Origin & Evolution of Life »

Last Updated on Sunday, 29 January 2012 06:10 Written by Sandesh Monday, 24 May 2010 03:00

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Origin of Life

The beginning of life on primordial earth is known as Origin of Life or protobiogenesis. The presence of carbonates suggests that life was present on earth some 3.9 billion years ago. However the oldest microfossils discovered so far are those of photosynthetic cyanobacteria which appeared 3.3 to 3.5 billion years ago.

Theories on Origin of Life:
Many theories have been advanced to explain the origin of life. The major theories accounting for origin of life are as follows:
1. Special Creation
2. Spontaneous generation (Abiogenesis)
3. Biogenesis
4. Steady-state Theory
5. Extraterrestrial origin (Cosmozoan Theory)
6. Theory of Chemical Evolution (Biochemical evolution)

1. Special Creation theory:

Different religions have put forward different concepts regarding the origin of life on earth. Almost every religion has a story of its own regarding the creation of life on earth by God. Archbishop Ussher in1650 A.D. calculated that God created the world in October 4004B.C, beginning on October first and finishing with man at 9.00a.m. on the morning of October 23. The scientific knowledge based on fossil rocks, comparative morphology and physiological data disprove the idea of special creation.

2. Spontaneous Generation or Theory of Abiogenesis( a=not + bios=life=genesis=origin) :

According to this theory living things are produced spontaneously and continuously from non-living things. This was the most convincing concept regarding the origin of life until seventeenth century. This concept was held by many people including early Greek philosophers such as Aristotle. He observed that animals arouse not only from other animals but often from non-living things as well. He believed that fire flies originated from morning dew, frogs and snakes from mud, maggots from the decaying meat, spontaneously.

Van Helmont (1557-1644), a much acclaimed and successful scientist, described an experiment three weeks. The raw materials for the experiment were a sweat soaked shirt, dark cupboard and a handful of grains.

3. Theory of Biogenesis:

The concept of Biogenesis is that life comes only from pre-existing life.
Scientists like Redi, Spallanzani and Pasteur conducted classic experiments to disprove the theory of Abiogenesis. However this theory does not explain the origin of first life on earth. It only explains the continuity of life.

Francesco Redi (1668), an Italian physician was the first to refute the theory of Abiogenesis. He used a simple experiment to show that maggots did not arise from the decaying meat but were produced from the eggs of flies which were laid in meat.

(Draw the diagrams of his experiment)

Lazzaro Spallanzani (1765), also an Italian, showed that even microorganisms come into existence from those already present in the air but not spontaneously.

Louis Pasteur (1862), a French biochemist gave death blow to the theory of Abiogenesis. He performed several experiments by which he disproved the spontaneous generation of life. The simplest and the most sophisticated one were with the use of Swan-Necked flask. In this flask he prepared some yeast and sugar solution and boiled for several hours to kill the bacteria. He then left the flask unsealed on the laboratory table. The flask being unsealed, there was free exchange of air with the environment, so the system did not lack oxygen. Still the swan-necked flask remained free of microbial contamination for months because its swan-neck was shaped to trap microbes and allow only air to enter the flask. After several months when he broke the flask, contamination by air and proliferation of microorganisms in liquid started. This simple experiment thus altogether disapproved the idea of spontaneous generation.
(Draw the diagrams of his experiment)

4. Extraterrestrial origin (Cosmozoan Theory):

According to this theory, life is believed to have come to Earth from some other planets or space through meteorites.

Ritcher (1865) described that cosmic dust “protoplasm” reached on earth in form of simple particles like spores or germs.

The theory of cosmic panspermia was supported by Arrhenius (1908 AD). This theory states that life could have arisen once or several times in various parts of our Galaxy or the Universe.

5. Steady-State Theory:
According to this theory earth had no origin and has always been able to support life. Earth has changed remarkably little and that species had no origin.

6. Theory of Chemical Evolution (Biochemical evolution):

This is the modern concept of origin of life which is also referred to as Physiochemical theory.

Origin of life by the means of chemical evolution was explained by Russian scientist A. I. Oparin in 1924 and by J.B.S. Halden in 1929. Oprain’s theory was published in his book in 1936, The Origin of Life.

