Category : 12th Class
Life is the part and parcel of the universe and both are very intimately associated with each other. We know that “Life is the most unique, complex organisation of molecules, expressing itself through chemical reactions which lead to growth, development, responsiveness, adaptation and reproduction” that matter has achieved in our universe. Origin of life is a unique event in the history of universe.
(i) Ancient theories of origin of life : Various theories have been put forward to explain the phenomenon of origin of life. A few of them were only speculations while others were based on scientific grounds. These theories are –
(a) Theory of special creation : According to a Spanish Priest Father Suarez (1548 – 1617 B.C.), the whole universe was created in six days by the God. First day Earth and heaven, second day sky, third day dry land and vegetation, fourth day Sun, Moon and other planets, fifth day fishes and birds, and sixth day human beings other animals were created by God. This theory was based on some supernatural power.
(b) Theory of spontaneous generation or Abiogenesis : This theory postulates that life originated from non-living matter spontaneously from time to time. This theory was supported by Plato, Aristotle, Anaximander, John Ray, Needham, Von Helmont, etc., upto the end of seventeen century. Huxley (1870) criticised this theory and propounded the theory “life originated from preexisting life only.”
(c) Biogenesis : Scientists like Redi (1668) Spallanzani (1767), Louis Pasteur (1866–1862) provided experimental support for the Biogenesis concept of Huxley.
Francesco Redi (1668) showed that maggots could not be created from meat. Actually, the smell of meat attracts flies which lay eggs on the flesh. These eggs hatched into flies.
Spallanzani (1767) showed that even primitive, unicellular organisms cannot arise from non-living matter.
Louis Pasteur (1860-62) obtained air samples in the flasks of broth (yeast and sugar solution) whose drawn-out necks were sealed cooling these contained a partial vaccum. Where a sample was required, the flask was opened. Air was drawn in and the flask was resealed. Flasks were incubated. These flasks which were opened in the streets became turbid while those exposed to dust-free air rarely contained bacteria.
Louis Pasteur also, used swan-necked flasks whose long, curved necks permitted exchange of air between outside and inside of the flask, but dust and bacteria were trapped along the wall of the neck. On tilting the flask, the bacteria got washed down into the broth, so that the latter became cloudy due to bacterial growth.
(d) Cosmozoic or Extraterrestrial or Interplanetary or Panspermiatic Theory : Richeter (1865), Preyer (1880), Arrhenius (1908), Hoyle (1950) and Bondi (1952) believed in eternity of life. According to Arrhenius life was transferred from “cosmozoa” (life of outer space) to different planets small units called ‘spores’. The spores were covered by a thick protective covering. When the spores got favourable conditions and temperature, the spore coat was dissolved and gave birth to initial living organisms. This theory does not explain as to how the life originated in space and how the life originated in spores remain impenetrable by ultraviolet and gama rays.
(e) Theory of Catastropism or Theory of sudden creation from inorganic material : Cuvier (1769-1832) believed in catastrophism. According to him, the catastrophy destroys the whole life on earth, and after that, new life originates called it as Mechanistic theory.
(ii) Modern Theory / Oparin Haldane Theory / Chemical Theory / Naturalistic Theory / Materialistic Theory : Haldane, a British scientist, stated that in the early atmosphere of gas mixture probably carbon dioxide, ammonia and water vapours were predominantly present. When ultraviolet rays reacted on them, organic molecules were formed. Gradually, quantity of these oceans which later gave rise to amino acids, proteins, carbohydrates, nucleic acids, etc.
Oparin's Modern Theory : Oparin (1924) proposed that “life could have originated from non-living organic molecules.” He believed in Biochemical origin of life. Haldane (1929) also stated similar views. Oparin greatly expended his ideas and presented them as a book “The origin of life” in 1936.
