Science
According to scientists, life on earth began about 3 billion years ago: During this time, simple organisms developed into complex life forms. However, it is still a mystery to scientists how life began on the planet, and they have put forward several theories to explain this phenomenon:
1. Electric sparks
In the famous Miller-Urey Experiment, scientists proved that lightning could contribute to the emergence of the basic substances necessary for the origin of life: electrical sparks form amino acids in an atmosphere consisting of huge amount water, methane, ammonia and hydrogen. More complex life forms then evolved from amino acids. This theory was changed somewhat after researchers found that the planet's atmosphere billions of years ago was poor in hydrogen. Scientists suggested that methane, ammonia and hydrogen were contained in volcanic clouds saturated with electrical charges.
2. Clay
Chemist Alexander Graham Cairns-Smith from the University of Glasgow, Scotland, put forward the theory that at the dawn of life, clay contained many organic components located close to each other, and that the clay helped organize these substances into structures similar to our genes.
DNA stores information about the structure of molecules, and DNA's genetic sequences indicate how amino acids should be built into proteins. Cairns-Smith suggests that clay crystals helped organize organic molecules into ordered structures, and later the molecules themselves began to do this, “without the help” of clay.
3. Deep sea vents
According to this theory, life began in underwater hydrothermal vents that spewed out hydrogen-rich molecules. On their rocky surface, these molecules could come together and become mineral catalysts for the reactions that led to the origin of life. Even now, such hydrothermal vents, rich in chemical and thermal energy, are home to quite a few large number living beings.
4. Icy start
3 billion years ago, the Sun did not shine as brightly as it does now, and, accordingly, less heat reached the Earth. It is quite possible that the surface of the earth was covered with a thick layer of ice, which protected fragile organic matter, located in the water underneath, from ultraviolet rays and cosmic exposure. In addition, the cold helped the molecules to exist longer, as a result of which the reactions that led to the origin of life became possible.
5. RNA world
DNA needs proteins to form, and proteins need DNA to form. How could they have formed without each other? Scientists have suggested that RNA, which, like DNA, stores information, was involved in this process. From RNA, proteins and DNA were formed, respectively., which replaced it due to their greater efficiency.
Another question arose: “How did RNA appear?” Some believe that it spontaneously appeared on the planet, while others deny this possibility.
6. "Simple" theory
Some scientists have suggested that life evolved not from complex molecules like RNA, but from simple ones that interacted with each other. They may have been contained in simple shells similar to cell membranes. As a result of the interaction of these simple molecules, complex, which reacted more efficiently.
7. Panspermia
In the end, life could not have originated on our planet, but was brought from space: In science this phenomenon is called panspermia. This theory has a very solid basis: due to cosmic influences, fragments of stones are periodically separated from Mars, which reach the Earth. After scientists discovered Martian meteorites on our planet, they assumed that these objects brought bacteria with them. If you believe them, then we are all martians. Other researchers have suggested that life was brought by comets from other star systems. Even if they are right, humanity will look for an answer to another question: “How did life originate in space?”
The generally accepted theory is that the entire universe was compressed to the size of a proton, but after a powerful explosion it expanded to infinity. This event occurred about 10 billion years ago and as a result, the resulting universe was filled with cosmic dust, from which stars and planets around them began to form. The Earth, by cosmic standards, is a very young planet, it was formed about five billion years ago, but how did life arise on it? Scientists still cannot find a definite answer to this question.
According to Darwin's theory, life on Earth arose as soon as suitable conditions were established, that is, an atmosphere appeared, a temperature that ensured the flow of life processes and water. According to the scientist, the first simple unicellular organisms appeared precisely under the influence of the Sun on water. Later, they evolved into brown algae and other plant species. Thus, if you follow this rule, all multicellular species on the planet originated from plants. The answer to the most important question has not been received: “How can life appear from nothing, even under the influence of the Sun?” It is enough to carry out a simple experiment - pour well water into a jar, then seal it hermetically and place it on sunlight. In any case, the liquid will remain the same as it was, microscopic changes may occur in its composition, but microorganisms will not appear there. If we carry out the same experiment with open jar, then within a few days it will be possible to notice how the walls begin to become covered with a layer of unicellular algae.
