The earth when it was formed about 4.8 billion years ago, was a hot revolving ball of gas consisting of atoms of various elements. Heavy elements such as iron and nickel were found in the center while comparatively lighter ones like those of aluminium and silicon formed the middle layer. The lightest elements like hydrogen, oxygen and carbon were found in the outermost layer. Due to the extremely high temperature, the atoms of these elements could not combine to form molecules.
As the earth started cooling gradually, the atoms started combining with one another to form molecules.
Water vapour present in the primitive atmosphere formed the clouds, which then resulted in rainfall continuously for several centuries. This rain water filled the hollows and basins of the earth's crust to form the oceans. Water in these oceans contained ammonia and methane. These compounds reacted among themselves to form the primitive organic compounds, which had carbon-carbon linkages. Thus, ocean water provided the basis for formation of organic compounds.
Haldane suggested that due to the accumulation of complex organic molecules, the sea ultimately became a sort of 'hot, dilute soup' where in, the molecules collided, reacted and aggregated to form more complex molecules.
Some of the proteins in protobionts are said to have developed the ability to catalyse chemical reactions, thereby functioning as the first enzymes. The formation of enzymes greatly enhanced the rate of synthesis of various molecules in the protobionts.In the course of time, the protobionts became enclosed by a protein lipid membrane, allowing the accumulation of some molecules and the exclusion of others. This property improved the ability of protobionts to survive and compete with others. With the processes of metabolism, growth and reproduction becoming regular, precise and regulated, the first cells or organisms were formed. The term progenote has been suggested by Carl Woese to describe the first cell which served as the ancestor of all the forms of life existing today.
The first forms of life developed among the organic molecules, in the oxygen free atmosphere. Hence, they presumably obtained energy by the fermentation of organic compounds. They were heterotrophs, requiring ready-made organic compounds as food.
They were prokaryotic like bacteria. They were anaerobes. They must have been dependent on the organic molecules present in the broth for body building and obtaining energy.
They were unable to synthesize organic molecules from inorganic raw materials, with the help of chemical energy obtained by the degradation of chemical compounds present in the sea.
The next step was to development of pigment molecules chlorophyll. It would absorb solar energy and convert it into chemical energy. This process is termed as photosynthesis. The earliest formed organisms were photoautotrophic bacteria. They were anaerobic and did not produce O2 as byproduct during photosynthesis, because they did not use water as a reagent.
They evolved 3300 to 3500 million years ago. They were like present day cyanobacteria and could release O2 into the atmosphere because they used water as the reagent. Thus, the whole reducing atmosphere changed to an oxidising atmosphere.Autotrophs are said to have arisen much later in the primitive earth due to a mutation in the primitive heterotrophs. The appearance of autotrophs, particularly photo autotrophs changed the situation. The appearance of photosynthetic organisms resulted in the release of free molecular oxygen into the atmosphere gradually transforming it into an oxidizing type from the existing reducing type.
The major steps in the origin of life can be summarized as follows
Steps Involved in Origin of Life