It is known that scientific journals try not to accept for publication articles devoted to problems attracting everyone’s attention but not having a clear solution — a serious edition on physics will not publish a draft perpetual motion machine. This theme was the origin of life on Earth. The question of the emergence of wildlife. The appearance of a person is disturbed by thinking people for thousands of years, and only creationists — supporters of the divine origin of all things — have found a definite answer for themselves, but this theory is not a scientific one that cannot be tested.
Views of the ancients
Ancient Chinese and ancient Indian manuscripts tell about the appearance of living creatures from water and rotting remnants, the Egyptian hieroglyphs and cuneiform writing of ancient Babylon tell us about the birth of amphibious creatures in the oozy sediments of large rivers. The hypotheses of the origin of life on Earth through spontaneous generation for sages of the distant past were obvious.
Ancient philosophers also gave examples of the appearance of animals from inanimate matter, but their theoretical foundations were of a different nature: materialistic and idealistic. Democritus (460-370 BC. E.) Found the cause of the emergence of life in a special interaction of the smallest, eternal and indivisible particles - atoms. Plato (428-347 BC) and Aristotle (384-322 BC. E.) The origin of life on Earth was attributed to the miraculous effect on lifeless matter of the higher principle, which infuses the soul into the objects of nature.
The idea of the existence of some kind of “life force”, contributing to the appearance of living beings, turned out to be very stable. It formed views on the origin of life on Earth for many scientists who lived in the Middle Ages and later, until the end of the XIX century.
Theory of spontaneous generation
Anthony van Leeuwenhoek (1632-1723) with the invention of the microscope made the smallest microorganisms discovered by him the main subject of controversy between scientists who shared the two main theories of the origin of life on Earth - biogenesis and abiogenesis. The former believed that all life could only be the creation of living matter, while the latter believed that spontaneous generation of organic matter in solutions placed in special conditions was possible. The essence of this dispute has not changed so far.
Experiments of some naturalists have proved the possibility of the spontaneous emergence of the simplest microorganisms, and supporters of biogenesis completely denied this possibility. Louis Pasteur (1822–1895), using strictly scientific methods and the high correctness of his experiments, proved the absence of the mythical life force transmitted through the air and giving rise to living bacteria. However, in his works he allowed the possibility of spontaneous generation in some special conditions, which scientists of future generations had to find out.
The writings of the great Charles Darwin (1809-1882) shook the foundations of many natural sciences. The proclaimed appearance of a huge variety of biological species from one common ancestor again made the origin of life on Earth the most important issue of science. The theory of natural selection and at the beginning hardly found its supporters, and now it is undergoing critical attacks that look reasonably reasonable, but it is Darwinism that underlies the modern natural sciences.
After Darwin, the origin of life on Earth biology could not be considered from their previous positions. Scientists of many branches of biological science were convinced of the truth of the evolutionary path of development of organisms. Let modern views on a common ancestor, placed by Darwin in the foundation of the Tree of Life, have changed in many ways, but the truth of the general concept is unshakable.
Theory of stationary state
The laboratory refutation of the spontaneous spontaneous generation of bacteria and other microorganisms, an awareness of the complex biochemical structure of the cell, along with the ideas of Darwinism, had a particular influence on the emergence of alternative versions of the theory of the origin of life on Earth. In 1880, one of the new propositions was proposed by William Preyer (1841-1897). He believed that there is no need to talk about the birth of life on our planet, since it exists forever, and it had no beginning as such, it is unchangeable and constantly ready for rebirth in any suitable conditions.
The ideas of Preyer and his followers are only of purely historical and philosophical interest, because later astronomers and physicists calculated the time for the final existence of planetary systems, fixed the constant, but steady expansion of the Universe, that is, it was never eternal or permanent.
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 of organic matter, spoke of the geological eternity of life. One of the ways in which life appeared on a 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 - 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 space 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 XVIII century French diplomat and geologist Benoît de Mayo (1656-1738) spoke about it. These ideas were reanimated by Svante August Arrhenius (1859-1927), Lord Kelvin William Thomson (1824-1907) and Hermann von Helmholtz (1821-1894).
The study of the cruel influence on living organisms of cosmic radiation and the temperature conditions of the interplanetary space made such hypotheses of the origin of life on Earth not very relevant, but with the beginning of the space age, interest in panspermia increased.
In 1973, the Nobel laureate Francis Crick (1916-2004) suggested an extraterrestrial production of molecular living systems and hitting them on Earth with meteorites and comets. At the same time, the chances of abiogenesis on our planet were rated as very low. The eminent scientist did not consider the origin and development of life on Earth by the self-assembly of high-level organic matter a reality.
Petrified biological structures were found in meteorites across the planet, similar traces were found in soil samples taken from the Moon and Mars. On the other hand, numerous experiments are being carried out on the treatment of biostructures with influences that are possible when they are located in outer space and when passing through an atmosphere similar to that of the Earth.
An important experiment was conducted in 2006 as part of the Deep Impact mission. Comet Tempel was rammed by a special probe-impactor, issued by an automatic apparatus. 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 the appearance of the theory of panspermia has changed significantly. Modern science treats 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 in interplanetary travel. All this makes the idea of extraterrestrial origin of earthly life relevant. The basic concepts of the origin of life on Earth are theories into which panspermia enters either as the main part, or as a method of delivering components to the Earth to create living matter.
