BIOCHEMISTRY FOR TEACHERS - F.F. BOYECHKO - 1985

THE ORIGIN OF LIVING SYSTEMS AND THEIR STRUCTURAL ORGANIZATION

FROM SIMPLE MOLECULES TO LIVING ORGANISMS

Living systems are characterized by specific patterns and features of Structure and function that emerged through a prolonged evolutionary process, progressing from simple molecules to highly organized, specialized, and ordered systems.

As Friedrich Engels defined it, "life is the mode of existence of protein bodies, the essential element of which is the constant metabolic exchange with the surrounding external nature; with the cessation of this METABOLISM, life likewise ceases, leading to the decomposition of protein." Building upon modern scientific achievements in the biological sciences, Engels' insights into The Essence of living phenomena have been corroborated and further developed.

In 1924, the Soviet scientist O. I. Oparin developed a theory on THE ORIGIN OF life on Earth, which gained support from scientists worldwide and remains the most comprehensive and scientifically grounded framework to date. According to this theory, The Emergence of life on Earth is an integral component of the general evolutionary Development of the material world. It represents a prolonged, unidirectional process of the gradual complexification of organic substances and their corresponding integral systems, evolving in constant interaction with the environment.

Oparin provided a detailed account of the initial formation of organic substances from inorganic molecules and their subsequent transformation into protein bodies possessing the Characteristic Features of life. According to Oparin's theory, the entire process of transitioning from lifeless to living matter consists of three consecutive, interdependent stages: abiotic synthesis of organic molecules from inorganic precursors → concentration of organic substances into microscopic multimolecular systems (macromolecules) → refinement of supramolecular protein-lipid-polynucleotide complexes (coacervates).

Fig. 1 illustrates the General scheme of the Main stages in the origin of life on Earth and their accompanying processes. As shown in the diagram, the fundamental elements whose compounds served as the initial material for synthesizing Organic compounds are carbon, oxygen, hydrogen, nitrogen, and phosphorus. These are the so-called biogenic elements, which constitute the bulk (99%) of the organic matter in living organisms. Through the involvement of these elements, a vast array of compounds was formed, which, through successive complexification, gave rise to organic substances that laid the foundation for complex supramolecular structures and systems.

The simplest representatives of organic compounds are Hydrocarbons—compounds of carbon and hydrogen. According to modern planetary cosmogony, hydrocarbons are quite widespread: they have been found on planetary surfaces, in interplanetary gas and dust matter, as well as in comets and meteorites.

Using spectral analysis, the simplest compounds of carbon with hydrogen (CH₂, CH₄), as well as with other elements, have been detected in the atmosphere of the Sun, whose surface Temperature reaches 5000–7000 °C. Numerous facts supporting the possibility of the abiotic origin of organic substances have also been gathered through The Study of meteorites falling to Earth. Aside from iron-nickel meteorites, stony meteorites are very common. Their composition is dominated by silicates and oxides of various metals (Mg, Al, Ca, Na). Among stony meteorites, carbonaceous chondrites deserve special attention, as they contain a significant amount of carbon (up to 5%) in the form of graphite and various organic compounds—low-molecular-weight hydrocarbons and aliphatic polymers similar to those found in modern living organisms. Some scientists believe that the synthesis of organic molecules and their high-molecular-weight polymers occurred directly on cosmic dust particles coated with ice and condensed gases, driven by solar radiation. These processes led to The formation of carbon-rich chondritic bodies. Since, According to the prevailing hypothesis, our planet formed at relatively low temperatures through the Condensation of compositionally heterogeneous interplanetary matter—a large portion of which consisted of accumulations of chondritic bodies—some researchers suggest that these pre-existing organic substances served as the foundation for the subsequent development of life. However, organic compounds formed prior to the planet's formation could not have played a major role in the origin of life, because as the Earth gradually warmed locally from the heat released during the decay of radioactive elements, a significant portion of these organic compounds would have broken down via pyrolysis. Therefore, it is more accurate to assert that the bulk of organic matter was formed on the Earth's surface from carbon compounds released into the primordial atmosphere during the Formation of the Earth's crust—the lithosphere—as the most volatile compounds transitioned from the Earth's outer layers.

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Fig. 1. Stages and main processes of the origin of life on Earth (After P. I. Gupalo).

The processes occurring in the lithosphere are closely linked to the formation of our planet's hydrosphere and atmosphere. It is known that the primordial interplanetary matter from which our planet formed contained 76% hydrogen. Part of it was consumed in the formation of various compounds and released during the shaping of the lithosphere, resulting in a terrestrial atmosphere with distinctly reducing properties.

