It is known that scientific journals they try not to accept for publication articles devoted to problems that attract general attention, but do not have a clear solution - a serious physics publication will not publish a perpetual motion project. This topic was the origin of life on Earth. The question of the emergence of living nature, the appearance of man has worried thinking people for many millennia, and only creationists - supporters of the divine origin of all things - have found a definite answer, but this theory is not scientific as it cannot be verified.
Views of the ancients
Ancient Chinese and ancient Indian manuscripts tell about the emergence of living creatures from water and rotting remains; the birth of amphibious creatures in the muddy sediments of large rivers is written in ancient Egyptian hieroglyphs and the cuneiform script of Ancient Babylon. The hypotheses of the origin of life on Earth through spontaneous generation were obvious to the sages of the distant past.
Ancient philosophers also gave examples of the emergence of animals from inanimate matter, but their theoretical justifications had different natures: materialistic and idealistic. Democritus (460-370 BC) found the cause of life in special interaction the smallest, eternal and indivisible particles - atoms. Plato (428-347 BC) and Aristotle (384-322 BC) explained the origin of life on Earth by the miraculous influence of a higher principle on lifeless matter, infusing souls into natural objects.
The idea of the existence of some kind of “life force” that contributes to the emergence of living beings has proven to be very persistent. It shaped the views on the origin of life on Earth among many scientists who lived in the Middle Ages and later, until the end of the 19th century.
Theory of spontaneous generation
Anthony van Leeuwenhoek (1632-1723), with the invention of the microscope, made the smallest microorganisms he discovered the main subject of dispute between scientists who shared two main theories of the origin of life on Earth - biogenesis and abiogenesis. The former believed that all living things could be the product of only living things, the latter believed that the spontaneous generation of organic matter in solutions placed under special conditions was possible. The essence of this dispute has not changed to this day.
The experiments of some naturalists proved the possibility of spontaneous emergence of the simplest microorganisms; supporters of biogenesis completely denied this possibility. Louis Pasteur (1822-1895) strictly scientific methods, with the high correctness of his experiments, proved the absence of a mythical vital force transmitted through the air and generating living bacteria. However, in his works he admitted the possibility of spontaneous generation in some special conditions, which scientists of future generations had to find out.
Evolution theory
The works of the great Charles Darwin (1809-1882) shook the foundations of many natural sciences. The emergence of a huge diversity of biological species from one common ancestor, proclaimed by him, again made the origin of life on Earth the most important question of science. The theory of natural selection had difficulty finding its supporters in the beginning, and is now subject to critical attacks that seem quite reasonable, but it is Darwinism that lies at the basis of modern natural sciences.
After Darwin, biology could not consider the origin of life on Earth from its previous positions. Scientists from many branches of biological science were convinced of the truth of the evolutionary path of development of organisms. Let them change in many ways modern views on the common ancestor placed by Darwin at the base of the Tree of Life, but the truth of the general concept is unshakable.
Steady State Theory
Laboratory refutation of the spontaneous generation of bacteria and other microorganisms, awareness of the complex biochemical structure of the cell, together with the ideas of Darwinism, had a special influence on the emergence alternative options theories of the origin of life on Earth. In 1880, one of the new judgments was proposed by William Preyer (1841-1897). He believed that there was no need to talk about the birth of life on our planet, since it exists forever, and it did not have a beginning as such, it is unchanging and constantly ready for rebirth in any suitable conditions.
The ideas of Preyer and his followers are of purely historical and philosophical interest, because later astronomers and physicists calculated the timing of the final existence of planetary systems, recorded the constant but steady expansion of the Universe, i.e. it was never eternal or constant.
The desire to view the world as a single global living entity echoed the views of the great scientist and philosopher from Russia, Vladimir Ivanovich Vernadsky (1863-1945), who also had his own idea of the origin of life on Earth. It was based on the understanding of life as an integral characteristic of the Universe, the cosmos. According to Vernadsky, the fact that science could not find layers that did not contain traces organic matter, spoke of the geological eternity of life. One of the ways in which life appeared on the young planet, Vernadsky called its contacts with space objects - comets, asteroids and meteorites. Here his theory merged with another version, which explained the origin of life on Earth by the method of panspermia.
The cradle of life is space
Panspermia (Greek - “seed mixture”, “seeds everywhere”) considers life to be a fundamental property of matter and does not explain the ways of its origin, but calls the cosmos a source of life germs that fall on celestial bodies with conditions suitable for their “germination”.
The first mention of the basic concepts of panspermia can be found in the writings of the ancient Greek philosopher Anaxagoras (500-428 BC), and in the 18th century the French diplomat and geologist Benoit de Maillet (1656-1738) spoke about it. These ideas were revived by Svante August Arrhenius (1859-1927), Lord Kelvin William Thomson (1824-1907) and Hermann von Helmholtz (1821-1894).
The study of the cruel influence of cosmic radiation and temperature conditions of interplanetary space on living organisms has made such hypotheses of the origin of life on Earth not very relevant, but with the beginning space age interest in panspermia intensified.
In 1973 Nobel laureate Francis Crick (1916-2004) expressed the idea of the extraterrestrial production of molecular living systems and their arrival on Earth with meteorites and comets. At the same time, he assessed the chances of abiogenesis on our planet as very low. The eminent scientist did not consider the origin and development of life on Earth by the method of self-assembly of high-level organic matter to be a reality.
Fossilized biological structures have been found in meteorites all over the planet, and similar traces have been found in soil samples brought back from the Moon and Mars. On the other hand, numerous experiments are being conducted on the treatment of biological structures with influences that are possible when they are in outer space and when passing through an atmosphere similar to the earth’s.
An important experiment was carried out in 2006 as part of the Deep Impact mission. Comet Tempel was rammed by a special impactor probe launched by an automatic device. Analysis of the cometary substance that was released as a result of the impact showed the presence of water and various organic compounds in it.
Conclusion: Since its inception, the theory of panspermia has changed significantly. Modern science interprets differently those primary elements of life that could have been delivered to our young planet by space objects. Research and experiments prove the viability of living cells during interplanetary travel. All this makes the idea of the extraterrestrial origin of earthly life relevant. The main concepts of the origin of life on Earth are theories that include panspermia either as the main part or as a method of delivering components to Earth to create living matter.
Oparin-Haldane theory of biochemical evolution
The idea of spontaneous generation of living organisms from inorganic substances has always remained almost the only alternative to creationism, and in 1924 a 70-page monograph was published, giving this idea the force of a well-developed and well-founded theory. This work was called “The Origin of Life”, its author was a Russian scientist - Alexander Ivanovich Oparin (1894-1980). In 1929, when Oparin’s works had not yet been translated into English language, similar concepts of the origin of life on Earth were expressed by the English biologist John Haldane (1860-1936).
Oparin proposed that if the primitive atmosphere of the young planet Earth was reducing (that is, containing no oxygen), a powerful burst of energy (such as lightning or ultraviolet radiation) could contribute to the synthesis of organic compounds from inorganic matter. Subsequently, such molecules could form clots and clusters - coacervate drops, which are proto-organisms, around which water jackets are formed - the rudiments of a shell-membrane, separation occurs, generating a charge difference, which means movement - the beginning of metabolism, the rudiments of metabolism, etc. Coacervates were considered to be the basis for the beginning of the evolutionary processes that led to the creation of the first life forms.
Haldane introduced the concept of the “primordial soup” - the initial earth’s ocean, which became a huge chemical laboratory connected to a powerful power source - sunlight. The combination of carbon dioxide, ammonia and ultraviolet radiation resulted in a concentrated population of organic monomers and polymers. Subsequently, such formations were combined with the appearance of a lipid membrane around them, and their development led to the formation of a living cell.
The main stages of the origin of life on Earth (according to Oparin-Haldane)
According to the theory of the emergence of the Universe from a clot of energy, Big Bang occurred about 14 billion years ago, and about 4.6 billion years ago the creation of the planets was completed solar system.
The young Earth, gradually cooling, gained hard shell, around which the formation of the atmosphere took place. The primary atmosphere contained water vapor and gases, which later served as raw materials for organic synthesis: carbon oxide and dioxide, hydrogen sulfide, methane, ammonia, and cyanide compounds.
Bombardment by space objects containing frozen water and condensation of water vapor in the atmosphere led to the formation of the World Ocean, in which various chemical compounds. Powerful thunderstorms accompanied the formation of an atmosphere through which strong ultraviolet radiation penetrated. Under such conditions, the synthesis of amino acids, sugars and other simple organic matter occurred.
At the end of the first billion years of the Earth's existence, the process of polymerization in water of the simplest monomers into proteins (polypeptides) and nucleic acids (polynucleotides) began. They began to form prebiological compounds - coacervates (with the rudiments of the nucleus, metabolism and membrane).