Based on this, we can say that for the origin of life and even its simplest forms, outside intervention is necessary. Of course, the version of the independent origin of species is very tempting because it allegedly proves the independence of humanity not beholden to God or aliens from other planets.
IN lately More and more supporters of cosmic origin are appearing, both for humans and for the entire biosphere. Oddly enough, however, researchers in their research combine appealing not only to artifacts already found or being found, but also to the Bible. If you interpret what is written there in ordinary language, then you can draw analogies not with miracles, but with completely explainable physical phenomena. Based on this material, there is a certain higher mind, which populated the planet with living beings, and also human race. The book says that God created man in his own image and likeness, that is, it is possible that we are a copy, in any case, outwardly we repeat our creator.
A person is a biorobot - that is, an artificially created organism with intelligence, with a built-in opportunity for self-improvement. It is possible that the moment of people settling the planet is precisely described in the episode when Adam and Eve were expelled from the Garden of Eden to Earth, where they had to independently adapt to harsh living conditions. It may well be that the Garden of Eden means the place where the biorobots made by the creator were tested in greenhouse conditions and, after checking their performance, they were released into harsh reality.
Of course, the question remains: “What about the diversity of animal species in this case? Surely the creator could not have created species, subspecies and orders, down to single-celled creatures?” It is assumed that evolution still took place here, but more accelerated and occurring under the control of the creators. It is impossible not to deny the fact that in each of the animal species there are still signs of a species that precedes it on the evolutionary ladder. Birds are very similar to reptiles, especially in the elongated shape of their beaks and the skin of their paws. The outlines of reptiles, in turn, strongly resemble fish, and many mammals have absorbed the characteristics of several previous species at once. Looking at a cat, you can easily guess the signs of both reptiles and amphibians. The love for a warm place was most likely passed on to cats in their genes, and despite the fact that they are warm-blooded, they always prefer to live where there is a source of heat. The same sign is characteristic of cold-blooded animals that are unable to generate heat on their own. Studying carefully cat eye you can see that it is very similar to the eyes of a crocodile, and the shape of the head, with minor changes, resembles a snake’s. Sometimes you get the impression that someone worked on creating the species in the same way as, for example, the designers of an automaker work, taking the chassis of a previous car as a basis and adding a few changes.
If this is so, then it is not surprising that some of the animal species simply cause bewilderment, being associated with a situation where there are not enough parts during assembly and they use what is available. There are especially many examples of such animals in Australia. In addition to the kangaroo, which is a rodent but has a powerful musculoskeletal system like a horse, there are other interesting species, such as the platypus. This animal is a mammal, but reproduces like birds - it lays eggs and has a bony beak similar to a goose. The structure of its body is very similar to a beaver, and the born cubs feed on milk not through the mother’s nipples, but by licking the liquid protruding on the surface of the belly. Whether the creators themselves did such painstaking work, or whether they set only the basic direction in development, and the formation of individual subspecies already occurred independently - today this question remains open.
Options for evolution can be considered from different angles, but most researchers still agree that evolution itself, if it took place, is just a consequence, but the cause remains to be found out. An equally popular opinion is that the reason for the appearance of life on Earth was the fall of a meteorite, on which the simplest single-celled organisms were in a frozen state. Since by that time a warm climate had already been established on the planet, and most of the surface was occupied by the ancient world ocean, all conditions were created for the subsequent development of life. There is also a version that the meteorite was actually sent by intelligent beings specifically for the purpose of populating the planet, which is also not without its right to exist.
Instead of a meteorite, there could be simply an optical information beam, for example, sent from another universe or even another dimension. In fact, why would such highly developed beings send something material across billions of light years? Given their level of development, they have long been able to discover the possibilities of teleportation and freely operate with space and time, appearing exactly where it is needed. The information transmitted using the beam materialized here on earth into the same organisms and thus the process of evolution was launched.