During the formation of the lithosphere, other organogenic elements were also produced and released. For instance, the interaction of nitrides with the hydrate Water of the Earth's interior led to the formation and release of ammonia. Sulfur-metal compounds became a source of hydrogen sulfide. As for oxygen, it is believed to have been present in a free state in the primordial atmosphere only in trace amounts. The bulk of the oxygen in our modern atmosphere, much like nitrogen, emerged much later through biogenic pathways As a result of organismal activity. At that time, The amount of water on the Earth's surface was only 10% of its current volume in the seas and oceans; the rest formed much later.

A detailed analysis of meteorite composition revealed that carbon within them exists as metal compounds—carbides and graphite. Natural carbides (the mineral cohenite)—compounds of carbon with iron, cobalt, and nickel—constitute a substantial portion of meteoritic material.

Cohenite has also been discovered in the deep-seated rocks of the Earth's crust. The interaction of these compounds with water produced various hydrocarbons, primarily methane, which accumulated in the primordial atmosphere of the Earth.

It is believed that hydrocarbons could have formed through other pathways as well—such as the direct reduction of graphite by free hydrogen produced by the radiochemical decomposition of water, or via the pyrolysis of primordial organic Materials present in interstellar matter. In addition to hydrocarbons, large amounts of ammonia, water vapor, hydrogen cyanide (HCN), carbon dioxide, and Other Compounds, as well as active free radicals, accumulated in the Earth's primordial atmosphere and actively reacted with one another. These reactions yielded organic substances, primarily hydrocarbons. These processes unfolded over the first two billion years. The abiotic origin of organic compounds is further corroborated by geochemical and radioastronomical data. Research has shown that even today, deep within the lithosphere where all Signs of Life are absent, abiotic hydrocarbon formation continues to take place. Gas and oil deposits have been discovered in strata of the Earth's interior that have no direct connection to sedimentary rocks. Consequently, petroleum is now considered to be of both biogenic and abiotic origin. According to radioastronomy and planetary cosmogony, ammonia, water vapor, methane, and even hydrogen cyanide and formaldehyde have been detected in interstellar space and in the atmospheres of many planets, having arisen abiotically through reactions that accompanied planetary formation. All these findings serve as undeniable proof that the abiotic formation of hydrocarbons and their derivatives occurred on Earth just as it did on other planets. It must be emphasized that the formation and subsequent chemical evolution of these compounds took place in the primordial atmosphere and on the Earth's surface under conditions drastically different from those of today.

Researcher M. Rutten notes that regarding the Earth's surface, a clear distinction must be made between two periods: the actualistic period, which is similar to the present day, and the pre-actualistic period, which preceded it and was fundamentally different. A defining feature of the pre-actualistic period was the strongly reducing nature of our planet's atmosphere. Due to the absence of free oxygen, direct oxidation of hydrocarbons could not occur, nor could the ozone layer form in the upper atmosphere—a layer of ozone gas (O₃) that absorbs short-wave ultraviolet rays lethal to all living things. Furthermore, thunderstorms raged across the Earth's surface during this epoch, with powerful electrical discharges piercing the atmosphere and torrential rains falling. Short-wave ultraviolet and radioactive radiation, spark discharges, and the searing heat of molten lava bursting through cracks and fissures in the Earth's crust served as vital Energy Sources driving Chemical Reactions and the chemical evolution of newly formed compounds. Today, such conditions no longer exist on Earth. The ozone layer, situated at an altitude of 30 km, reliably shields our planet from deadly solar radiation; the modern atmosphere, lithosphere, and hydrosphere are oxidizing in character. Living organisms inhabiting the planet consume organic substances, which prevents their prolonged evolution. Therefore, in Oparin's words, life cannot arise on our planet under present conditions precisely because it has already arisen.

With the emergence of life on the Earth's surface, environmental conditions shifted in a way that precluded the continued evolution of organic matter along the same pathways seen during the pre-actualistic period of our planet's existence. Recreating the conditions that prevailed on Earth prior to the appearance of life is only possible in scientific laboratories. In 1957, American biochemist S. Miller demonstrated the synthesis of Amino Acids from methane, hydrogen, ammonia, and water vapor to participants of an international symposium on the origin of life by passing electrical discharges through a mixture of these gases in a specially designed apparatus.

Around the same time, Soviet scientists achieved a similar synthesis using a slightly different gas mixture: replacing hydrogen with carbon monoxide (II).

