Municipal educational institution
Secondary school No. 45
Theories about the origin of life on Earth
Performed : student 11 "B" class
Nigmatullina Maria
Proveila : biology teacher
Trapueva L. S.
Chelyabinsk
2010
Introduction
Hypotheses about the origin of life
Genobiosis and holobiosis
Oparin–Haldane theory
The world of RNA as a forerunner of modern life
Panspermia
Spontaneous generation of life
Steady State Theory
creationism
Evolution theory
Darwinian theory
Conclusion
Introduction
Theories concerning the origin of the Earth and life on it, and indeed the entire Universe, are diverse and far from reliable. According to the steady state theory, the universe has existed forever. According to other hypotheses, the Universe could have arisen from a bunch of neutrons as a result of the Big Bang, was born in one of the black holes, or was created by the Creator. Contrary to popular belief, science cannot refute the thesis of the divine creation of the universe, just as theological views do not necessarily reject the possibility that life in the process of its development acquired features that can be explained on the basis of the laws of nature.
Hypotheses about the origin of life
At various times, the following hypotheses were put forward regarding the origin of life on Earth:
Hypothesis of biochemical evolution
Panspermia hypothesis
Stationary state of life hypothesis
Hypothesis of spontaneous generation
theories spontaneous generation and steady state are only of historical or philosophical interest, since the results of scientific research contradict the conclusions of these theories.
Theory panspermia does not solve the fundamental issue of the origin of life, it only moves it to an even more vague past of the Universe, although it cannot be excluded as a hypothesis about the beginning of life on Earth.
Genobiosis and holobiosis
Depending on what is considered primary, there are two methodological approaches to the question of the origin of life:
Genobiosis- a methodological approach to the issue of the origin of life, based on the belief in the primacy of the molecular system with the properties of the primary genetic code.
holobiosis- a methodological approach to the issue of the origin of life, based on the idea of the primacy of structures endowed with the ability for elemental metabolism with the participation of the enzymatic mechanism.
Oparin–Haldane theory
In 1924, the future academician Oparin published the article "The Origin of Life", which was translated into English in 1938 and revived interest in the theory of spontaneous generation. Oparin suggested that in solutions of macromolecular compounds, spontaneously zones of increased concentration are formed, which are relatively separated from the external environment and can maintain an exchange with it. He called them Coacervate drops, or simply coacervates.
According to his theory, the process that led to the emergence of life on Earth can be divided into three stages:
The emergence of organic matter
The emergence of proteins
The emergence of protein bodies
Astronomical studies show that both stars and planetary systems arose from gas and dust matter. Along with metals and their oxides, it contained hydrogen, ammonia, water and the simplest hydrocarbon - methane.
The conditions for the beginning of the process of formation of protein structures have been established since the appearance of the primary ocean. In the aquatic environment, derivatives of hydrocarbons could undergo complex chemical changes and transformations. As a result of this complication of molecules, more complex organic substances, namely carbohydrates, could be formed.
Science has proven that as a result of the use of ultraviolet rays, it is possible to artificially synthesize not only amino acids, but also other biochemical substances. According to Oparin's theory, the formation of coacervate drops could be a further step towards the emergence of protein bodies. Under certain conditions, the aqueous shell of organic molecules acquired clear boundaries and separated the molecule from the surrounding solution. Molecules surrounded by a water shell united, forming multimolecular complexes - coacervates.
Coacervate droplets could also arise from simple mixing of various polymers. In this case, the self-assembly of polymer molecules into multimolecular formations took place - drops visible under an optical microscope.
Drops were able to absorb substances from the outside in the manner of open systems. When various catalysts (including enzymes) were included in coacervate droplets, various reactions occurred in them, in particular, polymerization of monomers coming from the environment. Due to this, the drops could increase in volume and weight, and then break up into daughter formations. Thus, coacervates could grow, multiply, and carry out metabolism.
Similar views were also expressed by the British biologist John Haldane.
The theory was tested by Stanley Miller in 1953 in the Miller-Urey experiment. He placed a mixture of H 2 O, NH 3 , CH 4 , CO 2 , CO in a closed vessel (Fig. 1) and began to pass electric discharges through it. It turned out that amino acids are formed. Later, other sugars and nucleotides were obtained under different conditions. He concluded that evolution can occur in a phase-separated state from solution (coacervates). However, such a system cannot reproduce itself.
