If we analyze all the data that scientists were able to obtain in the course of various studies, it becomes obvious that life on Earth is an amazingly incredible fact. The chances of her appearing in our universe are negligible. All stages of the emergence of life contained the possibility of an alternative development of events, as a result of which the world would have remained a cold cosmic abyss without a hint, not only of the human mind, but even of the smallest microbe. Creationists attribute such an incredible event to divine intervention. However, it is impossible to prove or refute the existence of God, and modern ideas about the origin of life, like the whole science as a whole, are based on experimental data and theoretical developments that can be called into question or confirmed.
Vitalism
Human knowledge is undergoing evolution, somewhat similar in the main points to the process described by Darwin. Theories undergo natural selection and the strongest survive, able to withstand the onslaught of counterarguments or adapt, mutate to match them. The hypotheses of the origin of life also went a long way of formation, the completion of which has not even been outlined, since new facts are opened daily that force us to correct already established views.
A major milestone on this road was vitalism - the theory of the constant spontaneous generation of life. According to its provisions, mice appeared in old rags, worms in rotting food debris. Vitalism prevailed over science until the experiments of Louis Pasteur in 1860, when he proved the impossibility of spontaneous generation of living organisms. The results triggered paradoxical events: they strengthened faith in the divine principle and led scientists to seek evidence that they had recently refuted. Science sought to explain that the independent birth of life took place, but a very long time ago and took place in stages, taking millions of years.
Carbon synthesis
The situation seemed hopeless until the time when in 1864 A.M. Butlerov did not make an important discovery.
He managed to get
organic matter (carbon) from inorganic (in his experiment it was formaldehyde). The data obtained destroyed the impressive wall that distinguished between living organisms and the world of dead matter. After a while, scientists were able to get other options for organics from inorganic substances. From that moment, modern ideas about the origin of life began to take shape. They incorporated data not only from biology, but also from cosmology and physics.
Aftermath of the big bang
Theories of the emergence of life span a huge period: scientists find the first prerequisites for the future formation of organisms in the early stages of the emergence of the Universe. Modern physics counts the existence of the world from the Big Bang, when practically everything appeared from nothing. In the rapidly expanding and cooling Universe, atoms and molecules first formed, then they began to unite, forming stars of the first generation. They became the place of formation of most of the elements known to science today. New atoms filled the cosmos after explosions of stars and became the basis for the next generation of objects, including our Sun. Current data suggest that the first organic compounds could appear in protoplanetary clouds surrounding new stars. Planets soon formed from them. It turns out that the first stages of the emergence of life on Earth passed even before its formation.
Autocatalytic cycles
The processes that took place on the Blue Planet in its “childhood” were supported by substances that are part of its bowels and come from outer space as meteorites. The hypotheses of the emergence of life as one of the important foundations for the emergence of organics on Earth are called catalysts for chemical reactions that came here with fragments of these "aliens". They led to the fact that the fastest processes began to play an overwhelming role in the formation of new substances on the planet.
The next stage is autocatalytic cycles. In such processes, substances are formed that contribute to an increase in the reaction rate, as well as renewing the substrate - elements that enter into interaction. Thus, the cycle closed: the processes accelerated themselves and “prepared food” for themselves, that is, substances that reacted again, again catalyzing themselves and again forming a substrate, and so on.
Doubt
Modern ideas about the origin of life for a long time contained conflicting opinions. A stumbling block is the problem of chicken and eggs. What arose first: proteins that carry out all the processes in the cell, or DNA, which determines the structure of these proteins, which stores all the hereditary information. The former are necessary for the body, as they contribute to the self-maintenance of the system, without which life is impossible. DNA contains a cell structure record that also determines viability. The opinions of scientists were divided and there was no answer to the question until it became known that not DNA, but RNA, acts as a repository of hereditary information in viruses, the third class of organic compounds, which usually was assigned only a secondary role in the theory of the origin of life.
RNA world
Gradually, facts began to accumulate, and in the 80s of the last century, data appeared that turned ideas about the initial stages of the formation of living matter. Ribozymes, RNA molecules with the ability of proteins, in particular, to catalyze reactions, were discovered. The first life forms, therefore, could have arisen without the participation of proteins and DNA. In them, the function of storing information, as well as all internal work, was performed by RNA. Life on Earth now came from proto-organisms, which are autocatalytic cycles consisting of self-reproducing ribozymes. The theory was called "RNA-world."