Subsequently, under laboratory conditions mimicking those of the Earth's first billion years, the synthesis of Polypeptides, polynucleotides, and other compounds was successfully carried out. Utilizing polyphosphates and short-wave ultraviolet radiation, researchers achieved the abiotic synthesis of ATP—a crucial high-energy compound found in All living organisms. Thus, it was experimentally proven that during a certain stage of Earth's development, inorganic salts and organic compounds of varying complexity accumulated in the hydrosphere, capable of entering into chemical interactions. Calculations show that over the course of a billion years, the mass fraction of abiotically formed organic compounds in the waters of the World Ocean could have reached 1%. Particularly high concentrations of organic matter may have occurred in isolated bodies of water due to water evaporation.

Moreover, both quantitative and qualitative changes occurred continuously, governed primarily by general physicochemical laws. In other words, the decisive factor in development at that time was the physicochemical evolution of various organic compounds.

The biological evolution of organic compounds was certainly not teleological; therefore, at this stage, the chaos of uncontrolled chemical reactions could not produce substances with an ordered intramolecular structure and defined specific Properties and Functions. Their formation became possible only when, through prolonged development, distinct isolated precursor systems took shape, establishing an orderly mode of interaction with the external environment. It was precisely through this interaction and the associated Selection of individual precursor systems that their internal Organization adapted to external conditions, leading to more refined interactions between substances and processes, which ultimately resulted in an orderly, organized metabolism. The emergence of high-molecular-weight complex systems represents the second, higher stage in the evolution of organic compounds.

In the 1960s, American researcher S. Fox experimentally demonstrated the feasibility of synthesizing high-molecular-weight systems by applying thermal Treatment to a mixture of amino acids. The resulting particles precipitated from a 1% aqueous solution of sodium chloride in the form of microscopic spheres, leading to their designation as microspheres.

Numerous theories exist regarding the mechanisms by which high-molecular-weight complex systems (coacervates) formed and separated from a homogeneous primordial organic solution. The most widespread is the coacervate theory of multimolecular system formation, formulated by O. I. Oparin. Coacervates (from the Latin coacervo—to heap together, assemble) can easily be obtained in laboratory settings by mixing solutions of both natural polymers (Proteins, Nucleic Acids) and synthetic ones. According to scientists, such coacervate droplets formed gradually under natural pre-actualistic conditions. In The first phase, molecules of various substances concentrated and drew closer together in specific locations within the "primordial soup." These closely packed molecules united to form substantial aggregates. Once they reached a certain size, the Hydration shells of the molecules transformed, producing microscopic droplets surrounded by a shared envelope—coacervates (Fig. 2).

Coacervate droplets are capable of moving within a liquid and selectively adsorbing small molecules from their surrounding environment. Studies into their properties have revealed that ordered chemical processes can take place within coacervate droplets, particularly when active biocatalysts are added to their composition. O. I. Oparin added specific Enzymes to coacervate droplets consisting of polyglycoside and histone, in the presence of which, and with glucose-1-phosphate available in the environment, starch synthesis occurred. The volume of the droplet increased significantly as a result. Within 30 minutes, it grew 1.5-fold. Upon adding an enzyme that promotes the hydrolytic breakdown of Polysaccharides, maltose was formed, and the droplet size decreased significantly. By selecting appropriate enzymes, the scientist synthesized or decomposed the polynucleotides that made up the coacervate droplets. These studies indicate that the formation of coacervates in water bodies during the pre-actualistic period represented a novel, higher stage in the evolution of abiotically formed organic compounds. While organic substances had previously floated freely in the environment, forming a single continuous whole with it, the formation of coacervates caused organic molecules to separate from it and create distinct, holistic, high-molecular-weight individual systems capable of engaging in primitive metabolic reactions. Possessing the ability for selective adsorption of substances from the environment, various coacervate droplets differed significantly in their ionic composition, the quality and quantity of their polymeric compounds, and their internal structure, along with other individual traits. Consequently, the further development of such systems depended no longer solely on environmental conditions, but also on their specific internal organization. A distinct relationship was established between the individual ORGANIZATION OF THE system and The Nature of the transformations occurring within it. Therefore, under these conditions, only those systems could persist for long periods whose individual organization ensured a course of chemical reactions and processes favorable to their survival.

Fig. 2. Diagram of coacervate droplet formation:

1 — protein molecules surrounded by layers of water molecules; 2, 3 — reduction of water shells around protein molecules and their approximation;

4 — formation of the coacervate.

