Earth is said to have been formed about4.8 billion years ago.
· Earliest earth was a hot revolving ball of gas.
· The formation of first cells in planet earth is explained by the theory of chemical evolution proposed by Oparin and Haldane, independently.
· According to this theory, the reducing atmosphere of primitive earth helped in the formation of simple inorganic compounds followed by simple organic compounds. Then complex organic compounds and subsequently their interaction leading to the formation of self duplicating nucleic acids.
· The nucleic acids and other macro molecules became surrounded by membranes to form the protocells.
· The first forms of life were probably prokaryotic chemo-autotrophs.
· Anaerobic chemotrophs probably appeared subsequently followed by evolution of chlorophyll containing anaerobic photo-autotrophes.
· The first aerobic photo-autotrophs (cyanobacteria) are said to have appeared about 3.5 billion years ago.
· Life is said to have originated in water, because of its unique properties.
· The origin of life was followed by organic evolution with the appearance of well adapted newer form of life from the pre-existing simple forms of life.

Theories of origin of life

Several attempts have been made from time to time to explain the origin of life on earth. As a result, there are several theories which offer their own explanation on the possible mechanism of origin of life. Following are some of them:

Theory of Special Creation

According to this theory, all the different forms of life that occur today on planet earth, have been created by God, the almighty. This idea is found in the ancient scriptures of almost every religion. According to Hindu mythology, Lord Brahma, the God of Creation, created the living world in accordance to his wish. According to the Christian belief, God created this universe, plants, animals and human beings in about six natural days. The Sikh mythology says that all forms of life including human beings came into being with a single word of God. Special creation theory believes that the things have not undergone any significant change since their creation.
The theory of Special Creation was purely a religious concept, acceptable only on the basis of faith. It has no scientific basis.

Theory of Spontaneous Generation

This theory assumed that living organisms could arise suddenly and spontaneously from any kind of non-living matter. One of the firm believers in spontaneous generation was Aristotle, the Greek philosopher (384-322 BC). He believed that dead leaves falling from a tree into a pond would transform into fishes and those falling on soil would transform into worms and insects. He also held that some insects develop from morning dew and rotting manure. Egyptians believed that mud of the Nile river could spontaneously give rise to many forms of life. The idea of spontaneous generation was popular almost till seventeenth century. Many scientists like Descartes, Galileo and Helmont supported this idea. In fact, Von Helmont went to the extent stating that he had prepared a 'soup' from which he could spontaneously generate rats! The 'soup' consisted of a dirty cloth soaked in water with a handful of wheat grains. Helmont stated that if human sweat is added as an 'active principle' to this, in just 17 days, it could generate rats!
The theory of Spontaneous Generation was disproved in the course of time due to the experiment conducted by Fransisco Redi, (1665), Spallanzani (1765) and later by Louis Pasteur (1864) in his famous Swan neck experiment. This theory was disapproved, as scientists gave definite proof that life comes from pre-existing life.

Theory of Catastrophism

It is simply a modification of the theory of Special Creation. It states that there have been several creations of life by God, a catastrophe resulting from some kind of geological disturbance. According to this theory, since each catastrophe completely destroyed the existing life, each new creation consisted of life form different from that of previous ones.

A French scientist Georges Cuvier (1769-1832) and Orbigney (1802 to 1837) were the main supporters of this theory.
Cosmozoic Theory (Theory of Panspermia)
According to this theory, life has reached this planet Earth from other heavenly bodies such as meteorites, in the form of highly resistance spores of some organisms. This idea was proposed by Richter in 1865 and supported

Theory of Chemical Evolution

According to this theory,
· Spontaneous generation of life, under the present environmental conditions is not possible.
· Earth's surface and atmosphere during the first billion years of existence, were radically different from that of today's conditions.
· The primitive earth's atmosphere was a reducing type of atmosphere and not oxidising type.
· The first life arose from a collection of chemical substances through a progressive series of chemical reactions.
· Solar radiation, heat radiated by earth and lighting must have been the chief energy source for these chemical reactions.

Steps involved in Origin of life


The earth when it was formed about 4.8 billion years ago, was a hot revolving ball of gas consisting of atoms of various elements. Heavy elements such as iron and nickel were found in the center while comparatively lighter ones like those of aluminium and silicon formed the middle layer. The lightest elements like hydrogen, oxygen and carbon were found in the outermost layer. Due to the extremely high temperature, the atoms of these elements could not combine to form molecules.
As the earth started cooling gradually, the atoms started combining with one another to form molecules.

Formation of Inorganic Molecules and Compounds

With a considerable decrease in the earth's temperature over thousands of years, the atoms of different elements came together at random and formed inorganic molecules. Since the lighter elements (hydrogen, oxygen, carbon and nitrogen) were the most abundant in the outermost layer, their atoms reacted with each other to form the first inorganic molecules. Thus, the earliest molecules formed were those of hydrogen (H2), nitrogen (N2), ammonia (NH3), methane (CH4), carbon dioxide (CO2) and water vapour (H2O). All the atoms of oxygen probably combined with those of hydrogen and carbon to form water vapour and carbon dioxide. Hence, the lack of free molecular oxygen was responsible for the reducing type of atmosphere that existed on the primitive earth. The energy required for the configuration of these molecules must have come from the ultraviolet radiation in the sunlight.

