation, they must be corrected by enzymes before they can be used. Scholars believe that this optical rotation is an important criterion to distinguish living things from nonliving things. Some scholars believe that optical activities are completely accidental in the origin of life; Some scholars think that optical rotation may be related to parity non-conservation, and speculate that the existence of L-amino acids and D-sugars may be determined by the asymmetric nature of substances.

Is there life on other celestial bodies? The exploration and research so far show that there is no life on other planets in the solar system except the earth. But the solar system is only a tiny member of the whole universe, with a history of only about 5 billion years since its birth. According to some astronomers' estimates, the universe appeared at least 65.438+000 billion years after the Big Bang, and there were about 654.38+0020 stars similar to the sun. Some scientists estimate that life exists on at least 108 planets in the universe. In addition, in the right place, interstellar molecules may evolve into life through chemical evolution. Therefore, although life outside the earth has not been found yet, theoretically, the possibility of life on other celestial bodies cannot be ruled out.

As for the fact that aliens visit the earth from time to time or are visiting the earth, there is no valid evidence to prove it. So far, the so-called evidence is either forged or illusory, or the observation method is wrong or the equipment is limited.

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