The theory was substantiated, except for one problem, which for a long time turned a blind eye to almost all experts in the field of the origin of life. If spontaneously, by random template-free syntheses in a coacervate, single successful constructions of protein molecules arose (for example, effective catalysts that provide an advantage for this coacervate in growth and reproduction), then how could they be copied for distribution inside the coacervate, and even more so for transmission to descendant coacervates? The theory has been unable to offer a solution to the problem of exact reproduction - within the coacervate and in generations - of single, randomly appearing effective protein structures. However, it was shown that the first coacervates could form spontaneously from lipids synthesized abiogenically, and they could enter into symbiosis with "living solutions" - colonies of self-reproducing RNA molecules, among which were ribozymes catalyzing lipid synthesis, and such a community is already possible. call it an organism.
Alexander Oparin (right) in the laboratory
The world of RNA as a forerunner of modern life
By the 21st century, the Oparin-Haldane theory, which assumes the initial appearance of proteins, has practically given way to a more modern one. The impetus for its development was the discovery of ribozymes - RNA molecules with enzymatic activity and therefore capable of combining functions that in real cells are mainly performed separately by proteins and DNA, that is, catalyzing biochemical reactions and storing hereditary information. Thus, it is assumed that the first living beings were RNA organisms without proteins and DNA, and their prototype could be an autocatalytic cycle formed by the very ribozymes capable of catalyzing the synthesis of their own copies.
Panspermia
According to the theory of Panspermia, proposed in 1865 by the German scientist G. Richter and finally formulated by the Swedish scientist Arrhenius in 1895, life could be brought to Earth from space. The most likely hit of living organisms of extraterrestrial origin with meteorites and cosmic dust. This assumption is based on data on the high resistance of some organisms and their spores to radiation, high vacuum, low temperatures, and other influences. However, there are still no reliable facts confirming the extraterrestrial origin of microorganisms found in meteorites. But even if they got to Earth and gave rise to life on our planet, the question of the original origin of life would remain unanswered.
Francis Crick and Leslie Orgel proposed in 1973 another option - controlled panspermia, that is, the deliberate "infection" of the Earth (along with other planetary systems) with microorganisms delivered on unmanned spacecraft by an advanced alien civilization that may have been facing a global catastrophe or just hoping to terraform other planets for future colonization. In favor of their theory, they cited two main arguments - the universality of the genetic code (known other variations of the code are used much less frequently in the biosphere and differ little from the universal one) and the significant role of molybdenum in some enzymes. Molybdenum is a very rare element in the entire solar system. According to the authors, the original civilization may have lived near a star enriched in molybdenum.
Against the objection that the theory of panspermia (including controlled) does not solve the issue of the origin of life, they put forward the following argument: on planets of another type unknown to us, the probability of the origin of life may initially be much higher than on Earth, for example, due to the presence of special minerals with high catalytic activity.
In 1981, F. Crick wrote the book "Life itself: its origin and nature", in which he describes the hypothesis of controlled panspermia in more detail than in the article and in a popular form.
Spontaneous generation of life
This theory was circulated in ancient China, Babylon, and ancient Egypt as an alternative to the creationism with which it coexisted. Aristotle (384-322 BC), often hailed as the founder of biology, held to the theory of the spontaneous generation of life. According to this hypothesis, certain "particles" of matter contain some kind of "active principle", which, under suitable conditions, can create a living organism. Aristotle was right in thinking that this active principle is contained in a fertilized egg, but mistakenly believed that it is also present in sunlight, mud and rotting meat.
With the spread of Christianity, the theory of spontaneous generation of life fell out of favor, but this idea continued to exist somewhere in the background for many more centuries.
The famous scientist Van Helmont described an experiment in which he allegedly created mice in three weeks. For this, a dirty shirt, a dark closet and a handful of wheat were needed. Van Helmont considered human sweat to be the active principle in the process of the birth of a mouse.
In 1688, the Italian biologist and physician Francesco Redi approached the problem of the origin of life more rigorously and questioned the theory of spontaneous generation. Redi established that the small white worms that appear on rotting meat are fly larvae. After conducting a series of experiments, he received data confirming the idea that life can only arise from a previous life (the concept of biogenesis).
These experiments, however, did not lead to the rejection of the idea of spontaneous generation, and although this idea somewhat faded into the background, it continued to be the main version of the origin of life.
While Redi's experiments seemed to disprove the spontaneous generation of flies, the early microscopic studies of Antonie van Leeuwenhoek strengthened this theory in the case of microorganisms. Leeuwenhoek himself did not enter into disputes between supporters of biogenesis and spontaneous generation, but his observations under the microscope provided food for both theories.
In 1860, the French chemist Louis Pasteur took up the problem of the origin of life. Through his experiments, he proved that bacteria are ubiquitous and that non-living materials can easily be contaminated by living things if they are not properly sterilized. The scientist boiled various media in water in which microorganisms could form. Additional boiling killed the microorganisms and their spores. Pasteur attached a sealed flask with a free end to the S-shaped tube. Spores of microorganisms settled on a curved tube and could not penetrate into the nutrient medium. A well-boiled nutrient medium remained sterile; no life was found in it, despite the fact that air access was provided.
