(A) Earliest Atmosphere on Earth:

Earth development around 4.6 billion years back by condensation of ylem or cloud of consmic dust and gases. It was made of radioactive elements which decayed and generated heat. Condensation and cooling resulted in stratification with heavier elements (e.g. Nickel, Iron).

Passing to the core, intermediate to middle (e.g. silicon, Aluminium) while the lighter ones remained on the surface and the atmosphere. Since the surface was very hot, the lighter elements occurred only in their atomic state. They included hydrogen, carbon, nitrogen and oxygen.

(B) Formation of Early Molecules:

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With slight lowering of the surface temperature of the earth, the lighter elements interacted to form water (H2O), methane (CH4), ammonia (NH,), carbon monooxide (CO), carbondioxide (CO2) and hydrogen cyanide (HCN). Metal carbides, nitrides and oxides also formed on the surface. Free oxygen disappeared and the environment became reducing. Initially water existed as superheated steam.

On cooling, it came down as hot water rain but evaporated again. This continued for several centuries till the surface of earth came to have hot water oceans and lakes with large number of dissolved salts coming from atmosphere, bottom, shore-line and volcanoes.

(C) Formation of Simple Organic Molecules:

Surface of earth had a temperature of 50°-60°. It was constantly receiving UV radiations and cosmic rays. Lightning was frequent. This helped methane, ammonia, water and other molecules to form a whole array of simple organic substances like aldehydes, alcohols, organic acids, amino acids, purines, pyrimidines, pentose, hexose and other sugars, fatty acids, glycerol etc. Water bodies changed into a broth/ primordial soup/prebiotic soup.

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Experimental Proof:

Stanley Cloyed Miller (1953, Miller and Urey, 1953) took ammonia, water, methane and hydrogen in a sterilised apparatus having provisions for heating, cooling, circulation and electrical discharges.

The materials and conditions resemble those present on early earth. The experiment was continued for one week after which the contents of the apparatus were analysed. A number of simple organic compounds were present-amino acids (glycine, alanine, aspartic acid, glutamic acid), organic acids (formic, acetic, propionic, lactic acid, succinic), purines, pyrimidines and sugars.

(D) Formation of Larger Organic Molecules:

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In the priomordial/prebiotic hot, sterile soup, the simple organic molecules reacted amongst themselves to form newer and larger molecules. The environment was oxygen free. Radiations, lightning and volcanic eruptions were abundant. Simple sugars condensed to form polysaccharides, fatty acids and glycerol reacted to produce fats while amino acids condensed into peptides, polypeptides and proteins.

All these reactions occurred without the agency of enzymes, only anri rihncomes S W Fox (1957) obtained polypeptides by heating a mixture or amino acias 101 long period. Nucleotides developed due to condensation of nitrogen bases (purines, pyrimidines), phosphoric acid and pentose sugar.

(E) Formation of Complex Biomolecules:

Some of the nucleotides came to have more phosphates and functional as energy suppliers. Nucleotides also underwent polymerisation to form nucleic acids, DNA and RNA. DNA has the ability of replication and transcription.

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Enzymes and coenzymes came into existence. ATP prophyrin and other biomolecules have been synthesized under hot,oxygen free, iradiated environment stimulating the one present on early earth (Fox, 1964).

Biological Evolution

(F) Formation of Coacervates/Pre-cells/Pre- bionts:

On the shores, organic molecules tended to aggregate due to wave action and partial drying up. Some of these molecular aggregates formed Coacervates. Coacervates developed the ability to grow and divide giving rise to microspheres that were separated from their environment by lipid membrane.

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Microspheres could grow by absorbing material from outside. They could also increase in number by budding. Close association of biomolecules evolved into an organisation based on interactions and development of enzymes for the same.

(G) Formation of Protocells/Eobionts/ Protobionts:

Some of the microspheres having DNA and RNAs evolved further. They developed lipoprotein membrane and enzyme mediated reactions. They are called protocells, eobionts or protobionts. The latter developed their own internal environment distinct from outside. Nucleic acids could replicate and help in multiplication of eobionts.

It is probable that viruses developed simultaneously with the evolution of protocells. They could affect assembly of their parts by controlling reaction in the premordia/or prebiotic soup.

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(H) First Cells/Living Beings:

Protocells evolved into cells and living beings. They were procaryotic unicells which had naked DNA, protein manufacturing machinery, mode of energy liberation and its utilization. The first procaryotes were anaerobes and chemoautotrophs. They absorbed organic molecules from outside for body building and energy.

(I) Development of Chemautrophs:

Drops in temperature stopped synthesis of organic molecules in the water bodies. Organic content of primordial soup decreased due to continuous withdrawl by chemoautotrophs. Some of the early procaryotes got converted into chemoautotrophs.

They developed the ability to synthesise organic materials from inorganic raw materials with the help of energy obtained from reducing inorganic substances of the medium. Some present day chemoautotrophs are iron bacteria, nitrifying bacteria, sulphate reducing bacteria.

(J) Development of Phototrophs/Pigments evolved which could trap solar energy. The absorbed energy could be used in various reactions. Certain prokaryotes developed bacteriochlorophyll and later chlorophyll. The pigment could trap solar energy which could be used in synthesis of organic material from inorganic substances. The process is called ‘photosynthesis’.

The first photoautotrophs were anaerobic and performed ‘anoxygenic photosynthesis’, Hydrogen was obtained from sources other than water. All present day photoautotrophic bacteria are anoxygenic e.g. Chlorobium, Rhodoseudomonas. Aerobic photoautotrophs developed around 3300-3500 million years ago in the form of cyanobacteria or blue-green algae.

They performed ‘oxygenic photosynthesis’ using water as hydrogen donor and liberating oxygen. Fossils of their cyanobacteria or blue-green algae have been found in rocks as old as 2-9 billion years old, e.g. Archaeophoroides barhertonensis. In India 3.2 billion years old fossils of photosynthetic blue green algae have been found from Kashia region in Orissa. Oxygen accumulated in the atmosphere.

The latter became oxidising. Methane and ammonia disappeared because they react with oxygen to form C02 and N respectively. High energy radiations in the upper layers of atmosphere changed some of the oxygen into ozone. Ozone formed a shield that protects the living beings from UV radiations.

(K) Development of Eukaryotes:

Eukaryotes evolved around 1600 million years back. They came into existence due to (i) Evolution from prokaryotes through mutations (Raff and Mailer, 1972) (ii) Symbiotic association of different types of prokaryotes (Marguilis, 1970).