Formation of Simple Organic Compounds

As the earth cooled further, the primitive inorganic molecules interacted and combined with one another to form simple organic compounds. Simple sugars, fatty acids, glycerol, amino acids and nitrogen bases (purines and pyrimidines) were probably the simple organic compounds that resulted from the interactions of the inorganic molecules.
Water vapour present in the primitive atmosphere formed the clouds, which then resulted in rainfall continuously for several centuries. This rain water filled the hollows and basins of the earth's crust to form the oceans. Water in these oceans contained ammonia and methane. These compounds reacted among themselves to form the primitive organic compounds, which had carbon-carbon linkages. Thus, ocean water provided the basis for formation of organic compounds.

Formation of Complex Organic Compounds

The smaller and simpler organic compounds that were formed initially in the earth, gradually started combining among themselves to form complex organic compounds. Simple sugars combined among themselves to form complex polysaccharides such as starch and cellulose. Fatty acids and glycerol molecules combined to form lipids. Amino acids combined among themselves to form polypeptides and proteins. Purines and pyrimidines combined with simple sugars and phosphates to form nucleotides, which in turn combined to form nucleic acids. Heat of the sun probably provided the energy required for the formation of complex organic compounds.
Haldane suggested that due to the accumulation of complex organic molecules, the sea ultimately became a sort of 'hot, dilute soup' where in, the molecules collided, reacted and aggregated to form more complex molecules.

Formation of Molecular Aggregates

It is suggested that the large organic molecules formed abiotically in the primitive earth came together spontaneously and due to intermolecular attraction, formed large colloidal aggregates called Coacervates. An envelope of water molecules formed around each such aggregate due to the hydrophilic nature of some of these compounds. A membrane of fatty acids protected and enclosed these molecules, increasing the chances of chemical reactions. Gradually, breakdown and building up reactions started for which the energy required was provided by the breakdown reactions. The coacervates selectively absorbed proteins and other materials from the ocean resulting in their active growth. The coacervates not only started growing rapidly but also started multiplying.

Formation of First Cells (Protobionts)

The coacervates were in a state of dynamic equilibrium, constantly taking in new materials from the oceans and releasing degraded materials. Thus, they had all the basic properties of life such as metabolism, growth and reproduction. However, they lacked the complexity of molecular organization, catalytic proteins (enzymes) and precise control of nucleic acids. Later, the nucleic acids are said to have taken control of coacervate and the process of replication became precise in the due course of time. With the nucleic acids being established as the genetic material, the coacervates got transformed into the primitive living systems which have been called as protobionts or eobionts.
Some of the proteins in protobionts are said to have developed the ability to catalyse chemical reactions, thereby functioning as the first enzymes. The formation of enzymes greatly enhanced the rate of synthesis of various molecules in the protobionts.

In the course of time, the protobionts became enclosed by a protein lipid membrane, allowing the accumulation of some molecules and the exclusion of others. This property improved the ability of protobionts to survive and compete with others. With the processes of metabolism, growth and reproduction becoming regular, precise and regulated, the first cells or organisms were formed. The term progenote has been suggested by Carl Woese to describe the first cell which served as the ancestor of all the forms of life existing today.
The first forms of life developed among the organic molecules, in the oxygen free atmosphere. Hence, they presumably obtained energy by the fermentation of organic compounds. They were heterotrophs, requiring ready-made organic compounds as food.

Chemoheterotrophs

They were prokaryotic like bacteria. They were anaerobes. They must have been dependent on the organic molecules present in the broth for body building and obtaining energy.

Chemoautotrophs

They were unable to synthesize organic molecules from inorganic raw materials, with the help of chemical energy obtained by the degradation of chemical compounds present in the sea.

Photoautotrophic

The next step was to development of pigment molecules chlorophyll. It would absorb solar energy and convert it into chemical energy. This process is termed as photosynthesis. The earliest formed organisms were photoautotrophic bacteria. They were anaerobic and did not produce O2 as byproduct during photosynthesis, because they did not use water as a reagent.

Aerobic Photoautotrophs

They evolved 3300 to 3500 million years ago. They were like present day cyanobacteria and could release O2 into the atmosphere because they used water as the reagent. Thus, the whole reducing atmosphere changed to an oxidising atmosphere.

Autotrophs are said to have arisen much later in the primitive earth due to a mutation in the primitive heterotrophs. The appearance of autotrophs, particularly photo autotrophs changed the situation. The appearance of photosynthetic organisms resulted in the release of free molecular oxygen into the atmosphere gradually transforming it into an oxidizing type from the existing reducing type.