As a result of a series of experiments, Pasteur proved the validity of the theory of biogenesis and finally refuted the theory of spontaneous generation.
Steady State Theory
According to the steady state theory, the Earth never came into being, but existed forever; it has always been capable of sustaining life, and if it has changed, it has changed very little. According to this version, species also never arose, they always existed, and each species has only two possibilities - either a change in numbers or extinction.
However, the hypothesis of a stationary state fundamentally contradicts the data of modern astronomy, which indicate the finite time of existence of any stars and, accordingly, planetary systems around stars. According to modern estimates based on radioactive decay rates, 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 do not recognize that the presence or absence of certain fossil remains may indicate the time of appearance or extinction of a particular species, and cite as an example a representative of the lobe-finned fish - coelacanth (coelacanth). According to paleontological data, the crossopterans became extinct at the end of the Cretaceous. However, this conclusion had to be revised when living representatives of the crossopterygians were found in the Madagascar region. Proponents of the steady state theory argue that only by studying the living species and comparing them with fossil remains, one can conclude about extinction, and in this case it is very likely that it will turn out to be wrong. Using paleontological data to support the steady state theory, its proponents interpret the appearance of fossils in an ecological sense. Thus, for example, the sudden appearance of a fossil species in a particular stratum is explained by an increase in its population or its movement to places favorable for the preservation of remains.
creationism
Creationism (from the English. creation- creation) - a religious and philosophical concept, within which the entire diversity of the organic world, humanity, planet Earth, as well as the world as a whole, are considered as deliberately created by some supreme being or deity. Theory creationism, referring the answer to the question of the origin of life to religion (the creation of life by God), according to Popper's criterion, it is outside the field of scientific research (since it is irrefutable: it is impossible to prove by scientific methods both that God did not create life, and that God created it). In addition, this theory does not give a satisfactory answer to the question of the causes of the emergence and existence of the supreme being itself, usually simply postulating its beginninglessness.
Evolution theory
Until now, in the scientific and general educational environment, the theory of evolution was considered the main theory of the origin of life on Earth in all its diversity. This theory arose from the work of the Darwin family: the physician, naturalist and poet Erasmus Darwin (1731-1802), who proposed the theory of evolution in the 1790s, and, especially, his grandson naturalist Charles Darwin (1809-1882), who published in 1859 his now famous book On the Origin of Species by Means of Natural Selection, or the Preservation of Favored Breeds in the Struggle for Life.
The theory of evolution, which is often referred to as Darwinian theory or Darwinism, did not arise out of nowhere. By the time of Darwin, the cosmological theory of Immanuel Kant had become generally recognized, with its infinite in space and time universe, subject to the laws of mechanics described by Isaac Newton. In addition, the English scientist Charles Lyell (1797-1875) confirmed the theory of the so-called uniformitarianism proposed by the 18th century scientist Jason Hutton (1726-1797), according to which the Earth was formed over millions of years as a result of slow and gradual processes that continue today. Lyell substantiated this conclusion in 3 volumes of "Fundamentals of Geology", published in 1830-1833.
Thus, the foundation of the theory of evolution was created, on which Charles Darwin created the harmonious building of his theory by publishing books: "The Origin of Species", "Change of Domestic Animals and Cultivated Plants", "The Origin of Man and Sexual Selection" and others.
Darwinian theory
According to Darwin, evolution, i.e. the history of the development of the organic world of the Earth, is carried out as a result of the interaction of three main factors: variability, heredity and natural selection. Due to these factors, organisms in the process of development accumulate more and more new adaptive traits, which ultimately leads to the formation of new species.
In support of Darwin's theory, 2 arguments were immediately proposed: vestigial organs and the theory of embryonic recapitulation.
Thus, a list was compiled of 180 human rudiments - organs that lost their purpose in the process of its development from lower forms, i.e. organs that a person no longer needs and can be removed. However, as they studied these rudiments (for example, the appendix), scientists crossed out organ after organ from the list, until they crossed everything out. After 100 years, physiologists do not consider any of the human organs useless.
Pretty soon, the theory of embryonic recapitulation, proposed in 1868 by the German zoologist Ernst Haeckel, a supporter and propagandist of Darwin's teachings, gave a long life. This theory is based on the apparent similarity of human and dog embryos at the age of 4 weeks, as well as the presence of the so-called "gill slits" and "tail" in the human embryo.