3.5-3 billion years BC - the stage of formation of protobionts with self-reproduction, regulated exchange substances, a membrane with variable permeability.
3 billion years BC e. - appearance cellular organisms, nucleic acids, primary bacteria, the beginning of biological evolution.
Experimental evidence for the Oparin-Haldane hypothesis
Many scientists positively assessed the basic concepts of the origin of life on Earth based on abiogenesis, although from the very beginning they found bottlenecks and inconsistencies in the Oparin-Haldane theory. IN different countries Work began on conducting test studies of the hypothesis, of which the most famous is the classic experiment conducted in 1953 by American scientists Stanley Miller (1930-2007) and Harold Urey (1893-1981).
The essence of the experiment was to simulate in the laboratory the conditions of the early Earth, in which the synthesis of the simplest organic compounds could occur. A gas mixture similar in composition to the primary earth's atmosphere circulated in the device. The design of the device provided an imitation of volcanic activity, and electrical discharges passed through the mixture created the effect of lightning.
After circulating the mixture through the system for a week, the transition of a tenth of carbon into organic compounds was noted, amino acids, sugars, lipids and compounds preceding amino acids were discovered. Repeated and modified experiments fully confirmed the possibility of abiogenesis under simulated conditions of the early Earth. In subsequent years, repeated experiments were carried out in other laboratories. Hydrogen sulfide was added to the composition of the gas mixture as a possible component of volcanic emissions, and other non-drastic changes were made. In most cases, the experience of synthesizing organic compounds was successful, although attempts to go further and obtain more complex elements approaching the composition of a living cell were unsuccessful.
RNA world
By the end of the 20th century, many scientists who never ceased to be interested in the problem of the origin of life on Earth, it became clear that, with all the harmony of theoretical constructions and clear experimental confirmation, the Oparin-Haldane theory has obvious, perhaps insurmountable, flaws. The main one was the impossibility of explaining the appearance in protobionts of the properties that define a living organism - to reproduce while maintaining hereditary characteristics. With the discovery of genetic cellular structures, with the determination of the function and structure of DNA, with the development of microbiology, a new candidate for the role of the molecule of primordial life appeared.
It became a ribonucleic acid molecule - RNA. This macromolecule, which is part of all living cells, is a chain of nucleotides - the simplest organic units consisting of nitrogen atoms, a monosaccharide - ribose and a phosphate group. It is the sequence of nucleotides that is the code for hereditary information, and in viruses, for example, RNA plays the same role that DNA plays in complex cellular structures.
In addition, scientists have discovered unique ability Some RNA molecules introduce breaks in other chains or glue individual RNA elements together, and some play the role of autocatalysts - that is, they contribute to rapid self-reproduction. The relatively small size of the RNA macromolecule and its simplified structure compared to DNA (one strand) made ribonucleic acid the main candidate for the role of the main element of prebiological systems.
The new theory of the origin of living matter on the planet was finally formulated in 1986 by Walter Gilbert (born 1932), an American physicist, microbiologist and biochemist. Not all experts agreed with this view of the origin of life on Earth. Briefly called “RNA World,” the theory of the structure of the prebiological world of our planet cannot answer the simple question of how the first RNA molecule with the given properties appeared, even if a huge amount of “building material” was present in the form of nucleotides, etc.
PAH world
Simon Nicholas Platts tried to find the answer in May 2004, and in 2006 a group of scientists led by Pascale Ehrenfreund. Polyaromatic hydrocarbons have been proposed as starting materials for RNA with catalytic properties.
The world of PAHs was founded on the high abundance of these compounds in visible space(they were probably present in the “primordial soup” of the young Earth) and the features of their ring-shaped structure, which facilitates rapid connection with nitrogenous bases - the key components of RNA. The PAH theory once again speaks of the relevance of some provisions of panspermia.
Unique life on a unique planet
Until scientists have the opportunity to go back 3 billion years ago, the mystery of the origin of life on our planet will not be revealed - this is the conclusion that many of those who have studied this problem come to. The main concepts of the origin of life on Earth are: the theory of abiogenesis and the theory of panspermia. They may overlap in many ways, but, most likely, they will not be able to answer: how, in the midst of the vast cosmos, an amazingly precisely balanced system of the Earth and its satellite, the Moon, appeared, how life originated on it...
The origin of life on Earth is a key and unresolved problem in natural science, often serving as the basis for a clash between science and religion. If the presence in nature of the evolution of living matter can be considered proven, since its mechanisms have been revealed, archaeologists have discovered ancient, more simply structured organisms, then not a single hypothesis of the origin of life has such an extensive evidence base. We can observe evolution with our own eyes, at least in selection. No one has succeeded in creating living things from non-living things.
Despite a large number of hypotheses about the origin of life, only one of them has an acceptable scientific explanation. This is a hypothesis abiogenesis- long-term chemical evolution, which took place in the special conditions of the ancient Earth and preceded biological evolution. At the same time, from inorganic substances First, simple organic ones were synthesized, of which more complex ones were synthesized, then biopolymers appeared, the next stages are more speculative and hardly provable. The abiogenesis hypothesis has many unsolved problems and different views on certain stages of chemical evolution. However, some of its points have been confirmed experimentally.
Other hypotheses for the origin of life - panspermia(bringing life from space), creationism(creation by the creator), spontaneous generation(living organisms suddenly appear in inanimate matter), steady state(life has always existed). The impossibility of spontaneous generation of life in nonliving things was proven by Louis Pasteur (19th century) and a number of scientists before him, but not so categorically (F. Redi - 17th century). The panspermia hypothesis does not solve the problem of the origin of life, but transfers it from Earth to outer space or to other planets. However, it is difficult to refute this hypothesis, especially those of its representatives who claim that life was brought to Earth not by meteorites (in this case, living things could burn in the layers of the atmosphere, be subjected to the destructive effects of cosmic radiation, etc.), but by intelligent beings. But how did they get to Earth? From the point of view of physics (the enormous size of the Universe and the impossibility of overcoming the speed of light), this is hardly possible.
For the first time possible abiogenesis was substantiated by A.I. Oparin (1923-1924), later this hypothesis was developed by J. Haldane (1928). However, the idea that life on Earth could have been preceded by the abiogenic formation of organic compounds was already expressed by Darwin. The theory of abiogenesis has been refined and is being refined by other scientists to this day. Its main unresolved problem is the details of the transition from complex non-living systems to simple living organisms.
In 1947, J. Bernal, based on the developments of Oparin and Haldane, formulated the theory of biopoiesis, identifying three stages in abiogenesis: 1) abiogenic emergence of biological monomers; 2) formation of biopolymers; 3) the formation of membranes and the formation of primary organisms (protobionts).
Abiogenesis
The hypothetical scenario for the origin of life according to the theory of abiogenesis is described below in general terms.
The age of the Earth is about 4.5 billion years. According to scientists, liquid water on the planet, so necessary for life, appeared no earlier than 4 billion years ago. At the same time, 3.5 billion years ago, life already existed on Earth, which is proven by the discovery of rocks of such ages with traces of the vital activity of microorganisms. Thus, the first simplest organisms arose relatively quickly - in less than 500 million years.
When the Earth first formed, its temperature could reach 8000 °C. As the planet cooled, metals and carbon, the heaviest elements, condensed and formed the earth's crust. At the same time, volcanic activity occurred, the crust moved and compressed, folds and breaks formed on it. Gravitational forces led to compaction of the crust, which released energy in the form of heat.
Light gases (hydrogen, helium, nitrogen, oxygen, etc.) were not retained by the planet and went into space. But these elements remained in the composition of other substances. Until the temperature on Earth dropped below 100 °C, all water was in a vapor state. After the temperature dropped, evaporation and condensation were repeated many times, and there were heavy downpours and thunderstorms. Hot lava and volcanic ash, once in the water, created different environmental conditions. In some, certain reactions could occur.
Thus, physical and chemical conditions on the early Earth were favorable for the formation of organic and inorganic substances. The atmosphere was of a reducing type, there was no free oxygen and no ozone layer in it. Therefore, ultraviolet and cosmic radiation penetrated the Earth. Other sources of energy were heat earth's crust, which has not yet cooled down, erupting volcanoes, thunderstorms, radioactive decay.
The atmosphere contained methane, carbon oxides, ammonia, hydrogen sulfide, cyanide compounds, and water vapor. A number of simple organic substances were synthesized from them. Next, amino acids, sugars, nitrogenous bases, nucleotides and other more complex organic compounds could be formed. Many of them served as monomers for future biological polymers. The absence of free oxygen in the atmosphere favored the occurrence of reactions.
Chemical experiments (first in 1953 by S. Miller and G. Ury), simulating the conditions of the ancient Earth, proved the possibility of abiogenic synthesis of organic substances from inorganic ones. By passing electric discharges through a gas mixture that simulated the primitive atmosphere, in the presence of water vapor, amino acids, organic acids, nitrogenous bases, ATP, etc. were obtained.
