Modern theories about the origin of life on earth. Theories and hypotheses of the origin of life on earth
Philosophers and historians, biologists and chemists have been thinking about how life arose on our planet for centuries and even millennia, but there is still no consensus on this issue, so modern society There are several theories, all of which have a right to exist.
Spontaneous generation of life
This theory was formed in ancient times. In its context, scientists argue that living beings arose from inanimate matter. Many experiments were carried out to confirm or refute this theory. Thus, L. Pasteur received a prize for his experiment of boiling broth in a flask, as a result of which it was proven that all living organisms can only originate from living matter. However, there is new question: Where did the organisms from which life originate on our planet come from?
Creationism
This theory assumes that all life on Earth was created almost simultaneously by some higher being with superpowers, be it a deity, the Absolute, a superintelligence or a cosmic civilization. This hypothesis has been relevant since ancient times, and it is the basis of all world religions. It has not yet been refuted because scientists have not been able to find a reasonable explanation and confirmation of all complex processes and phenomena occurring on the planet.
Stationary state and panspermia
These two hypotheses allow us to present a general vision of the world in such a way that outer space exists constantly, that is, eternity (stationary state), and there is life in it, which periodically moves from one planet to another. Life forms travel with the help of meteorites (panspermia hypothesis). Acceptance of this theory is impossible, since astrophysicists believe that the universe appeared about 16 billion years ago due to a primordial explosion.
Biochemical evolution
This theory is most relevant in modern science and is considered accepted in the scientific community in many countries around the world. It was formed by A.I. Oparin, Soviet biochemist. According to this hypothesis, the origin and complication of life forms occurs due to chemical evolution, due to which the elements of all living things interact. First, the Earth was formed as a cosmic body, then atmospheres appeared, and the synthesis of organic molecules and substances took place. After this, over the course of millions and billions of years, various living beings appear. This theory is confirmed by a number of experiments, however, in addition to it, there are a number of other hypotheses that should be taken into account.
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 we see today. But how exactly did life arise 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 very metaphorical spark of life could be a completely literal spark or many sparks, the source of which was lightning. Electrical sparks entering 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 well have been 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 a sufficient amount of 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-sea vents could have become a necessary source of hydrogen for the formation of the first living organisms on their rocky surfaces. Even today, a variety of ecosystems develop around hydrothermal vents, even at great depths.
Clay
The first organic molecules could have been found 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 effective structures, similar to DNA.
In fact, DNA is a kind of map for amino acids, indicating exactly how they should be organized in the cells of complex fats. A group of biologists from the University of Glasgow in Scotland argue that clay could provide such a map for the simplest polymers and fats until they learned to “self-organize.”
Panspermia
This theory makes us think about the possibility of the cosmic origin of life. That is, according to its postulates, life did not originate on Earth, but was simply brought here with the help of a meteorite, for example, from Mars. Enough fragments have been found on earth that presumably came to us from the red planet. Another way of “space taxi” for unknown life forms is comets, which are capable of traveling between star systems.
Even if this is true, panspermia is still unable to answer the question of how exactly life originated where it was brought to planet Earth.
Under the ice cover
It is possible that the oceans and continents three billion years ago were covered with a thick layer of ice, because the Sun did not shine as brightly as it does today. Ice could become protective layer for fragile organic molecules, preventing ultraviolet rays and cosmic bodies colliding with the surface from harming the first and weakest forms of life. In addition, lower temperatures could have caused the first molecules to evolve into stronger and longer lasting 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 self-reproduce without the very map embedded in the DNA. So how did life arise if one cannot appear without the other, but both exist perfectly in the present? The answer may be RNA, a ribonucleic acid that can store information like DNA and serve as protein enzymes. Based on RNA, 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 functioning of certain genes. This theory is quite logical, but it does not answer the question of what served as the 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 beginning
This theory is called holobiosis and comes from the idea that life began not from complex RNA molecules and primary genetic code, but from simple particles interacting with each other for the sake of metabolism. Perhaps these particles developed over time containment, like a membrane, and then evolved into one, more complex, organism. This model is called the "enzyme model of metabolism", while the RNA world theory is called the "primary genetic code model".
Municipal educational institution
Secondary school No. 45
Theories of the origin of life on Earth
Completed : student of 11th grade "B"
Nigmatullina Maria
Proveila : biology teacher
Trapueva L. S.
Chelyabinsk
2010
1. Introduction
2. Hypotheses about the origin of life
3. Genobiosis and holobiosis
4. Oparin–Haldane theory
5. The RNA world as a precursor to modern life
6. Panspermia
7. Spontaneous generation of life
8. Steady State Theory
9. Creationism
10. Theory of evolution
11. Darwinian theory
12. Conclusion
Introduction
Theories concerning the origin of the Earth and life on it, and indeed the entire Universe, are varied 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 different times, the following hypotheses were put forward regarding the origin of life on Earth:
- Biochemical evolution hypothesis
- Panspermia hypothesis
- Stationary State of Life Hypothesis
- Spontaneous generation hypothesis
Theories spontaneous generation And steady state are of historical or philosophical interest only, since the results scientific research contradict the conclusions of these theories.