In fact, it turned out that Haeckel forged the illustrations (retouched them), for which the academic council of the University of Jena found Haeckel guilty of scientific fraud, and his theory was untenable. But in the USSR, almost before its collapse, textbooks stubbornly cited pictures of embryos, supposedly confirming the theory of recapitulation, long rejected by embryologists in the rest of the world.
Conclusion
Many of these "theories" and the explanations they offer for existing species diversity use the same data but emphasize different aspects of it. Scientific theories can be super-fantastic on the one hand, and super-skeptical on the other. Theological considerations can also find a place within this framework, depending on the religious views of their authors. One of the main points of contention, even in pre-Darwinian times, was the question of the relationship between scientific and theological views on the history of life.
(9) life on the Earth in the historical past as a result ... a huge number of different hypotheses and theories about the cause of life on the Earth none of them...
The origin of life on Earth is a key and unresolved problem of natural science, often serving as a ground for a clash between science and religion. If the existence of the evolution of living matter in nature can be considered proven, since its mechanisms were discovered, archaeologists discovered ancient, more simply arranged organisms, then no 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 been able to create a living thing from an inanimate one.
Despite the large number of hypotheses about the origin of life, only one of them has an acceptable scientific explanation. It's a hypothesis abiogenesis- a long chemical evolution that took place in the special conditions of the ancient Earth and preceded biological evolution. At the same time, simple organic substances were first synthesized from inorganic substances, of which more complex ones, then biopolymers appeared, the following stages are more speculative and hardly proven. The hypothesis of abiogenesis has many unresolved problems, different views on certain stages of chemical evolution. However, some of its points were confirmed empirically.
Other hypotheses for the origin of life - panspermia(introduction of 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 the inanimate was proved by Louis Pasteur (XIX century) and a number of scientists before him, but not so categorically (F. Redi - XVII 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 out in the layers of the atmosphere, be subjected to the destructive action of cosmic radiation, etc.), but by intelligent beings. But how did they get to Earth? From the point of view of physics (the huge size of the Universe and the inability to overcome 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 be preceded by the abiogenic formation of organic compounds was expressed by Darwin. The theory of abiogenesis has been finalized and is being finalized 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, distinguishing three stages in abiogenesis: 1) the abiogenic occurrence of biological monomers; 2) formation of biopolymers; 3) the formation of membranes and the formation of primary organisms (protobionts).
Abiogenesis
The hypothetical scenario of 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. Liquid water on the planet, so necessary for life, according to scientists, appeared no earlier than 4 billion years ago. At the same time, 3.5 billion years ago, life on Earth already existed, which is proved by the discovery of rocks of such ages with traces of the vital activity of microorganisms. Thus, the first simple organisms arose relatively quickly - in less than 500 million years.
When the Earth first formed, its temperature could reach 8000 °C. When the planet cooled, metals and carbon, as the heaviest elements, condensed and formed the earth's crust. At the same time, volcanic activity was taking place, the crust was moving and contracting, folds and ruptures formed on it. Gravitational forces led to the compaction of the crust, while energy was released in the form of heat.
Light gases (hydrogen, helium, nitrogen, oxygen, etc.) were not retained by the planet and escaped 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 repeated many times, there were heavy showers with thunderstorms. Hot lava and volcanic ash, once in the water, created different environmental conditions. In some, certain reactions could take place.
Thus, the physical and chemical conditions on the early Earth were favorable for the formation of organic substances from inorganic ones. The atmosphere was of a reducing type, there was no free oxygen and no ozone layer. Therefore, ultraviolet and cosmic radiation penetrated the Earth. Other sources of energy were the warmth of the earth's crust, which has not yet cooled down, erupting volcanoes, thunderstorms, radioactive decay.
Methane, carbon oxides, ammonia, hydrogen sulfide, cyanide compounds, and water vapor were present in the atmosphere. A number of the simplest organic substances were synthesized from them. Further, 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 reactions.
Chemical experiments (for the first time in 1953 by S. Miller and G. Urey), 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 imitated 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, contained in volcanic dust. Moreover, it could be quite large amounts of organic matter.
Low molecular weight organic compounds accumulated in the ocean, creating the so-called primordial soup. 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. Experience has proven the possibility of synthesizing a polypeptide from amino acids in contact with pieces of hot lava.
The next important step towards the origin of life is the formation of coacervate drops in water ( coacervates) from polypeptides, polynucleotides, other organic compounds. Such complexes could have a layer on the outside that imitated a membrane and preserved their stability. Coacervates were obtained experimentally in colloidal solutions.
Protein molecules are amphoteric. They attract water molecules to themselves so that a shell forms around them. Colloidal hydrophilic complexes are obtained, isolated from the water mass. As a result, an emulsion is formed in water. Further, the colloids merge with each other and form coacervates (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 of different chemical composition were formed. Some of them were more stable and could, to a certain extent, carry out selective metabolism with the environment. There was a kind of biochemical natural selection.