It should be noted that in the ancient atmosphere of the Earth, the simplest organic substances could be formed not only abiogenically. They were also brought from space and contained in volcanic dust. Moreover, these could be quite large amounts of organic matter.
Low molecular weight organic compounds accumulated in the ocean, creating the so-called primordial soup. The substances were adsorbed on the surface of clay deposits, which increased their concentration.
Under certain conditions of the ancient Earth (for example, on clay, the slopes of cooling volcanoes), polymerization of monomers could occur. This is how proteins and nucleic acids were formed - biopolymers, which later became the chemical basis of life. In an aqueous environment, polymerization is unlikely, since depolymerization usually occurs in water. Experiments have proven the possibility of synthesizing a polypeptide from amino acids in contact with pieces of hot lava.
The next important step on the path to the origin of life is the formation of coacervate droplets in water ( coacervates) from polypeptides, polynucleotides, and other organic compounds. Such complexes could have a layer on the outside that imitates a membrane and maintains their stability. Coacervates were experimentally obtained in colloidal solutions.
Protein molecules are amphoteric. They attract water molecules to themselves so that a shell forms around them. The resulting colloidal hydrophilic complexes are isolated from the water mass. As a result, an emulsion is formed in water. Next, the colloids merge with each other and coacervates are formed (the process is called coacervation). The colloidal composition of the coacervate depended on the composition of the medium in which it was formed. In different reservoirs of the ancient Earth, coacervates with different chemical compositions were formed. Some of them were more stable and could, to a certain extent, carry out selective metabolism with environment. A kind of biochemical natural selection took place.
Coacervates are capable of selectively absorbing certain substances from the environment and releasing into it certain products of chemical reactions occurring in them. It's like metabolism. As the substances accumulated, the coacervates grew, and when they reached critical sizes, they disintegrated into parts, each of which retained the features of the original organization.
In the coacervates themselves there could be chemical reactions. Enzymes could be formed when metal ions were absorbed by coacervates.
In the process of evolution, only those systems remained that were capable of self-regulation and self-reproduction. This marked the onset of the next stage in the origin of life - the emergence protobionts(according to some sources, this is the same as coacervates) - bodies that have a complex chemical composition and a number of properties of living beings. Protobionts can be considered as the most stable and successfully obtained coacervates.
The membrane could be formed in the following way. Fatty acids combined with alcohols to form lipids. Lipids formed films on the surface of reservoirs. Their charged heads face the water, and their non-polar ends face outward. Protein molecules floating in water were attracted to the lipid heads, resulting in the formation of double lipoprotein films. The wind could bend such a film, and bubbles would form. Coacervates may have been accidentally trapped in these vesicles. When such complexes again appeared on the surface of the water, they were covered with a second lipoprotein layer (due to hydrophobic interactions with the nonpolar ends of the lipids facing each other). The general layout of the membrane of today's living organisms is two layers of lipids inside and two layers of proteins located at the edges. But over millions of years of evolution, the membrane has become more complex due to the inclusion of proteins immersed in the lipid layer and penetrating it, protrusion and invagination of individual sections of the membrane, etc.
Coacervates (or protobionts) could contain already existing nucleic acid molecules capable of self-reproduction. Further, in some protobionts such a restructuring could occur that the nucleic acid began to encode a protein.
The evolution of protobionts is no longer chemical, but prebiological evolution. It led to an improvement in the catalytic function of proteins (they began to act as enzymes), membranes and their selective permeability (which makes the protobiont a stable set of polymers), and the emergence of template synthesis (transfer of information from nucleic acid to nucleic acid and from nucleic acid to protein).
| Evolution | results | |
|---|---|---|
| 1 | Chemical evolution - synthesis of compounds |
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| 2 | Prebiological evolution – chemical selection: the most stable protobionts capable of self-reproduction remain |
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| 3 | Biological evolution - biological selection: struggle for existence, survival of those most adapted to environmental conditions |
|
One of the biggest mysteries of the origin of life remains the question of how RNA came to encode the amino acid sequence of proteins. The question involves RNA, not DNA, since it is believed that at first ribonucleic acid played not only a role in the implementation of hereditary information, but was also responsible for its storage. DNA replaced it later, arising from RNA by reverse transcription. DNA is better suited for storing information and is more stable (less prone to reactions). Therefore, in the process of evolution, it was she who was left as the keeper of information.
In 1982, T. Check discovered the catalytic activity of RNA. In addition, RNA can be synthesized under certain conditions, even in the absence of enzymes, and also form copies of itself. Therefore, it can be assumed that RNAs were the first biopolymers (RNA-world hypothesis). Some sections of RNA could accidentally encode peptides useful for the protobiont; other sections of RNA became excised introns in the process of evolution.
A feedback loop has arisen in protobionts - RNA encodes enzyme proteins, enzyme proteins increase the amount of nucleic acids.
Beginning of biological evolution
Chemical evolution and the evolution of protobionts lasted more than 1 billion years. Life arose and its biological evolution began.
From some protobionts primitive cells emerged, which included the entire set of properties of living things that we observe today. They implemented the storage and transmission of hereditary information, its use for the creation of structures and metabolism. Energy for vital processes was provided by ATP molecules, and membranes typical of cells appeared.
The first organisms were anaerobic heterotrophs. They obtained the energy stored in ATP through fermentation. An example is glycolysis - the oxygen-free breakdown of sugars. These organisms fed on organic matter from the primordial broth.
But the reserves of organic molecules were gradually depleted, as conditions on Earth changed, and new organic matter was almost no longer synthesized abiogenically. In conditions of competition for food resources, the evolution of heterotrophs accelerated.
The bacteria that were able to fix carbon dioxide with the formation of organic substances gained an advantage. Autotrophic synthesis nutrients more complex than heterotrophic nutrition, therefore it could not have arisen in early forms of life. From some substances, under the influence of solar radiation energy, compounds necessary for the cell were formed.
The first photosynthetic organisms did not produce oxygen. Photosynthesis with its release most likely appeared later in organisms similar to modern blue-green algae.
The accumulation of oxygen in the atmosphere, the appearance of an ozone screen, and a decrease in the amount of ultraviolet radiation have led to the almost impossibility of abiogenic synthesis of complex organic substances. On the other hand, the emerging forms of life became more stable under such conditions.
Oxygen breathing has spread on Earth. Anaerobic organisms have survived only in certain places (for example, there are anaerobic bacteria living in hot underground springs).
The question of the origin of life on Earth is one of the most complex issues modern natural science, to which there is still no clear answer.
There are several theories about the origin of life on Earth, the most famous of which are:
- theory of spontaneous (spontaneous) generation;
- creationism (or creation) theory;
- steady state theory;
- theory of panspermia;
- theory of biochemical evolution (theory of A.I. Oparin).
Let us consider the main provisions of these theories.
Theory of spontaneous generation
The theory of the spontaneous origin of life was widespread in the Ancient world - Babylon, China, Ancient Egypt and Ancient Greece(this theory was adhered to, in particular, by Aristotle).
Scientists Ancient world and medieval Europe believed that living beings constantly arise from inanimate matter: worms from dirt, frogs from mud, fireflies from morning dew, etc. Thus, the famous Dutch scientist of the 17th century. Van Helmont quite seriously described in his scientific treatise an experience in which, over 3 weeks, he obtained mice directly from a dirty shirt and a handful of wheat in a locked dark closet. For the first time, the Italian scientist Francesco Redi (1688) decided to subject a widespread theory to experimental testing. He placed several pieces of meat in vessels and covered some of them with muslin. In open vessels, white worms—fly larvae—appeared on the surface of the rotting meat. In the vessels covered with muslin, there were no fly larvae. Thus, F. Redi was able to prove that fly larvae do not appear from rotting meat, but from eggs laid by flies on its surface.
In 1765, the famous Italian scientist and doctor Lazzaro Spalanzani boiled meat and vegetable broths in sealed glass flasks. Broths in sealed flasks did not spoil. He concluded that the high temperature killed all living creatures that could cause the broth to spoil. However, the experiments of F. Redi and L. Spalanzani did not convince everyone. Vitalist scientists (from lat. vita- life) believed that spontaneous generation of living beings does not occur in boiled broth, since a special “ life force", which cannot penetrate into a sealed container because it is carried through the air.
Disputes about the possibility of spontaneous generation of life intensified in connection with the discovery of microorganisms. If complex living things cannot spontaneously generate, perhaps microorganisms can?
