The chemical evolution of life is the most widely accepted theory of origin of life. It is based on the fact that primitive earth was different from the earth which is present today. It was proposed by Russian bio-chemist, A. I Oparin (1923) and was supported by Englishman J. B. S Haldane (1928). This theory can be well-explained under the following headings:
Formation of the earth
When the earth was formed, it was extremely hot with temperature of 5000 centigrades. Everything in the atmosphere was in molten state. Gradually, the constituents of the earth cooled down and formed primitive atmosphere.
Formation of simple molecules
Even after the formation of the earth, there existed certain simple molecules like hydrogen, nitrogen, etc. These elements were arranged under different strata according to their densities in the atmosphere. Soon, these reactive elements combined to form simple molecules like water, methane, ammonia, hydrogen cyanide, etc. These compounds settled in the atmosphere when the earth began to cool and condensation of water occurred to give rise to water cycle. These molecules got mixed with the rain water and settled in the water resources formed by the water cycle.
Formation of simple organic compounds
The chemical synthesis of the organic compounds was supposed to occur in the warm primeval oceans or primordial soup. The energy for the synthesis of these chemicals were abundantly provided by the UV radiation from the sun as well as the rocks, heat energy from the volcanoes and electric energy from the lightening.
Different organic molecules like amino acids, simple sugars and nucleic acid were formed spontaneously from the methane, ammonia and water. Since there were no organisms to consume these compounds and no oxygen for the oxidation of intermediate compounds, the organic compounds slowly accumulated in the primordial soup.
Formation of complex organic compounds
The polymerization of the simple organic compounds gave rise to complex molecules of these compounds. Some of the compounds formed then were polysaccharides, fats, nucleotides, purine, pyrimidine, polypeptides, etc. It is thought that nucleotide is the basic unit of the constituent of the firstly evolved cell-nucleic acid.
Formation of first cell-like aggregates
These organic molecules got aggregated in a compact colloidal mass covered by water layer which is termed as coecervates or microspheres. Coecervates have limiting membranes and can absorb nutrient from any medium. They were called coecervates by Oparin and microspheres by Sydney Fox. These aggregates could reproduce by budding, fragmentation and binary fission.
Formation of primitive cell
The coecervates may have produced the first cell-like structures by absorbing nutrients from the primordial soup or broth. ii. The first cell-like structure which possessed the power of division were called eobionts or pre-cell. Oparin called it protobionts which originated about 3800-4200 million years ago.
Evolution of the cells
The firstly formed cells were anaerobic, prokaryotic and chemoheterotrophs. Their number became huge such that the food supply didn’t meet there requirement. As a result, they started to seek alternatives due to which photoautotrophic, anaerobic and prokaryotic organisms were formed. The first oxygenic (aerobic) photoautotrophs were cyanobacteria which were evolved around 3300-3500 million years ago. These underwater dwellers passed out oxygen to the atmosphere through photosynthesis. Hence, the atmosphere became oxidizing type.
This led to the evolution of the first terrestrial photoautotrophs. It also signified the end of abiosynthesis. The chemical evolution was followed by biological evolution. For a long period of time, the dominant and perhaps the only forms of life were bacteria, moulds and cyanobacteria. Gradually blue green algae evolved into other forms of algae. The slow evolution of the prokaryotes gave rise to the eukaryotes via mutation. Therefore, life existed from slow changes in the earth’s atmosphere giving rise to the first cell upto today’s extent.