The modern theory of origin of life was put forwarded by Haeckel in crude form. Thus, it required further exploration and experimentation. The exploration was done by Oparin and Haldane while its experimentation was done by Miller and Urey.
AI (Alexander Ivanovich) Oparin(1894-1980) Was a Russian biochemist in 1923; he had given his view regarding the origin of life. He wrote book “Origin of life” in which he explained his view regarding origin of life.
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| AI Oparin |
JBS (John Burdon Sanderson) Haldane (1892-1964) was an English biologist. In 1929; he had given his view regarding the origin of life. He wrote book “The enzyme”.
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| JBS Haldane |
Though they had given their view independently, their views were quite similar with each other. Thus, their view is collectively called as Oparin-Haldane View.
According to them the origin of life started along with the origin and evolution of earth and its atmosphere and the conditions on primitive atmosphere were entirely different from present day conditions. There was reducing atmosphere i.e. no free oxygen was available. Whatever O2 was present, it was in bound form i.e. metal oxides. There was no ozone. Temperature was very high. There was abundant methane, ammonia, hydrogen and water vapours. The simple molecules combine to form complex macromolecules in water in presence of energy sources like heat from volcanic eruption, lightning during thunderstorms, ultraviolet rays of solar radiation. Probably Life originated in water because water is an excellent solvent and provides maximum chances for molecular interaction. According to this theory Primary abiogenesis occurred only once and the life originated on primitive earths primordial soup through a series of chemical reactions, followed by biological evolution and diversity. Abiogenesis first but Biogenesis ever since.
It can be explained under following two main headings; chemogeny and biogeny.
*Chemogeny or Chemical evolution:-
It refers to all the processes by which the simple molecules present in the primitive earth combined with each other to form complex macromolecules. It can be explained under following headings;
* Atomic phase or conditions of primitive earth
* Formation of simple molecules and simple inorganic compounds
* Formation of simple organic molecules and
* Formation of complex macromolecules
* Atomic phase or conditions of primitive earth:
The primitive earth was very hot and elements were present in atomic forms in molten or gaseous forms. Early earth had innumerable atoms of all those elements (e.g., hydrogen, oxygen, carbon, nitrogen, sulphur, phosphorus, etc.) which are essential for the formation of protoplasm. The free atoms present during that time were stratified in three layers according to their density. The heaviest atoms of iron, nickel, copper, etc. were found in the centre or core of the earth. ) Medium weight atoms of sodium, potassium, silicon, magnesium, alu-minium, phosphorus, chlorine, fluorine, sulphur, etc. were collected in the lithosphere ( outer surface) of the earth. The lightest atoms of nitrogen, hydrogen, oxygen, carbon etc. formed the primitive atmosphere.
* Formation of simple molecules and simple inorganic compounds:
Free atoms combined to form inorganic molecules such as H2 (Hydrogen), N2 (Nitrogen), H20 (Water vapour), CH4 (Methane), NH3 (Ammonia), C02 (Carbon dioxide). Hydrogen atoms were most numerous and most reactive in primitive atmosphere.
First hydrogen atoms combined with all oxygen atoms to form water and leaving no free oxygen. Thus, primitive atmosphere was reducing atmosphere (without free oxygen) unlike the present oxidizing atmosphere (with free oxygen).
Due to high temperature water was present in vapour form. Hydrogen atoms also combined with nitrogen, forming ammonia (NH3). So, water and ammonia were probably the first molecules of primitive earth. Nitrogen and Carbon combined with metallic atoms to form nitrides and carbides. Substances like carbondi-oxide, carbonmono-oxide, nitric oxide, nitrogen dioxide etc were also formed.
* Formation of simple organic molecules:
The water vapor and metallic carbides combined with each other to form methane, first organic compound. The atmosphere was with water vapour , carbondioxide ,methane and ammonia.
The temperature of earth started to decrease and water vapour formed clouds and torrential rain fall occurred continuously for long period of time. As the water rushed down, it must have dissolved away and carried with it salts and minerals and ultimately accumulated in the form of oceans. Thus, ancient oceanic water contained large amounts of dissolved NH3, CH4, HCN, nitrides, carbides, various gases and elements.
The early inorganic molecules interacted and produced simple organic molecules such as simple sugars (e.g., ribose, deoxyribose, glucose, etc.), nitrogenous bases (e.g., purines, pyrimidines), amino acids, glycerol, fatty acids, etc.
CH4 + C02 + H20 —> Sugars + Glycerol + Fatty Acids
CH4 + HCN + NH3 + H20 —> Purines + Pyrimidines
CH4 + NH3 + C02 + H20 —> Amino Acids
Some external sources of energy must have been acting on the mixture for reactions. These external sources might be ultra-violet light, X-rays, etc. from solar radiations, electrical discharges from lightning and heat during volcanic eruptions/ radioactive break down of elements, high energy radiations etc.
