In the second stage, biological macromolecules are produced by small organic molecules. This process takes place in the primitive ocean, that is, after long-term accumulation and interaction, small organic molecules such as amino acids and nucleotides form original protein molecules and nucleic acid molecules through condensation or polymerization under appropriate conditions (such as adsorption of clay).
In the third stage, a multi-molecular system is formed by biological macromolecules. How is this process formed? Oberlin, a scholar in the former Soviet Union, put forward the aggregate hypothesis. His experiments show that protein, polypeptide, nucleic acid and polysaccharide can be automatically concentrated and aggregated into dispersed spherical droplets when put into a suitable solution, and these droplets are aggregates. Oberlin and others believe that aggregates can show life phenomena such as synthesis, decomposition, growth and reproduction. For example, the aggregate has a boundary similar to a membrane, and its internal chemical characteristics are significantly different from the external solution environment. Agglomerates can absorb some molecules from the external solution as reactants, and can also undergo specific biochemical reactions under the catalysis of enzymes, and the products of the reactions can also be released from the agglomerates. In addition, some scholars have put forward other hypotheses, such as microspheres and lipid spheres, to explain the process of organic polymers forming multi-molecular systems. Fig. 7 Schematic diagram of simple metabolism of aggregates In the fourth stage, the organic multi-molecular system evolved into primitive life. This stage, formed in the primitive ocean, is the most complicated and decisive stage in the origin of life. At present, people can't verify this process in the laboratory. The origin and evolution of life are closely related to that of the universe. The constituent elements of life, such as carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur, come from the evolution of elements after the Big Bang. The data show that the chemical evolution in the pre-biological stage is not limited to the earth, and the products of chemical evolution exist widely in the universe. In interstellar evolution, some biomolecules, such as amino acids, purines and pyrimidines, may be formed in interstellar dust or condensed nebulae, and then under certain conditions, biopolymers such as peptides and polynucleotides are produced on the planet surface. Through several transitional forms of pre-biological evolution, the most primitive biological system, that is, life with original cell structure, was finally formed on the earth. At this point, the evolution of life began, and countless complex life forms have been produced on the earth until today.
3.8 billion years ago, a stable land mass was formed on the earth, and various evidences show that the liquid hydrosphere is hot and even boiling. Some extremely thermophilic archaea and methanogens may be closest to the oldest life forms on earth, and their metabolic patterns may be chemical and inorganic autotrophs. 3.5 billion years ago, microorganisms in the Wallavo group in Western Australia may be the earliest evidence of life on earth.
The appearance of the primitive crust marks that the earth has entered the era of geological development from the era of astronomical planets, and life with primitive cell structure has gradually formed. However, for a long time, there were not many creatures. It was not until the Cambrian 540 million years ago that a large number of metazoans with shells appeared, so the geological age after the Cambrian was called Phanerozoic.
First, the origin of life, the first mystery is the time of life, the time of origin. In the Middle Ages, people believed the story that God created man in the Bible. 1650, an Irish archbishop calculated that the exact time of God's creation was 4004 BC according to the description in the Bible, and another priest even calculated the time of creation more accurately to 9: 00 am on June 23, BC 10. That is to say, the origin of life is 6 thousand years ago, which is certainly not true, but what is it? That's a really scientific answer. How does science answer the time of the origin of life? That is to say, to answer with fossils, fossils are preserved in rocks. We know that after the death of living things, their remains are preserved in rocks under appropriate conditions. We call it a fossil. Rock formations formed in geological history, like a chronicle, are buried deep in these rocks, and older biological fossils are preserved at the bottom of the rock formations.
