The main viewpoints and basis of the origin of life

There have been two different cognitive routes to the origin of life for a long time: idealism and materialism. Idealism includes two different explanations: life is created by God (such as God), and life is the expression of spiritual power beyond material existence. According to this view, the origin of life is a question that neither needs to be raised nor studied. The basic view of materialism is that life is not a phenomenon that the material world already exists and will always exist. Life has a process of emergence and development, has its own laws, and can be recognized by people. This part discusses the origin of life from the materialist point of view, including the following parts.

Creationism "-life is created by God, and the Bible describes in detail the story that God created all kinds of creatures and finally created people.

Spontaneous theory)-Ancient China, Egypt and Greece all said "rotten grass turns into fireflies", "carrion begets maggots" and "mud begets rats". It is believed that organisms naturally appear in abiotic environments.

"Cosmic Theory"-Life on Earth comes from other planets in space. However, we will still encounter the question of how life originated from alien space.

It can be said that since the beginning of human history, the understanding of the origin of life has gone through a long and arduous road in the debate between materialism and idealism, and continues to this day. Strictly speaking, due to the limitation of knowledge at that time, it was impossible to really explore the origin of ancient life. Therefore, all discussions in this respect can only be confined to the category of ideas and philosophy. In short, people who held a more or less simple materialistic view in ancient times once put forward a view, which was called "autogenous theory" by later generations. They believe that life is naturally produced by inanimate matter. For example, in ancient China, people believed that "rotten grass breeds fireflies"; Egyptians believe that frogs and eels are produced by the mud of the Nile being irradiated by sunlight. Aristotle (384-322 BC) said that tapeworms come from carrion and excrement, while crabs, fish, frogs and salamanders come from mucus. Until the17th century, the Dutchman J. Van Helmont also proved through "experiments" that a mouse would be born in 2 1 day after putting grain and worn-out shirts in a bottle and standing in the dark. Corresponding to the theory of "autogenesis", idealists believe that all living things are created by God or designed by super-intelligent people. This view occupied a dominant position in the long Middle Ages in the West, and its typical view was expressed in the teachings of Christianity. Genesis in the Old Testament tells the story that God created everything in the world including land, sea, man, woman, animals and plants in six days.

/kloc-Since the 6th century, a systematic life science system has been gradually established, starting from anatomy. Some scientists have tried to prove through experiments that living things can "spontaneously" from inanimate substances, but all the results are negative. The most famous is the "Goose Neck Bottle Experiment" (1822- 1895) conducted by louis pasteur in the 9th century. He put the broth into a bottle connected with a slender elbow and communicated with the outside world, boiled it to kill all the microorganisms in it, and then cooled it and let it stand. As a result, there are no more microorganisms in the bottle. In contrast, if the connection between the elbow and the bottle is interrupted after heating, bacteria will multiply rapidly in the bottle. The explanation of this experiment is that microorganisms in the bottle can only be brought in by the outside air, and the "precipitation" effect of the elbow prevents the entry of cell spores, thus avoiding microbial pollution. From this, people get a concept that life can only come from life. So where did the earliest life on earth come from? It was not until the 20th century that the discussion on this issue gradually entered a substantive stage.

1. Oberlin hypothesis

1924, a.i. Oparin, a Soviet biologist, published the book The Origin of Life. Through the mixed experiment of gelatin (protein) and Arabic gum (sugar), he put forward the hypothesis that life originated from lumps formed by primitive protein. He and later British geneticist Haldane (T.B.S.Haldane, 1929) further proposed that the initial conditions for the early life of the earth to originate from the surface were not "mild", but this thought inspired people to think about whether it was possible for protein and other basic substances that make up life to be first produced in an inanimate form and then enter the construction of life systems.

2. Miller experiment and the discovery of meteorites

1953, American biologist S.Miller used a closed device filled with methane, ammonia, nitrogen and water to heat and discharge, simulating the environmental conditions of the primitive earth. After a week of cyclic treatment, it was found that many amino acids (such as glycine, alanine and aspartic acid), organic acids (such as acetic acid and lactic acid) and urea were produced. Later, many important biomacromolecules such as purine, pyrimidine, ribonucleotide, deoxynucleotide and fatty acid were obtained by the same method. Then in September 1959, a variety of amino acids and organic acids were found in a carbonaceous meteorite falling from Australia. Because the conformation of these amino acids exists in the mixed form of D-type and L-type, it shows that they will not be polluted by the surface of the earth (the racemic configuration of amino acids on the earth is L-type). These discoveries shocked the whole biological and scientific circles at that time, and people began to realize that primitive life could be produced under natural conditions without life, and inferred that primitive life could also be produced on this basis.

3. Physical evolution, chemical evolution and biological evolution

In the 20th century, with the progress of natural science, people gradually put the appearance and phenomenon of life in the background of the development of the universe. The popular view now is that the universe we observed was born in the Big Bang 654.38+0.5 billion years ago. Later, from micro to macro, the whole universe experienced the stages of physical evolution and chemical evolution, that is, the form of matter realized the gradual evolution of high-energy physics (such as layers, elementary particles, nuclei and atoms), from various elements (such as hydrogen, oxygen and carbon), small interstellar molecules (such as water, hydrogen and methane) and small biological molecules (such as amino acids, pyrimidines and purines) to. In the process of the expansion of the universe, the types of biological small molecules and macromolecules may increase sharply under certain conditions, because this process is directly related to the amount of operable information carried by the system. On this basis, the development of the universe has entered the stage of life evolution. Although the ways and means of chemical evolution of various substances, especially biological macromolecules, need further study. However, the basic material components of life can exist independently of the premise of life system, and life is the continuation of the evolution of these materials, which is undoubtedly an important progress for human beings to understand the origin of life.