In this regard, in 1859, the French Academy announced the award of a prize to the one who would finally decide the question of the possibility or impossibility of the spontaneous generation of life. This prize was received in 1862 by the famous French chemist and microbiologist Louis Pasteur. Just like Spalanzani, he boiled the nutrient broth in a glass flask, but the flask was not an ordinary one, but with a neck in the form of a 5-shaped tube. Air, and therefore the “life force,” could penetrate the flask, but the dust, and with it the microorganisms present in the air, settled in the lower leg of the 5-shaped tube, and the broth in the flask remained sterile (Fig. 1). However, as soon as the neck of the flask was broken or the lower leg of the 5-shaped tube was rinsed with sterile broth, the broth began to quickly become cloudy - microorganisms appeared in it.
Thus, thanks to the work of Louis Pasteur, the theory of spontaneous generation was recognized as untenable and the theory of biogenesis was established in the scientific world, a brief formulation of which is: “everything living is from living things.”
Rice. 1. Pasteur flask
However, if all living organisms in the historically foreseeable period of human development descend only from other living organisms, the question naturally arises: when and how did the first living organisms appear on Earth?
Creation theory
Creation theory assumes that all living organisms (or only their simplest forms) were created (“designed”) by some supernatural being (deity, absolute idea, supermind, supercivilization, etc.) at a certain period of time. It is obvious that this is the point of view that followers of most of the leading religions of the world, in particular the Christian religion, have adhered to since ancient times.
The theory of creationism is still quite widespread today, not only in religious but also in scientific circles. It is usually used to explain the most complex issues of biochemical and biological evolution that currently have no solution, related to the emergence of proteins and nucleic acids, the formation of the mechanism of interaction between them, the emergence and formation of individual complex organelles or organs (such as the ribosome, the eye or brain). Acts of periodic “creation” also explain the absence of clear transitional links from one type of animal
to another, for example, from worms to arthropods, from monkeys to humans, etc. It must be emphasized that the philosophical dispute about the primacy of consciousness (supermind, absolute idea, deity) or matter cannot be resolved in principle, however, since an attempt to explain any difficulties of modern biochemistry and evolutionary theory fundamentally incomprehensible supernatural acts of creation takes these questions beyond the scope of scientific research, the theory of creationism cannot be classified as scientific theories origin of life on Earth.
Theories of steady state and panspermia
Both of these theories represent complementary elements of a single picture of the world, the essence of which is as follows: the universe exists forever and life exists in it forever (stationary state). Life is transferred from planet to planet by “seeds of life” traveling in outer space, which can be part of comets and meteorites (panspermia). Similar views on the origin of life were held, in particular, by the founder of the doctrine of the biosphere, Academician V.I. Vernadsky.
However, the steady state theory, which assumes an infinitely long existence of the universe, does not agree with the data of modern astrophysics, according to which the universe arose relatively recently (about 16 billion years ago) through a primary explosion.
It is obvious that both theories (panspermia and stationary state) do not offer an explanation at all for the mechanism of the primary origin of life, transferring it to other planets (panspermia) or pushing it back in time to infinity (stationary state theory).
Theory of biochemical evolution (theory of A.I. Oparin)
Of all the theories of the origin of life, the most widespread and recognized in the scientific world is the theory of biochemical evolution, proposed in 1924 by the Soviet biochemist Academician A.I. Oparin (in 1936 he outlined it in detail in his book “The Emergence of Life”).
The essence of this theory is that biological evolution - i.e. emergence, development and complication various forms living organisms were preceded by chemical evolution - a long period in the history of the Earth associated with the emergence, complication and improvement of the interaction between elementary units, the “building blocks” of which all living things are composed - organic molecules.
Prebiological (chemical) evolution
According to most scientists (primarily astronomers and geologists), the Earth formed as a celestial body about 5 billion years ago. by condensation of particles of a gas and dust cloud rotating around the Sun.
Under the influence of compression forces, the particles from which the Earth is formed release enormous amounts of heat. Thermonuclear reactions begin in the depths of the Earth. As a result, the Earth is heating up greatly. Thus, 5 billion years so-called. The Earth was a hot ball rushing through outer space, the surface temperature of which reached 4000-8000 ° C (laughter. 2).
Gradually, due to the radiation of thermal energy into outer space, the Earth begins to cool. About 4 billion years so-called. The earth cools so much that a solid crust forms on its surface; at the same time, light, gaseous substances erupt from its depths, rising upward and forming the primary atmosphere. The composition of the primary atmosphere was significantly different from the modern one. There was apparently no free oxygen in the atmosphere of the ancient Earth, and its composition included substances in a reduced state, such as hydrogen (H 2), methane (CH 4), ammonia (NH 3), water vapor (H 2 O ), and possibly also nitrogen (N 2), carbon monoxide and carbon dioxide (CO and CO 2).
The reducing nature of the Earth's primary atmosphere is extremely important for the origin of life, since substances in a reduced state are highly reactive and, under certain conditions, are able to interact with each other, forming organic molecules. The absence of free oxygen in the atmosphere of the primary Earth (almost all of the Earth’s oxygen was bound in the form of oxides) is also an important prerequisite for the emergence of life, since oxygen easily oxidizes and thereby destroys organic compounds. Therefore, in the presence of free oxygen in the atmosphere, the accumulation of significant amounts of organic substances on the ancient Earth would have been impossible.
About 5 billion years etc.— the emergence of the Earth as a celestial body; surface temperature - 4000-8000°C
About 4 billion years so-called. - formation of the earth's crust and primary atmosphere
At a temperature of 1000°C- synthesis of simple organic molecules begins in the primary atmosphere
Energy for synthesis is provided by:
The temperature of the primary atmosphere is below 100°C - the formation of the primary ocean -
Synthesis of complex organic molecules - biopolymers from simple organic molecules:
- simple organic molecules - monomers
- complex organic molecules - biopolymers
Scheme. 2. Main stages of chemical evolution
When the temperature of the primary atmosphere reaches 1000°C, the synthesis of simple organic molecules begins in it, such as amino acids, nucleotides, fatty acids, simple sugars, polyhydric alcohols, organic acids, etc. The energy for synthesis is supplied by lightning discharges, volcanic activity, hard space radiation and, finally, ultraviolet radiation from the Sun, from which the Earth is not yet protected by an ozone screen, and it is ultraviolet radiation that scientists consider the main source of energy for abiogenic (i.e., taking place without the participation of living organisms) synthesis of organic substances.
Recognition and wide dissemination of the theory of A.I. Oparin was largely promoted by the fact that the processes of abiogenic synthesis of organic molecules are easily reproduced in model experiments.
The possibility of synthesizing organic substances from inorganic ones has been known since the beginning of the 19th century. Already in 1828, the outstanding German chemist F. Wöhler synthesized an organic substance - urea from inorganic - ammonium cyanate. However, the possibility of abiogenic synthesis of organic substances under conditions close to the conditions of the ancient Earth was first shown in the experiment of S. Miller.
In 1953, a young American researcher, a graduate student at the University of Chicago, Stanley Miller, reproduced in a glass flask with electrodes sealed into it the primary atmosphere of the Earth, which, according to scientists of that time, consisted of hydrogen methane CH 4, ammonia NH, and water vapor H 2 0 (Fig. 3). S. Miller passed electric discharges through this gas mixture for a week, simulating thunderstorms. At the end of the experiment, α-amino acids (glycine, alanine, asparagine, glutamine), organic acids (succinic, lactic, acetic, glycolic), y-hydroxybutyric acid and urea were found in the flask. By repeating the experiment, S. Miller was able to obtain individual nucleotides and short polynucleotide chains of five to six units.
Rice. 3. Installation of S. Miller
In further experiments on abiogenic synthesis, carried out by various researchers, not only electrical discharges were used, but also other types of energy characteristic of the ancient Earth - cosmic, ultraviolet and radioactive radiation, high temperatures, inherent in volcanic activity, as well as various options gas mixtures simulating the primary atmosphere. As a result, almost the entire spectrum of organic molecules characteristic of living things was obtained: amino acids, nucleotides, fat-like substances, simple sugars, organic acids.
Moreover, abiogenic synthesis of organic molecules can occur on Earth at the present time (for example, in the process of volcanic activity). At the same time, in volcanic emissions one can find not only hydrocyanic acid HCN, which is a precursor of amino acids and nucleotides, but also individual amino acids, nucleotides and even such complex organic substances as porphyrins. Abiogenic synthesis of organic substances is possible not only on Earth, but also in outer space. The simplest amino acids are found in meteorites and comets.
When the temperature of the primary atmosphere dropped below 100°C, hot rains fell on the Earth and the primary ocean appeared. With the flow of rain, abiogenically synthesized organic substances entered the primary ocean, which turned it, in the figurative expression of the English biochemist John Haldane, into a diluted “primary broth.” Apparently, it is in the primary ocean that the processes of formation from simple organic molecules - monomers - of complex organic molecules - biopolymers begin (see Fig. 2).
However, the processes of polymerization of individual nucleotides, amino acids and sugars are condensation reactions; they occur with the elimination of water, therefore, the aqueous environment does not promote polymerization, but, on the contrary, the hydrolysis of biopolymers (i.e., their destruction with the addition of water).