Theory of Oparin-Haldane's biochemical evolution
Oparin suggested that if the primitive atmosphere of a young planet Earth was reducing (that is, not containing oxygen), a powerful burst of energy (for example, lightning or ultraviolet radiation) could contribute to the synthesis of organic compounds from inorganic matter. Subsequently, such molecules could form clots and accumulations — coacervate drops, which are the proto-organisms around which water shirts are formed — the primordia of the membrane membrane; The koatservatov was considered the basis for the beginning of the evolutionary processes that led to the creation of the first life forms.
Haldane introduced the concept of "primary broth" - the initial ocean of the earth, which became a huge chemical laboratory, connected to a powerful source of food - sunlight. The combination of carbon dioxide, ammonia and ultraviolet radiation has led to the emergence of 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 (Oparin-Haldane)
According to the theory of the emergence of the universe from a bunch of energy, the Big Bang occurred about 14 billion years ago, and about 4.6 billion years ago the creation of the planets of the solar system was completed.
The young Earth, gradually cooled, acquired a solid shell around which the formation of the atmosphere occurred. 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, cyanide compounds.
The bombardment of space objects containing water in a frozen state, and the condensation of water vapor in the atmosphere led to the formation of the World Ocean, in which various chemical compounds were dissolved. Powerful thunderstorms accompanied the formation of the atmosphere, through which penetrated strong ultraviolet radiation. In such conditions, the synthesis of amino acids, sugars and other simplest organics took place.
At the end of the first billion years of the existence of the Earth, 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 beginnings of the nucleus, metabolism, and membrane).
3.5-3 billion years BC - the stage of formation of protobionts with self-reproduction, regulated metabolism, membrane with variable permeability.
3 billion years BC e. - the emergence of cell 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 reservations in the Oparin-Haldane theory. In various countries, work began on conducting test studies of the hypothesis, of which the classical experiment, conducted in 1953 by American scientists Stanley Miller (1930–2007) and Harold Urey (1893–1981), is most famous.
The essence of the experiment was to model 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 the electrical discharges transmitted through the mixture created a lightning effect.
After a weekly circulation of the mixture through the system, a tenth of carbon was transferred to organic compounds, amino acids, sugars, lipids and compounds preceding amino acids were found. Repeated and modified experiments fully confirmed the possibility of abiogenesis in simulated conditions of the early Earth. In subsequent years, other laboratories conducted repeated experiments. Hydrogen sulfide was added to the composition of the gas mixture as a possible component of volcanic emissions, and other non-cardinal changes were made. In most cases, the experience of the synthesis of organic compounds was successful, although attempts to go further and obtain more complex elements approaching the living cell in composition did not succeed.
By the end of the 20th century, many scientists who had not ceased to be interested in the problem of the origin of life on Earth, it became clear that with all the symmetry of theoretical constructions and distinct experimental confirmation the Oparin-Haldane theory has obvious, perhaps irresistible, flaws. The main one was the impossibility of explaining the appearance of properties in protobionts that determine for a living organism - to multiply while preserving hereditary traits. With the discovery of genetic cellular structures, with the definition of the function and structure of DNA, with the development of microbiology, a new candidate for the role of a first-life molecule has emerged.
It has become 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, the monosaccharide — the ribose, and the phosphate group. The sequence of nucleotides is the code of hereditary information, and in viruses, for example, RNA plays the role that DNA plays in complex cellular structures.
In addition, scientists have discovered the unique ability of some RNA molecules to insert breaks into other chains or glue together individual elements of RNA, and some play the role of autocatalysts — that is, they contribute to the rapid reproduction. The relatively small size of the RNA macromolecule and its simplified, as compared to DNA, structure (in one strand) made ribonucleic acid the main candidate for the role of the main element of prebiological systems.
The final theory of the emergence of living matter on the planet was formulated in 1986 by Walter Gilbert (born 1932) - an American physicist, microbiologist and biochemist. Not all experts agreed with this view on the origin of life on Earth. Briefly named “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 appeared with desired properties, even if there was a huge amount of “building material” in the form of nucleotides, etc.
The answer tried to find in May 2004, Simon Nicholas Platts, and in 2006, a group of scientists led by Pascal Ehrenfreund. Polyaromatic hydrocarbons have been proposed as a starting material for RNA with catalytic properties.
The world of PAHs was based on the high prevalence of these compounds in the visible space (they must have been present in the “primary broth” of the young Earth) and the peculiarities of their ring-shaped structure, which promotes rapid connection with nitrogenous bases, the key components of RNA. The PAH theory once again speaks of the topicality of some of the provisions of panspermia.
Unique life on a unique planet
Until scientists have the opportunity to return 3 billion years ago, the secret of the origin of life on our planet will not be revealed - many of those who dealt with this problem come to this conclusion. The basic concepts of the origin of life on Earth are: the theory of abiogenesis and the theory of panspermia. They can intersect in many ways, but, most likely, they will not be able to answer: how among the vast cosmos a wonderfully balanced system emerged from the Earth and its satellite, the Moon, how life originated on it ...