Of course, life could not only have been triggered by an accidentally flying meteorite; the version that Mars could have become the donor also has many supporters. The mystery of this planet still cannot be solved. All that scientists have on hand are photographs of a red surface thinned by deep depressions, a mysterious face, most likely a feature of the relief, and insignificant soil samples. Billions of dollars have been spent on designing and launching devices, but most of these attempts have failed. It seems that some force on this planet stubbornly does not want to have contact with earthlings.
It is assumed that Mars was once inhabited and rich natural resources, like the Earth, but subsequently, its magnetic field weakened. This led to the fact that most of the atmosphere and moisture evaporated into space, as a result, the body of the planet was left without protection from the hard ultraviolet radiation. It is possible that the inhabitants of Mars had the necessary knowledge and were able to move some species of animals to the neighboring planet, move themselves, or send a capsule with microorganisms.
The search for the original source of life will continue for a very long time, because with each new discovery in science and especially genetics, it is possible to only slightly lift the veil of secrecy about the origin of humanity, which in turn leads to the emergence of new hypotheses. Still, whatever the answer to this question, it is unlikely that it will be known until a person learns to feel responsibility for his unique planet, where he was lucky enough to live.
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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 have 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 geological processes and movements 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 in the atmosphere maintained 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 closely 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, on earth's surface various chemicals, necessary for the synthesis of organic molecules. 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, then 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 a 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, in generations of descendants the consequences of mutations must manifest themselves - 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 protein molecules are synthesized, in which the third type of RNA molecules take 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 structure, more protected from external influences, served as the prototype of 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 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 it will be possible to talk about the extraterrestrial origin of life and the possibility of bringing life from Space a larger share confidence (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 conditions 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 that are 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, the greatest dish - life - was prepared in this “chemical kitchen”. 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 harmful, 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 advent of sexual reproduction and marked by the appearance 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 modern concept of the origin of life on Earth is the result of a broad synthesis of natural sciences, many theories and hypotheses put forward by researchers of various specialties.
For the emergence of life on Earth, the primary atmosphere (of the planet) is important.
The Earth's primary atmosphere contained methane, ammonia, water vapor and hydrogen. By exposing the mixture of these gases to electric charges and ultraviolet radiation, scientists were able to obtain complex organic substances that are part of living proteins. The elementary “building blocks” of living things are: chemical elements like carbon, oxygen, nitrogen and hydrogen.
A living cell, by weight, contains 70% oxygen, 17% carbon, 10% hydrogen, 3% nitrogen, followed by phosphorus, potassium, chlorine, calcium, sodium, magnesium, and iron.
So, the first step towards the emergence of life is the formation of organic substances from inorganic ones. It is associated with the presence of chemical “raw materials”, the synthesis of which can occur under certain radiation, pressure, temperature and humidity.
The emergence of the simplest living organisms was preceded by a long chemical evolution. From a small number of compounds (as a result of natural selection), substances with properties suitable for life arose. Compounds arising from carbon formed the “primary broth” of the hydrosphere. Substances containing nitrogen and carbon originated in the molten depths of the Earth and were brought to the surface during volcanic activity.
The second step in the emergence of compounds is associated with the emergence of biopolymers in the primary ocean of the Earth: nucleic acids, proteins. If we assume that during this period all organic compounds were in the primary ocean of the Earth, then complex organic compounds could have formed on the surface of the ocean in the form of a thin film and in shallow water heated by the sun. The anaerobic environment facilitated the synthesis of polymers from inorganic compounds. Simple organic compounds began to combine into large biological molecules.
Enzymes are formed - protein substances - catalysts that contribute to the formation or disintegration of molecules. As a result of the activity of enzymes, the “primary elements” of life arose - nucleic acids, complex polymeric substances consisting of monomers.
Monomers in nucleic acids are arranged in such a way that they carry certain information, code,
which consists in the fact that each amino acid included in the protein corresponds to a specific protein of 3 nucleotides (triplet). Proteins can be built on the basis of nucleic acids and exchange with external environment matter and energy.
The symbiosis of nucleic acids formed “molecular genetic control systems.”