Coacervates are able to selectively absorb certain substances from the environment and release into it some products of chemical reactions occurring in them. It's like metabolism. As the substances accumulated, the coacervates grew, and when they reached a critical size, they broke up into parts, each of which retained the features of the original organization.
In the coacervates themselves, chemical reactions could take place. During the absorption of metal ions by coacervates, enzymes could be formed.
In the process of evolution, only such 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 successful coacervates.
The membrane could be formed in the following way. Fatty acids combine with alcohols to form lipids. Lipids formed films on the surface of water bodies. Their charged heads face into the water, while the non-polar ends face out. Protein molecules floating in water were attracted to the heads of lipids, resulting in the formation of double lipoprotein films. From the wind, such a film could bend, and bubbles formed. Coacervates may have been accidentally trapped in these vesicles. When such complexes again appeared on the water surface, they were already covered with a second lipoprotein layer (due to hydrophobic interactions of the nonpolar ends of 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 became more complex due to the inclusion of proteins immersed in the lipid layer and penetrating it, protrusion and protrusion of individual sections of the membrane, etc.
Coacervates (or protobionts) could get already existing nucleic acid molecules capable of self-reproduction. Further, in some protobionts, such a rearrangement could occur that the nucleic acid began to encode the 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 play the role of enzymes), membranes and their selective permeability (which makes the protobiont a stable set of polymers), the emergence of matrix synthesis (transfer of information from nucleic acid to nucleic acid and from nucleic acid to protein).
| Evolution | results | |
|---|---|---|
| 1 | Chemical evolution - synthesis of compounds |
|
| 2 | Prebiological evolution - chemical selection: the most stable, self-reproducing protobionts remain |
|
| 3 | Biological evolution - biological selection: the struggle for existence, the survival of the most adapted to environmental conditions |
|
One of the biggest mysteries about the origin of life is how RNA came to code for the amino acid sequence of proteins. The question refers to 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, emerging from RNA by reverse transcription. DNA is better at storing information and is more stable (less prone to reactions). Therefore, in the process of evolution, it was she who was left as the custodian of information.
In 1982, T. Chek 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 themselves. Therefore, it can be assumed that RNAs were the first biopolymers (the RNA world hypothesis). Some sections of RNA could accidentally encode peptides useful for the protobiont, while other sections of RNA became excised introns in the course of evolution.
A feedback appeared 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.
Some protobionts gave rise to primitive cells, which include the totality of the properties of living things that we observe today. They implemented the storage and transmission of hereditary information, its use to create 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 ate at the expense of organic substances of the primary broth.
But the reserves of organic molecules were gradually depleted, as the conditions on the Earth changed, and the new organics were almost no longer synthesized abiogenically. Under conditions of competition for food resources, the evolution of heterotrophs accelerated.
The advantage was gained by bacteria, which turned out to be able to fix carbon dioxide with the formation of organic substances. Autotrophic synthesis of nutrients is more complex than heterotrophic nutrition, so it could not have arisen in early life forms. From some substances, under the influence of the energy of solar radiation, 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 the current 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 led to the almost impossibility of the abiogenic synthesis of complex organic substances. On the other hand, emerging life forms have become more resilient under such conditions.
Oxygen respiration spread on Earth. Anaerobic organisms have survived only in a few places (for example, there are anaerobic bacteria living in hot underground springs).
Life on Earth began three billion years ago. Since then, evolution has transformed elementary single-celled organisms into the variety of shapes, colors, sizes, and functions that we see today. But how exactly did life originate in the primordial soup - water contained in shallow springs and saturated with amino acids and nucleotides?
There are many theoretical answers to the question of what exactly caused the emergence of life, from a lightning strike to a cosmic body. Here are just a few of them.
spark of electricity
That same metaphorical spark of life could be a completely literal spark or a lot of sparks, the source of which was lightning. Electric sparks falling into the water could cause the formation of amino acids and glucose, converting them from an atmosphere rich in methane, water, hydrogen and ammonia. This theory was even confirmed experimentally in 1953, proving that lightning could very well be the cause of the formation of the basic elements necessary for the emergence of the first forms of life.
After conducting the experiment, scientists were able to prove that the early atmosphere of our planet could not contain enough hydrogen, but the volcanic clouds covering the surface of the Earth could include all the necessary elements and, accordingly, enough electrons to cause lightning.
Underwater hydrothermal vents
Relatively strong deep-water vents could become a necessary source of hydrogen for the formation of the first living organisms on their rocky surfaces. Even today, diverse ecosystems develop around hydrothermal vents, even at great depths.
