Oparin was a Russian Scientist of biochemistry, who suggested a theory of the origin of life in his published book ‘Origin of life’, in 1963. According to this theory, the primitive oceans contained a large quantity of organic compounds, these would react with one another to form structures of increasing complexity. This theory can be divided into five stages. 

1. First stage: The origin of the earth and its primitive atmosphere- The earth has been presumed to have originated about five billion years ago. It was as hot as the Sun. Initially, there existed a hot rapidly rotating ball of gases, made up of free molecules of carbon, hydrogen, oxygen and nitrogen, along with other free molecules of elements. Gradual cooling led to the formation of the water molecule, which provided a suitable medium for chemical reactions, due to which several compounds such as methane (CH₄), ammonia (NH₃) and water were formed. They were dissolved in water, that was collected in the form of oceans, lakes, rivers etc. According to oparin, it is in this water that life originated first. 

2. Second Stage: Synthesis of simple compounds- The formation of water, ammonia and methane- the large bodies of water having minerals and salts was the key event in the origin of life. Methane being reactive, having fused with other compounds to form simple organic compounds such as Ethane, Butane, Propane, Ethyl Alcohol (C₂H₅OH), Methyl alcohol (CH₃OH), Glycerol, Fats, Sugars, etc. The dissolved ammonia forms the nitrogenous organics like amino acids, purines, pyramidines and finally, nucleic acid. Hence, in this phase all the chemical compounds are formed in the oceanic water, required for the origin of life.

3. Third Stage: Formation of macro-molecules or complex organic molecules- Pyramidine and purines, combined with phosphorus and sugars of oceanic water and formed nucleotides ultimately from DNA and RNA. This sort of chemical reactions went on. Nucleic acids, proteins and fats on synthesis form nucleoproteins. In short, in this step, the formation of nucleoproteins takes place by polymerisation of nucleotides. These molecules were like naked genes. Their formation indicates the step for differentiating between life and no-life. Hundreds and thousands of these nucleotides were linked together in different combinations, producing the complete nucleic acids. 

4. Fourth Stage: Formation of Nucleoproteins- Due to the aggregation tendency of molecules, giant molecules of nucleoproteins were formed, by the union of nucleic acid and protein molecules. It is presumed that such giant nucleoproteins had certain characteristics of a free-living gene. A number of such genes might have aggregated to form a single large unit. 

Thus, by aggregation tendency of molecules, large or macromolecules of nucleoproteins are formed, around which a coat of the macromolecules of carbohydrates(like cellulose), fats and minerals were formed, in the form of a membranous structure. The central core around which these membranes developed is referred to as organic soup, which has free entry and consists of molecules. In this way, the membrane-bound ‘soup’ acquired the properties of incorporating the absorbed molecules and to split a part of a portion of itself. This represents the primitive cell, which is still without a distinct and definite nucleus. Such cells are still found, in certain algal forms. This could be compared with the present day, virus and was named as proto-virus. This shows that the nucleus developed later than the cell. Thus, the virus is regarded as a connecting link between the living and non-living forms. 

5. Fifth Stage: Formation of the primary organism called coacervates- The big organic macro-molecules has a tendency to aggregate in various combinations to form huge, colloidal particles called “Coacervates”. These coacervates with nucleoprotein molecules surrounded by nutrient shells and limiting membrane were supposed to be the first living cell. Natural selection must be operated at this level, and the coacervates with more favourable internal arrangements became more abundant. Those with nutritive shells proved to be more successful and became abundant and finally, led to the development of cells.