At this stage, nucleic acid molecules acquired the properties of self-reproduction of their own kind and began to control the process of formation of protein substances.
At the origins of all living things were revertase and matrix synthesis from DNA to RNA, the evolution of the r-RNA molecular system into a DNA one. This is how the “genome of the biosphere” arose.
Heat and cold, lightning, ultraviolet reaction, atmospheric electric charges, gusts of wind and water jets - all this ensured the beginning or attenuation of bio chemical reactions, the nature of their course, gene “bursts”.
Towards the end of the biochemical stage, structural formations such as membranes appeared, limiting the mixture of organic substances from the external environment.
Membranes played main role in the construction of all living cells. The bodies of all plants and animals are made up of cells.
Modern scientists have concluded that the first organisms on Earth were single-celled prokaryotes. In their structure, they resembled bacteria or blue-green algae that currently exist.
For the existence of the first “living molecules”, prokaryotes, as for all living things, an influx of energy from the outside is necessary. Each cell is a small “energy station”. The direct source of energy for cells is ATP and other phosphorus-containing compounds. Cells receive energy from food; they are able not only to spend, but also to store energy.
Scientists suggest that many of the first lumps of living protoplasm arose on Earth. About 2 billion years ago, a nucleus appeared in living cells. Eukaryotes arose from prokaryotes. There are 25 - 30 species of them on Earth. The simplest of them are amoebas. In eukaryotes, the cell has a formed nucleus with a substance containing the code for protein synthesis.
By this time, there was a “choice” of a plant or animal way of life. The differences between these lifestyles are associated with the method of nutrition and the emergence of photosynthesis, which consists of creating organic substances (for example, sugars from carbon dioxide and water using light energy).
Thanks to photosynthesis, plants produce organic substances, due to which the plant mass increases, and produce large amounts of organic substances.
With the advent of photosynthesis, oxygen began to enter the Earth's atmosphere, and a secondary atmosphere of the Earth with a high oxygen content was formed.
The appearance of oxygen and the intensive development of land plants is the greatest stage in the development of life on Earth. From this moment, the gradual modification and development of living forms began.
Life with all its manifestations has made profound changes in the development of our planet. Improving in the process of evolution, living organisms spread more and more widely across the planet, taking a large part in the redistribution of energy and substances in the earth's crust, as well as in the air and water shells of the Earth.
The emergence and spread of vegetation led to a radical change in the composition of the atmosphere, initially containing very little free oxygen, and consisting mainly of carbon dioxide and probably methane and ammonia.
Plants assimilating carbon from carbon dioxide resulted in an atmosphere containing free oxygen and only traces of carbon dioxide. Free oxygen in the atmosphere served not only as an active chemical agent, but also as a source of ozone, which blocked the path of short ultraviolet rays to the Earth's surface (ozone screen).
At the same time, carbon, which had accumulated for centuries in plant remains, formed energy reserves in the earth's crust in the form of deposits of organic compounds (coal, peat).
The development of life in the oceans led to the creation of sedimentary rocks consisting of skeletons and other remains of marine organisms.
These deposits, their mechanical pressure, chemical and physical transformations changed the surface of the earth's crust. All this testified to the presence on Earth of a biosphere in which life phenomena unfolded and continue to this day.
How did life originate on Earth? The details are unknown to humanity, but the cornerstone principles have been established. There are two main theories and many minor ones. So, according to the main version, organic components came to Earth from space, according to another - everything happened on Earth. Here are some of the most popular teachings.
Panspermia
How did our Earth appear? The biography of the planet is unique, and people are trying to unravel it in different ways. There is a hypothesis that life existing in the Universe spreads 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 fall into debris and dust, which are thrown into space after preserving, thus, life after the death of small bodies of the Solar system. Bacteria can travel in a dormant state for long periods of time before another chance encounter with other planets.
They may also mix with protoplanetary disks (a dense cloud of gas around a young planet). If in a new place the “steadfast but sleepy soldiers” find themselves in favorable conditions, they become active. The process of evolution begins. The story is unraveled with the help of probes. Data from instruments that have been inside comets indicate: in the overwhelming majority of cases, the probability is confirmed that we are all “a little aliens,” since the cradle of life is space.
