It has always been intriguing hat how life originated on the earth. Of the many theories of the origin of the life, perhaps the oldest is the theory of spontaneous generation, according to which even complicated forms of Life might arise from non-living matter.

Until about 300 years ago, most people assumed that spontaneous generation was reasonable and even complex animals like rats were supposed to arise spontaneously in 21 days from a dirty shirt placed in contact with kernels of wheat.

Francisco Reid, an Italian physician criticized the theory of spontaneous generation and give experimental proofs in support of his criticism. He proved that the maggots which ordinarily appeared in the decaying meat were developed from eggs laid in the meat by the adult flies. Reid thus proved that life can arise only from pre-existing life.

This concept is also known as biogenesis. Later the experiments of Spallanzani also criticize the authenticity of spontaneous generation. Later Pasteur (1860) demonstrated that air-borne bacteria ordinarily infected exposed broth or other suitable media, and that bacteria arose only from preexisting bacteria. The various theories of origin of life have been given below:

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1. Theory of Spontaneous Generation:

According to it all forms of life might arise spontaneously from non-living matter. This theory was criticized by Francisco Redi and Louis Pasture.

2. Theory of Special Creation:

This is a mythological explanation. According to it some super natural being (God) created all the living organisms in the same form as they exist today, right at the time of creation of the earth. This theory was rejected due to its lack of experimental proof.

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3. Cosmologic Theory:

According to this theory the original spores of the life reached the earth accidentally from some other part of the universe. This theory is rejected for two reasons – First because of the intense radiations of the interstellar space no form of life can with stand, second there is no evidence that living things other than well protected space travelers, have been able to survive in outer space.

4. Autotroph & Heterotroph Theory:

An organism capable of making food is called an autotrophy, such as all green plants. This hypothesis assumes that an organism of one of these types of Foodmaker was the first form of life. This hypothesis has a serious flaw because chemical reactions involved in making food are very complex and require a complex organism.

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If early organisms were able to synthesize food, we must then assume that autotrophy were complex organisms from the very beginning. The theory of heterotrophy assumes that a simple organism originated in complex surroundings. According to this theory the earliest from of life developed form non­living and it was not able to make its food.

5. Modern Synthetic Theory:

The proponent of this theory was A.I. Oaring and J.B.S. Haldane. Since then a number of researches have contributed towards this theory. According to this theory, earth originated as a result of condensation of cosmic clouds made up of vapors of atoms of different gases and also suspended molecules of metals and compounds, with heavier elements and compounds going towards the centre and lighter ones towards the surface.

The earth’s atmosphere contained H2, N2, and water vapors. The gradual cooling resulted in the formation of newer inorganic compounds, CH4, H2O, H2, CO2, NH3 and HCN, etc. From these molecules of new simple organic compounds were formed in the due passage of time.

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For example, sugars, fatty acids, glycerin etc. are formed form methane and water, amino acids from methane, water and ammonia and nitrogen bases from methane, water, ammonia and cyanides. All these were experimentally prepared under simulated conditions in the laboratory by Stanley Miller (1953).

These molecules assembled into small spherical structures (Coacervates or micosphere) with a membrane of several molecules in thickness and containing concentrated solution of similar molecules. These also contained perhaps a system of reflexive catalysis. After attaining a certain maximum size these divided into two. These can be called first precellular life.

A further association of these molecules produced big complex molecules of greater functional significance, more probably inside the coacervates. For example, polysaccharides are formed from sugar molecules; fats from amino acids; nucleotides from nitrogen bases, sugar and phosphates and nucleic acids from nucleotides.

Further changes in these chemical molecules resulted in the evolution of first informed nucleic acid and protein based genetic system. Thus bacteria like prokaryotic cell originated which has the following composition. Water + Minerals + Carbohydrates + Fats + Proteins + Nucleic acid + Adenosine Phosphates.

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The atmosphere was still reducing type. From prokaryote cell evolved eukaryote type of cell. The next most important incident was the origin of chloroplast molecule having ability to utilize solar energy in the synthesis of food and this in turn produced free atmospheric oxygen.

The atmosphere, thus, became oxidizing type and aerobic respiration came into existence. From this point onward the origin of Metazoa and metaphyta took place.