CBSE Class 9 Answered

During the earth's first billion years, simple organic molecules were formed under the harsh conditions prevailing then. It is presumed that  gases such as CO₂, CH₄, NH₃, and H₂ when energized by electrical discharge or by UV radiation, react to form small organic molecules, some of which are crucial to life such as amino acids, nucleotides, sugars, and fatty acids.

Organic molecules such as amino acids and nucleotides can interact to form polymers. The polymer of amino acids and nucleotides are called polypeptides and polynucleotides respectively. These polymers are capable of directing their own synthesis. For example, a polynucleotide is able to influence the replication of other polynucleotides by acting as a template. Also, specific portions of a polynucleotide can also pair with free complementary nucleotides within itself. This self-complementary binding allow the polynucleotide to form specific structures. Much like proteins, this structure determines a polynucleotide's stability and ability to replicate.

Errors during replication of polynucleotides could influence the polynucleotide sequence and subsequently result in a different structure. Therefore, as polynucleotides undergo numerous cycles of replication, the ones that have received more beneficial mutations will survive over the ones that received non-favorable mutations.

It has been hypothesized that between 3.5 and 4 billion years ago, self-replicating systems of polynucleotides began the process of evolution. Different strands of polynucleotides competed for resources to ensure their own reproduction. To ensure the accuracy and efficiency of their own replication, polynucleotides adapted polypeptides as catalysts to aid their reproduction. Therefore polynucleotides and polypeptides became intertwined in their evolutionary struggle for survival. 

In order to ensure the survival of a specie of polynucleotide, newly synthesized polynucleotides must be retained in the same area. Also, the polypeptide that was used to facilitate the synthesis of polynucleotide must also be kept in the same vicinity. To achieve this purpose, present-day cells are surrounded by a plasma membrane that envelops the internal organelles of the cell. Current research has suggested that the first cell was formed when phospholipid molecules spontaneously assembled into a membrane structure under the primitive conditions. The oligonucleotides were then able to evolve and natural selection was able to select the cells with the most efficient system of replication machineries.