1. The atomic stage -
The earth was originated about 4.6 billion years ago. Early earth had free atoms of all those elements whichare essential for the formation of protoplasm.The lightest atoms like carbon, hydrogen, nitrogen and oxygen formed the primitive atmosphere.
- Most abundant of all of them was hydrogen.

2. Molecular stage (Origin of molecules and simple Inorganic compounds)-
Free atoms combined to form molecules and simple inorganic compounds. Due to presence of high temperature, active hydrogen
atoms combined with all oxygen atoms to form water and leaving no free oxygen.Thus the primitive atmosphere was reducing (without free oxygen) unlike present oxidising atmosphere (with free oxygen). Hydrogen atoms also combined withnitrogen to from Ammonia.
(The first molecular compounds formed were probably water and Ammonia).
These Lighter elements also formed C02, CO, N2, H2 etc.

3. Origin of early organic compounds -
The nitrogen and carbon of the atmosphere combined with metallic atoms formiog nitrides and carbides. Water vapour and metallic carbides reacted to form the first organic compound Methane{CH4). Later on hydrogen cyanide(HCN) was formed.
Water which formed on earth due to high temperature evapourated so clouds were formed. Water vapour changed into rain drops and by the collection of water on earth primiiiw: oceans were formed.

4. Origin of simple organic compounds -
Water of primitive oceans contained large amount of methane, ammonia, hydrogen, cyanides, carbides, nitrides.
These early compounds interacted and formed simple organic compounds like, aldehyde, Ketones, Alcoholes, Pentose and hexose sugar, Amino Acids, Glycerol, Fatty Acids, Purines, Pyrimidines etc.
Energy was obtained from U.V. Rays of sunlight, cosmic rays and heat of volcanic eruptions.

5. Origin of complex organic compounds -
The small simple organic molecules combined to form large complex organic molecules.
- Amino acids Joined to form polypeptides and proteins, which were non-enzymatic and non structural.
-Simple sugar units combined to form polysaccharides.
- Fatty acids and glyceroles united to form fats and lipids.
- Sugar, nitrogenous bases, phosphates combined into nucleotides which polymerized into nucleic acid, which
unable to replicate.
These macromolecules forms main component of protoplasm hence the possibility of origin of life in primitive
oceans could be established.
After long time the water of primitive oceans became rich mixture of organic compounds as a result of chemical
evolution.
Haldane called this saturated water of oceans as prebiotic soup or hot dilute soup.

(a) Origin of Protobionts (Coacervates) :

Macromolecules which were synthesized abiolically in primitive ocean later came together and formed large colloidal drop like struclures named as Protobionts Each protobiont was a cluster of macromolecules. They contain proteins,nucleic acids, lipids, polysaccharides etc. They grew by absorbing molecules from their environment. Were unable to reproduce many chemical reactions including the decomposition of glucose look place inside the protobionts.
According to oparin coacervates were the first sole living molecules which gave rise to cell.

(b) Origin of protocells [Eobiont] :

The first living form named protocell originated in the primitive oceans.The protocell were clusters of nucleo proteins which formed by composition of nucleic acids and enzymatic proteins. Nucleoproteins had the property of self duplication. Nucleo proteins were first sign of life. The protocell represented the beginning of life. From prolocells or eobionts few core of nucleoproteins gets sepera!ed free in oceans and became inactive but when they enter in another eobionts they became active so virus like structures were formed.
Origin of virus like structure is an example of retrogressive evolution (complex to simple). While evolution which inhibits earth surface was progressive means simple to complex.

(c) Origin of Prokaryotes :
As a result of mutation the protocells became more complex and efficient to use the materials available in the surrounding medium and condensed themselves into prokaryotic cells. Thus the firs! living being were prokaryotic like bacteria they were single celled and consisted of naked DNA. Nutritionally they were chemoheterotrophs (saprotrophs), respiration was anaerobic.

Origin of Autotropism -
It includes the origin of chemosynthesis and photosynthesis.
(a) Origin of chemosynthesis :
- Due to continue withdrawal of organic molecules by chemoheterotrophs organic material decreased in oceans.
- Before the organic material disappeared in sea, new modes of nutrition developed, one of them was chemosynthesis.
- The organism which perform chemosynthesis are called as chemoautotrophs. They were anaerobic and synthesise organic molecules from inorganic material. The energy was obtained by oxidising inorganic materials present in the sea.
- Such mode of nutrition is found in Bacteria e.g. sulphur bacteria, iron bacteria, nitrifying bacteria.

(b) Origin of Photosynthesis :
- After some time bacteriochlrophyll developed in some autotrophic bacteria like organism. They could absorb solar energy and convert it into chemical form. These organism called photoautotrophs.
- They utilise solar energy for synthesizing organic compounds. This process is called photosynthesis.
- They were anaerobic and utilised hydrogen from sources other than water like H2S. Therefore, no oxygen was evolved and atmosphere remained reducing.
- This stage of photosynthetic autotrophism is represented by planktonic sulphur bacteria of today.

During the origin and evolution of life liberation of free 02 by blue green algae like prokaryotes due to photosynthesis was a revolutionary change in the history of earth. It is called oxygen revolution.
It includes important changes like -
(1) Atmosphere of earth changed from reducing to oxidising, hence possibilities of further chemical evolution and abiogenesis finished, because chemical evolution always take place in reducing environment.
(2) Free O2 oxidized CH4 and NH3 to form gases like C02, N2 and H2O.
(3) Accumulation of free 02 formed a layer of 0 3 (ozone) above the atmosphere of earth. Which started absorbing most of the UV rays of sunlight.

- Billion of years ago conditions became suitable for aerobic respiration with the release of free O2 . Aerobic respiration yields about 20 times more energy then anaerobic respiration hence the themselves for aerobic mode of respiration.
- Nucleus, mitochondria and other cell organelles developed in the cell and thus free living eukaryotlc organism originated about 1.5 billion years ago in the primitive ocean.

1. Though life originated by chemical evolution on primitive earth, was later replaced by organic evolution.
2. Organic evolution states "Descent with modification". i.e. the present day complex organism have evolved from earlier simpler organism by small but gradual changes which have occurred over millions of years.
3. Though living organisms show great diverstiy in size, structure, funclion, behaviour etc. they also show
basically similar metabolic processes indicating common ancestory.