The formation of biopolymers (in particular, proteins from amino acids) could occur in the atmosphere at a temperature of about 180°C, from where they were washed into the primary ocean with precipitation. In addition, it is possible that on ancient Earth, amino acids were concentrated in drying up reservoirs and polymerized in dry form under the influence of ultraviolet light and the heat of lava flows.
Despite the fact that water promotes the hydrolysis of biopolymers, in a living cell the synthesis of biopolymers occurs precisely in the aquatic environment. This process is catalyzed by special catalyst proteins - enzymes, and the energy necessary for synthesis is released during the breakdown of adenosine triphosphoric acid - ATP. It is possible that the synthesis of biopolymers in the aqueous environment of the primordial ocean was catalyzed by the surface of some minerals. It has been experimentally shown that a solution of the amino acid alanine can polymerize in an aqueous medium in the presence of special type alumina This produces the peptide polyalanine. The polymerization reaction of alanine is accompanied by the breakdown of ATP.
The polymerization of nucleotides is easier than the polymerization of amino acids. It has been shown that in solutions with high salt concentrations, individual nucleotides spontaneously polymerize, turning into nucleic acids.
The life of all modern living beings is a process of continuous interaction of the most important biopolymers of a living cell - proteins and nucleic acids.
Proteins are “worker molecules,” “engineer molecules” of a living cell. When characterizing their role in metabolism, biochemists often use figurative expressions such as “protein works,” “enzyme conducts a reaction.” Essential Function protein-catalytic. As you know, catalysts are substances that accelerate chemical reactions, but are not themselves included in the final reaction products. Catalyst tanks are called enzymes. Enzymes bend and speed up metabolic reactions thousands of times. Metabolism, and therefore life, is impossible without them.
Nucleic acids- these are “computer molecules”, molecules are the keepers of hereditary information. Nucleic acids store information not about all substances of a living cell, but only about proteins. It is enough to reproduce in the daughter cell the proteins characteristic of the mother cell so that they accurately recreate all the chemical and structural features of the mother cell, as well as the nature and rate of metabolism characteristic of it. Nucleic acids themselves are also reproduced due to the catalytic activity of proteins.
Thus, the mystery of the origin of life is the mystery of the origin of the mechanism of interaction between proteins and nucleic acids. What information does modern science have about this process? Which molecules were the primary basis of life—proteins or nucleic acids?
Scientists believe that despite the key role of proteins in the metabolism of modern living organisms, the first “living” molecules were not proteins, but nucleic acids, namely ribonucleic acids (RNA).
In 1982, American biochemist Thomas Check discovered the autocatalytic properties of RNA. He experimentally showed that in a medium containing high concentrations of mineral salts, ribonucleotides spontaneously polymerize, forming polynucleotides - RNA molecules. On the original polynucleotide chains of RNA, as on a template, RNA copies are formed by pairing of complementary nitrogenous bases. The RNA template copying reaction is catalyzed by the original RNA molecule and does not require the participation of enzymes or other proteins.
What follows is fairly well explained by a process that could be called “natural selection” at the molecular level. When self-copying (self-assembling) RNA molecules, inaccuracies and errors inevitably arise. The RNA copies containing errors are copied again. When copying again, errors may occur again. As a result, the population of RNA molecules in a certain area of the primary ocean will be heterogeneous.
Since RNA decay processes occur in parallel with the synthesis processes, molecules that have either greater stability or better autocatalytic properties will accumulate in the reaction medium (i.e., molecules that copy themselves faster “multiply” faster).
On some RNA molecules, as on a matrix, self-assembly of small protein fragments - peptides - can occur. A protein “cover” is formed around the RNA molecule.
Along with autocatalytic functions, Thomas Check discovered the phenomenon of self-splicing in RNA molecules. As a result of self-splicing, sections of RNA that are not protected by peptides are spontaneously removed from the RNA (they are, as it were, “cut out” and “thrown out”), and the remaining sections of RNA encoding protein fragments are “fused,” i.e. spontaneously combine into a single molecule. This new RNA molecule will already code for a large, complex protein (Figure 4).
Apparently, initially protein covers were performed first of all, protective function, protecting RNA from destruction and thereby increasing its stability in solution (this is the function of protein covers in the simplest modern viruses).
It is obvious that at a certain stage of biochemical evolution, RNA molecules encoding not only protective proteins, but also catalyst proteins (enzymes) that sharply accelerate the speed of RNA copying received an advantage. Apparently, this is precisely how the process of interaction between proteins and nucleic acids, which we currently call life, arose.
In progress further development, thanks to the appearance of a protein with the functions of an enzyme - reverse transcriptase, deoxyribonucleic acid (DNA) molecules consisting of two chains began to be synthesized on single-stranded RNA molecules. The absence of an OH group in the 2" position of deoxyribose makes DNA molecules more stable with respect to hydrolytic cleavage in weakly alkaline solutions, namely, the reaction of the environment in primary reservoirs was weakly alkaline (this reaction of the environment has been preserved in the cytoplasm of modern cells).
Where did the complex process of interaction between proteins and nucleic acids develop? According to the theory of A.I. Oparin, the so-called coacervate drops became the birthplace of life.
Rice. 4. Hypothesis of the occurrence of interaction between proteins and nucleic acids: a) during the process of self-copying RNA, errors accumulate (1 - nucleotides corresponding to the original RNA; 2 - nucleotides not corresponding to the original RNA - errors in copying); b) on part of the RNA molecule due to its physical and chemical properties amino acids “stick” (3 - RNA molecule; 4 - amino acids), which, interacting with each other, turn into short protein molecules - peptides. As a result of the self-splicing characteristic of RNA molecules, the sections of the RNA molecule unprotected by peptides are destroyed, and the remaining ones “grow together” into a single molecule encoding a large protein. As a result, an RNA molecule appears, covered with a protein cover (the most primitive modern viruses, for example, the tobacco mosaic virus, have a similar structure)
The phenomenon of coacervation is that under certain conditions (for example, in the presence of electrolytes), high molecular weight substances are separated from the solution, but not in the form of a precipitate, but in the form of a more concentrated solution - coacervate. When shaken, the coacervate breaks up into individual small droplets. In water, such drops are covered with a hydration shell (a shell of water molecules) that stabilizes them - Fig. 5.
Coacervate drops have some semblance of metabolism: iodine, under the influence of purely physical and chemical forces, they can selectively absorb certain substances from a solution and release their decay products into the environment. Due to the selective concentration of substances from the environment, they can grow, and when they reach a certain size they begin to “multiply”, budding small droplets, which, in turn, can grow and “bud”.
Coacervate droplets that arise as a result of concentrating protein solutions during mixing under the influence of waves and wind can become covered with a shell of lipids: a single shell, reminiscent of soap micelles (when a drop is lifted off the surface of water covered with a lipid layer once), or a double shell, reminiscent of a cell membrane ( when a drop covered with a single-layer lipid membrane repeatedly falls onto a lipid film covering the surface of a reservoir - Fig. 5).
The processes of the emergence of coacervate droplets, their growth and “budding”, as well as their “dressing” with a membrane of a lipid bilayer are easily simulated in laboratory conditions.
For coacervate droplets, there is also a process of "natural selection" in which the most stable droplets are retained in solution.
Despite the external resemblance of coacervate droplets to living cells, coacervate droplets do not have main feature living - the ability for accurate self-reproduction, self-copying. Obviously, the precursors of living cells were such coacervate droplets, which included complexes of replicator molecules (RNA or DNA) and the proteins they encode. Possibly RNA-protein complexes long time existed outside the coacervate droplets in the form of a so-called “free-living gene”, and perhaps their formation took place directly inside some coacervate droplets.
A possible path of transition from coacervate drops to primitive flares:
a) formation of a coacervate; 6) stabilization of coacervate droplets in an aqueous solution; c) - formation around the drop of a double lipid layer, similar to a cell membrane: 1 - coacervate drop; 2 - monomolecular layer of lipid on the surface of the reservoir; 3—formation of a single lipid layer around the drop; 4 - formation of a double lipid layer around the droplet, similar to a cell membrane; d) - a coacervate drop surrounded by a double lipid layer with a protein-nucleotide complex included in its composition - the prototype of the first living cell
The extremely complex process of the origin of life on Earth, not fully understood by modern science, passed from a historical point of view extremely quickly. Already 3.5 billion years so-called. chemical evolution ended with the appearance of the first living cells and biological evolution began.
Lesson objectives:
Expanding and generalizing students’ knowledge about different views on the origin of life on Earth;
Creation of a problem-oriented developmental environment as a condition for revealing the intellectual potential of a high school graduate.