So far, we have found that the oldest biological fossils are rocks from western Australia, about 3.5 billion years ago. These fossils are similar to modern cyanobacteria. They are some primitive life, invisible to the naked eye. Its size is only a few microns, to dozens of microns. Therefore, we can say that life originated no later than 3.5 billion years. At the same time, we know that the earth was formed about 4.6 billion years ago. With these two data, we can see the age of the origin of life, which can be roughly defined between 4.6 billion and 3.5 billion years. Today, with the development of science, geologists believe that in the early days of the earth's formation, the earth was hit by a large number of asteroids and meteorites, which was not suitable for life. It was not so much that there was life on the earth at that time as that it was destroying life, so the origin of life on the earth was not earlier than 4 billion years. In addition, carbon was found in 3.85 billion-year-old rocks in Greenland. As we know, there are two kinds of carbon, one is inorganic carbon and the other is organic carbon. In addition, this kind of carbon can be divided into heavy carbon and light carbon, so we can infer the source of these carbons according to the ratio of light carbon to heavy carbon in this kind of carbon. According to the isotope analysis of carbon, scientists speculate that it is organic carbon and comes from organisms. That is to say, in this way, we have greatly shortened the time of the origin of life, that is, between 4 billion and 3.8 billion years, from the origin of life on earth to the present, it is an endless evolutionary history of life.
From 65438 to 0859, with the publication of Darwin's On the Origin of Species, great changes have taken place in biological science, and at the same time, it has brought a ray of light to reveal the eternal mystery of the origin of life, which is the modern chemical evolution theory. The chemical evolution theory of the origin of life was first confirmed by American scholar Miller in 1953. Since you said that the temperature of the earth was relatively high in the early days, reducing gases and water were everywhere, I put these gases and water in a bottle to see if life or organic compounds could be produced. 1953, Miller put ammonia, hydrogen, water and carbon monoxide in a sealed bottle, inserted metal rods at both ends in the bottle, and then turned on the power. Through this lightning action, a large number of amino acids were indeed produced in a few days. That is to say, on the earth, under lightning and at room temperature, it can also become inorganic molecules and synthesize organic molecules. We know that your amino acid is the most important substance that constitutes protein, and it can be said that it is the most important substance that constitutes the origin of life. So, what should Miller's description of the origin of life look like? That is, in the early days, the earth contained a large number of reducing primitive atmospheres, such as methane, ammonia, water, hydrogen and primitive oceans. When lightning struck the earth in the early days, these gases were polymerized into a variety of amino acids, which may be partially concentrated at normal temperature and pressure, and then further evolved into polysaccharides such as protein and protein, as well as polymer lipids, which may be pregnant and developed into life at a certain time.
However, this kind of warm pool also encountered some problems, including two problems. The first problem is that geologists now believe that the early atmosphere of the earth does not contain a lot of reducing gases, but contains a lot of carbon dioxide and nitrogen, which is more inert than Miller gas. In the case of lightning, you can't form a lot of amino acids. Second, it is impossible to form a warm pool in the early days of the earth for a long time. Why? Because at that time, in the early days of the earth, as I said just now, there were a lot of meteorites and meteors, and the temperature was very high with the radioactivity of the earth itself. Once life is born in your warm pool and a meteorite hits it, the temperature can reach thousands of degrees or even thousands of degrees in an instant, and life is extinct, so you can only regain the origin of life. But now we think so. Are there any relatively high temperatures, reducing gases and living things on the earth today? Then, two works can be said to be of epoch-making significance. One is 1967, an American scholar, Blake, who found a large number of thermophiles in the hot springs of Yellowstone National Park. We know that protein generally freezes when it exceeds 60 degrees, and eggs will be cooked at 60 degrees or above 70 degrees. But can living things survive above 60 degrees? I didn't dare to think about it before
Modern biologists have compared some thermophilic archaea in hot springs with ordinary bacteria through the study of biomolecules, and found that their genetic similarity is less than 60%. That is to say, these archaea contain a lot of ancient genes, which means that this type is probably the origin of life. It should be said that the best evidence for us to study the origin of life is the information contained in rocks and fossils between 4 billion and 3.8 billion years on earth. However, after 4 billion years of changes, the earth has changed beyond recognition. Even if you have rocks from 4 billion to 3.8 billion years old, it has entered a large number of varieties with little information.