1. protein origin theory of life and nucleic acid origin theory.

(1) protein's theory of origin

In 1960s, S.W.Fox of the United States basically inherited the route of Oberlin, emphasizing the key role of protein in the origin of life. They mixed and heated the dried amino acid powder to form "protein-like microspheres" in water, and regarded them as models of primitive cells. This hypothesis is called microsphere theory. This kind of "protocell" simulated in the laboratory has a membrane boundary, which can exchange substances with the outside world, increase the volume (growth), germinate and divide (reproduction), and reflect some characteristics of life. In 1990s, Zhao, a biologist in China, discovered that amino acids became extremely active after phosphorylation, and they could self-polymerize into peptides, esters and other biochemical reactions. This discovery is undoubtedly beneficial to protein's theory of the origin of life.

(2) the theory of nucleic acid origin

As early as the 1920s, the geneticist H.S.Muller put forward the "naked gene theory", thinking that life should start with genes. In 1980s, American researcher T.Cech found that introns cut from the precursor rRNA molecule of Tetrahymena have enzymatic activity, which can polymerize monomer nucleotides into polynucleotides and cut the polynucleotides into fragments of different lengths while keeping themselves unchanged. The discovery of RNase activity and the superiority of RNA as a template for DNA synthesis make people think of the possibility that RNA originated from life. This hypothesis holds that the existence of RNase activity initiated the early appearance of primitive life system with nucleic acid as the main body, and RNA established DNA system through reverse transcription. Later, protein's intervention accelerated the development of this system, which led to the birth of life DNA-RAN- protein system.

2. The rise of system theory and new thinking on the origin of life order.

In the 1940s, Austrian biologist L.V. Bertalanffy (1901-1971) proposed that life is an orderly system with integrity, dynamics and openness, thus opening a new era of system theory. In the past few decades, system theory has developed rapidly, including the discovery of ordered self-organization phenomenon of dissipative systems by Belgian physicist I I.llyaPrigogine (19 17-), the publication of dynamic analysis of life forms based on catastrophe theory by French mathematician R.Thom (1923-) and the publication of German scholar H. Haken (R.Thom). 1927-), the establishment of German physical chemist M. Egan's hypercycle theory (1929-), American meteorologist E. Lorenz's discovery of order in chaos (1963), and French mathematician Mond Lebrau (1963).

Under the guidance of systematic thought and theory, in 1984, Schuster, an Austrian scholar who discovered silicate-mediated DNA synthesis, proposed a gradual transition model from chemical evolution to biological evolution, trying to decompose the dynamic process of small biological molecules into six sequence transitions.

For a long time, people have always held the concept of single evolution, that is, the most primitive cells must appear in a very mild and superior environment. The birth of cells is set as the following procedure: prokaryotic heterotrophic cells appear first, and then autotrophic cells and eukaryotic cells appear through the process of cell energy utilization and cell fusion (symbiosis). Now it seems that this view should be revised.

1. New understanding of cell birth environment

In recent years, the exploration of early life on earth has greatly exceeded people's expectations. Fossils found in Australia and South Africa show that cells appeared at least 3.5 billion years ago. Then, the occurrence and evolution of cell life can only be pushed back to an earlier historical era (3.8-4 billion years ago) that was almost synchronous with the formation of the crust. This shows that the conditions for the birth of life on earth will never be as mild as Obalin and Haldane imagined, nor will it be a "warm pool" environment. In recent years, the research results of extreme environmental life on earth have greatly exceeded people's expectations. The discovery of some extreme environmental organisms strongly shows that it is not inconceivable that early cells were born in harsh conditions, but from the analysis of fossil age and geological conditions at that time, the hypothesis that the earliest cells were born in "warm pools" is unrealistic.

2. Possibility of early cell polymorphism.

Recent research not only broke through the traditional understanding of cell birth conditions, but also exceeded the expectations of many biologists. The geological records of photosynthesis and photosynthetic autotrophs (the existence of stromatolites and the analysis of carbon and sulfur isotopes) can be traced back to 3.5 billion years ago, or even earlier. The comparative study of molecular evolution has also reached a similar conclusion: many non-photosynthetic autotrophs may actually originate from photosynthetic autotrophs (Fox, 1980). Evidence from geology, paleontology and molecular biology shows that photosynthetic autotrophy, chemoautotrophy and heterotrophs originated almost simultaneously in the early Archean (Zhang Yun, 1998). This may completely change the thinking mode that photosynthetic autotrophs should appear much later than chemoautotrophs and heterotrophs, and this thinking mode only comes from complexity comparison, and challenges the traditional concept of single-branch hierarchical evolution of organisms.

In addition to life on earth, exploring alien celestial bodies or life that once existed is no longer a science fiction thing today. In 1996, it is reported that a meteorite from Mars preserved a fossil that may be a cell (Mckay et al.). On February 4, 65438 of the same year, the United States launched the "Mars Pathfinder", and the research work of the Mars probe continued. Today, if someone guesses that life once happened on Mars, they were interrupted for some reason, or that life still exists in some environments on Mars today, such an assumption should not be regarded as a fantasy. At present, many people have volunteered to join the search for cosmic intelligence information. At the same time, people are also thinking about the possibility that primitive life substances and even primitive life forms were produced in other cosmic environments, which were captured by the earth and then developed into life on the earth today, that is, the extraterrestrial source of life on the earth. People pay attention to the analysis of cosmic dust and comet matter or the composition of celestial bodies that broke into the earth. For example, in the Tunguska region of Siberia, Russia, in 1908, researchers found that ammonia and other substances were left behind. At the same time, people are trying their best to spread the information of life on the earth, including launching systematic aircraft.