The Theory of Abiogenesis
The theory of abiogenesis explains that life arose from non-living things more than 3.5 million years ago. In the early history of science, this idea was proposed to convince how organic molecules originated from inorganic molecules by the input of great amounts of energy. These molecules, with the passage of time, formed the self-replicating molecule which with the help of other molecules, originated from abiogenesis, eventually created the basic structures of life such as the cell.
This theory further proposes that first originated life forms were simple but gradually increased in complexity. Just as the populations evolve with time, so are the molecules. Scientists propose that the first self-replicating molecules were RNA molecules. As in almost all life form on earth, the RNA molecules have the ability to catalyze the formation of new RNA molecules. So, the one RNA molecule that formed just right, started replicating and produced identical RNA molecules.
Over the time, the RNA molecules evolved and acquired such mutations which allowed it to synthesize proteins which were able to make more RNA. Further mutations caused such proteins to be created which started making strands of DNA from RNA. So, the major constituent of the modern genome was created. Over millions of years of evolution, these molecules increased in complexity and gave rise to the modern life forms we see today.
Biogenesis (the idea which proposes that life is derived from the reproduction of other life) was preceded by abiogenesis, as abiogenesis became impossible after the earth got its present composition. Many scientists that study abiogenesis argue at the point where abiogenesis switches to biogenesis.