Theory panspermia does not solve the fundamental question of the origin of life, it only pushes it into the even more nebulous 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 question of the origin of life, based on the belief in the primacy of a molecular system with the properties of a primary genetic code.
Holobiosis- a methodological approach to the question of the origin of life, based on the idea of the primacy of structures endowed with the ability to elementary metabolism with the participation of an enzymatic mechanism.
Oparin–Haldane theory
In 1924, the future academician Oparin published an 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 high molecular weight compounds they can spontaneously zones of increased concentration are formed, which are relatively separated from the external environment and can maintain exchange with it. He called them Coacervate drops, or just 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 substances
- 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 were established from the moment the primary ocean appeared. In the aquatic environment, hydrocarbon derivatives could undergo complex chemical changes and transformations. As a result of this complication of molecules, more complex organic substances could be formed, namely carbohydrates.
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 bio chemicals. According to Oparin's theory, a further step towards the emergence of protein bodies could be the formation of coacervate droplets. At certain conditions the water 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 simply mixing different polymers. In this case, self-assembly of polymer molecules into multimolecular formations occurred - droplets visible under an optical microscope.
The drops were capable of absorbing substances from outside like open systems. When various catalysts (including enzymes) were included in coacervate droplets, various reactions occurred in them, in particular the polymerization of monomers coming from the external environment. Due to this, the drops could increase in volume and weight, and then split into daughter formations. Thus, coacervates could grow, multiply, and carry out metabolism.
British biologist John Haldane also expressed similar views.
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 electrical discharges through it. It turned out that amino acids are formed. Later in different conditions other sugars and nucleotides were obtained. He concluded that evolution can occur in a phase-separated state from solution (coacervates). However, such a system cannot reproduce itself.
The theory was justified, except for one problem, to which almost all experts in the field of the origin of life had long turned a blind eye. If spontaneously, through random template-free syntheses, single successful designs of protein molecules arose in the coacervate (for example, effective catalysts that provide an advantage for a given coacervate in growth and reproduction), then how could they be copied for distribution within the coacervate, and even more so for transmission to descendant coacervates? The theory turned out to be unable to offer a solution to the problem of exact reproduction - within a coacervate and in generations - of single, randomly appearing effective protein structures. However, it was shown that the first coacervates could be formed spontaneously from lipids synthesized abiogenically, and they could enter into symbiosis with “living solutions” - colonies of self-replicating RNA molecules, among which were ribozymes that catalyze the synthesis of lipids, and such a community is already possible call it an organism.
Alexander Oparin (right) in the laboratory
The RNA World as a Precursor to Modern Life
By the 21st century, the Oparin-Haldane theory, which assumes the initial emergence 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 those 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 have been brought to Earth from space. Living organisms of extraterrestrial origin are most likely to enter 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 another option in 1973 - 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, which may have been facing a global catastrophe or simply hoped to terraform other planets for future colonization. They gave two main arguments in favor of their theory - 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 throughout 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 question 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 sets out the hypothesis of controlled panspermia in more detail than in the article and in a popular form.
The question of when life appeared on Earth has always worried not only scientists, but also all people. Answers to it
almost all religions. Although there is still no exact scientific answer to this question, some facts allow us to make more or less reasonable hypotheses. Researchers found a rock sample in Greenland
with a tiny splash of carbon. The age of the sample is more than 3.8 billion years. The source of carbon was most likely some kind of organic matter - during this time it completely lost its structure. Scientists believe this lump of carbon may be the oldest trace of life on Earth.
What did the primitive Earth look like?
Let's fast forward to 4 billion years ago. The atmosphere does not contain free oxygen; it is found only in oxides. Almost no sounds except the whistle of the wind, the hiss of water erupting with lava and the impacts of meteorites on the surface of the Earth. No plants, no animals, no bacteria. Maybe this is what the Earth looked like when life appeared on it? Although this problem has long been of concern to many researchers, their opinions on this matter vary greatly. Rocks could indicate conditions on Earth at that time, but they were destroyed long ago as a result of geological processes and movements of the earth's crust.
In this article we will briefly talk about several hypotheses for the origin of life, reflecting modern scientific ideas. According to Stanley Miller, a well-known expert in the field of the origin of life, we can talk about the origin of life and the beginning of its evolution from the moment when organic molecules self-organized into structures that were able to reproduce themselves. But this raises other questions: how did these molecules arise; why they could reproduce themselves and assemble into those structures that gave rise to living organisms; what conditions are needed for this?