Coacervates
Today it is difficult to imagine the life of that period, since it did not have one important feature - a shell or border. In fact, it was a solution containing autocatalytic cycles from RNA. The problem of the lack of boundaries necessary for the correct course of processes was solved by improvised methods. Protoorganisms found shelter near zeolite minerals with a network structure of the crystal lattice. Their surface was able to catalyze the formation of RNA chains and give them a specific configuration.
More is more: coacervates or water-lipid drops appear on the scene. The hypotheses of the origin of life both in recent times and in modern times are largely based on the theory of A.I. Oparina, who studied the properties of such formations. Coacervates are drops of solution enclosed in a shell of fats (lipids). Their membranes are characterized by selective permeability and the ability to metabolize. Some of them, apparently, combined with chains of self-reproducing RNAs, including those that catalyzed the synthesis of lipids themselves. So new forms of life arose that overcame the path from the preorganismal level to the organically proper. The possibility of such formations has been confirmed recently: scientists have experimentally confirmed the ability of RNA in combination with calcium ions to attach to lipid membranes and regulate their permeability.
Skillful helpers
The emergence of life at the next stage went through the process of improving the functions of the formed organisms. RNA acquired the ability to catalyze the synthesis of amino acid polymers, initially quite simple. The crown of tuning a new mechanism was the ability to synthesize proteins. The resulting formations were several times more effective in coping with biological processes than ribozymes.
Initially, the synthesis of peptides was not ordered. The process took place “anyhow”, leaving the occasion to guide the sequence of amino acids in new chains. Over time, exact copying has been fixed, since it was it that contributed to greater stability of the entire system. So there was a genetic code that allows you to synthesize certain proteins with the necessary functions.
Improvement
Hone the ability to synthesize the necessary proteins was gradual. The first step was the emergence of a special type of RNA that could combine amino acids. The next phase was accompanied by the construction of the process of formation of peptide molecules using the bases arranged in a certain order. The sequence was set by the RNA template. The correlation of the “instruction” of informative RNA and the elements of future proteins took up a new type of RNA, called transport. Like the information one, to this day it is an important part of the synthesis of peptides.
DNA
The complication of organisms further went along the path of improving the methods of storing information. It is believed that DNA was originally one of the phases in the life cycle of RNA colonies. She had a more stable structure. Its degree of information protection was an order of magnitude higher, therefore, after some rather long time, DNA became the main repository of the genetic code.
One of the properties of the new formation, which at one time did not allow putting DNA at the head of the theory of the origin of life, was the inability to take action. It has become a kind of payment for advanced information storage functions. All “work” remained with proteins and RNA.
Symbiosis
Modern ideas about the origin of life do not display a closed and fenced off organism as an ancestor. Scientists are more inclined in favor of the assumption that in the first stages there were communities of microscopic similarities of cells that performed different functions. Such a symbiosis is not difficult to meet in nature today. The simplest example is cyanobacterial mats, which are both a community of microorganisms and a single living being.
Biology sees the emergence of life on Earth at the present stage of its development as a process characterized not by constant struggle and competition, but rather by the ever-growing consolidation of certain diverse structures, which ultimately led to the appearance of a living cell, as we present it today.
Generalization
Summing up, we can briefly list all the stages of the formation of life, which, within the framework of modern theories, are the most likely version of the appearance and development of organisms on Earth:
The formation of primary organic compounds in protoplanetary clouds.
The gradual emergence of self-accelerating reactions and autocatalytic cycles.
The appearance of autocatalytic cycles consisting of RNA.
Union of RNA and lipid membranes.
Acquisition of RNA ability to synthesize protein.
The emergence of DNA and its approval as the main repository of information.
The formation of the first unicellular organisms based on symbiosis.
Understanding the processes that led to the emergence of life is still imperfect. Scientists have a lot of questions. It is not known exactly how RNA originated; many intermediate phases remain only theoretical. However, new experiments are put in place every day, facts and hypotheses are checked. It is safe to say that our century will give the world a lot of discoveries related to the prehistoric era.