Equipment:
Portraits of outstanding scientists and philosophers of the past;
Presentations: “Creationism”, “Development of ideas about the origin of life”;
Card for performing laboratory work: “Analysis and evaluation of various hypotheses of the origin of life”;
Card “Brief Glossary of Terms”;
Computer, projector, screen.
During the classes
1. Updating knowledge.
Differences between living and nonliving and the definition of “life”. (short conversation).
2. Introductory speech by the teacher.
Life has existed on Earth for 4.5 billion years. It fills all corners of our planet. Lakes, rivers, seas, oceans, mountains, plains, deserts, even the air are inhabited by living beings. It is estimated that over the entire history of life on Earth there have been about 4.5 billion species of animals and plants.
How did life arise and develop on our planet? The problem of the origin of life has captivated human thought since ancient times. From ancient times to our time, many hypotheses have been put forward about the origin of life on Earth. But to this day there is no definitive answer. By exploring the history of the development of ideas about the origin of life, we can only familiarize ourselves with the scientific theories proposed by scientists and the results of their research on this issue.
From ancient times to our time, many hypotheses have been put forward about the origin of life on Earth. However, all their diversity is reduced to two mutually exclusive points of view.
Proponents of the theory of biogenesis (from the Greek bio - life and genesis - origin) believed that all living things come only from living things. Their opponents defended the theory of abiogenesis and believed that the origin of living things from non-living things was possible, i.e., to one degree or another, they allowed the spontaneous generation of life.
We can observe elements of materialistic and idealistic views that permeate the entire history of the formation of views on the origin of life from ancient times to the present day.
Emergence of the Earth
From point of view modern science The sun and planets arose simultaneously from interstellar matter - particles of dust and gas. This cold substance gradually became denser, compressed, and then broke up into several unequal clumps. One of them, the largest, gave rise to the Sun. Its substance, continuing to compress, heated up, and a rotating cloud of gas and dust formed around it, which had the shape of a disk. Planets emerged from the dense clumps of this cloud. The earth was formed approximately 4.5 billion years ago. Scientists determined this by the age of the oldest rocks.
The theory of stationary (constant) state
As the steady state theory states, the Earth never came into being, but existed forever; environmental conditions were always possible to support life, and if they changed, it was not by much. According to this version, species of living beings also never formed, they have always existed, and each species has only two possible realities - either a change in numbers or extinction. But the hypothesis of a stationary state fundamentally contradicts the data of modern science, in particular astronomy; these data indicate the finite existence of the lifetime of any stars and, accordingly, planetary systems around these luminaries. According to modern estimates, based on taking into account the rates of radioactive decay, the age of the Earth, the Sun and the Solar System is ~4.6 billion years. Therefore, this hypothesis is not usually considered by academic science.
Proponents of this theory refuse to admit that the presence or absence of certain fossil remains (remains) can specifically focus attention on the time of the emergence or extinction of individual, different species, and cite as an example a representative of lobe-finned fish - coelacanth (coelacanth).
Theory of spontaneous generation of life
The theory of spontaneous generation originated in ancient China, Babylon, and Greece as an alternative to creationism, with which it coexisted. Aristotle was also a proponent of this theory. Her followers believed that certain substances contained an “active principle” that, under suitable conditions, could create a living organism.
Among seafarers, views on the appearance of the Bernakel goose were known. This goose grows on fragments of pine trees, rushing through the depths of the sea. At first it looks like a drop of resin. It attaches itself to a tree with its beak and secretes a hard shell for safety, in which it lives calmly and carefree. After some time, the goose grows feathers, and then it leaves the piece of bark into the water and begins to swim. And one fine day it flaps its wings and flies away.
For many centuries, while firmly believing in the act of Divine creation, people were also firmly convinced that life constantly arises spontaneously. The ancient Greek philosopher Aristotle wrote that not only plants, worms, insects, but even fish, frogs and mice can be born from wet soil or rotting silt. Dutch scientist Jan Van Helmont in the 17th century. described his experience, claiming that living mice allegedly arose from dirty laundry and a handful of wheat locked in a closet. Another naturalist, Grindel von Ach, spoke about the spontaneous generation of a living frog that he allegedly observed: “I want to describe the birth of a frog, which I was able to observe using a microscope. One day I took a drop of May dew and, carefully observing it under a microscope, noticed that some kind of creature was forming. Observing diligently on the second day, I noticed that the body had already appeared, but the head still seemed not clearly formed; continuing my observations on the third day, I became convinced that the creature I was observing was nothing more than a frog with a head and legs. The attached drawing explains everything.”
“These are the facts,” Aristotle wrote in his work, “living things can arise not only as a result of the mating of organisms, but also as a result of the decomposition of the soil, spontaneously generating under the influence of the forces of nature from the decaying earth.”
4. Teacher's comment on the assessment of research into the problem of the origin of life in the 18th and 19th centuries.
The Italian naturalist Francesco Redi opposed this approach to the problem of the origin of life. “Conviction would be in vain,” he wrote, “if it could not be confirmed by experiment. So I took 2 vessels and placed the eel in it. One vessel was closed and the other remained open. It could be seen that fly larvae appeared only in the open vessel. This means that the larvae are not born spontaneously, but from eggs laid by flies.”
But Redi’s opponents, the so-called vitalists (from the Latin vitas - life) - supporters of the all-pervading vital force - argued that air could not enter into a closed pot, and with it the “vital force”, therefore the fly larvae in a closed vessel could not appeared.
Then Redi staged an experiment that was brilliant in its simplicity. He placed dead snakes In 2 vessels, one was left open, the other was closed with muslin. After some time, fly larvae appeared only in the open vessel. Experience convinced us that plants and animals appear only from seeds or eggs formed by parent individuals, but cannot arise from inanimate nature. What about microorganisms? The debate between proponents of biogenesis and abiogenesis continued.
In 1859, the French Academy of Sciences awarded a prize to anyone who would put an end to the debate about the spontaneous generation of life. In 1862, Louis Pasteur received the Prize. He conducted an experiment that rivaled Redi's in simplicity. He boiled meat broth in flasks in which microorganisms could develop. When boiled, they and their spores died. Pasteur attached a curved tube to the flask; microbial spores settled in it and could not penetrate the nutrient medium, and access to the notorious “vital force” was ensured. The nutrient medium remained sterile, but as soon as the tube was broken off, the medium rotted. Subsequently, based on Pasteur’s experience, methods were created: pasteurization, preservation, the doctrine of asepsis and antiseptics. These were the practical results of the theoretical dispute.
5. Presentations by students on the analysis of other hypotheses of the origin of life on Earth.
Hypotheses of the eternity of life in the Universe. Panspermia
L. Pasteur's refutation of the theory of the spontaneous origin of life played a dual role. On the one hand, representatives of idealistic philosophy saw in his experiments only direct evidence of the fundamental impossibility of the transition from inorganic matter to living beings as a result of the action of only natural forces of nature. This was quite consistent with their opinion that the emergence of life requires the intervention of an immaterial principle - the creator. On the other hand, some materialistically minded natural scientists have now lost the opportunity to use the phenomenon of spontaneous generation of life as the main proof of their views. The idea of the eternity of life in the universe arose. This is how the hypothesis of panspermia appeared, which was put forward by the German chemist J. Liebig (1803 - 1873).
According to the panspermia hypothesis, life exists forever and is transferred from planet to planet by meteorites. The simplest organisms or their spores (“seeds of life”), arriving on a new planet and finding favorable conditions here, multiply, giving rise to evolution from the simplest forms to complex ones. A supporter of the panspermia hypothesis was the outstanding Russian naturalist V.I. Vernadsky (1863 – 1945)
The Swedish physical chemist S. Arrhenius (1859-1927) was especially active in developing the theory of panspermia. In the experiments of the Russian physicist P.N. Lebedev (1866-1912), who discovered the pressure of the light flux, S. Arrhenius saw evidence of the possibility of transferring microorganism spores from planet to planet. Life is transported, he suggested, not in the form of microorganisms on meteorites that heat up when entering the dense layers of the atmosphere - the spores themselves can move in cosmic space, driven by the pressure of sunlight!
This view was later rejected. In space conditions, the beginnings of life in those forms that are known to us on Earth, apparently, cannot exist, and all attempts to detect any forms of life in space have not yet yielded positive results. Nevertheless, some modern scientists express hypotheses about the extraterrestrial origin of life. Thus, American scientists F. Crick and L. Orgel believe that the Earth was “seeded” by some intelligent creatures, inhabitants of those planetary systems, the development of life on which was billions of years ahead of our Solar System. Having equipped a rocket and placed a container with simple organisms in it, they launched it towards the Earth, having previously established that there is the necessary conditions for life. Of course, this cannot be proven and it is not possible to categorically refute it.
