Published in Zoology
Thursday, 08 June 2017 22:52
The origin of life on earth and organic evolution are two fundamental aspects in modern biology. Origin of life means the origin of first living organism. Organic evolution means the formation of complex organism from simple organism over a period of time.
It means the formation of life on the planet Earth. Both living and non living things contain the same chemical elements. These are carbon, hydrogen, oxygen and nitrogen. But living things differ from non living things in having energy flow in an ecosystem, metabolism, reproduction, etc.
Many theories were formulated to explain the origin of life before modern times. They are special creation, cosmozoic theory, spontaneous generation and biogenesis.
A. Special Creation theory:
It is the oldest theory. Special creation theory states that all living organisms are created by super natural power. It is a relegious view. Special creation theory was discarded as it did not stand in scientific arguments.
B. Cosmozoic theory:
According to this theory, life did not arise in our planet. But life was descended from other planet where life existed previously. The believers of this theory assumed that life was brought to the earth through some agencies like meteorite (mass of stone or metal that has reached the earth from outer space). Some meteorites do contain organic molecules. However, simple organic molecules do not constitute life. This theory explains the existence of life on earth. But the cosmozoic theory does not explain the formation of first life.
C. Spontaneous generation theory:
This theory states that life arose suddenly from non living matter. Hence spontaneous generation is also called abiogenesis (a - no; bio-Life, genesis - formation). It was proposed by Aristotle. According to this theory, the fishes and frogs arose from moist soil, maggots and worms arose from decaying organic meat, field mice arose from mud of Nile river, etc.
Why abiogenists was believed to be correct for such a long time? It is because the complex life cycles of animals were not observed and many microscopic stages were unobserved. So, scientific information was not available in that period.
D. Biogenesis theory:

It was proposed by Francisco Redi and Louis Pasteur. According to this theory life arose from pre existing life. Redi practically proved that life never arose from non living matter. Maggots will arise from decaying organic meat if it contains eggs only. If eggs are absent on such meat, never maggots will arise. 

Even though biogenesis explains the formation of living organism from pre existing life, it never explains the origin of first living organism. But it laid the foundation for proposing the theories of organic evolution.

The first modern theory of origin of life was proposed by A.I. Oparin and Haldane independently. Oparin explained the origin of life from non living materials through the chemical evolution of life. Oparin’s theory is called naturalistic theory or theory of coacervates.
The approximate age of earth is 4,600 million years. Where as the age of first life is 3,500 million years.
The origin of sun, earth and other planets is explained by cosmic evolution. It was proposed by Sir James Jeans. According to him, before 4,600 million years ago, the nebula was a ball like mass. It contained high temprature and everything is in the form of a gas. One star was attracted by the nebula. As the star speedily colloided with nebulous mass, the outer atmosphere was broken into nine pieces. These pieces were sprayed in the different distances. As these pieces moved away they gradually lost the temperature. These nine pieces were formed into planets. The remaining nebulous mass is being considered as the sun. As the earth began to cool, the cosmic dust was condensed. Hence on earth solids, liquids and gases are co existing.

The different steps in the formation of first living organism are explained below. 

A. Environment of early earth:
Before 3,500 million years, the earth contained six elements. They are hydrogen, oxygen, carbon, nitrogen, sulphur and phosphorus. These are present through out the solar system. Among these hydrogen is most prominent. According to Oparin, these elements react with one another due to lightinings and formed into methane, ammonia and water vapour. This hypothesis was correct as the scientists observed methane, ammonia, hydrogen sulphide and water vapour in Jupiter and Neptune. So early atmosphere of earth is a reducing atmosphere. Now the environment of earth is an oxidising atmosphere.
B. Production of polymers:
It is the formation of complex molecules, like glycogen, fats and proteins. According to Oparin all these were formed from the above simple molecules. The polymerisation of aminoacids lead to the formation of essential proteins. Like wise sugars united and formed into glycogen. Glycerol and fatty acids united and formed into fats. All these polymerisations were occured due to the ultraviolet rays and inorganic catalysts of marine water.
C. Origin of nucleic acids:
The nitrogenous bases, phosphates and sugars united and formed into nucleosides. The nucleotides were polymerised and formed into long poly nucleotide chains. This caused the formation of nucleic acids. The polymerisation may be due to inorganic catalysts of marine water.
Once nucleic acids were formed, transcription and translations were also formed. This caused the formation of essential proteins. The nucleic acids were also developed the property of self duplication. The early proteins and nucleic acids were formed by some non enzymatic processes.
The proteins, nucleic acids and other materials might have been formed into groups in marine water. Each group developed a small transparent membrane around. These water like globules are called coacervates. These developed a blue coloured pigment. This caused the &igin of photosynthesis. Now this is called first organism. This organism resembles to present day Cyanobacterium. So, the life first arose in marine water as microscopic, autotropic and anaerobic. This prokaryote later developed the membrane around the nucleic acids. This caused the formation of first eukaryotic cell. 
Miller and Urey experimentally proved the formation of complex molecules in the laboratory. They supported Oparin’s hypothesis.
Miller made an apparatus to synthesize complex materials, amino acids. Miller’s apparatus contains two rounded glass flasks and glass tubes. The rounded flasks are connected by glass tubes. In one flask, Miller took fresh water. He took C, H, N and phosphorus in one connected tube. In the another flask, Miller took marine water. He boiled the fresh water. Hence steam was produced. The steam was passed over the elements and finally mixture was sent into marine water flask. Miller and Urey observed ammonia, methane and phsophates in the marine water flask. These chemicals sent into a glass tube, which was connected to electrodes. The electrodes produced sparks. Then the chemical mixture was cooled and sent into the ‘u’ shaped tube. After one week, urey and Miller observed amino acids in the ‘u’ tube. •This experiment proved that the amino acids are produced from inorganic chemicals. It also proved the hypothesis of Oparin.

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