Clay
The first organic molecules could meet on a clay surface. Clay always contains a sufficient amount of organic components, in addition, it could become a kind of organizer of these components into more complex and efficient DNA-like structures.
In fact, DNA is a kind of map for amino acids, indicating exactly how they should be organized in complex fat cells. A group of biologists from the University of Glasgow in Scotland argue that clay could be such a map for the simplest polymers and fats, as long as they do not learn to "self-organize".
Panspermia
This theory raises questions about the possibility of a cosmic origin of life. That is, according to its postulates, life did not originate on Earth, but was only brought here with the help of a meteorite, for example, from Mars. Enough fragments were found on the ground, which supposedly came to us from the red planet. Another way to "space taxi" for unknown life forms are comets, which are able to travel between star systems.
Even if this is true, panspermia is still not able to answer the question of how exactly life originated from where it was brought to planet Earth.
Under the ice
It is quite possible that the oceans and continents were covered with a thick layer of ice three billion years ago, because the Sun did not shine as brightly as it does today. Ice could become a protective layer for fragile organic molecules, preventing ultraviolet rays and cosmic bodies colliding with the surface from harming the first and weakest life forms. In addition, a lower temperature could cause the evolution of the first molecules into stronger and more durable ones.
RNA World
The RNA world theory is based on the philosophical question of the egg and the chicken. The fact is that for the formation (doubling) of DNA, proteins are needed, and proteins cannot reproduce themselves without the very map embedded in DNA. So how did life come into existence if one cannot come into existence without the other, but both exist beautifully in the present? The answer may be RNA - ribonucleic acid, which is able to store information like DNA and serve as protein enzymes. On the basis of RNA, a more perfect DNA was formed, then more efficient proteins completely replaced RNA.
Today, RNA exists and performs several functions in complex organisms, for example, it is responsible for the operation of some genes. This theory is quite logical, but it does not answer the question of what served as a catalyst for the formation of ribonucleic acid itself. The assumption that it could have appeared by itself is rejected by most scientists. The theoretical explanation is the formation of the simplest acids PNA and TNA, which then developed into RNA.
The simplest start
This theory is called holobiosis and comes from the idea that life did not begin from complex RNA molecules and the primary genetic code, but from the simplest particles interacting with each other for the sake of metabolism. Perhaps these particles eventually developed a protective shell, like a membrane, and then evolved into one, more complex, organism. This model is called the "enzyme model of metabolism", while the theory of the RNA world is called the "primary genetic code model".
How did life originate on Earth? The details are unknown to mankind, but the cornerstone principles have been established. There are two main theories and many minor ones. So, according to the main version, the organic components came to Earth from outer space, according to another, everything happened on Earth. Here are some of the most popular teachings.
Panspermia
How did our Earth come about? The biography of the planet is unique, and people are trying to unravel it in different ways. There is a hypothesis that the life that exists in the universe is distributed with the help of meteoroids (celestial bodies intermediate in size between interplanetary dust and an asteroid), asteroids and planets. It is assumed that there are life forms that can withstand exposure (radiation, vacuum, low temperatures, etc.). They are called extremophiles (including bacteria and microorganisms).
They get into debris and dust, which are thrown into space after saving, thus, life after the death of small bodies of the solar system. Bacteria can travel at rest for long periods of time before another random collision with other planets.
They can also mix with protoplanetary disks (dense gas cloud around a young planet). If in a new place "persistent but sleepy soldiers" fall into favorable conditions, they become active. The process of evolution begins. History is unraveled with the help of probes. Data from instruments that have been inside comets indicate that in the vast majority of cases, the likelihood is confirmed that we are all "a little alien", since the cradle of life is space.
Biopoiesis
And here is another opinion on how life originated. On Earth there is living and non-living. Some sciences welcome abiogenesis (biopoesis), which explains how, in the course of natural transformation, biological life emerged from inorganic matter. Most amino acids (also called the building blocks of all living organisms) can be formed using natural chemical reactions that are not related to life.
This is confirmed by the Muller-Urey experiment. In 1953, a scientist ran electricity through a mixture of gases and produced several amino acids in laboratory conditions that mimic those of the early Earth. In all living beings, amino acids are transformed into proteins under the influence of nucleic acids, the genetic memory custodians.
The latter are synthesized independently by biochemical means, and proteins accelerate (catalyze) the process. Which of the organic molecules is the first? And how did they interact? Abiogenesis is in the process of finding an answer.
Cosmogonic trends
This is the doctrine of space. In a certain context of space science and astronomy, the term refers to the theory of creation (and study) of the solar system. Attempts to gravitate toward naturalistic cosmogony do not stand up to scrutiny. First, the existing scientific theories cannot explain the main thing: how did the Universe itself appear?
