Modern or natural or chemosynthetic theory of origin of life

Modern or natural or chemosynthetic theory of origin of life

That theory was proposed by the Russian biochemist Alexander Ivanovich Oparin (1923) and supported by JBS Haldane (1928). This theory is popularly known as Oparin Haldane theory. This theory explained in detail in Oparin’s book “the origin of life on the earth” published in 1936.

This theory states that primitive life originated in the water bodies on the primitive earth by the chemical evolution through a series of chemical reaction about 4200 million years ago.

Origin of earth (Biopoiesis)

The earth is supposed to be originated about 4.6 billion years ago. The sphere was filled with cloud masses of dust particles called xylem. The rotation and gravity caused condensation and formed entire solar system. The earth when formed was very hot (4000-8000), like a ball of fire. Gradually cool in next few million of years. They started condensing forming solids, liquids, gases. Heavy elements shank at the center forming the core. The lighter one (Aluminum, silicon, sodium, potassium) formed the middle core of the earth. Thus, lithosphere was formed. Lightest gases N₂ H₂ CO₂ He etc. including water vapor with dust particle rose above the land surface and turned finally into the atmosphere.

Origin of life on earth

It is expected that life originated on earth nearly 3.7 billion years ago and involved following changes.

Chemogeny

Volcanic gases like H₂, N₂, CO₂, H₂O, and He underwent many changes (chemical evolution) which took place in following steps.

Formation of original gases

The original gases like NH₂, CH₂, HCN etc. would have formed by the interaction of volcanic gases when the primitive earth cooled down from 100. The formation of the prime component was the start of life in primitive earth.

N₂ +3H₂ → 2NH₃

C +2H₂ → CH₄

2C +N₂ + H₂ → 2HCN

Formation of key bio micro compounds

Simple key bio micro compounds like pentose sugar, glycerol, fatty acids, amino acids, nitrogen bases (adenine, cytosine, thymine, guanine etc.) would have formed by the interaction of original gases (NH₃, CH₄, HCN) when the temperate further went down to about 50 to 60 . Such reaction took place under the action heat coming from energy radiation of sun lightning lava, radioactive elements like radio, uranium of rocks etc.

6CH₄ +H₂O → C₆H₁₂O₆ +12H₂

CO₂ +2NH₃ → CO(NH₂)₂ + H₂O

CH₄ + H₂O +NH₃ + HCN → Nitrogen bases

CH₄ saturated hydrocarbon reacted with water thereby forming unsaturated hydrocarbon like acetylene, ethylene, etc. From these, aldehydes, ketones, alcohol, acid was formed spontaneously, still, more complex organic compounds like glucose, amino acid, fatty acids, etc. and purine and pyrimidine were formed as key bio micro compounds.

• Formation of complex bio-macro compounds

Complex bio-macro compounds like starch, protein, lipid, nucleic acid would have formed by polymerization of key simple bio micro compounds in the presence of water.

C₆H₁₂O₆ + C₆H₁₂O₆ + C₆H₁₂O₆ → Polysaccharides

Fatty acids + glycerol → Lipid (fat)

Amino acids + amino acids + amino acids → protein

Complex bio-macro compounds thus formed did not undergo oxidation because the primitive earth was reducing, no free O₂ and no microbes present.

1. Formation of coacervates

When earth further cooled down, water vapour turns into rain, earth surface shrunk and formed depression where rain water filled up and turned into lakes, sea, as hydrosphere. Uplands turned into mountains. In sea, bio-macro compounds when mixed with sea water formed thin hot soup called “broth”. To this Haldine 1992 called ‘prebiotic soup’. Oparin in 1923 called it as coacervate (Latin Coacervus = clump or heap) which is an aggregate of an insoluble viscous colloid of complex biomacromolecules. These had the power of self-growing and dividing by budding like bacteria.

Some protein droplets in coacervate with metallic ions like magnesium, iron, and manganese etc. and water acted as the catalytic agent called enzyme and helped in breaking down organic molecules. The formation of protein for the first time is the crucial start of a transformation of non-living into a living one.

2. Biogeny

It is believed that the coacervates would have undergone further series of chemical changes due to the formation of enzymes. Such enzymes after combining with the nitrogen bases and sugar phosphate molecules formed nucleotides. Later on, nucleotides underwent polymerization and formed nucleic acids.

N₂ bases + sugar phosphate → Nucleotide

Nucleotide + nucleotide + nucleotide → Nucleic acid

Nucleoprotein of nucleic acid when surrounded by the thin fold of phospholipid as limiting membrane developed cytoplasm. This pre-cell like structure is called eobiont and Oparin called it as ‘protobiont’. The pre-cell stage was developed in the same way as coacervate which developed in water of primitive earth in the total reducing environment. The nucleic acid ( the pre-cell component of RNA and DNA) can manufacture another molecule like itself. This regarded as the first sign of life. Eobiont or protobiont formed life (living cell) on this earth for the first time. In this way, biogeny is the developer-cell into living ell.

3. Cosmogeny

The formation of life (living cell) on this earth was anaerobic, prokaryotic, and chemoheterotrophic. This type of cell used the energy present in the chemicals. Later, on such type of cell changed into anaerobic, prokaryotic, chemoheterotrophic. This means these cells changed the available chemical to suit themselves in the production of energy. Later on some of these cells developed the chlorophyll molecules within them. Then they changed into anaerobic prokaryotic and photoautotrophic. With this, there is release of oxygen in the atmosphere. Now the anaerobic prokaryotic photoautotrophic organisms changed into the aerobic prokaryotic photoautotrophic. They gave rise to protozoans and then to metazoans and various types of animals and higher forms of plants which we can see today around us. Thus cognogeny is the evolution in the living organisms.

Anaerobic, prokaryotes and chemoheterotrophic

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Anaerobic, prokaryotes chemoheterotrophic

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Anaerobic, prokaryotes and photoautotrophic

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Anaerobic, prokaryotes and photoautotrophic

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Unicellular animals and plate

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Multicellular animals and plants

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Higher forms and animals and plants

Miller-Urey experiment

Stanley Miller and Harold C Urey in 1953 performed series of experiments and tested Oparin and Haldane theory. They devised an apparatus that simulated the conditions of the surface of pre biological earth and analyzed the molecular forms that arose. The apparatus consisted exposing water vapor to electric spark of 75,000 volts (to represent the lightning of thunder storms) in an ammonia, methane and hydrogen atmosphere of the early earth. After two weeks of treatment, miller chromatographed the final solution and found many organic compounds including both carbohydrates and amino acids. Many of the compounds were important metabolic intermediates of present day organisms.

Since free oxygen destroys most of the intermediate products, the presence of intermediate products supported the Oparin and Haldane’s hypothesis of reducing atmosphere of primitive earth. Abiotic synthesis of molecules is possible only on reducing atmosphere and abundance of free oxygen (as in earth today) prevents the abiotic origin of life. So, life is originated from the earth’s inorganic atmosphere in the past but is no longer happens today.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.