According to one hypothesis, life began in a piece of ice. Although many scientists believe that carbon dioxide present in the atmosphere ensured the maintenance greenhouse conditions, others believe that winter reigned on Earth. At low temperatures, all chemical compounds are more stable and can therefore accumulate in larger quantities than at high temperatures. Meteorite fragments brought from space, emissions from hydrothermal vents, and chemical reactions occurring during electrical discharges in the atmosphere were sources of ammonia and organic compounds such as formaldehyde and cyanide. Getting into the water of the World Ocean, they froze along with it. In the ice column, molecules of organic substances came close together and entered into interactions that led to the formation of glycine and other amino acids. The ocean was covered with ice, which protected the newly formed compounds from destruction by ultraviolet radiation. This icy world could melt, for example, if a huge meteorite fell on the planet (Fig. 1).
Charles Darwin and his contemporaries believed that life could have arisen in a body of water. Many scientists still adhere to this point of view. In a closed and relatively small reservoir, organic substances brought by the waters flowing into it could accumulate in the required quantities. These compounds were then further concentrated on the inner surfaces of layered minerals, which could catalyze the reactions. For example, two molecules of phosphaldehyde that met on the surface of a mineral reacted with each other to form a phosphorylated carbohydrate molecule, a possible precursor to ribonucleic acid (Fig. 2).
Or maybe life arose in areas of volcanic activity? Immediately after its formation, the Earth was a fire-breathing ball of magma. During volcanic eruptions and with gases released from molten magma, a variety of chemicals necessary for the synthesis of organic molecules were carried to the earth's surface. Thus, carbon monoxide molecules, once on the surface of the mineral pyrite, which has catalytic properties, could react with compounds that had methyl groups and form acetic acid, from which other organic compounds were then synthesized (Fig. 3).
For the first time, the American scientist Stanley Miller managed to obtain organic molecules - amino acids - in laboratory conditions simulating those that were on the primitive Earth in 1952. Then these experiments became a sensation, and their author gained worldwide fame. He currently continues to conduct research in the field of prebiotic (before life) chemistry at the University of California. The installation on which the first experiment was carried out was a system of flasks, in one of which it was possible to obtain a powerful electric discharge at a voltage of 100,000 V.
Miller filled this flask with natural gases - methane, hydrogen and ammonia, which were present in the atmosphere of the primitive Earth. The flask below contained a small amount of water, simulating the ocean. The electric discharge was close to lightning in strength, and Miller expected that under its action chemical compounds were formed, which, when they got into the water, would react with each other and form more complex molecules.
The result exceeded all expectations. Having turned off the installation in the evening and returning the next morning, Miller discovered that the water in the flask had acquired a yellowish color. What emerged was a soup of amino acids, the building blocks of proteins. Thus, this experiment showed how easily the primary ingredients of life could be formed. All that was needed was a mixture of gases, a small ocean and a little lightning.
Other scientists are inclined to believe that the ancient atmosphere of the Earth was different from the one that Miller modeled, and most likely consisted of carbon dioxide and nitrogen. Using this gas mixture and Miller's experimental setup, chemists attempted to produce organic compounds. However, their concentration in water was as insignificant as if a drop of food coloring were dissolved in a swimming pool. Naturally, it is difficult to imagine how life could arise in such a dilute solution.
If indeed the contribution of earthly processes to the creation of reserves of primary organic matter was so insignificant, where did it even come from? Maybe from space? Asteroids, comets, meteorites and even particles of interplanetary dust could carry organic compounds, including amino acids. These extraterrestrial objects could provide sufficient amounts of organic compounds for the origin of life to enter the primordial ocean or small body of water.
The sequence and time interval of events, starting from the formation of primary organic matter and ending with the appearance of life as such, remains and, probably, will forever remain a mystery that worries many researchers, as well as the question of what. in fact, consider it life.
Currently, there are several scientific definitions of life, but all of them are not accurate. Some of them are so wide that inanimate objects such as fire or mineral crystals fall under them. Others are too narrow, and according to them, mules that do not give birth to offspring are not recognized as living.
One of the most successful defines life as self-sustaining chemical system, capable of behaving in accordance with the laws of Darwinian evolution. This means that, firstly, a group of living individuals must produce descendants similar to themselves, which inherit the characteristics of their parents. Secondly, generations of descendants must show the consequences of mutations - genetic changes that are inherited by subsequent generations and cause population variability. And thirdly, it is necessary for a system of natural selection to operate, as a result of which some individuals gain an advantage over others and survive in changed conditions, producing offspring.
What elements of the system were necessary for it to have the characteristics of a living organism? Large number biochemists and molecular biologists believe that RNA molecules had the necessary properties. RNA - ribonucleic acids - are special molecules. Some of them can replicate, mutate, thus transmitting information, and, therefore, they could participate in natural selection. True, they are not capable of catalyzing the replication process themselves, although scientists hope that in the near future an RNA fragment with such a function will be found. Other RNA molecules are involved in “reading” genetic information and transferring it to ribosomes, where the synthesis of protein molecules occurs, in which the third type of RNA molecules takes part.