Therefore, we should not confine our eyes to the earth. If life is a common phenomenon in the universe, is there an environment similar to the early earth on other celestial bodies besides the earth? If so, it may open a new window for studying the origin of life. Where is our first goal? Not Mars, but the moon. Now geologists believe that the moon was 4 billion years ago, when a big planet hit the earth and disappeared from it. Today's moon was formed, which happened to be 4 billion years. If there is the origin of life on the earth, let's look at it on the moon, then this problem will be solved. China ancient mythology has the Goddess Chang'e flying to the moon's saying that there are laurel trees, moon rabbits and romantic love stories on the moon. But in the 1960s and 1970s, with the successful landing of astronauts from the former Soviet Union and the United States, this myth was completely shattered. The moon is actually a desert planet without life, water and oxygen, which is not suitable for life.
So what's our second goal? The second goal is Mars, because Mars probably had an Earth-like experience 4 billion years ago. Its material composition is very similar to that of the earth, and its orbit is also very similar to that of the earth. So is there life on Mars? What are we going to do on Mars? When we look for the origin of life, where should we start? Generally speaking, it is three o'clock. Are there creatures on Mars? If there is living life, that's good. Life may really have originated in the universe, or life on earth may have come from Mars or other comets. Secondly, we look for liquid water, because we know that water is the source of everything and water is the source of life. Now all the life forms we understand on the earth are inseparable from water, so finding liquid water is also a very important indicator. The third is to find compounds related to life. If we don't have living things now, did we have them in the past? Did the creatures of the past form some compounds? Are they preserved in these rocks in the form of fossils? So we go to Mars to find life for three purposes.
1957 when the American Viking spacecraft sent back information to the earth, there was no life and no liquid water on Mars. It is a barren and thirsty red planet. However, mankind is not discouraged. In 1990s, NASA accelerated its exploration of Mars. Images obtained by Mars probe, Mars Pioneer spacecraft and Hubble telescope, as well as other astrophysical information, indicate that liquid water may have existed on Mars in the past. According to some space data, in front of the mountain where we had a big flood, there were structures similar to alluvial fans on Mars, as well as traces of water, rivers, rivers like the dry riverbed on the earth, and water eroding rocks. In addition, there is something very special. At the poles of Mars, a phenomenon similar to the thawing of frozen soil on the earth was found. This is our spatial information.
So our research on Mars is helpless? At least not at this stage. We have meteorites from Mars. Fortunately, in 1984, a meteorite was found on the Antarctic ice sheet. After this meteorite was brought back, we analyzed its elements and gas chemistry and found this meteorite. Its gases and isotopes are very similar to those on Mars. So they think this meteorite came from Mars. This meteorite fell on the ice sheet 10 thousand years ago, the ice sheet in Antarctica.
According to the radioisotope dating of this meteorite, this meteorite has a history of 4 billion years, which is about 4 billion years from now, which is completely consistent with the age of the origin of life on earth. So for decades, scientists have done a lot of research on this meteorite. Some researchers believe that this meteorite contains signs of life. What evidence is there? There are three. The first one contains several sedimentary minerals. Because sedimentary minerals are formed in the presence of water, scientists infer that there may be water on Mars, especially one of these minerals is a magnet mineral. He believes that this kind of magnetite can only exist in the form of life, which is the first evidence. Secondly, through the chemical analysis of the surface of this meteorite, a variety of polycyclic aromatic hydrocarbons were obtained. He believes that this polycyclic aromatic hydrocarbon is related to life forms. Thirdly, through careful observation by scanning electron microscope, biological fossils very similar to bacteria were found. This fossil is not very big, only a few hundred nanometers. Therefore, in 1996, NASA announced to the world that there was life on Mars 4 billion years ago. Of course, this is a family statement. Is the information about the existence of life in this meteorite true? Of course, many scholars have questioned the evidence. First, take magnetite as an example. You think it can only survive by life, and I agree with you. You think this sedimentary mineral is also survived by life, and I agree with you. Only life with water can precipitate, and I agree. But you should know that this meteorite was found on the Antarctic ice sheet. The ice is full of water. When you hit the ice sheet, a lot of water will melt. When a meteorite hits the earth, many cracks may be formed. If liquid water and melted water enter through this crack, won't it also form a sedimentary mineral of its own? In addition, if you think this magnetite, you may also, if some people think that magnetite is not unique to life and can be formed under other material conditions, then if the first evidence is given, many scientists think that it cannot be occupied. The second is the problem of polycyclic aromatic hydrocarbons. Similarly, if you look like the Antarctic ice sheet, it is MINUS 40 degrees or 50 degrees, and there are also a lot of bacteria and algae. Is it polluted? The pollution now may have been ten thousand years ago. So this evidence, you can't say it is a very reliable evidence, 100% evidence. The third evidence, especially the third evidence, is even more unreliable. Is to split the meteorite. Look at these so-called bacterial fossils. These fossils, the first one, are too small, only a few tens of nanometers in diameter. We know that if you are like an iron core, it may be 0.6 nanometer, then your so-called biological fossils may be hundreds or even thousands of cores. So this basic word, the smallest form of primitive cells that we understand now, is unimaginable. So whether there is life on meteorites, or whether there is life on Mars, needs further study.
