Origin of Life on Earth
According to Nebular hypothesis about 10000 to 20000 millions of years ago . There was a huge mass of highly condensed cosmic material. It exploded (Big bang) to form many nebulae . From one of the nebulae our solar system evolved . Sun evolved from the dense central part and planets evolved from the less denser peripheral part. Our earth evolved about five millions years ago. In the beginning it was the spinning ball of hot gases. Its temperature was about 6000 C. Gradually, it cooled down and heavy elemnt like Fe, Ni formed the central core and lightest elements like H, C, N, He etc formed the atmosphere but there was no free oxygen therefore the atmospheric condition wa s reducing.
Origin of life
Different philosophers and scientist have proposed different explanation for the origin of life . The main theories for the origin of life are
Theory of spontaneous generation
According to the Nebular hypothesis, all the living organisms originated spontaneously or automatically from the non-living materials and they remain unchanged . For example : Aristotle proposed that fish, frog, insect etc were developed from the mud filth, tapeworm developed from the excreta of animals.
But the theory of Abiogenesis was experimently rejected by Fransisco Ready and Lois Pasteur and they proposed theory of biogenesis (i.e. lifeoriginated from life).
According to this theory, all the living organisms were created by super natural power (God) in the form as they are present today either at a time or at regular interval.
Theory of panspermia or cosmozoic theory
According to this theory life came on the earth from other planets in the form of seeds and spores.
Theory of catastropism and sudden origin of life
Many violent and destructive changes occured on the earth time to time which destroyed existing life and formed inorganic materials life originated suddenly.
Modern theory or chemosysthetic theory :
It was proposed by Oparin and supported by Haldane. It states that the primitive life originated in the water bodies on the primitive earth by chemical evolution through a series of chmical reactions. According to this theory , atom combined to form molecules which reacted to form simple organic compounds like alcohol, aldyhides, fatty acids, lycerol, simple sugar, amino acids, purines, pyrimidines, etc. These simple organic compunds reacted to form complex organic compounds like polysaccharides, proteins, fats, nucleotides, nucleic acids etc . which accumulate to form coacervates aand the coacervates finally organised to form living cells.
Oparin explained his thory in his book “The Origin of life on the earth” published in 1936 A.D. This explanation of origin of life can be described stepwise as follows :
Chemogany: (chemical evolution)
Conditions on the primitive earth were such which favoured the chemical evolution. When the surface temperature of the earth was 100c , it atmosphere had nitrogen in the form of NH3 , carbon in the form of CH4 and oxygen in the form of H2O vapour but there was no free oxygen. So, the primitive atmosphere was reducing.
As the earth cooled down it developed a solid earth crust which later formed allevations and depressions. The atmospheric water vapour condensed and finally come on the surface as rain. Rain water collected in the depressions dissolved the minerals and finally organised to form large water bodies called oceans. The minerals and simple molecules present in the water of ocean combined and recombined by various ways to form the simple organic compounds like alcohol, aldyhides, fatty acids, glycerol, simple sugar, amino acids, purines, pyrimidines, etc.The energy for the reaction was provided by the heat of the earth and the electrical energy produced during lightening. The simple organic compounds through the process of polymerization fromed complex organic coumpunds like polysaccharides, proteins, fats, nucleotides, nucleic acids etc . The complex organic compounds accumulated to form colloidal shell like aggregates called coacervates having self – duplicating molecules and capable of growth and division.
Biogany ( Formation of Primitive lives):
Some of the proteins of cacervates started acting as enzymes (bio – catalyst). Nucleic acid associates with the proteins to form nucleoproteins which had unique quality bof duplication and behaving like chromosomes. The self duplicating capacity in nucleo proteins is due to the nucleic acids , which carrys hereditary information. Thus, nucleic acid had shown the first sign of life. Later , a thin walled cell like structure developed called protobionts or eobionts. Eobionts gave rise to prokaryots and eukaryots.
Nature of primitive life and its evolution
The first formed cells were anaerobic, prokaryotic and chemoheterotropic. These started taking organic compounds from sea water as the source of energy and to make protoplasm. They started mitosis and increased their numbers . Now to face with the decreasing supply of organic compounds. Some of those evolved into chemoautotrophs and started synthesizing own organic food from simple inorganic compounds in presence of chemical energy and enzymes. For example : Nitrifying bacteria, sulphate reducing bacteria etc.
Again now , to face the problem of decreasing inorganic compounds , some of those developed chlorophyll and started photosysthesis whcih lead to the evolution of photo autotrophs and in the process of photosynthesis O2 gas was evolved as a byproduct so aerobic autotrophs like cyanobacteria were developed. Due to the photosynthesis and evolution of oxygen the atmosphere of the earth become oxidizing . For long period the only forms of life on the earth were bacteria, moulds , cyanobacteria, etc. Gradually eukaryots like algae , fungi, protozoans were developed.
Miller and Urey Experiment :
Miller and Urey experimentally proved the formation of simple organic compounds from simpler molecules under reducing condition. They subjected mixture of CH4, NH3, H2O,H2 for about 1 week over an electric spark between two tungsten electrodes in a gas chamber placed under the reducing condition . CH4, NH3 andH2 were taken in the ratio of 2:1:2. They passed hot products through a condenser then chemically analyzed the products by chromatographic method. They found that many organic compounds like amino acids, purines, pyrimidines which are essential for the formation of nucleoproteins were present in the product. This experiment supports the Oparin theory of Origin of life.