Thus, the most primitive living system could be represented by RNA molecules duplicating, undergoing mutations and being subject to natural selection. In the course of evolution, based on RNA, specialized DNA molecules arose - the custodians of genetic information - and no less specialized protein molecules, which took on the functions of catalysts for the synthesis of all currently known biological molecules.
At some point in time, a “living system” of DNA, RNA and protein found shelter inside a sac formed by a lipid membrane, and this one was more protected from external influences the structure served as the prototype for the very first cells that gave rise to the three main branches of life, which are represented in the modern world by bacteria, archaea and eukaryotes. As for the date and sequence of appearance of such primary cells, this remains a mystery. In addition, according to simple probabilistic estimates, there is not enough time for the evolutionary transition from organic molecules to the first organisms - the first simplest organisms appeared too suddenly.
For many years, scientists believed that it was unlikely that life could have arisen and developed during the period when the Earth was constantly subject to collisions with large comets and meteorites, a period that ended approximately 3.8 billion years ago. However, recently, traces of complex cellular structures dating back at least 3.86 billion years have been discovered in the oldest sedimentary rocks on Earth, found in southwestern Greenland. This means that the first forms of life could have arisen millions of years before the bombardment of our planet by large cosmic bodies stopped. But then a completely different scenario is possible (Fig. 4).
Space objects falling to Earth could have played a central role in the emergence of life on our planet, since, according to a number of researchers, cells similar to bacteria could have arisen on another planet and then reached Earth along with asteroids. One piece of evidence supporting the theory of extraterrestrial origins of life was found inside a meteorite shaped like a potato and named ALH84001. This meteorite was originally a piece of Martian crust, which was then thrown into space as a result of an explosion when a huge asteroid collided with the surface of Mars, which occurred about 16 million years ago. And 13 thousand years ago, after a long journey within the solar system, this fragment of Martian rock in the form of a meteorite landed in Antarctica, where it was recently discovered. A detailed study of the meteorite revealed rod-shaped structures resembling fossilized bacteria inside it, which gave rise to heated scientific debate about the possibility of life deep in the Martian crust. It will be possible to resolve these disputes no earlier than 2005, when the US National Aeronautics and Space Administration will implement a program to fly an interplanetary spacecraft to Mars to take samples of the Martian crust and deliver samples to Earth. And if scientists manage to prove that microorganisms once inhabited Mars, then we can speak with a greater degree of confidence about the extraterrestrial origin of life and the possibility of life being brought from outer space (Fig. 5).
Rice. 5. Our origin is from microbes.
What have we inherited from ancient life forms? The comparison below of single-celled organisms with human cells reveals many similarities.
 1. Sexual reproduction |
 2. Eyelashes |
 3. Capture other cells |
 4. Mitochondria |
 5. Flagella |
The evolution of life on Earth: from simple to complex
At present, and probably in the future, science will not be able to answer the question of what the very first organism that appeared on Earth looked like - the ancestor from which the three main branches of the tree of life originated. One of the branches is eukaryotes, whose cells have a formed nucleus containing genetic material and specialized organelles: energy-producing mitochondria, vacuoles, etc. Eukaryotic organisms include algae, fungi, plants, animals and humans.
The second branch is bacteria - prokaryotic (prenuclear) single-celled organisms that do not have a pronounced nucleus and organelles. And finally, the third branch is single-celled organisms called archaea, or archaebacteria, whose cells have the same structure as prokaryotes, but a completely different chemical structure of lipids.
Many archaebacteria are able to survive in extremely unfavorable environmental conditions. Some of them are thermophiles and live only in hot springs with temperatures of 90 ° C or even higher, where other organisms would simply die. Feeling great in such conditions, these single-celled organisms consume iron and sulfur-containing substances, as well as a number of chemical compounds, toxic to other life forms. According to scientists, the thermophilic archaebacteria found are extremely primitive organisms and, in evolutionary terms, close relatives of the most ancient forms of life on Earth.
It is interesting that modern representatives of all three branches of life, most similar to their ancestors, still live in places with high temperatures. Based on this, some scientists are inclined to believe that, most likely, life arose about 4 billion years ago on the ocean floor near hot springs, erupting streams rich in metals and high-energy substances. Interacting with each other and with the water of the then sterile ocean, entering into a wide variety of chemical reactions, these compounds gave rise to fundamentally new molecules. So, for tens of millions of years in this “ chemical kitchen“The greatest dish was being prepared – life. And about 4.5 billion years ago, single-celled organisms appeared on Earth, whose lonely existence continued throughout the Precambrian period.
