The modern view of the origin of life stresses on the idea of chemical evolution. According to Oparin and Haldane, the origin of first life had been initiated from the time of the existence of the solar system (the sun with its nine planets). The earth, like the sun was made up of light and heavy elements. Heavy elements like iron, nickel etc were present in the nucleus of the earth, while the light elements and compounds like hydrogen, methane, nitrogen, carbon, ammonia, nitrogen oxide, etc in the form of vapours existed on the surface of the earth. These light elements and compounds were responsible for the first life on earth.
The earth had high temperature and radiation and had frequent and abundant discharges. In these conditions, the first life originated. Oparin and Haldane suggested that simple inorganic molecules slowly and gradually combined to produce complex organic molecules from which the simplest form of life (bacteria) came into existence. This process took a long time.
Haldane proposed that primitive earth’s atmosphere had only carbon dioxide, ammonia and water vapors. If a mixture of these gases is exposed to ultraviolet radiation, it leads to the formation of organic compounds like sugar and amino acids. As free oxygen was not available to check the radiation from reaching the earth so substances like sugar and amino acids went on accumulating under such conditions.
About 15 billion years ago, there was a huge explosion (Big Bag). The universe started expanding and the temperature dropped drastically. In time, about 4.6 billion years ago our earth and other planets appeared as part of the solar system. The primitive atmosphere of the earth was rich in hydrogen.
With the passage of time, the atmospheric temperature gradually dropped. This allowed condensation and heavy rains, which caused formation of oceans. Thunder and lightning sparks together with ultraviolet radiation caused reactions of the atmospheric gases resulting in the formation of simple organic molecules. These molecules came down with the rains and accumulated in the seas, oceans, lakes, rivers and the soil over a very long period of time. These molecules interacted and produced amino acids and proteins which are the body building substances.
The fossil evidence indicates that the earliest forms of organisms lived about 3.8 billion years ago. From this it is speculated that the origin of life started about 4 billion years ago.
The earliest organisms were heterotrophs. The depletion of the pre-existed food from the environment led to the evolution of organisms capable of making their own food. They became autotrophs, and added free oxygen into the atmosphere.
For at least the first 2 billion years of life on earth, all organisms were bacteria. About 1.5 billions years ago, the first eukaryotes appeared.
The idea of organic evolution was supported by scientists like Lamarck and Charles Darwin.