Biopoiesis
Here is another opinion regarding how life began. There are living and nonliving things on Earth. 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 have nothing to do with life.
This is confirmed by the Muller-Urey experiment. In 1953, a scientist passed electricity through a mixture of gases and obtained several amino acids in laboratory conditions that simulated the conditions of the early Earth. In all living things, amino acids are transformed into proteins under the influence of genetic memory keepers, nucleic acids.
The latter are synthesized independently biochemically, and proteins accelerate (catalyze) the process. Which organic molecule is the first? And how did they interact? Abiogenesis is in the process of finding an answer.
Cosmogonic trends
This is the doctrine of in space. In the specific context of space science and astronomy, the term refers to the theory of the creation (and study) of the solar system. Attempts to gravitate toward naturalistic cosmogony do not stand up to criticism. Firstly, 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. The mentioned theory does not contain the concept of quantum gravity. Although string theorists state that elementary particles arise from the vibrations and interactions of quantum strings), exploring the origins and consequences big bang(loop quantum cosmology), we 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 homogeneity 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 Russian botanist Konstantin Merezhkovsky in 1905. He believed that some organelles arose as free-living bacteria and were adopted 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 to form a eukaryotic cell (eukaryotes are cells of living organisms containing a nucleus). Horizontal transfer of genetic material between bacteria is also facilitated by symbiotic relationships.
The emergence of diversity in life forms may have been preceded by the Last Common Ancestor (LUA) of modern organisms.
Spontaneous generation
Until the early 19th century, people generally rejected "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 life forms 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 observed 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 emergence of microbes in order to be able to combat infectious diseases. However, Pasteur's evidence was no longer controversial, but strictly scientific in nature.
The Clay Theory and Sequential Creation
The emergence of life based on clay? Is this 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, once between layers of clay and interacting with them, adopted a method of storing information and growing. Thus, the scientist considered the “clay gene” to be primary. Initially, the mineral and nascent life existed together, but at a certain stage they “scattered.”
The idea of destruction (chaos) in the emerging world paved the way for the theory of catastrophism as one of the predecessors of the theory of evolution. Its proponents believe that the Earth has been affected by sudden, short-lived, violent events in the past, and the present is the key to the past. Each successive catastrophe destroyed existing life. The subsequent creation revived it already different from the previous one.
Materialistic doctrine
And here is another version regarding how life began on Earth. It was put forward by materialists. They believe that life emerged as a result of gradual chemical transformations extended over time and space, which, in all likelihood, occurred 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 movement, the doctrine arose in the 1960s, when active research was carried out affecting molecular and evolutionary biology, and population genetics. Scientists then tried to understand and confirm recent discoveries regarding nucleic acids and proteins.
One of the key themes that stimulated the development of this field of knowledge was the evolution of 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
Supporters of this doctrine talk about how life appeared on Earth 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 existed). Probably, at first there was a slow and gradual process of transformation, and then a rapid (within the Universe) stage of improvement began, the transition from one static state to another under the influence of existing conditions.
Evolution, known as biological or organic, is the process of change over time in one or more heritable traits found in populations of organisms. Hereditary traits are special distinctive characteristics, including anatomical, biochemical and behavioral, that are passed on from one generation to the next.
Evolution has led to diversity and diversification of all living organisms (diversification). Charles Darwin described our colorful world as “infinite forms, most beautiful and 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 were 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. The three religions agreed that God created the universe in seven days, making the sixth day the culmination of his work: 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 him to tend the garden called Eden. In the center grew the Tree of Life and the Tree of the Knowledge of Good. God gave permission to eat the fruit of all the trees in the garden except the Tree of Knowledge (“for in the day that you eat of it you will die”).
But people disobeyed. The Koran says that Adam suggested trying the apple. God forgave the 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 clear answer. Although modern scientists are increasingly inclined to the abiogenic (inorganic) theory of the origin of all living things.