According to this theory, the Earth originated about 4,500 million years ago. When the earth was cooling down, it had a reduced atmosphere. In this primitive atmosphere nitrogen, hydrogen, ammonia, methane, carbon mono-oxide and water were present. Energy was available in the form of electric discharges by lightening and ultraviolet rays. As soon as the earth crust was formed, it was very much folded. Torrential rains poured over the earth for centuries and were deposited in deep places.
The atmospheric compounds, inorganic salts and minerals also came in deep places oceans, these molecules gave rise to a variety of compounds and finally to the self-duplicating molecules. Ultimately these molecules were enclosed in membranes derived from lipids and proteins, along with water and chemical compounds, giving rise to cell like units. Again random combinations may have led to the formation of chlorophyll– containing organisms which could produce their own food (autotrophs) by a process called photosynthesis. These organisms had a better chance to live because they synthesise starch from carbon dioxide and water in presence of sunlight. Starch could be used as further source of energy. During photosynthesis, oxygen was produced. The oxygen was used by other organisms for respiration. Also oxygen, when acted upon by ultraviolet rays, formed ozone layer through which ultraviolet rays cannot pass. This layer is formed about 25 km. from earth's surface. After the formation of ozone layer, organisms could come to the surface of the ocean and could survive even on land, if thrown out of oceans. The Oparins's and Haldane's theory of origin of life is most accepted these days as it is supported by Miller's experiment duly supported by David Buhal, Melvin Kelvin's experiment etc.
Miller's Experiment : An American scientist (Biologist) Stanley Miller (1953) performed an experiment under support Oparin's theory of origin of life. He believed that basic compounds which are essential for life can be synthesised in the laboratory by creation in the laboratory, on a small scale, the conditions which must have existed at the time of origin of life on earth.
Miller took a flask and filled it with methane, ammonia and hydrogen in proportion of 2:1:2 respectively at 0°C. This proportion of gases probably existed in the environment at time of origin of life. This flask was connected with a smaller flask, that was filled with water, with the help of glass tubes. In the bigger flask, two electrodes of tungsten were fitted. Then a current of 60,000 volts was passes, through gases containing bigger flask for seven days. At the end of seven days, when the vapours condensed, a red substance was found in the U-tube. When this red substance was analyzed, it was found to contain amino acids, Glycine and nitrogenous bases which are found in the nucleus of a cell.
• An experiment to prove that organic compounds were the basis of life, was performed by miller.
From the above theory we conclude that life first originated in water. Therefore, water still continues to be an essential constituent of life.
The entire process of the origin of life, as proposed by Oparin, can be summarised as under :
(a) The Chemical Evolution
(1) Step 1 : Formation of simple molecules : The reactions between simple atoms like carbon, hydrogen oxygen and nitrogen in the primitive atmosphere led to the formation of simple compounds like water, ammonia and methane. But since the earth was very hot, all these substances remained in the form of vapours in the atmosphere. Gradually, as the earth started cooling down, the dense clouds began raining on the earth. But the liquid was still very hot. Therefore, as the liquid water touched the earth's surface, it again got vaporized to be returned to the atmosphere. This process continued for millions of years. As a result of these heavy downpours, the earth's surface got filled with water to form rivers and oceans. Ammonia and methane got dissolved in the oceanic water. The mineral elements, which were dissolved in rivers, were also carried into the oceans when rivers joined it.
The scientists have found that simple unicellular organisms (resembling modern cyanobacteria) were present on this earth about 3600 million years ago. It is believed, therefore, that life must have originated on this earth about 4600 to 3600 million years ago.
It must be clear that the earth's atmosphere at that time was quite different from as it exists today. The earth's atmosphere at that time was reducing, not oxidizing (as it is today). The primitive atmosphere of earth had hydrogen, nitrogen, water vapours, carbon dioxide, methane and ammonia abundance. Oxygen was not available in free state in sufficient quantities.
(2) Step 2 : Formation of simple organic compounds : Continuous rains provided opportunities for different types of molecules to collide with each other and react. Methane is an active compound, and it reacted with other compounds to form organic compounds like ethane, butane, propane, ethyl alcohol. From such organic compounds which were formed in the ocean and which played a role in the origin of life include –
(i) Sugars, glycerol and fatty acids : These were formed by the combination of carbon, hydrogen and oxygen.