Thus, at this stage in The Development of organized multi-molecular systems, a kind of pre-biological selection emerged, under the control of which their further evolution proceeded, acquiring a specific directional trend. As a result of this directed evolution, individual coacervate droplets acquired the character of dynamic open systems capable of self-preservation and prolonged existence through constant interaction with the external environment, the capacity for growth and mass increase at the expense of the environment, and The ability to reproduce and divide not only under the Influence of External factors, but also driven by internal processes. Consequently, through the interaction of the initial systems with the environment, the number of these systems steadily increased. The systems became not only dynamically stable, but also faster-acting. Those systems that acquired the ability to rapidly transform substances—meaning they were more dynamic—while simultaneously executing reactions that did not disrupt the dynamic equilibrium of the system, possessed a greater capacity to survive and evolve. O. I. Oparin referred to such systems as protobionts, the precursors of living matter, thereby emphasizing that despite their more complex and advanced organization compared to static coacervates, these systems could by no means be classified as living creatures (eubionts). Only as a result of subsequent prolonged evolution and pre-biological natural selection could protobionts transform into the primary living organisms that gave rise to life on Earth.

The refinement of complex dynamic systems (protobionts), which occurred over hundreds of millions of years under The Influence of pre-biological natural selection, constitutes, according to O. I. Oparin's theory, The final stage in the emergence of life on Earth. Some scientists have denied the possibility of natural selection in complex multi-molecular systems and protobionts, believing it to be a property exclusive to living organisms. In O. I. Oparin's view, the origin of life cannot be reduced solely to the operation of the laws of inorganic nature, and therefore the establishment of natural selection preceded the origin of life. Such pre-biological natural selection ensured that any disruption in the coordination of processes within the system led to its death and disappearance. Conversely, enhanced coordination and efficiency of processes ensured the preservation and multiplication of more advanced systems.

Thus, on The basis of pre-biological natural selection, the organization of a vast number of protobionts underwent continuous improvement. At the Initial Stages of protobiont evolution, coacervate droplets grew through the polymerization of constituent components in an environment rich in energy-yielding phosphates, which had formed abiotically in the primeval environment via the energy of short-wave radiation. However, over time, due to the increasing number of coacervates and the intensive consumption of high-energy compounds, those coacervate droplets gained a competitive advantage in more favorable environments that had developed the evolutionary capacity to independently supply themselves with energy through specific internal processes, primarily oxidation-reduction reactions.

However, simple catalysts present in the environment—such as Metal Ions Zn, Cu, and Fe—were incapable of sustaining such complex processes. Therefore, for the further development of protobionts, the improvement of catalysis—the core mechanism in organizing metabolism—was of vital importance. This improvement occurred during the evolutionary development of protobionts on the basis of natural selection, through the incorporation of catalysts into the supramolecular System of the protobionts and their subsequent fixation within it. Selectively taken up from the environment, individual constituent PARTS OF THE catalytically active molecule established a specific catalytically active complex within the protobiont. If this complex exerted a positive influence on certain polymerization reactions or other processes that determined the dynamic Stability of the System and Its growth capacity, the number of such systems increased significantly, securing a prominent place in protobiont evolution. Systems with less refined catalytic mechanisms gradually died out. Thus, out of numerous combinations of catalytically competent molecular assemblies, only the most efficient ones were selected, which would later become universal for living organisms. One such universal compound ensuring oxido-reduction processes in all living organisms without exception is complex enzymes in which nicotinamide adenine dinucleotide (NAD) serves as the active group. These enzymes facilitate the complex process of hydrogen transfer along the Respiratory Chain, The oxidation of organic compounds, and the synthesis of energy-rich NUCLEOTIDES (ATP). The universality of these enzymes for all living systems indicates that NAD was selected by nature from all analogous compounds during the early Stages of development amid the refinement of protobionts. Consequently, in the course of pre-biological selection, a complex system of enzymatic processes gradually formed, in which Coenzymes played the primary role, proving superior to inorganic catalysts.

The synthesis of coenzymes represented a major step forward along the evolutionary path of protobionts. This established the conditions for protobionts to exist in an environment of a specific composition. All of this required the coordinated execution of A large number of reactions necessary for synthesis. Therefore, concurrently with the refinement of catalytic processes resulting from the pre-biological natural selection of protobionts, metabolic reactions were improved and complicated: their number increased, the chains they formed lengthened, and metabolic cycles branched and closed.