Oparin proposed that life must have come into existence as a result of chemical evolution which took place on the primordial earth under the impact of certain favourable conditions which no longer exists today. According to this theory, oceans of early earth were full of organic molecules, which over a long period of time became associated with each other to form complexes and such complexes gradually transformed into life.

It is generally accepted that earth originated as hot gaseous mass some five billion years ago. The atmosphere of early earth contained hydrogen, oxygen, nitrogen and carbon elements. As the early earth was very hot (4000 to 9000oC), the high temperature did not permit the formation of chemical bonds between these elements and hence they existed in form of atoms only.

(Chemogeny):

i. Formation of molecules:
As the earth started cooling, chemical bonds were formed between the atoms to form molecules. Hydrogen being active and abundance, readily combined with other atoms to form simple compounds like methane (CH4), water vapour (H2O), ammonia (NH3), hydrogen molecule ( H2 ), hydrogen cyanide (HCN). All the oxygen that was present was in combination with other elements: some of it was in water(H2O) and some in carbon dioxide (CO2). All biologists agree that there was no free molecular oxygen O2 in the early atmosphere of the earth and therefore, the earth had a reducing atmosphere.

Atoms Molecules

ii. Formation of simple organic molecules:
As the temperature of the earth came down further, say below 100 oC, the water vapour condensed into liquid and torrential rains began accompanied by violent burst of lightning. This resulted in the formation of lakes, river and great oceans on the earth’s surface. the rain water dissolved some of the atmospheric methane, ammonia, hydrogen, hydrogen cyanide and carbon dioxide. Running water contained dissolved salts and minerals of rocks. There was high energy ultraviolet radiation from the sun; radiating heat from earth’s centre and frequent electrical discharges in form lightning. Thus availability of simple compounds; reducing atmosphere: sterile environment; water and energy were responsible for the formation of simple organic compounds on the primordial earth.

The earliest organic compounds formed were as follows:

Formation of formaldehyde was an important step in the production of amino acids. The primitive oceans were thus full of organic compounds. Such water was referred to as hot dilute soup of organic compounds or primitive broth by J.B.S. Halden.

iii. Formation of complex organic compounds:
In the sterile and reducing atmosphere of the primordial earth, various reactions of the simple organic compounds together with the inorganic molecules in the oceans must have led to the formation of more complex organic compounds.
Some of these simple organic compounds would undergo polymerization to produce large molecules.
• Sugars would combine to form polysaccharides. Fatty acids could combine with glycerol to form lipids.
• Polysaccharides and lipids are important building materials and sources of energy in living organisms.
• Amino acids could combine to form proteins (formation of protein was an important landmark)
• Purines, pyramides with sugar and phosphate could develop nucleotides.
• Different nucleotides would combine to form nucleic acids, which would combine to form nucleoproteins, which are the main components of all living cells.

Thus chemical evolution took place in hot dilute soup of organic substances (prebiotic soup).

(Biogeny):

iv. Formation of Coacervates:
According to Oparin, many of these organic compounds would tend to form colloidal solutions in water. Thus protein aggregated and developed electrical charges.They formed colloidal masses as insoluble droplets which finally aggregated to form denser colloidal system. Minute water drops collected on their surfaces. Such droplets with opposite electrical charges aggregated into larger colloidal system. Oparin called such larger colloidal system as Coacervates. Coacervates are the highly organized colloidal system having different types of proteins, nucleoproteins etc. Protoplasm is also colloidal in nature, and hence coacervates were considered as perbiotic structure or protocells. Thus they may have been forerunners of life.

vii. Formation of primitive living system (protocells or protobionts or prebiotic structures):
The Coacervates would absorb further proteins from the ocenic soup (nutrition) to increase in size (growth) and then divide (reproduction). Thus the coacervates developed properties of life and eventually transformed into the primitive living things. However the transition from non living to living was not sudden but gradual.

Many scientists believe that first organism came into existence with the formation of nucleic acids because they were responsible for the inheritance of hereditary characters and reproduction in living systems.

Some scientists also believe that nucleoprotein molecules aggregated to form virus-like structure called the protovirus. The protovirus attracted other organic molecules to exhibit the phenomenon of nutrition. They increased in size which is the phenomenon of growth, and replicated, which is reproduction.