The third celestial body we observed is Jupiter's satellite, especially the second satellite Europa, which is very similar in size to the diameter of the earth. 1997, American Galileo spacecraft observed Europa, and they found a lot of cracks on its surface, and many cracks. Through the research of astrophysics, this planet is actually made up of water, and this kind of water is. It turned into solid ice, and we can see from these cracks that many cracks seem to indicate that the planet may have melted in the past or at some time or at some time. In other words, it once had liquid water, and liquid water existed. Does it have life? But this is still an unknown, and we need further research. In short, with the further development of space science and other related technologies, the exploration of extraterrestrial life has opened up a new way for us to study the origin of life.
However, no matter how life originated, these three processes can't run away: the first one is from inorganic substances to small organic molecules. In this process, such as carbon monoxide, carbon dioxide, water, hydrogen, ammonia and methane, you can't get away from synthesizing small organic molecules, such as amino acids, purines, pyridine, nucleotides, high-energy compounds, fatty acids and chlorine. A process can't run anywhere, such as the bottom of the sea, hot springs, Mars and Europa, so the process of the origin of life is studied first.
The second is the process from organic small molecules to organic macromolecules, that is, the amino acid purine pyrimidine mentioned just now, and organic macromolecules such as protein polysaccharide nucleic acid, because protein is the main substance that constitutes organisms, polysaccharides and sugars are the main components of bones and cell walls of many cells, and nucleic acids are genetic materials, so this process cannot be run away.
The third largest molecule of these creatures has evolved into the life of primitive single cells, which cannot escape. A primitive single cell is wrapped in a membrane and contains genetic material, which needs metabolic exchange. So the process of the origin of life can actually be simply divided into three processes: which step do we take now for these three processes? What haven't we solved yet? First, let's look at the process from inorganic substances to organic small molecules. In fact, in this process, we can synthesize small organic molecules in hot springs, in deep-sea "black chimneys" or in the laboratory. Miller's experiment is the most classic experiment, which is to synthesize small organic molecules from inorganic substances.
The second process, let's take a look again. The second process is from small organic molecules to large organic molecules. If this process is carried out, it will actually be in hot springs, such as undersea hot springs, as well as on land, such as Huangshi National Park, and hot springs in Yunnan, our country. Because this temperature is very high, thermal polymerization dehydration reaction can be carried out to form protein. We are in the laboratory, and this process can be repeated.