The burst of evolution that gave rise to multicellular organisms occurred much later, a little over half a billion years ago. Although microorganisms are so small that a single drop of water can contain billions, the scale of their work is enormous.
It is believed that initially there was no free oxygen in the earth’s atmosphere and the oceans, and under these conditions only anaerobic microorganisms lived and developed. A special step in the evolution of living things was the emergence of photosynthetic bacteria, which, using light energy, converted carbon dioxide into carbohydrate compounds that served as food for other microorganisms. If the first photosynthetics produced methane or hydrogen sulfide, then the mutants that appeared once began to produce oxygen during photosynthesis. As oxygen accumulated in the atmosphere and waters, anaerobic bacteria, for which it is destructive, occupied oxygen-free niches.
Ancient fossils found in Australia dating back 3.46 billion years have revealed structures believed to be the remains of cyanobacteria, the first photosynthetic microorganisms. The former dominance of anaerobic microorganisms and cyanobacteria is evidenced by stromatolites found in shallow coastal waters of unpolluted salt water bodies. In shape they resemble large boulders and represent an interesting community of microorganisms living in the limestone or dolomite rocks formed as a result of their life activity. To a depth of several centimeters from the surface, stromatolites are saturated with microorganisms: photosynthetic cyanobacteria that produce oxygen live in the uppermost layer; deeper bacteria are found that are to a certain extent tolerant of oxygen and do not require light; in the lower layer there are bacteria that can only live in the absence of oxygen. Located in different layers, these microorganisms form a system united by complex relationships between them, including food relationships. Behind the microbial film is a rock formed as a result of the interaction of the remains of dead microorganisms with calcium carbonate dissolved in water. Scientists believe that when there were no continents on the primitive Earth and only archipelagos of volcanoes rose above the surface of the ocean, the shallow waters were replete with stromatolites.
As a result of the activity of photosynthetic cyanobacteria, oxygen appeared in the ocean, and approximately 1 billion years after that, it began to accumulate in the atmosphere. First, the resulting oxygen interacted with iron dissolved in water, which led to the appearance of iron oxides, which gradually precipitated at the bottom. Thus, over millions of years, with the participation of microorganisms, huge deposits of iron ore arose, from which steel is smelted today.
Then, when the bulk of the iron in the oceans was oxidized and could no longer bind oxygen, it escaped into the atmosphere in gaseous form.
After photosynthetic cyanobacteria created a certain supply of energy-rich organic matter from carbon dioxide and enriched the earth's atmosphere with oxygen, new bacteria arose - aerobes, which can exist only in the presence of oxygen. They need oxygen for the oxidation (combustion) of organic compounds, and a significant part of the resulting energy is converted into a biologically available form - adenosine triphosphate (ATP). This process is energetically very favorable: anaerobic bacteria, when decomposing one molecule of glucose, receive only 2 molecules of ATP, and aerobic bacteria that use oxygen receive 36 molecules of ATP.
With the advent of oxygen sufficient for an aerobic lifestyle, eukaryotic cells also made their debut, which, unlike bacteria, have a nucleus and organelles such as mitochondria, lysosomes, and in algae and higher plants - chloroplasts, where photosynthetic reactions take place. There is an interesting and well-founded hypothesis regarding the emergence and development of eukaryotes, expressed almost 30 years ago by the American researcher L. Margulis. According to this hypothesis, the mitochondria that function as energy factories in the eukaryotic cell are aerobic bacteria, and the chloroplasts of plant cells in which photosynthesis occurs are cyanobacteria, probably absorbed about 2 billion years ago by primitive amoebae. As a result of mutually beneficial interactions, the absorbed bacteria became internal symbionts and formed a stable system with the cell that absorbed them - a eukaryotic cell.
Studies of fossil remains of organisms in rocks of different geological ages have shown that for hundreds of millions of years after their origin, eukaryotic life forms were represented by microscopic spherical single-celled organisms such as yeast, and their evolutionary development proceeded at a very slow pace. But a little over 1 billion years ago, many new species of eukaryotes emerged, marking a dramatic leap in the evolution of life.
First of all, this was due to the emergence of sexual reproduction. And if bacteria and single-celled eukaryotes reproduced by producing genetically identical copies of themselves and without the need for a sexual partner, then sexual reproduction in more highly organized eukaryotic organisms occurs as follows. Two haploid sex cells of the parents, having a single set of chromosomes, fuse to form a zygote that has a double set of chromosomes with the genes of both partners, which creates opportunities for new gene combinations. The emergence of sexual reproduction led to the emergence of new organisms, which entered the arena of evolution.
Three quarters of the entire existence of life on Earth was represented exclusively by microorganisms, until a qualitative leap in evolution occurred, leading to the emergence of highly organized organisms, including humans. Let's trace the main milestones in the history of life on Earth in a descending line.