Secondly, there is no physical model that explains the earliest moments of the existence of the universe. In the mentioned theory, there is no concept of quantum gravity. Although string theorists say that elementary particles arise from the vibrations and interactions of quantum strings), those who study the origin and consequences of the Big Bang (loop quantum cosmology) do not agree with this. They believe they have formulas to describe the model in terms of field equations.
With the help of cosmogonic hypotheses, people explained the uniformity of the movement and composition of celestial bodies. Long before life appeared on Earth, matter filled all space and then evolved.
Endosymbiont
The endosymbiotic version was first formulated by the Russian botanist Konstantin Merezhkovsky in 1905. He believed that some organelles originated as free-living bacteria and were taken into another cell as endosymbionts. Mitochondria evolved from proteobacteria (specifically Rickettsiales or close relatives) and chloroplasts from cyanobacteria.
This suggests that multiple forms of bacteria entered into symbiosis with the formation of a eukaryotic cell (eukaryotes are cells of living organisms containing a nucleus). The horizontal transfer of genetic material between bacteria is also facilitated by symbiotic relationships.
The emergence of a variety of life forms may have been preceded by the Last Common Ancestor (LUA) of modern organisms.
Spontaneous birth
Until the early 19th century, people generally dismissed "suddenness" as an explanation for how life began on Earth. The unexpected spontaneous generation of certain forms of life from inanimate matter seemed implausible to them. But they believed in the existence of heterogenesis (a change in the method of reproduction), when one of the forms of life comes from another species (for example, bees from flowers). Classical ideas about spontaneous generation boil down to the following: some complex living organisms appeared due to the decomposition of organic substances.
According to Aristotle, this was an easily observable truth: aphids arise from dew that falls on plants; flies - from spoiled food, mice - from dirty hay, crocodiles - from rotting logs at the bottom of reservoirs, and so on. The theory of spontaneous generation (refuted by Christianity) secretly existed for centuries.
It is generally accepted that the theory was finally refuted in the 19th century by the experiments of Louis Pasteur. The scientist did not study the origin of life, he studied the appearance of microbes in order to be able to fight infectious diseases. However, Pasteur's evidence was no longer controversial, but strictly scientific.
Clay Theory and Sequential Creation
The emergence of life on the basis of clay? Is that possible? A Scottish chemist named A.J. Kearns-Smith from the University of Glasgow in 1985 is the author of such a theory. Based on similar assumptions by other scientists, he argued that organic particles, being between the layers of clay and interacting with them, adopted the way of storing information and growing. Thus, the scientist considered the “clay gene” to be primary. Initially, the mineral and the nascent life existed together, but at a certain stage they “ran up”.
The idea of destruction (chaos) in the emerging world paved the way for the theory of catastrophism as one of the forerunners of the theory of evolution. Its proponents believe that the Earth has been affected by sudden, short-lived, turbulent events in the past, and that the present is the key to the past. Each next catastrophe destroyed the existing life. The subsequent creation revived it already different from the previous one.
materialistic doctrine
And here is another version of how life originated on Earth. It was put forward by the materialists. They believe that life appeared as a result of gradual chemical transformations extended in time and space, which, in all likelihood, took place almost 3.8 billion years ago. This development is called molecular, it affects the area of deoxyribonucleic and ribonucleic acids and proteins (proteins).
As a scientific trend, the doctrine arose in the 1960s, when active research was carried out affecting molecular and evolutionary biology, population genetics. Scientists then tried to understand and validate recent discoveries regarding nucleic acids and proteins.
One of the key topics that stimulated the development of this field of knowledge was the evolution of the enzymatic function, the use of nucleic acid divergence as a "molecular clock". Its disclosure contributed to a deeper study of the divergence (branching) of species.
organic origin
About how life appeared on Earth, supporters of this doctrine argue as follows. The formation of species began a long time ago - more than 3.5 billion years ago (the number indicates the period in which life exists). Probably, at first there was a slow and gradual process of transformation, and then a fast (within the Universe) stage of improvement began, a transition from one static state to another under the influence of existing conditions.
Evolution, known as biological or organic, is the process of changing over time one or more inherited traits found in populations of organisms. Hereditary traits are special distinguishing features, including anatomical, biochemical and behavioral, that are transmitted from one generation to another.
Evolution has led to the diversity and diversification of all living organisms (diversification). Our colorful world was described by Charles Darwin as "endless forms, the most beautiful and the most wonderful." One gets the impression that the origin of life is a story without beginning or end.
special creation
According to this theory, all forms of life that exist today on planet Earth are created by God. Adam and Eve are the first man and woman created by the Almighty. Life on Earth began with them, believe Christians, Muslims and Jews. Three religions agreed that God created the universe within seven days, making the sixth day the culmination of labor: he created Adam from the dust of the earth and Eve from his rib.
