Life is a dynamic physio-chemical functional system and it starts when a cell is formed. The basic issue of evolution is chemical evolution and its product. The problems of molecular evolution and origin of life is called biogenesis and bioprocess or protobiogenesis respectively.

Theory of Special Creation:

Life on earth was created by super natural power, the GOD. According to Hindu mythology, the world and its plants and animals were created by God Brahma. Father Saurez (1548-1617) said the world was made in six natural days. It is not accepted said the world was made in six natural days, It is not accepted.

Theory of A biogenesis / Spontaneous Generation (Aristotle and Lamarck)


Life has originated from non-living organic matters abiogenetically from time to time. E.g. Development living things from slime, sea ooze and manure.

Theory of Biogenesis:

Redi (1626-1697) experimentally demonstrated that life can only arise from the pre-existing life. E.g. flies arose from the eggs but not spontaneously from dead organic matters (meat) Louis Pasteur (1862) supported this theory.

Theory of Eternity (Lord Kelvin & W. Preyer)


Life is eternal as it has existed from very beginning Preyer (1880) viewed that life comes only from the life.

Theory of Cosmozoa (Ritcher, 1865):

A few bacteria have come to earth along with meteorites from outer space.

Theory of Cosmozoa (Ritcher, 1865):


A few bacteria have come to earth along with meteorites from outer space.

Theory of Panspermia:

Arrhenius (1908), winner of Nobel Prize in chemistry. According to him organisms existed throughout universe and their spores could travel through space from one star to another.

Modern Concepts of Origin of Life:

All the above theories are unable to answer the question of origin of life on earth. A.I. Oparin, a Russian biochemist in 1924 proposed abiotic theory i.e. life arose from the abiotic (lifeless) substances. In his book origin of life (1936) he explained molecular explanation of origin of life. J.B. Haldane (1928) also made similar observation regarding the origin of life. The hypothesis of Oparin and Haldane together is known as Oparine-Haldane theory. This is the most widely accepted scientific theory of origin of life.


The Aspects of Oparine-Haldane theories are:

(1) Origin of earth and its primitive atmosphere.

(2) Chemical origin of life.

(3) Evolution of primary organisms.


Origin of Earth and its Primitive Atmosphere:

The earth originated about 4 to 6 billion years ago, due to condensation of cosmic clouds. At that time it was sphere of hot gases. The gases are condensed into a molten core. The heavy metals like iron, nickel and radioactive elements formed the core. The lighter elements like Silica, Aluminium and magnesium formed the mantel and crust. The lightest elements such as hydrogen, nitrogen, oxygen and carbon formed the gaseous atmosphere. The hydrogen was dominant element in the primitive atmosphere.

As earth got cooled gradually hydrogen combined with nitrogen, oxygen, carbon to form ammonia, methane and water respectively. The super heated steam condensed into water droplets. Torrents of boiling water would have flooded the surface of earth. The water cycle is formed. In this condition one can visualize the “hot dilute soap of Haldane” or the “warm little pond” of Darwin. The Primitime Ocean would have been alkaline. The crust is formed due to igneous rocks produced by volcanic eruptions. The atmosphere was reducing one. It was rich in methane, water vapour, hydrogen and no oxygen.

In the primitive earth, the elements such as H, C, O, N, P, and Mg are variously combined to form micromolecules (amino acids, fatty acids, glycerols, nucleotides, monosaccharides). These are polymerized and form macromolecules such as form proteins, lipids, nucleic acids and carbohydrates. These macromolecules further aggregated to form cellular membrane and organelles. These organelles together form the structural unit of life called cell. Life starts when cell is formed.


Scientific Hypothesis:

In 1923 A.I. Oparin, the Russian biochemist, stated in his book “Origin of life on earth” that living body is formed from lifeless molecules (abiotic theory). He synthesized a colloidal protenous body, the coacervates, from water, methane and ammonia in laboratory condition. He opined that a primitive cell may have formed when the coacervates were coated with lipids. Oparin, Hopkin and A.L.Herrera synthesized sulphobes from ammonia thiocyanate and formalin. Sulphobes are similar to appearance of cells.

Miller and Urey’s experiment- In 1953 Stanley Millor and Harlod Urey synthesized carbohydrates and amino acids from H2, CH4, NH4 and H2O in the laboratory condition applying 75000 volt electric discharge in sterilized apparatus made upon of closed gas tubes.

Sydney Fox (1955) could produce small yeast like cells by heating a mixture of 20 amino acids on volcanic rocks.

Formation of micromolecules- The micromolecules are formed from methane. Water and ammonia.

CH4 + H2O – Sugar, fatty acids

CH4 + H2O + NH3 – Amino acids

CH4 +H2O + NH3 +HCN – Nitrogen bases (purine & pyramidine)

– AMP (Nucleotide)


Formation of Macromolecules:

Amino acids – polymerization – Proteins

Glycerol & fatty acids – Lipids

Monosaccharides – Carbohydrates

Formation of Nucleic Acids:

Sugar + Phosphoric acid + Purine/Pyrimidine – Nucleotides

Nucleotides- Polymerisation – Nucleic acids

Formation of Nucleoproteins:

Nucleic acids agglutinated to form nucleoproteins, viruses such as non-living nucleo proteins.

Nucleic acids + Proteins – Nucleoproteins

Formation of Microphore:

Proteins – Polymerization – Proteinoids

Proteinoids – Polymerization – Microspheres

Formation of Coacervates or Coacervation:-

The proteinoids, nuceo proteins and other organic and inorganic molecules aggregated and precipited in sea form an organized structure called coacervates. These are semiliquid colloidal gel. They grow and change constantly absorbing new substances. Some protein acts as enzymes to keep the coacervates functioning. Oparine considered that the coacervates gave rise to primitive cells or Eubionts.

Formation of Earlier Type of Cells-

The Eubionts gave rise to the earlier cell types, Prokaryotes and Eukaryotes.

(a) Monerans: The nucleo proteins aggregated together in a single nucleoprotein core embedded on nutritive cellular substances. These are ancestors of modern bacteria and blue green algae (prokaryotes).

(b) Protistans: These cells have a nucleus. Nucleoproteins condensed in the center and were surrounded by a nuclear membrane. E.g. Eukaryotes.

The life supposed to have originated between 3600 to 4600 million years ago. The early cells arose in seas and oceans in form of coacervates.

Energy Source for Maintenance of Life-

The early unicellualar cells and present day micro-organisms have capacity to absorb organic substances from the environment. It is presumed that early cells absorbed organic substances in sea. Thus many methods evolved to obtain food.

Parasitism – Viruses can not multiply without a host.

Saprophytes- These organisms absorbed food from dead and decaying matters.

Chemosynthesis- Primitive bacteria nitrifying bacteria, sulphur bacteria firm bacteria derived. Energy from the inorganic substances and synthesized organic molecules. Some present day bacteria adapt this method.

Photosynthesis – evolution of chlorophyll molecule enabled certain protocells to utilize light energy and synthesize carbohydrates. These were photosynthetic cells. Organisms with such capacity are called autotrophs.

The Oxygen Evolution

The oxygen in present atmosphere is evolved from photosynthetic activities by autotrophs.