One piece of evidence in favor of the hypothesis of the extraterrestrial origin of life was the discovery inside the meteorite, named ALH 84001, of rod-shaped formations resembling fossilized bacteria in shape. The meteorite itself was a piece of Martian crust, which was thrown into space 16 million years ago as a result of an explosion on this planet. And 13 thousand years ago it fell to Earth, in Antarctica, where it was recently discovered. To definitively answer the question “Is there life on Mars?” will succeed in the near future, when the reports of the American National Aeronautics and Space Administration NASA are published. This organization launched a satellite to Mars to take samples of Martian soil and is now processing the resulting material. If research shows that microorganisms inhabited Mars, then we can speak with more confidence about the introduction of life from space.
The theory of panspermia takes us away from resolving the question of the origin of life on Earth: if life did not originate on Earth, then how did it originate outside of it? This theory has not found recognition among many scientists (does not explain the origin of life)
Creationism hypothesis
The creationism hypothesis is a view of the origin of life from the point of view of believers. According to this hypothesis, life arose as a result of some supernatural event in the past. It is adhered to by followers of all religious concessions of the world - Islam, Christianity, Buddhism, Judaism. From the point of view of these religions, the Universe consists of material and spiritual components. Living matter, that is, animal, vegetable world and man was born by the spiritual component, in other words, by God. Proponents of this hypothesis give examples of features of living matter that cannot be explained by modern science and, from the point of view of religion, demonstrate the existence of a Supreme Mind. For example: viruses consist of a protein shell and DNA. In the host cell, in order to reproduce, the virus needs to double the DNA molecule, but this requires enormous energy; who initiates this process? Within the natural sciences, the question is still unanswered.
Does this mean that the stereotypical view among many that science and religion are inherently contradictory is correct? Many researchers believe that science and religion are ways of understanding two sides of a single world - material and spiritual reality. In practice, they should not be opposed, but complement and support each other. That is why Albert Einstein said: “Science without religion is defective, religion without science is blind.” Presentation 2
Biochemical evolution hypothesis
The theory of biochemical evolution has the largest number of supporters among modern scientists. The earth originated about five billion years ago; Initially, its surface temperature was very high. As it cooled, a solid surface (lithosphere) formed. The atmosphere, originally consisting of light gases (hydrogen, helium), could not be effectively contained by the insufficiently dense Earth, and these gases were replaced by heavier ones: water vapor, carbon dioxide, ammonia and methane. When the Earth's temperature dropped below 100°C, water vapor began to condense, forming the world's oceans. At this time, complex organic substances were formed from primary compounds; energy for fusion reactions was supplied by lightning discharges and intense ultraviolet radiation. The accumulation of substances was facilitated by the absence of living organisms - consumers of organic matter - and the main oxidizing agent - oxygen.
Primary organic substances (proteins) could be created from inorganic ones under the conditions of a reducing atmosphere due to the energy of powerful electrical discharges. Due to amphotericity, protein structures (protobionts, in Oparin’s terminology) formed colloidal hydrophilic complexes (attracted water molecules) with a common water shell. These complexes could be separated from the entire mass of water and merged with each other, forming coacervate droplets (coacervation is the spontaneous separation of an aqueous solution of polymers into phases with different concentrations). In coacervates, substances entered into further chemical reactions (selective absorption of metal ions and the formation of enzymes occurred). The complication of protobionts was achieved by selecting such coacervate droplets, which had the advantage of better utilization of substances and energy of the environment. At the boundary between coacervates and external environment a primitive membrane was formed from lipids, which led to the emergence of the first cell.
Modern science considers the abiogenic origin of life on Earth, considering this theory the most probable. Abiogenesis consists of three main stages in the development of life:
1. Abiogenic occurrence of biological monomers.
2. Formation of biological polymers.
3. Formation of membrane structures and primary organisms (probionts).
At present, the problem of the origin of life has not been solved. Scientists continue to look for ways to solve it.
7. Performing laboratory work
Laboratory work
“Analysis and evaluation of various hypotheses for the origin of life”
Purpose of the study Characterize the mythological ideas of ancient scientists, the first scientific attempts to explain the essence and process of the origin of life, characterize the experimental evidence of hypotheses: the experiments of F. Redi, the views of V. Harvey, the experiments of L. Pasteur, the theory of the eternity of life, materialistic ideas about the origin of life on Earth. Get acquainted with the statements of supporters of panspermia, the hypothesis of the eternity of life in the Universe. Explain why these theories have not found acceptance among many scientists.
Are the hypotheses presented evidence-based? Do they allow for the evolutionary development of nature? Can these hypotheses be considered scientific? Indicate with (+) or (-)
| Hypotheses about the origin of life | Evidence of the hypothesis |
Evolutionary development |
The scientific nature of the hypothesis |
|
| 1 | Creationism | |||
| 2 | Vitalism - the theory of spontaneous generation of life | |||
| 3 | Panspermia theory | |||
| 4 | Steady State Theory | |||
| 5 | Theory of biochemical evolution |
Based on your analysis, draw a conclusion about which hypothesis of the origin of life on Earth is more likely.
Terminological dictionary
Life is one of the forms of existence of matter, which naturally arises under certain conditions in the process of its development. Organisms differ from inanimate objects in their metabolism, irritability, ability to reproduce, grow, develop, regulate composition and functions, various forms of movement, adaptability to the environment, etc.
Abiogenesis is the theory that living things can arise from non-living things.
In a broad sense, abiogenesis is an attempt to imagine the emergence of living things from non-living things.
Biogenesis is the theory that living things can only arise from living things.
Vitalism is a theory according to which there is a “life force” everywhere, which you just need to “breathe in”, and the inanimate will become alive.
Creationism is the theory that life arose as a result of some supernatural event in the past, which most often means divine creation.
Panspermia is a theory according to which the “seeds of life” were brought to Earth from space along with meteorites or cosmic dust.
Coacervates are protein complexes isolated from the mass of water, capable of exchanging substances with the environment and selectively accumulating various compounds.
Probionts are primitive heterotrophic organisms that arose in the “primordial broth.”
8. Summing up
Life is just a spark in endless darkness: it will appear, flicker and disappear forever.
Compared to the infinity of time, the duration of human life is only a vanishingly brief moment, but that is all that is given to us here.
Therefore, we must lead our lives in the light of eternity and spend our time and talents on things of eternal value.
Homework. Prepare answers to the following questions in presentation form:
1. What is the value of life?
2. What is the meaning of human life?
3. Why is it necessary to protect life?
Life is the greatest miracle that exists on our planet. The problems of its study are currently occupied not only by biologists, but also by physicists, mathematicians, philosophers and other scientists. Of course, the most difficult mystery is the very origin of life on Earth.
Researchers are still arguing about how this happened. Oddly enough, philosophy has made a significant contribution to the study of this phenomenon: this science allows one to draw correct conclusions by summarizing huge amounts of information. What versions are guiding scientists around the world today? Here are the current theories of the origin of life on Earth:
- The concept of spontaneous generation.
- Creationism, or the theory of divine creation.
- The principle of stationary state.
- Panspermia, whose proponents claim the natural “productivity” of any planet where suitable conditions exist. In particular, this idea was once developed by the well-known academician Vernadsky.
- Biochemical evolution according to A.I. Oparin.
Let us consider all these theories of the origin of life on Earth in a little more detail.
Materialism and idealism
Back in the Middle Ages and earlier, in the Arab world, some scientists, even at the risk of their own lives, assumed that the world could be created as a result of some natural processes, without the participation of a divine essence. These were the first materialists. Accordingly, all other points of view that provided for Divine intervention in the creation of all things were considered idealistic. Accordingly, it is quite possible to consider the origin of life on Earth from these two positions.
Creationists argue that life could only have been created by God, while materialists promote the theory of the emergence of the first organic compounds and life from inorganic substances. Their version is based on the complexity or impossibility of understanding those processes that resulted in life in its modern form. Interestingly, the modern Church only partially supports this hypothesis. From the point of view of the most scientist-friendly figures, it is truly impossible to understand the main Plan of the Creator, but we can determine the phenomena and processes due to which life arose. However, from truth scientific approach it's still very far away.
Currently, the materialist point of view prevails. However, they did not always put forward modern theories origin of life. Thus, the hypothesis that the origin and evolution of life on Earth occurred spontaneously was initially popular, and supporters of this phenomenon were found back in the early 19th century.
Proponents of this concept argued that there are certain laws of natural nature that determine the possibility of the arbitrary transition of inorganic compounds into organic ones with the subsequent arbitrary formation of life. This also includes the theory of the creation of a “homunculus,” an artificial person. In general, the spontaneous origin of life on Earth is still considered seriously by some “experts”... At least it’s good that they talk about bacteria and viruses.