The oceanic water rich in mixture of organic compounds was termed hot dilute soup or primordial soup or pre-biotic soup by J.B. Haldane (1920) from which living cells are believed to have appeared. It is also called as broth. Thus, the stage was set for combination of various chemical elements. Once formed, the organic molecules accumulated in water because their degradation was extremely slow in the absence of any life or enzyme catalysts.
*Formation of Complex Organic Molecules (Macromolecules):
A variety of amino acids, fatty acids, hydrocarbons, purines and pyrimidines, nitrogen bases, simple sugars and other organic compounds accumulated in the ancient seas. In the primitive atmosphere electrical discharge, lightning, solar energy, heat, radiations might have provided the source of energy for polymerisation reactions of organic synthesis.
Thus, the small simple organic molecules combined to form large complex organic molecules, e.g., amino acid units joined to form polypeptides and proteins; simple sugar units combined to form polysaccharides; fatty acids and glycerol united to form fats; sugars, nitrogenous bases, and phosphates combined into nucleotides which polymerized into nucleic acids in the ancient oceans.
Sugar + Sugar ———-> Polysaccharides
Fatty Acides + Glycerol ———-> Fats
Aminoacids- + Aminoacids ———–> Proteins
Nitrogenous Bases + Pentose Sugars + Phosphates ———> Nucleotides
Nucleotides + Nucleotides ———–> Nucleic Acids
The giant nucleoprotein molecules were formed by the union of nucleic acid and protein molecules. These nucleoprotein particles were described as free living genes. Probably Nucleoproteins gave the first sign of life.
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| Stages in chemical evolution |
Conditions for the Origin of Life:
For origin of life, at least three conditions are needed.
(a) There must have been a supply of replicators, i.e., self-producing molecules.
(b) Copying of these replicators must have been subject to error through mutation.
(c) The system of replicators must have required a continuous supply of free energy and partial isolation from the general environment.
The high temperature in early earth would have fulfilled the requirement of mutation.
* Biogeny or Biological Evolution:
It includes all the processes by which the macromolecules get associated with each other to form life. It can be explained under following headings:
*Isolation of organic molecules
*Origin of prokaryotes
*Evolution of modes of nutrition
*Formation of ozone layer and
* Formation of eukayotes
* Isolation of organic molecules:
There are two hypotheses regarding the formation of prebionts. These prebionts led to the formation of first living cell from which the more complex cells evolved.
(i) Coacervates:
The first hypothesis was proposed by Oparin (1920). According to this hypothesis early prebionts could have been a coacervate. Oparin gave the term coacervates.
Oparin speculated that a coacervate consisted carbohydrates, proteins, lipids and nucleic acids that accumulated to form a colloidal mass and was surrounded by a thin layer (film) of water.
This arrangement of water molecules, although not a membrane, could have functioned as a physical barrier between the organic molecules and their surroundings. They could selectively take in materials from their surroundings and incorporate them into their structure.
Coacervates have been synthesized in the laboratory. They can selectively absorb chemicals from the surrounding water and incorporate them into their structure. Some coacervates contain enzymes that direct a specific type of chemical reaction.
Because they lack a definite membrane, no one claims coacervates are alive, but they do exhibit some life like characters. They have a simple but persistent organization. They can remain in solution for extended periods. They have the ability to increase in size.
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| Coacervates |
(ii) Microspheres:
Another hypothesis is that early prebionts could have been a microsphere. A microsphere is a non-living collection of organic macromolecules with double layered outer boundary. The term microsphere was given by Sydney Fox (1958-1964).
The microspheres have RNA, proteins, polysaccharides etc. Sidney Fox demonstrated the ability to build microspheres from proteinoids. Proteinoids are protein like structures consisting of branched chains of amino acids. Proteinoids are formed by the dehydration synthesis of amino acids at a temperature of 180°C.
Microspheres swell or shrink depending on the osmotic potential in the surrounding solution. They also display a type of internal movement (streaming) similar to that exhibited by cells and contain some proteinoids that function as enzymes. Using ATP as a source of energy, microspheres can direct the formation of polypeptides and nucleic acids. They can absorb material from the surrounding medium.
They have the ability of motility, growth, binary fission into two particles and a capacity of reproduction by budding and fragmentation. Superficially, their budding resembles with those of bacteria and fungi.
According to some investigators, microspheres can be considered first living cells.