While the initial forms of protobionts were entirely heterotrophic and dependent on environmental composition, subsequent evolution led to the emergence of systems featuring a high number of complex multi-step processes, which reduced the protobionts' dependence on the external environment. As the network of metabolic reactions grew more complex, there arose a need for new, highly specific catalysts superior to coenzymes, whose intramolecular structure would be well-suited to the functions they performed. Since all enzymes are proteinaceous in nature—meaning they are polypeptides with a specific Primary Structure characteristic of each—their synthesis could only be achieved given the presence of sufficiently complex mechanisms capable of ensuring the strictly regular assembly of the entire molecule. Furthermore, this regularity pertains not only to the precise sequence of amino acid residues in the polypeptide chain, but also to the spatial arrangement and folding of the entire molecule into a globule, as this is a vital condition for ensuring their catalytic activity. In other words, for the subsequent progressive evolution of protobionts, the emergence of a protein-synthesizing apparatus capable of synthesizing proteins with a strictly specific primary structure was of paramount importance. There is no doubt that the complex and sophisticated ribosomal-template protein-synthesizing apparatus of modern organisms was formed as a result of the prolonged evolutionary development of protobionts and represents its highest stage. The pathways and directions of the pre-biological evolution of this apparatus remain largely hypothetical. It is believed that as a result of evolution and pre-biological selection, the polymerization of Peptides and nucleotides intensified significantly within protobionts. This was especially facilitated by the emergence of coupled energy reactions and the presence of inorganic catalysts and coenzymes. Gradually, a specific interaction began to manifest between the molecules of various polymers, establishing a system in which the polymerization of nucleotide molecules was linked to the formation of a specific Amino Acid Sequence—the Introduction/19.html">Primary structure of polypeptides. This can be understood by looking at certain modern microorganisms in which polypeptide synthesis occurs outside the ribosomal-template apparatus, with the participation of nucleoside phosphates. The scheme of this process is as follows: nucleoside triphosphate + amino acid → nucleoside diphosphate + peptide + orthophosphate.

In this process, for each of the four nucleoside phosphates (ATP, GTP, CTP, UTP), there exists a specific group of amino acids that they can selectively attach to the second carbon atom of ribose.

If the combination of amino acid residues generated under the Influence of the polynucleotide favored the occurrence of reactions necessary for the protobiont's survival, that protobiont gained an advantage over other analogous systems in terms of its GROWTH AND DEVELOPMENT rate. Conversely, that particular amino acid combination, along with the protobiont in which it arose, was eliminated by natural selection. Thus, natural selection operated not on individual polynucleotides or polypeptides, but on holistic systems—protobionts—that met the prevailing conditions. Polynucleotides played an important role here as stabilizing factors of evolution. Within them, the spatial fixation of a specific order of mononucleotide units took place, under whose participation the synthesis of catalytically advantageous amino acid combinations occurred.

At the next stage of improving the protein-synthesizing apparatus, the functions of polynucleotides gradually grew more complex and diversified, leading to the emergence of Different types of polynucleotides: DNA and RNA. It is believed that initially, the capacity for information storage and transmission during polypeptide synthesis was characteristic of RNA-like polynucleotides. A subsequent division of functions occurred between two Types of Nucleic acids—double-stranded DNA and single-stranded RNA. The former, being less metabolically active, "specialized" in self-preservation and acquired the capacity for Replication, while the latter specialized in direct participation in Protein Synthesis. This was a progressive step in the course of development. With the emergence of self-replicating complex supramolecular open systems, a transition took place from non-living matter to a qualitatively new form of its existence, which we call life.

At this stage, as a result of chemical evolution, the simplest primitive living organisms formed from non-living molecules, possessing all the characteristic features of living creatures that distinguish them from the inanimate realm. These include the ability for selective uptake of substances from the environment and The excretion of metabolic waste, the capacity for growth, development, reproduction and self-replication, movement, and irritability. Subsequent biological evolution gave rise to the entire diversity of living organisms inhabiting our planet.

Modern science is penetrating ever deeper into the secrets of life, unraveling the pathways of its origin and development from inorganic molecules to living systems. Soviet scientists have made a significant contribution to solving this problem. Experimental research is being widely conducted in our country, on the basis of which the realistic Pathways of the origin and development of life on our planet are becoming increasingly clear. The theory developed by O. I. Oparin concerning the transformation of non-living matter through complex chemical and biological evolution into a qualitatively new, higher form of existence of matter holds not only profound worldview significance, but also demonstrates the victory of the dialectical-materialist worldview over religious and idealistic views. This theory is vital for the development of biological science and natural history as a whole, since elucidating the mystery of the origin and development of life is a necessary condition for understanding its essence. This, in turn, opens up broad opportunities for utilizing the achievements of modern science to manage vital processes in the interests of humanity. The question of the origin of life is closely intertwined with the question of its essence, which stands as one of the most critical problems in biological science. Solving this problem will secure unprecedented progress in Human Development and grant humanity immense power over nature.