Theories of Origin of Life

Theories of Origin of Life:

1) Big Bang theory - Proposed by Abbe Lemaitre. 

• According to it, the universe originated about 20 billion years ago due to a thermonuclear explosion of a dense entity. This single huge explosion which is unimaginable in physical terms, is called as big bang. 
• The universe expanded and hence, the temperature came down. 
• The gaseous clouds which were formed by big bang condensed under gravitation and converted into many flat discs like structures called nebula, made up of atoms and small particles. Solar nebula was one of them, which formed our solar system. 
• The very hot central part of solar nebula became still hotter and converted into the sun. 
• Later, on due to condensation of atoms and dust particles moving around the sun other planets were formed. 
• In the solar system of the Milky Way galaxy, earth was supposed to have been formed about 4.5 billion years back. 
• There was no atmosphere on early earth. It was formed later. 

2) Theory of special creation:

• The greatest supporter of this theory was Father Suarez. This is a mythology-based theory.  
• This theory has three connotations- 
• All living organisms (Species or types) that we see today were created as such. 
• Diversity was always the same since creation and will be the same in future. 
• The earth is about 4000 years old. 

All these ideas were strongly challenged during the nineteenth century based on observations of Charles Darwin, Wallace, etc. They believed that life forms varied over periods of time. From fossils records and their dating, we can conclude that earth is very old, not thousands of years as was thought earlier but billions of years old. 

3) Cosmic panspermia theory: 

• Some scientists believe that life came from outer space. 
• Early Greek thinkers thought units of life called spores were transferred to different planets including earth. 
• ‘Panspermia’ is still a favourite idea for some astronomers. 

4) Theory of spontaneous generation (Abiogenesis/ Autogenesis):

•  This hypothesis was supported by ancient Greek philosophers. 
•  According to this theory, life came out of decaying and rotting matter like straw, mud, etc. Spontaneously. 
• They believed that the mud of the Nile River could give rise to fishes, frogs, crocodiles etc when warmed by light rays. 

5) Theory of biogenesis - Proposed by Harvey and Huxley 

• They stated "Omnis Vivum ex ovo or vivo", which means "New life can be originated on earth only by pre-existing life."  
• Experiments of Francesco Redi, Lazzaro Spallanzani, Louis Pasteur etc supported the theory of biogenesis and disproved abiogenesis.
• The experiment of Louis Pasteur is the most renowned among all of these. 
• Hence spontaneous generation theory was dismissed once and for all. However, this did not answer how the first life form came on the earth. 

6) Experiment of Louis Pasteur:  

• His experiment is also known as the 'Swan neck flask experiment'. 
• He prepared sterilized syrup of sugar and killed yeast by boiling them in flasks. 
• He took two flasks one of broken neck and another of curved neck (swan neck flask/ “S” shaped neck flask). 
• He showed that in pre-sterilized swan neck flasks, life did not come from killed yeast because germ-laden dust particles in the air were trapped by the curved neck which serves as a filter while in another flask open to air (broken neck), new living organisms arose. 

7) Oparin- Haldane theory (Modern theory) 

• Oparin of Russia and Haldane of England proposed that the first form of life could have come from pre-existing non-living organic molecules (e.g., RNA, Protein, etc.) and that formation of life was preceded by chemical evolution. i.e., formation of diverse organic molecules from inorganic constituents. 
• Oparin's theory was published in his book 'ORIGIN OF LIFE'. 
• First life originated in seawater, so water is essential for the origin of life. 

8) Chemical Evolution (chemogeny): 

• The primitive conditions on earth were high temperature, volcanic storms, lightening and reducing atmosphere.  
• Early earth had free atoms of all those elements which are essential for formation of protoplasm (C, H, O, N etc.).  
• Hydrogen was maximum among all of them.  
• Due to high-temperature hydrogen reacted with oxygen to form water and no free oxygen was left, which made the atmosphere reduced. 
• Hydrogen also reacted with nitrogen and formed ammonia.  
• Hence Water and ammonia were probably the first inorganic compounds formed on earth. 
•  Methane (CH₄) was the first organic compound. 
• As the earth cooled down, the water vapour fell as rain, to fill all the depressions and form primitive oceans. During this, molecules continued to react with each other and formed various simple complex organic compounds. 
• Now, the water of oceans became a rich mixture of macromolecules/ complex organic compounds. Haldane called it Hot dilute soup/pre-biotic soup. 
• Hence the possibilities of life were established in the water of primitive oceans because these macromolecules (Proteins, polysaccharides, fats/lipids, nucleic acids) form the main components of protoplasm. 