Many modern scientists are of opinion that life originated in primitive oceans as free, naked genes. These genes joined together to form long chain macromolecules of nucleic acids. Later, these molecules gathered proteins around themselves forming virus like organisms.

In this way formation of protocells began on primordial earth some 3.7 billion years ago. They were simple in structure, lacking nucleus and other cell organelles. Such structures are called Prokaryotes.

(Cognogeny):
viii. Formation of heterotrophs:
The early cells were unable to synthesize their own food. They absorbed nutrients from the organic soup. Therefore they were heterotrophs. It must be remembered that they were all anaerobes as free molecular oxygen was lacking at that time. Iron bacteria of today can be compared to such heterotrophs.

ix. Formation of chemoautotrophs:
Later some of these heterotrophs acquired enzymes which were used for synthesis of simple carbohydrate molecules from inorganic substances of the oceanic water. However in the absence of chlorophyll, they could not use the solar energy. The energy used was obtained form anaerobic breakdown. Such process is called chemoautotropism and such organisms are called chemoautotrophs which can be compared to present day deep-sea sulphur bacteria.

6CO2 + 12H2S ———-> C6H12O6 + 6H2O + 12S

x. Formation of autotrophs:
Gradually, some of these chemoautrophs synthesized chlorophyll like green pigments from magnesium prophyrin of the oceanic soup. Such pigments are called bacteriochlorophylls which could absorb sunlight and provide solar energy for synthesis of carbohydrates. Thus photosynthetic autotrophs came into existence which could be compared to certain planktonic sulphur bacteria of today.

6CO2 + 12H2S ——> C6H12O6 + 6H2O + 12S

Bacteriochlorophylls underwent certain changes in their molecular structure to give rise to true chlorophyll. Such cholorophyll bearing prokaryotes are comparable to present day blue-green algae.

6CO2 + 12H2O solar energy > C6H12O6 + 6O2

Recently, in the year 1968 some scientists have excavated a 3.2 billion year old fossil of such blue-green alga like prokaryote which has been named as Archaeospheroides barbertonesis.

As time passed by, primitive autotrophs became quiet efficient in synthesizing their own food by photosynthesis and started liberating free oxygen (O2 ) into the atmosphere. Liberation of free oxygen into the atmosphere by the autotrophic prokaryotes was revolutionary change in the early history of earth. It is referred to as oxygen revolution.

xi. Formation of eukaryotes:
With the liberation of free oxygen into the atmosphere, conditions suitable for aerobic respiration was established on the earth, presumably about 2.7 billion years ago. Gradually there was the development of free-living eukaryotic cell-like organisms in the primitive ocean, presumably about 1.5 billion years ago.

Miller-Urey Experiment:

An article tilted ” Production of amino-acids under the possible primitive earth conditions’ by Stanley Miller under the guidance of his supervisor prof. Harold Urey

has been published in the scientific journal SCIENCE (May 1953). Here he has given bit evidence in the support of biochemical evolution proposed by Oparin and Halden.

Miller devised an experiment to stimulate the conditions of primordial earth to demonstrate that simple organic molecules could formed from inorganic molecules.
Miller designed an apparatus called Spark Discharged Apparatus to conduct the experiment. He took water in the round bottom flask of the apparatus. All the air from the apparatus was removed. Methane, ammonia and hydrogen was passed in the apparatus in the ratio 2:1:2 by volume without allowing air. Water in the apparatus was then boiled to produce between the electrodes in the sparkling chamber. Convention currents produced by the circulation of steam carried the gases across the electrodes. Steam was cooled below the high energy chamber by using condenser. The products of high energy chamber were collected in U-Tube.

In this experiment, the apparatus was sterile; oxygen was lacking; apparatus filled with methane, ammonia and hydrogen; steam was provided by boiling water and electric sparks served as sources of energy.

Miller allowed experiment to run for a week and then analyzed the products in U-trap. He found number of organic compounds such as amino acids, acetic acid etc. Formation amino acid is an important step in the origin. This experiment of Miller clearly justifies the ideas projected through the theory of chemical evolution that organic molecules could be formed from the primitive gases on primordial earth.