1.2 billion years ago there was an explosion of evolution, caused by the emergence of sexual reproduction and marked by the emergence of highly organized life forms - plants and animals.
The formation of new variations in the mixed genotype that arises during sexual reproduction manifested itself in the form of biodiversity of new life forms.
2 billion years ago, complex eukaryotic cells appeared when single-celled organisms complicated their structure by absorbing other prokaryotic cells. Some of them - aerobic bacteria - turned into mitochondria - energy stations for oxygen respiration. Others - photosynthetic bacteria - began to carry out photosynthesis inside the host cell and became chloroplasts in algae and plant cells. Eukaryotic cells, which have these organelles and a clearly distinct nucleus containing genetic material, make up all modern complex life forms - from molds to humans.
3.9 billion years ago, single-celled organisms appeared that probably looked like modern bacteria and archaebacteria. Both ancient and modern prokaryotic cells have a relatively simple structure: they do not have a formed nucleus and specialized organelles, their jelly-like cytoplasm contains DNA macromolecules - carriers of genetic information, and ribosomes on which protein synthesis occurs, and energy is produced on the cytoplasmic membrane surrounding cell.
4 billion years ago, RNA mysteriously emerged. It is possible that it was formed from simpler organic molecules that appeared on the primitive earth. It is believed that ancient RNA molecules had the functions of carriers of genetic information and protein catalysts, they were capable of replication (self-duplication), mutated and were subject to natural selection. In modern cells, RNA does not have or does not exhibit these properties, but plays a very important role as an intermediary in the transfer of genetic information from DNA to ribosomes, in which protein synthesis occurs.
A.L. Prokhorov
Based on an article by Richard Monasterski
in National Geographic magazine, 1998 No. 3
The problem of life and living things is the object of study in many natural disciplines, starting with biology and ending with philosophy, mathematics, which considers abstract models of the phenomenon of living things, as well as physics, which defines life from the standpoint of physical laws.
All other more specific problems and questions are concentrated around this main problem, and philosophical generalizations and conclusions are also built.
In accordance with two ideological positions - materialistic and idealistic - even in ancient philosophy, opposing concepts of the origin of life developed: creationism and materialistic theory of origin organic nature from inorganic.
Supporters creationism claim that life arose as a result of an act of divine creation, evidence of which is the presence in living organisms of a special force that controls all biological processes.
Proponents of the origin of life from inanimate nature argue that organic nature arose due to the action of natural laws. Later, this concept was concretized in the idea of the spontaneous generation of life.
Concept of spontaneous generation, despite the fallacy, played a positive role; experiments designed to confirm it provided rich empirical material for the developing biological science. The final rejection of the idea of spontaneous generation occurred only in the 19th century.
In the 19th century was also nominated hypothesis of eternal existence of life and its cosmic origin on Earth. It has been suggested that life exists in space and is transferred from one planet to another.
At the beginning of the 20th century. idea cosmic origin biological systems on Earth and the eternity of life in space were developed by the Russian scientist academician V.I. Vernadsky.
Hypothesis of Academician A.I. Oparina
A fundamentally new hypothesis of the origin of life was presented by academician A.I. Oparin in the book "Origin of Life"", published in 1924. He made the statement that Redi principle, which introduces a monopoly of biotic synthesis of organic substances, is valid only for the modern era of the existence of our planet. At the beginning of its existence, when the Earth was lifeless, abiotic syntheses of carbon compounds and their subsequent prebiological evolution took place on it.
The essence of Oparin's hypothesis is as follows: the origin of life on Earth is a long evolutionary process of the formation of living matter in the depths of nonliving matter. This happened through chemical evolution, as a result of which the simplest organic substances were formed from inorganic ones under the influence of strong physicochemical processes.
He viewed the emergence of life as a single natural process, which consisted of the initial chemical evolution that took place under the conditions of the early Earth, which gradually passed to a qualitative new level- biochemical evolution.
Considering the problem of the origin of life through biochemical evolution, Oparin identifies three stages of transition from inanimate to living matter.
The first stage is chemical evolution. When the Earth was still lifeless (about 4 billion years ago), abiotic synthesis of carbon compounds and their subsequent prebiological evolution.
This period of the Earth's evolution was characterized by numerous volcanic eruptions with the release of huge amounts of hot lava. As the planet cooled, water vapor in the atmosphere condensed and rained down on the Earth, forming huge expanses of water (the primary ocean). These processes continued for many millions of years. Various inorganic salts were dissolved in the waters of the primary ocean. In addition, various organic compounds, continuously formed in the atmosphere under the influence of ultraviolet radiation, also entered the ocean. high temperature and active volcanic activity.
The concentration of organic compounds constantly increased, and, eventually, the ocean waters became " broth» from protein-like substances - peptides.