On the seventh day God rested. Then he breathed in and sent to look after the garden called Eden. In the center grew the Tree of Life and the Tree of the Knowledge of Good. God allowed the fruits of all the trees in the garden to be eaten, except for the Tree of Knowledge (“for on the day that you eat them you will die”).
But the people disobeyed. The Qur'an says that Adam offered to taste the apple. God forgave sinners and sent both of them to earth as his representatives. And yet... Where did life come from on Earth? As you can see, there is no single answer. Although modern scientists are increasingly inclined towards the abiogenic (inorganic) theory of the origin of all living things.
It's no secret that the eternal question of when life originated on Earth has always worried not only scientists, but all people. In this article, we will try to superficially get acquainted with all the alleged theories of the origin of all life on our planet. We will try to sort through the stages of its development and describe what the history of the development of life on Earth was like.
The origin of life on Earth in science
From a scientific point of view, there are several versions of the origin of life. Consider how life appeared on Earth according to scientists who have been struggling with this mysterious question for many centuries, putting forward new hypotheses.
- The theory says that life originated in a piece of ice. Pretty ridiculous idea, but anything is possible. Some scientists believe that the carbon dioxide present in the air supplied the maintenance of greenhouse conditions, others believe that there was a constant winter season on earth at that time.
- The science that studies the origin of life on Earth is biology. She adheres to the theory of Charles Darwin. He and his contemporaries believed that life began to form in a reservoir. This theory is followed by most scientists today. Organic substances delivered by the waters flowing into it could accumulate in the required quantities in a closed and rather shallow reservoir. Further, these compounds were even more concentrated on the inner surfaces of layered minerals. They could be catalysts for reactions.
- Water is the source of life on Earth for all living beings on Earth - humans, flora and fauna. It is an extremely important and expensive resource on our planet. All the waters of the earth are in continuous relationship with the rocks and the atmosphere. Water is self-purifying thanks to the continuous flow that supplies existence on our earth. The ancient and universal symbol of the fertility of purity is water. Man consists of 80% water, animals 75% and plants 89-90% of the total body weight. Water is an indispensable product, since it is the main building material for the human body. It is much more valuable than iron, gas, coal and oil. Without water, life on earth would never have been able to originate, be maintained, and could not exist at all. Water is life itself.
- What if life appeared in zones of volcanic activity? Immediately after its formation, the Earth was a fire-breathing ball of magma. With gases released from molten magma, various chemicals needed for the synthesis of organic molecules were brought to the earth's surface - this happened during volcanic eruptions.
The origin of life on earth in religion
Consider how life on earth originated from the point of view of religion. Another hypothesis about the origin of life on earth finds an explanation in different religions. Consider Christian:
The main dogma of the creation of all living things in Christianity is the phrase "creation from nothing", in which God acts as the Creator in his volitional action. At the same time, the Lord also appears to be the root cause of being. At the same time, God was not obliged to create the world; for the Divine essence, it is not determined by any “internal need”. It was His free choice, a gift to mankind "from an excess of love." The path and stages of the creation of the world are described in the first three chapters of Genesis.
The main stages of life on Earth
One can talk endlessly about the history of the development of life on earth. This topic is quite extensive and immense, we list only the main stages of the origin of life:
- Life originated in the seas.
- The existence of the simplest marine organisms.
- Multicellular living creatures arise in the seas
- Numerous invertebrates appear in the seas. Among the invertebrates we find the ancestors of modern mollusks and arthropods.
- The first marine vertebrate armored, modern fish are born. Life develops on emerging land areas. The first settlers are: fungi, bacteria, mosses and small invertebrates, followed by amphibians.
- The earth is covered with powerful forests of ferns and other plants that have disappeared by our time. Insects appear.
- The origin of reptiles.
- The era of reptiles, animals also spread in the seas. Some species reach considerable sizes.
- Mammals and birds appear. The first flowering plants spread. The first angiosperms appear.
- Dinosaurs and other large reptiles are dying out.
- Mammals are spreading throughout the earth, displacing reptiles, whose numbers are rapidly declining.
- Various types of mammals are born: carnivores, bats, and the ancestors of today's monkeys and humans. Herbivores are born.
- Individual mammals inhabit the seas. For example: whales.
- There is a progenitor of man - Australopithecus.
- Individual large mammals are disappearing. Man becomes the absolute owner of the Earth.
Now you know what the Earth looked like in ancient times. Life without people was very different.
