ORIGIN OF LIFE
Huge clusters of galaxies comprise the universe. Galaxies contain stars and clouds of gases and dust. The Big Bang theory attempts to explain to us the origin of universe. It talks of a singular huge explosion unimaginable in physical terms. The universe expanded and hence, the temperature came down. Hydrogen and Helium formed sometime later. The gases condensed under gravitation and formed the galaxies of the present day universe. There was no atomosphere on early earth.
Water vapour, methane, carbon dioxide and ammonia released from molten mass covered the surface. The UV rays from the sun brokeup water into hydrogen and oxygen the lighter H2 escaped. Oxygen combined with ammonia and methane to form water, CO2 and others. The ozone layer was formed. As it cooled, the water vapour fell as rain, to fill all the depressions and form oceans.
Conventional religious literature tells us about the theory of special creation. This theory has three connotations. One, that all living organisms (species or types) that we see, today were created as such. Two, that the diversity was always the same since creation and will be the same in future also. Three, that earth is about 4000 years old.
For a long time it was also believed that life came out of decaying and rotting matter like straw, mud, etc. This was the theory of spontaneous generation. Louis Pasteur by careful experimentation demonstrated that life comes only from pre-existing life. He showed that in pre-sterilised flasks, life did not come from killed yeast while in another flask open to air, new living organisms arose form 'killed yeast'. Spontaneous generation theory was dismissed once and for all.
Early Greek thinkers thought units of life called spores were transferred to different planets including earth. 'Panspermia' is still a favourite idea for some astronomers.
Oparin of Russia and Haldane of England proposed that the first form of life could have come from pre-existing non-living organic molecules (e.g., RNA, protein, etc.) and that formation of life was preceded by chemical evolution, i.e., formation of diverse organic constituents. The conditions on earth were - high temperature, volcanic storms, reducing atmosphere containing CH4, NH3 etc.
The entire process of the origin of life, as proposed by Oparin, can be summarized as ` under :
Step 1. Formation of simple inorganic molecules : The earth's atmosphere at that time was reducing, not oxidizing (as it is today). The primitive atmosphere of earth had hydrogen, nitrogen, water vapours, carbon dioxide, methane and ammonia in abundance. Oxygen was not available in free state in sufficient quantities.
Step 2.Formation of simple organic compounds : Sugars, glycerol, fatty acids, amino acids, pyrimidines and purines etc.
Step 3.Formation of complex organic compound : The simple organic compounds combined in different ratios to form complex organic compounds lide polysaccharides, fats and proteins.
Step 4.Formation of nucleic acids and nucleoproteins.
Step 5. Formation of coacervates.
Step 6. Fromation of primitive cell : Prokaryotic cells.
Step 7.Origin of autotrophism : Prokaryotic cells which were chemoautotrophs, became photoautotrophic thus, release of O2 in the atmosphere and its free availability was the result of photosynthesis.
Step 8.Origin of eukaryotic cells.
In 1953, S.L. Miller, and American scientist created similar conditions in a laboratory scale. He created electric discharge in a closed flask containing DH4, NH3, H2 in the ratio of 2 : 1 : 2 and water vapour at 8000C. He observed formation of amino acids. In similar experiments other observed formation of sugars, nitrogen bases, pigment and fats.
The origin of life is considered a unique event in the history of universe. The universe is very old - almost 20 billion years old. In the solar system of the milky way galaxy, earth was supposed to have been formed about 4.5 billion years back.
Life appeared 500 million years after the formation of earth, i.e., almost four billion years back. The first non-cellular forms of life could have originated 3 billion years back. They would have been giant molecules (RNA, Proteins, Polysaccharides, etc.) . These capsules reproduced their molecules perhaps. The first cellular form of life did not possibly originate till about 2000 million (2 billion) years ago. These were probably single-cells. All life forms were in water environment only.