The second stage is the appearance of protein substances. As conditions on Earth softened, under the influence of electrical discharges, thermal energy and ultraviolet rays on the chemical mixtures of the primary ocean, it became possible to form complex organic compounds - biopolymers and nucleotides, which, gradually combining and becoming more complex, turned into protobionts(precellular ancestors of living organisms). The result of the evolution of complex organic substances was the appearance coacervates, or co-acervate drops.
Coacervates- complexes of colloidal particles, the solution of which is divided into two layers: a layer rich in colloidal particles and a liquid almost free of them. Coacervates had the ability to absorb various substances dissolved in the waters of the primary ocean. As a result internal structure coacervates changed towards increasing their stability in constantly changing conditions.
The theory of biochemical evolution considers coacervates as prebiological systems, which are groups of molecules surrounded by a water shell.
For example, coacervates are able to absorb substances from environment, interact with each other, increase in size, etc. However, unlike living beings, coacervate droplets are not capable of self-reproduction and self-regulation, therefore they cannot be classified as biological systems.
The third stage is the formation of the ability to reproduce itself, the appearance of a living cell. During this period, natural selection began to operate, i.e. In the mass of coacervate droplets, the selection of coacervates that were most resistant to the given environmental conditions occurred. The selection process went on for many millions of years. The preserved coacervate drops already had the ability to undergo primary metabolism—the main property of life.
At the same time, having reached certain sizes, the mother drop disintegrated into daughter drops that retained the features of the mother structure.
Thus, we can talk about the acquisition by coacervates of the property of self-production - one of the most important signs of life. In fact, at this stage, coacervates turned into the simplest living organisms.
Further evolution of these prebiological structures was possible only with increasing complexity metabolic processes inside the coacervate.
The internal environment of the coacervate needed protection from environmental influences. Therefore, layers of lipids arose around the coacervates, rich in organic compounds, separating the coacervate from the surrounding aqueous environment. During the process of evolution, lipids were transformed into the outer membrane, which significantly increased the viability and stability of organisms.
The appearance of the membrane predetermined the direction of further biological evolution along the path of increasingly perfect autoregulation, culminating in the formation of the primary cell - the archecell. A cell is an elementary biological unit, the structural and functional basis of all living things. Cells carry out independent metabolism, are capable of division and self-regulation, i.e. have all the properties of living things. The formation of new cells from non-cellular material is impossible; cell reproduction occurs only through division. Organic development is considered as a universal process of cell formation.
The structure of the cell includes: a membrane that separates the contents of the cell from the external environment; cytoplasm, which is brine with soluble and suspended enzymes and RNA molecules; the nucleus containing chromosomes consisting of DNA molecules and proteins attached to them.
Consequently, the beginning of life should be considered the emergence of a stable self-reproducing organic system (cell) with a constant sequence of nucleotides. Only after the emergence of such systems can we talk about the beginning of biological evolution.
The possibility of abiogenic synthesis of biopolymers was experimentally proven in the middle of the 20th century. In 1953, an American scientist S. Miller simulated the primary atmosphere of the Earth and synthesized acetic and formic acids, urea and amino acids by passing electric charges through a mixture of inert gases. Thus, it was demonstrated how the synthesis of complex organic compounds is possible under the influence of abiogenic factors.
Despite its theoretical and experimental validity, Oparin’s concept has both strengths and weaknesses.
The strength of the concept is its fairly accurate experimental substantiation of chemical evolution, according to which the origin of life is a natural result of the prebiological evolution of matter.
A convincing argument in favor of this concept is also the possibility of experimental verification of its main provisions.
The weak side of the concept is the impossibility of explaining the very moment of the leap from complex organic compounds to living organisms.
One of the versions of the transition from prebiological to biological evolution is proposed by a German scientist M. Eigen. According to his hypothesis, the emergence of life is explained by the interaction of nucleic acids and proteins. Nucleic acids are carriers of genetic information, and proteins serve as catalysts for chemical reactions. Nucleic acids reproduce themselves and transmit information to proteins. A closed chain arises - a hypercycle, in which the processes of chemical reactions are self-accelerated due to the presence of catalysts and congestion.
In hypercycles, the reaction product simultaneously acts as both a catalyst and a starting reactant. Such reactions are called autocatalytic.
Another theory within which the transition from prebiological to biological evolution can be explained is synergetics. The patterns discovered by synergetics make it possible to clarify the mechanism of the emergence of organic matter from inorganic matter in terms of self-organization through the spontaneous emergence of new structures during the interaction of an open system with the environment.
Notes on the theory of the origin of life and the emergence of the biosphere
Modern science has accepted the hypothesis of the abiogenic (non-biological) origin of life under the influence of natural causes as a result of a long process of cosmic, geological and chemical evolution - abiogenesis, the basis of which was the hypothesis of Academician A.I. Oparin. The abiogenesis concept does not exclude the possibility of the existence of life in space and its cosmic origin on Earth.