Of course, this approach was later proven to be wrong, but it played an important role, providing a huge amount of valuable empirical material. Note that the final rejection of the version of the independent origin of life occurred only in the middle of the 19th century. In principle, the impossibility of such a process was proven by Louis Pasteur. For this, the scientist even received a considerable prize from the French Academy of Sciences. Soon the main theories of the origin of life on Earth come to the fore, which we will describe below.
Academician Oparin's theory
Modern ideas about the origin of life on Earth are based on a theory that was put forward by a domestic researcher, Academician Oparin, back in 1924. He refuted Redi's principle, which spoke about the possibility of only biogenic synthesis of organic substances, pointing out that this concept is valid only for the current state of affairs. The scientist pointed out that at the very beginning of its existence, our planet was a giant rocky ball, on which, in principle, there was no organic matter.
Oparin's hypothesis was that the origin of life on planet Earth is a long-term biochemical process, the raw materials for which are common compounds that can be found on any planet. The academician suggested that the transition of these substances into more complex ones was possible under the influence of extremely strong physical and chemical factors. Oparin was the first to put forward a hypothesis about the continuous transformation and interaction of organic and inorganic compounds. He called it "biochemical evolution." Below are the main stages of the origin of life on Earth according to Oparin.
Stage of chemical evolution
About four billion years ago, when our planet was a huge and lifeless rock in the depths of space, the process of non-biological synthesis of carbon compounds was already underway on its surface. During this period, volcanoes emitted titanic amounts of lava and hot gases. Cooling in the primary atmosphere, the gases turned into clouds, from which torrential rains fell incessantly. All these processes took place over millions of years. But, excuse me, when did the origin of life on Earth begin?
At the same time, the showers gave rise to huge primary oceans, the waters of which were extremely saturated with salts. The first organic compounds got there, the formation of which took place in the atmosphere under the influence of strong electrical discharges and UV irradiation. Gradually their concentration increased until the seas turned into a kind of “broth” saturated with peptides. But what happened next and how did the first cells arise from this “soup”?
Formation of protein compounds, fats and carbohydrates
And only at the second stage do true proteins and other compounds from which life is built appear in the “broth”. Conditions on Earth softened, carbohydrates, proteins and fats, the first biopolymers, and nucleotides appeared. This is how coacervate droplets formed, which were the prototype of real cells. Roughly speaking, this was the name given to drops of proteins, fats, and carbohydrates (as in soup). These formations could absorb and absorb those substances that were dissolved in the waters of the primary oceans. At the same time, a kind of evolution took place, the result of which were drops with increased resistance and stability to environmental influences.
Appearance of the first cells
Actually, at the third stage, this amorphous formation turned into something more “meaningful.” That is, in living cell capable of self-reproduction. The natural selection of drops, which we have already discussed above, became more and more stringent. The first “advanced” coacervates already had, albeit primitive, metabolism. Scientists suggest that the drop, having reached a certain size, disintegrated into smaller formations that had all the features of the mother “cell”.
Gradually, a layer of lipids appeared around the core of the coacervate, giving rise to a full-fledged cell membrane. This is how the primary cells, archecells, were formed. It is this moment that can rightfully be considered as the origin of life on Earth.
Is non-biological synthesis of organic matter real?
As for the hypothesis of the origin of life on Earth from Oparin... Many people immediately have a question: “How realistic is the formation of organic matter from inorganic matter under natural conditions?” Many researchers have had such thoughts!
In 1953, American scientist Miller modeled the Earth's primordial atmosphere, with its incredible temperatures and electrical discharges. In this environment simple inorganic compounds. As a result, acetic and formic acids and other organic compounds were formed there. This is how the origin of life on Earth took place. Briefly, this process can be characterized by the philosophical law of “Transition of quantity into quality.” Simply put, with the accumulation of a certain amount of proteins and other substances in the primary ocean, these compounds acquire different properties and the ability to self-organize.
Strengths and weaknesses of Oparin's theory
The concept we have considered has not only strong but also weak points. Strength theory is its logic and experimental confirmation of the abiotic synthesis of organic compounds. In principle, this could be the origin and development of life on Earth. A huge weakness is the fact that so far no one can explain how coacervates were able to degenerate into a complex biological structure. Even supporters of the theory admit that the transition from a protein-fat droplet to a full-fledged cell is very doubtful. We are probably missing something by not taking into account factors unknown to us. Currently, all scientists recognize that there was some kind of sharp jump, as a result of which the self-organization of matter became possible. How could this even happen? It is still unclear... What other main theories of the origin of life on Earth exist?
Theory of panspermia and steady state
As we have already said, at one time this version was ardently supported and “promoted” by the famous academician Vernadsky. In general, the theory of panspermia cannot be discussed in isolation from the concept of a stationary state, since they consider the principle of the origin of life from the same point of view. You should know that this concept was first proposed by the German Richter at the end of the 19th century. In 1907, he was supported by the Swedish researcher Arrhenius.
Scientists who adhere to this concept believe that life simply existed in the Universe and will always exist. It is transferred from planet to planet with the help of comets and meteorites, which play the role of peculiar “seeds”. The disadvantage of this theory is that the Universe itself is believed to have formed approximately 15-25 billion years ago. It doesn't look like Eternity at all. Considering that the planets potentially suitable for the formation of life are many times smaller than ordinary rocky planetoids, it is quite natural for the question to arise: “When and where did life form and how did it spread throughout the Universe at such a speed, taking into account the unrealistic distances?”
It should be remembered that the age of our planet is no more than 5 billion years. Comets and asteroids fly much slower than the speed of light, so they simply might not have enough time to plant the “seeds” of life on Earth. Proponents of panspermia suggest that certain seeds (spores of microorganisms, for example) are transported “on light rays” at an appropriate speed... But decades of spacecraft have made it possible to prove that there are quite a few free particles in space. The probability of this method of spreading living organisms is too low.
Some researchers today suggest that any planet that is suitable for life may eventually form protein bodies, but the mechanism of this process is unknown to us. Other scientists say that in the Universe, perhaps, there are some “cradles”, planets on which life can form. It sounds, of course, like some kind of science fiction... However, who knows. IN last years here and abroad, a theory gradually began to take shape, the provisions of which speak of the information initially encoded in the atoms of substances...
Allegedly, these data provide the very impetus that leads to the transformation of the simplest coacervates into archecells. If we think logically, then this is the same theory of the spontaneous origin of life on Earth! In general, the concept of panspermia is difficult to consider as a complete scientific thesis. Its supporters can only say that life was brought to Earth from other planets. But how did it form there? There is no answer to this.
"Gift" from Mars?
Today it is known for certain that there was indeed water on the Red Planet and there were all conditions favorable to the development of protein life. The data that confirms this was obtained thanks to the work on the surface of two landers at once: Spirit and Curiosity. But scientists are still passionately arguing: was there life there? The fact is that information received from the same rovers indicates the short-term (in geological aspect) existence of water on this planet. How high is the probability that full-fledged protein organisms? Again, there is no answer to this question. Again, even if life came to our planet from Mars, this in no way explains the process of its development there (which we have already written about).
So, we have examined the basic concepts of the origin of life on Earth. Which of them are absolutely true is unknown. The problem is that there is not yet a single experimentally confirmed test that could confirm or refute at least Oparin’s concept, not to mention other theses. Yes, we can synthesize protein without any problems, but we cannot obtain protein life. So scientists have work in store for many decades to come.
There is another problem. The fact is that we are intensely looking for carbon-based life and trying to understand exactly how it came to be. What if the concept of life is much broader? What if it could be based on silicon? In principle, this point of view does not contradict the principles of chemistry and biology. So on the way to finding answers we are met with more and more new questions. Currently, scientists have put forward several fundamental theses, guided by which people are looking for potentially habitable planets. Here they are:
- The planet should orbit in the so-called “comfort zone” around the star: its surface should not be too hot or too cold. In principle, at least one or two planets in each star system meet this requirement (Earth and Mars, in particular).
- The mass of such a body should be average (within one and a half times the size of the Earth). Planets that are too large either have unrealistically high gravity or are gas giants.
- More or less highly organized life can only exist near fairly old stars (at least three to four billion years).
- The star should not seriously change its parameters. It is useless to look for life near white dwarfs or red giants: if it was there, it would have died long ago due to extreme unfavorable conditions environment.
- It is desirable that the star system be single. In principle, modern researchers object to this thesis. It is possible that a binary system with two stars located at opposite ends could contain even more potentially habitable planets. Moreover, today there is more and more talk that somewhere on the outskirts of the solar system there is a gas-dust cloud, the forerunner of the unborn second Sun.
Final conclusions
So, what can we say in conclusion? First, we urgently lack data on the exact environmental conditions on the newly formed Earth. To obtain this information, ideally one should observe the development of a planet that is similar to ours in other respects. In addition, researchers are still finding it difficult to say exactly what factors stimulate the transition of coacervate archecapelles into full-fledged cells. Perhaps further in-depth studies of the genome of living beings will provide some answers.
