The Planet Earth came into existence about 4.5 billion years ago from a large rotating cloud of gas and dust. Life multiplies about 3.7 billion years ago. This is indicated by the discovery of micro-fossils of cyanobactena, like organisms about 3.3 to 3.5 billions years ago and presence of carbonates in about 3.9 billion years’ old rocks.

The early theory of spontaneous generation postulating the origin of life from nonliving matter was disproved by Spallanzini, Ridi and Pastur. Oparin and Ilaldane put the modern and most widely accepted theory of origin of life, named naturalistic or chemical theory. The primal earth had very high temperature and strongly reducing atmosphere having free gases such as hydrogen, nitrogen, carbon dioxide, water vapor. There was no free oxygen. As earth cooled, much of water vapor condensed into droplets and came down as rain, forming rivers, lakes, and seas. The gases interacted forming ammonia, methane and hydrogen cyanide. The primitive inorganic molecules by interaction produced simple organic molecules such as sugars, fatty-acids, glycerol, amino-acids and organic bases. The inorganic and organic molecules dissolved in sea water, forming a dilute broth or sludge. In the broth or sludge, the various molecules interacted to form complex organic compounds, namely, starches, fats, proteins and nucleic acids.

The synthesis of first proteins was perhaps non-enzymatic. The abiotically formed large organic molecules came together, and by intermolecular attraction, formed large colloidal aggregates, the cocervates. The later turned into first primitive living systems called eobionts. By developing enzymes, nucleic acid control and membrane, the eobionts became the first living cells or organisms.

Energy for chemical reactions was originally supplied by solar radiation, lightning and volcanic eruptions. Miller and Urey, by recreating conditions of primitive earth, produced amino-acids, fatty-acids and bases abiotically from simple inorganic molecules (H2O, NH3, CH4, H2 ) in the laboratory. This supported Oparin-Haldane chemical theory of origin of life.

The first organisms were heterotrophs. The presumably obtained energy by formation of organic compounds. Later, some of them became autotrophs by acquiring capacity for chemosynthesis or photosynthesis. The phototrophs released oxygen. The ultraviolet light changed some of the oxygen into ozone. The ozone layer blocked the UV light, leaving the visible light as the main source of energy. Photosynthesis changed the composition of the atmosphere to that of the present air. Because of this change, the conditions in which life can originate are unlikely to return.