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| Microspheres |
* Origin of Prokaryotes:
Prokaryotes were originated from proto cells ( prebionts) about 3.5 billion years ago in the sea. They were anaerobic heterotrophs. These were similar to viruses. Oparin called them protobionts or ebionts. The atmosphere was anaerobic because free oxygen was absent in the atmosphere. Prokaryotes do not have nuclear membrane, cytoskeleton or complex organelles. They divide by binary fission. The oldest known fossil cells are stromatolites.
Further processes can also be explained under heading Cognogeny. The process of diversification of prokaryotes over time is called cognogeny.
* Evolution of Modes of Nutrition:
(i) Heterotrophs:
The earliest prokaryotes presumably obtained energy by the fermentation of organic molecules from the sea broth in oxygen free atmosphere (reducing atmosphere). They required readymade organic material as food and thus they were heterotrophs.
(ii) Autotrophs:
With the gradual increase in number of anaerobic heterotrophs in Primitive Ocean, they consumed the organic nutrients, and it caused shortage of natural food in the oceanic water. Thus, these heterotrophs adopted for different methods to obtain food.
That led to the evolution of autotrophs. These organisms were capable of producing their own organic molecules by chemosynthesis or photosynthesis.
(a) Chemoautotrophs:
Drop in temperature stopped synthesis of organic molecules in the sea water.
In course of time; some prokaryotes acquired enzyme to catalyze the synthesis of simple carbohydrate from inorganic substances. It was supposed to be the first form of autotrophism. The energy for the process was from anaerobic break down of substances. Thus, it was chemo-autotrophism.
Chemical energy
Carbondioxide + hydrogensulphide-------------------------> Glucose + water+ sulphur
These anaerobic chemoautotrophs were like present day anaerobic bacteria.
(b) Photoautotrophs:
In course of time; certain bacteria developed pigment (Magnesium porphyrine) which can absorb solar radiation and utilized light energy to synthesize carbohydrates. These were anaerobic photoautotrophs. They did not use water as a hydrogen source. They were similar to present day sulphur bacteria in which hydrogen sulphide split into hydrogen and sulphur. Hydrogen was used in food manufacture and sulphur was released as a waste product.
Solar energy
Carbondioxide + hydrogensulphide-----------------------> Glucose + water+ sulphur
In course of time, in certain bacteria the magnesium porphyrine get changed into chlorophyll and used water as a source of hydrogen and carbon dioxide as source of carbon to synthesize carbohydrate in the presence of solar energy.
The first aerobic photoautotrophs were cyanobacteria (blue green algae) like forms which had chlorophyll. They released oxygen in the atmosphere as the byproduct of photosynthesis. They were evolved about 2.5 billion years ago.
Oxygen Revolution:
As the number of photoautotrophs increased, oxygen was released in the sea and atmosphere. Free oxygen than reacted with methane and ammonia present in the primitive atmosphere and transformed methane and ammonia into carbon dioxide and free nitrogen.
CH4 + 202 ————- > CO2 + 2H2O
4NH3 + 3O2 ———– > 2N2 + 6H2O
The oldest fossil belonging to blue green algae, named Archaeospheroides barbertonensis which is 3.2 billion years old. Oxygen releasing prokaryotes first appeared at least 2.5 billion years ago.
* Formation of Ozone Layer:
As oxygen accumulated in the atmosphere, the ultraviolet light changed some of oxygen into ozone.
2O2 + O2 ———- > 2O3
The ozone formed a layer in the atmosphere, blocking the ultraviolet light and leaving the visible light as the main source of energy.
Then the prokaryotes migrated to land from water and also anaerobic mode changed into aerobic mode as aerobic mode forms much more energy than anaerobic mode.
* Origin of Eukaryotes:
The eukaryotes developed from primitive prokaryotic cells about 1.5 billion years ago.
The supporters of Oparin’s theory say that coacervates evolved into prokaryotes which later evolved into eukaryotes by gene mutation and differentiation.
There are two views regarding the origin of eukaryotes.
(i) Symbiotic Origin: According to Margulis (1970-1981) of Boston University, some anaerobic predator host cells engulfed primitive aerobic bacteria but did not digest them. These aerobic bacteria established themselves inside the host cells as symbionts. Such predator host cells became the first eukaryotic cells.
The predator host cells that engulfed aerobic bacteria evolved into animal cells while those that captured both aerobic bacteria and blue-green algae became eukaryotic plant cells. The aerobic bacteria established themselves as mitochondria and blue green algae as chloroplasts.
The discovery of DNA in chloroplast by Ris and Plaut in 1962 and in mitochondria by Nass and Nass in 1963 further give support to this view.
(ii) Origin by Invagination:
According to this view cell organelles of eukaryotic cells might have originated by invagination of surface membrane of primitive prokaryotic cells.