The most difficult thing is the third process of the origin of life, that is, the process from biological macromolecules to primitive single cells. It can be said that this process is the biggest problem that scientists have encountered in their research so far. It is also an insurmountable gap between inorganic life and life, inorganic compounds and organic life. What are the parts of this process? In other words, how many parts should we start to study biological macromolecules and primitive single-cell life? First of all, we should study our own genetic system. Genetic system is the establishment of self-replicating biological macromolecules and the establishment of DNA and RNA systems. How was it established? How is it synthesized? How do they have genetic functions? Second, the synthesis of protein should be brought under the control of the self-replicating system. What does this mean? It is its metabolism, that is, the exchange of energy and substances in cells, receiving sunlight and chemical energy, producing organic matter, and then decomposing organic matter to produce energy, just like a motor to run cells. This is the process. This process is also very difficult. The third process, the formation of biofilm system, that is to say, systems such as cell wall, cell membrane and biofilm, why is it important? Because we know that the inorganic world is not isolated, there is no such isolation, but it has a membrane isolated from the outside world in biology, and this membrane is not absolutely isolated, but exchanges substances with the outside world. It has some small gaps, so this biofilm system is also a very sophisticated biological institution, so these three stages or steps are indispensable and very difficult in the origin of life.
So far, we can describe the origin of life as follows: on the earth 4 billion years ago, organic small molecules synthesized by inorganic molecules gathered in hot springs or hot water near the crater and formed biological macromolecules through polymerization. These macromolecules self-replicate, self-select, then self-organize, self-replicate and mutate, thus forming nucleic acids and active proteins, and simultaneously generating isolated structures. Finally, the metabolic reaction under gene control provides energy for gene replication and protein synthesis, thus producing a self-replicating protocell wrapped in a biofilm on the earth. This primitive cell may be heterotrophic or chemoautotrophic, and it may be similar to thermophilic archaea of modern organisms near hot springs. This description describes the origin of life in just a few hundred words. But it has four insurmountable gaps, one is self-selection, because you are composed of biological macromolecules or RNA and DNA, which are very limited molecules. It forms many such molecules under inorganic conditions, or in the case of lightning, or in hot water. How do these molecules choose themselves, synthesize DNA and RNA and discard other macromolecules? We don't know this process. Why? The second is self-replication. DNA and RNA can replicate themselves and pass them on to the next generation. We don't know this process either. The third is the separation structure, that is, the cell membrane, such as the cell membrane, or the membrane structure inside the cell. We are not very clear about this process. How is it formed? It is not clear how fine biological structures such as phospholipids are formed. The other is metabolism. We haven't solved the problem of how you absorb external energy first, but anyway, this hypothesis of the origin of hot spring life does have a lot of favorable evidence to support it, especially in recent years, it has made a series of most important progress.
We know that hot springs contain a lot of carbon monoxide, hydrogen sulfide and sulfide metal minerals, especially pyrite and sulfur. On the one hand, iron sulfide and sulfur have metabolic effects. Iron sulfide is a very important catalyst. Many chemical reactions proceed very smoothly on its surface or crystal skeleton. Some important compounds were found in hot springs. For example, an active substance, such as sulfide, is found in hot springs. It is very similar to a very important compound and some compounds, providing a way of energy metabolism.
Therefore, this metabolic pathway may be related to pyrite and sulfur in hot springs and their polymers. On the other hand, the appearance of genetic material ribonucleic acid and RNA is closely related to the chemical process of sulfurizing fat and sulfur. And deoxyribonucleic acid, DNA, it can also be directly deoxygenated by RNA and evolved. There is another word, like the polymer of pyrite, which is the polymer of pyrite in this hot spring mouth. In fact, it exists in the center of many important biochemical enzymes, and those biochemical enzymes may be produced in hot springs containing a lot of sulfur. From this perspective, life on earth may have been born in these sulfur-filled hot pools or mud 3.8 billion to 4 billion years ago. However, we should be soberly aware that we still have a long way to go to uncover the eternal mystery of the origin of life. The evolution from inorganic matter to organic matter, from organic compounds to organic life is also very accidental. It is not such an environment. Only with such conditions can life be produced. Someone once said that these inorganic substances are like a garbage dump, which contains everything, such as plastics, plastic bottles, iron, scrap metal and oil. Life, a single cell, is like a beautiful Mercedes-Benz. After a typhoon, these wastes were assembled into a Mercedes-Benz. Therefore, we can imagine the process of the origin of life is very, very difficult. Therefore, maybe we are the only paradise for life on this blue planet, so please protect our earth and cherish life on it. We can't expect the second origin of life on earth. Thank you.