The details of the origin of life are unknown, but the broad principles have been established. There are two schools of thought about the origin of life. One suggests that organic components arrived on Earth from space (see “Panspermia”), while the other argues that they originated on Earth. Nevertheless, both schools propose similar mechanisms by which life initially arose. 

पृथ्वीवरील सजीवसृष्टीची सुरुवात नक्की कधी झाली याबद्दल मतांतरे आहेत. Perhaps it arose around 4 Ga. 
In the energetic chemistry of early Earth, a molecule gained the ability to make copies of itself–a replicator. (More accurately, it promoted the chemical reactions which produced a copy of itself.) The replication was not always accurate: some copies were slightly different from their parent. 

If the change destroyed the copying ability of the molecule, the molecule did not produce any copies, and the line “died out”. On the other hand, a few rare changes might make the molecule replicate faster or better: those “strains” would become more numerous and “successful”. This created evolution. As choice raw materials (“food”) became depleted, strains which could exploit different materials, or perhaps halt the progress of other strains and steal their resources, became more numerous.

The nature of the first replicator is unknown because its function was long since superseded by life’s current replicator, DNA. Several models have been proposed explaining how a replicator might have developed. Different replicators have been posited, including organic chemicals such as modern proteins, nucleic acids, phospholipids, crystals, or even quantum systems. 
There is currently no way to determine whether any of these models closely fits the origin of life on Earth. 

One of the older theories, and one which has been worked out in some detail, will serve as an example of how this might occur. The high energy from volcanoes, lightning, and ultraviolet radiation could help drive chemical reactions producing more complex molecules from simple compounds such as methane and ammonia. Among these were many of the relatively simple organic compounds that are the building blocks of life. As the amount of this “organic soup” increased, different molecules reacted with one another. Sometimes more complex molecules would result—perhaps clay provided a framework to collect and concentrate organic material. 

The presence of certain molecules could speed up a chemical reaction. All this continued for a very long time, with reactions occurring more or less at random, until by chance it produced a replicator molecule. In any case, at some point, the function of the replicator was superseded by DNA; all known life (except some viruses and prions) use DNA as their replicator, in an almost identical manner (see "Genetic code").

Modern life has its replicating material packaged inside a cellular membrane. It is easier to understand the origin of the cell membrane than the origin of the replicator, because a cell membrane is made of phospholipid molecules which often form a bilayer spontaneously when placed in water. Under certain conditions, many such spheres can be formed (see “The bubble theory”). 

The prevailing theory is that the membrane formed after the replicator, which perhaps by then was RNA (the RNA world hypothesis), along with its replicating apparatus and maybe other biomolecules. Initial protocells may have simply burst when they grew too large; the scattered contents may then have recolonized other “bubbles”. Proteins that stabilized the membrane, or that later assisted in an orderly division, would have promoted the proliferation of those cell lines. 

RNA is a likely candidate for an early replicator, because it can both store genetic information and catalyze reactions. At some point DNA took over the genetic storage role from RNA, and proteins known as enzymes took over the catalysis role, leaving RNA to transfer information and modulate the process. There is increasing belief that these early cells evolved in association with underwater volcanic vents known as "black smokers" or even hot, deep rocks. 

It is believed that of this multiplicity of protocells, only one survived. Current evidence suggests that the last universal common ancestor lived during the early Archean eon, perhaps roughly 3.5 Ga or earlier. This “LUCA” cell is the ancestor of all life on Earth today. It was probably a prokaryote, possessing a cell membrane and probably ribosomes, but lacking a nucleus or membrane-bound organelles such as mitochondria or chloroplasts. 

Like all modern cells, it used DNA as its genetic code, RNA for information transfer and protein synthesis, and enzymes to catalyze reactions. Some scientists believe that instead of a single organism being the last universal common ancestor, there were populations of organisms exchanging genes in lateral gene transfer.

Proterozoic eon
भूगर्भशास्त्रीय कालदर्शिका
सजीवसृष्टीची सुरुवात
Late Proterozoic climate and life