(ii) Amino acids : These were formed by the combination of carbon, hydrogen, oxygen and nitrogen.
(iii) Pyrimidines and Purines : These were formed by the combination of carbon, hydrogen and nitrogen.
These compounds were formed at the time when sunlight could not reach earth because of dense clouds in the sky. Under such circumstances, the energy required for the synthesis of above–mentioned chemical substances must have been obtained from the cosmic rays and lightening in the sky. Haldane proposed that these simple organic compounds gradually accumulated in the water bodies and finally a 'hot thin soup' or 'prebiotic soup' or broth was formed. This set the stage for the chemical reactions.
(3) Step 3 : Formation of complex organic compounds : The simple organic compounds combined in different ratios to form complex organic compounds like polysaccharides, fats and proteins.
(i) Simple sugars combined in different ratios to form polysaccharides like starch, cellulose, glycogen etc. The formation of such compounds had been very important for the origin of life because cell walls are made up of cellulose and energy–giving molecules are stored in the form of starch and glycogen.
(ii) The reaction between glycerol and fatty acids yielded fats.
(iii) Different types of combinations between a variety of amino acids yielded different types of proteins. The formation of protein was a very important step in the origin of life because proteins are not only structural components of cell organelles, but many proteins, called enzymes, work as catalysts for biochemical reactions.
In fact, the present day organisms synthesise their complex molecules from simple organic substances with the help of enzymes only. However, enzymes themselves are proteins. Therefore, first of all protein must have been synthesized without the help of any enzyme.
(4) Step 4 : Formation of nucleic acids and nucleoproteins : The reaction between methane, ammonia and water resulted in the formation of purines and pyrimidines. Some of the purines and pyrimidines combined with sugar and phosphorus to form nucleotides. Many molecules of nucleotides combined to form nucleic acids–DNA and RNA. The formation of nucleic acid in the oceanic water was a big steps in the direction of origin of life. Nucleic acids combined with the proteins to form nucleoproteins. Some of the nucleoproteins developed the capability to synthesise molecules similar to themselves, from organic and inorganic substance present in the ocean. In other words, the capability to reproduce had evolved. As a result of continuos reproduction, the number of nucleoproteins went on increasing. Since, organic substance were required for this, the organic substances started being depleted resulting in competition between the nucleoproteins. Physical and chemical changes sometimes led to the changes in the competition of nucleoproteins, and new types of nucleoproteins came into existence by mutations. Those new nucleoproteins which were successful in the competition, increased in number.
(b) Organic Evolution (Biological Evolution)
(1) Step 5 : Formation of Coacervates : Oparin believed that the formation of protein was a very important step in towards the origin of life. The Zwitterionic nature of the protein molecules enabled these colloidal structures to maintain their identify inspite of being surrounded by water molecules–forming a type of emulsion. The coalescence of these colloidal structures led to the formation of structures called coacervates. These coacervates had the ability to exchange substances with the surrounding water and accumulating required substances within them.
Sydney F. Fox's experiment : Sydney F. Fox of Florida University, obtained some complex molecules by heating upto 90°C a dry mixture of many amino acids found in living organisms. The molecules so obtained very much resembled the proteins. He heated these molecules in water and allowed the mixture to cool down. In the fluid so obtained, he could observe minute structures resembling the cells. He called them microspheres. The microspheres are surrounded by membranes, and these also reproduce vegetatively just like yeast. Surprisingly the biochemical processes like breakdown of glucose also occur their. However, electron microscopic examination of these does not reveal any cellular structure. On keeping in distilled water, these become turgid, but these get shrunk if immersed in salt solution. Oparin's coacervates and Fox's microspheres are infact, similar structures or Protocells or Protobionts or Eobionts.