However, based on modern scientific achievements, the hypothesis of A.I. Oparin suggests the following clarifications.
Life could not have arisen on the surface (or near it) of the ocean water, since in those distant times the Moon was much closer to the Earth than it is now. The tidal waves must have been of enormous height and great destructive power. Protobionts simply could not form under these conditions.
Due to the absence of the ozone layer, protobionts could not exist under the influence of hard ultraviolet radiation. This suggests that life could only appear in the water column.
Due to special conditions, life could only appear in the water of the primordial Ocean, but not on the surface, but at the bottom in thin films of organic matter adsorbed by the surfaces of pyrite and apatite crystals, apparently near geothermal springs. Since it has been established that organic compounds are formed in the products of volcanic eruptions, and volcanic activity under the Ocean in ancient times was very active. There was no dissolved oxygen in the ancient Ocean capable of oxidizing organic compounds.
Today it is believed that the protobionts were RNA molecules, but not DNA, since it has been proven that the process of evolution went from RNA to protein, and then to the formation of a DNA molecule in which the C-H bonds were stronger than the C-OH bonds in RNA. However, it is clear that RNA molecules could not arise as a result of smooth evolutionary development. Probably, there was a jump with all the features of self-organization of matter, the mechanism of which is currently not clear.
The primary biosphere in the water column was likely to be rich in functional diversity. And the first appearance of life should have occurred not in the form of any one type of organism, but in a collection of organisms. Many primary biocenoses should have appeared immediately. They consisted of the simplest single-celled organisms capable of performing all the functions of living matter in the biosphere without exception.
These simplest organisms were heterotrophs (they fed on ready-made organic compounds), they were prokaryotes (organisms without a nucleus), and they were anaerobes (they used yeast fermentation as a source of energy).
Due to the special properties of carbon, life arose precisely on this basis. However, no current evidence contradicts the possibility of the emergence of life other than carbon-based.
Some Future Directions for the Study of the Origin of Life
In the 21st century In order to clarify the problem of the origin of life, researchers are showing increased interest in two objects - to the satellite of Jupiter, opened back in 1610 G. Galileo. It is located at a distance from Earth of 671,000 km. Its diameter is 3100 km. It is covered with many kilometers of ice. However, under the cover of ice there is an ocean, and in it the simplest forms of ancient life may have been preserved.
Another object - East Lake, which is called a relict reservoir. It is located in Antarctica under a four-kilometer layer of ice. Our researchers discovered it as a result of deep-sea drilling. An international program is currently being developed with the goal of penetrating the waters of this lake without disturbing its relict purity. It is possible that relict organisms several million years old exist there.
There is also great interest in cave discovered in Romania, without access to light. When they drilled the entrance to this cave, they discovered the existence of blind living organisms such as bugs that feed on microorganisms. These microorganisms use for their existence inorganic compounds containing hydrogen sulfide coming from inside the bottom of this cave. No light penetrates into this cave, but there is water there.
Of particular interest are microorganisms, recently discovered by American scientists during research one of the salt lakes. These microorganisms are exceptionally resistant to their environment. They can live even in a purely arsenic environment.
Organisms living in so-called “black smokers” also attract a lot of attention (Fig. 2.1).
Rice. 2.1. “Black smokers” of the ocean floor (jet of hot water shown by arrows)
“Black smokers” are numerous hydrothermal vents operating on the ocean floor, confined to the axial parts of mid-ocean ridges. Of these, into the oceans under high pressure of 250 atm. highly mineralized hot water(350 °C). Their contribution to the Earth's heat flow is about 20%.
Hydrothermal ocean vents carry dissolved elements from the oceanic crust into the oceans, altering the crust and making very significant contributions to the chemistry of the oceans. Together with the cycle of generation of oceanic crust at ocean ridges and its recycling into the mantle, hydrothermal alteration represents a two-stage system for the transfer of elements between the mantle and the oceans. The oceanic crust recycled into the mantle is apparently responsible for some of the mantle heterogeneities.
Hydrothermal vents at mid-ocean ridges are home to unusual biological communities that obtain energy from the decomposition of hydrothermal fluid compounds (black jet).
The oceanic crust apparently contains the deepest parts of the biosphere, reaching a depth of 2500 m.
Hydrothermal vents make a significant contribution to the Earth's heat balance. Under the median ridges, the mantle comes closest to the surface. Sea water penetrates through cracks into the oceanic crust to a considerable depth, due to thermal conductivity it is heated by mantle heat and concentrated in magma chambers.
An in-depth study of the “special” objects listed above will undoubtedly lead scientists to a more objective understanding of the problem of the origin of life on our planet and the formation of its biosphere.
However, it should be pointed out that to date it has not been possible to obtain life experimentally.
