Life is a dynamic physio–chemical functional system and it starts with formation of a single cell.
Life on Earth began about 3.5 billion years ago. Life is a continuum extending from the earliest organisms to the great variety of species that exist today.
It is difficult for us to appreciate exactly how far back life has existed on Earth. The origin of life is a necessary precursor for biological evolution. The current scientific consensus is that the complex biochemistry that makes up life came from simpler chemical reactions, but it is unclear how this occurred. Consequently, there is no scientific consensus on how life began, but proposals include self–replicating molecules such as RNA, and the assembly of simple cells.
Scientists have showed that the electrical discharges of lightning, radioactivity, and ultraviolet light caused the elements in the early Earth atmosphere to form the basic molecules of biological chemistry, such as nucleotides, simple proteins and ATP. It seems likely, then, that the Earth was covered in a hot, thin soup of water and organic materials. Over time, the molecules became more complex and began to collaborate to run metabolic processes.
RNA World is the first proposed step in transitioning from non–living to living.
RNA World is the first proposed step in transitioning from non–living to living. Several hypotheses describe the assimilation of chemicals into an order that responds to the environment in a living sort of way. If we assume those ideas are correct, the next step is the world of the living.
Origin of Prokaryotes and Eukaryotes: Prokaryotes are the first to origin. Prokaryotes are the organisms that lack a cell nucleus(= karyon), or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Prokaryotes are mostly bacteria, and their advancements led to more complex living organisms, called eukaryotes. Eukaryotic cells differ in many respects from the generally smaller cells of bacteria and archaea. Even the smallest single–celled eukaryote is far more complex in structure than any prokaryote.