(2) Step 6 : Formation of Primitive living system : A primitive cell membrane was formed by the arrangement of lipid molecules between the surface of coacervates and external watery medium. This provided stability to the coacervates. It is believed that a primitive cell was formed when–
(A) Nucleic acids having the property of self-duplication entered the coacervates.
(B) Rearrangement of molecules occurred inside the coacervate surrounded by lipid molecules.
(3) Step 7 : Formation of first cell : Evidences available so far indicate that the cells of the earliest organisms did not contain either nucleus or cell organelles. The molecules of nucleic acid were surrounded by a colloidal mixture (may be called protoplasm) of proteins and organic compounds. This, in turn, was surrounded by a thin protein-lipid membrane. Water and soluble substances would pass through this membrane. Some proteins achieved the ablity to act as enzyme. Such cells which lacked nuclei were called prokaryotic cells. In 1966, some fossils have been discovered from 300 million years old rocks. These fossils are of prokaryotic organisms. Earliest organism is chemoheterotrops.
(4) Step 8 : Origin of autotrophism : In the primitive organisms, the process of metabolism began because all the substances required for reduction were available in water. Of course, oxygen was not available which was required for oxidation of substances to yield energy. Thus the first living organisms were anaerobes and heterotrophs. The primitive cells respired anaerobically i.e., these used to obtain energy by fermenting the organic compounds obtained form the water, with the help of enzymes due to fast nutrition, growth and multiplication, their number in the ocean increased greatly. As a result, scarcity of organic substances developed in the ocean. A struggle started between the cells for obtaining nutrition.
At such a time, some of these organisms developed the capability of synthesizing organic substances. Such organisms began synthesizing energy– giving substances (carbohydrates) from simple inorganic substances abundantly available in the environment. In this way, evolution of autotrophs from heterotrophs took place.
This was the beginning of autotrophic nutrition. However, it was quite different from the photosynthesis which is carried out by green plants, because it utilized energy obtained by anaerobic respiration (not solar energy). Therefore, such type of nutrition is also called chemoautotrophic nutrition. Such type of nutrition is observed even today in the sulphur bacteria.
Till then, oxygen was not freely available in the atmosphere. However, gradually molecular changes in the bacteriocholorophyll led to the evolution of present chlorophyll, so that these cells started utilizing H2O instead of H2S for photosynthesis. Thus they performed oxygenic photosynthesis using water as hydrogen donar.
In this way, the prokaryotic cells which were chemoautotrophs, became photo autotrophic. These cells resembled modern cyanobacteria. In 1968, the forms of such types of cells have been recovered from 320 million years old rocks. These have been given the name Archaeospheroides barbertonensis. Due to the absence of well-defined nuclei in them, these have been included under the kingdom ‘Monera’. Thus, release of O2 in the atmosphere and its free avilability was the result of photosynthesis. This was a revolutionary change which greatly affected the course of organic evolution.
(5) Step 9 : Origin of Eukaryotic cells : As a result of photosynthesis, oxygen was released in the atmosphere which started reacting with methane and ammonia in the atmosphere. Its reaction with methane yielded CO2 and H2O. On the other hand, reaction between oxygen and ammonia resulted in the formation of CO2 and nitrogen. In the course of these changes, Ozone (O3) gas was formed from oxygen; the ozone spread in the form of an envelope surrounding the earth, the distance between the ozone layer and the earth's surface being approximately 15 miles. Thus free oxygen changed the reducing atmosphere into oxidizing atmosphere. As free oxygen became available on the earth, gradual changes took place in cell structure also. Membrane bound organelles i.e., mitochondria, chloroplasts, golgi bodies, lysosomes evolved. Thus, prokaryotic cells. Most of the organisms on the earth today are eukaryotic.Gradual changes in the earth's atmosphere led to gradual changes in the eukaryotic cells also. Instead of living separately, the cells started living together in the form of colonies. Simultaneously, multinucleation of multicellular structures forming tissues. Different types of tissue combined to form special organs. From the organs, organ systems and ultimately complex bodies of organisms were formed.
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