9) Biological Evolution (Biogeny): 

i) Origin of protobionts:  

• Macromolecules which were synthesized abiotically in primitive oceans later came together and formed large colloidal drop like structures named as protobionts.  

• It is believed that they were the clusters of proteins, polysaccharides, lipids, nucleic acids etc. 
• These protobionts were unable to reproduce but they could grow by absorbing molecules from their surroundings and can exhibit simple metabolism. 

ii) Origin of protocells (Eobionts)-  

• Nucleic acid developed the ability of self-duplication due to a sudden change called mutation.  
• Nucleic acid and proteins combined to form nucleoproteins. Nucleoproteins were the first sign of life. 
• Clusters of nucleoproteins are surrounded by lipid coat called protocell, the first form of life. 
•  These first non-cellular forms of life could have originated 3 billion years ago.  
• They would have been giant molecules (RNA, Protein, Polysaccharides, etc.). These capsules reproduced their molecules perhaps.  
• Altman (1980) discovered that some RNA molecules have enzymatic activity, called as ribozymes. It means at the time of origin of life, RNA molecule could carry out all the processes of life (replication, protein formation etc) without the help of either protein or DNA. Hence this concept called as RNA World.  

iii) Origin of first cellular form (Prokaryotes) -  

• As a result of mutation protocells became more complex and efficient to use the materials available in the surrounding medium and evolved into prokaryotic cells. 
• This first cellular form of life did not possibly originate till about 2000 million years ago. 
• The first living beings were single celled bacteria like prokaryotes with naked DNA.  
• They were probably chemoheterotrophs and anaerobic.  
• Some of the chemoheterotrophic bacteria evolved into chemoautotrophs. They were anaerobic and synthesized organic food from inorganic material; this mode of nutrition is called as chemosynthesis. e.g., Iron bacteria, Nitrifying bacteria etc. 
  
  
• When bacteriochlorophyll was developed in some chemoautotrophic bacteria, they started to convert light energy into chemical energy, this mode of nutrition is called photosynthesis. They used H₂S as source of hydrogen instead of H₂O hence they were non-oxygenic photosynthetic bacteria. e.g., Planktonic sulphur bacteria 
• Some molecular changes occurred in bacteriochlorophyll, and it transformed into true chlorophyll. Such organisms used H₂O as source of hydrogen and released oxygen in the environment, they were oxygenic photosynthetic bacteria.  e.g., Cyanobacteria (Blue green algae) 
  
  

iv) Oxygen Revolution: 

The liberation of free oxygen by cyanobacteria was a revolutionary change in the history of the earth. It includes some major changes like 

1. The atmosphere of the earth changed from reducing to oxidizing, and hence possibilities of further chemical evolution are finished because chemical evolution always takes place in a reducing environment. 
2. Free O₂ oxidized CH₄ and NH₃ to form gases like CO₂, N₂ and H₂O. 
3. Accumulation of free oxygen formed a layer of ozone outside the atmosphere of earth which started to absorb most of the UV rays of sunlight. 
4. Some prokaryotes adapted themselves for an aerobic mode of respiration which provides approx. 20 times more energy than anaerobic respiration.

v) Origin of Eukaryotic cell:

• Nucleus, mitochondria and other cell organelles developed in the cell and metabolically it became more active.
  
• These free-living unicellular eukaryotic organisms originated about 1.5 billion years ago in the primitive ocean.  

10) Evidence in Favour of Chemical Evolution: 

i) Harold Urey and Stanley Miller Experiment: 

• In 1953, S.L. Miller, an American scientist created similar conditions at a laboratory scale which were thought to be on the primitive earth. 
• He took CH₄, NH₃, H₂ and water vapour at 800° C in a large flask. 
• He created electric discharge by using two tungsten electrodes as a source of energy.
• He observed the formation of simple amino acids like glycine, alanine, and aspartic acid.
• In similar experiments other scientists observed, the formation of sugars, nitrogen bases, pigment and fats. 

ii) Evidence from meteorites:  

• Analysis of meteorite contents also revealed similar compounds indicating that similar processes are occurring elsewhere in space.
• Evolution of giant organic molecules from simpler inorganic constituents - Chemical evolution. 
• Evolution from macromolecule aggregates/coacervate to simple cell - Biological evolution 
•  Evolution from simple cell to recent - Organic evolution  
• Oparin's theory is also known as primary abiogenesis and it is based on artificial synthesis, so also called as artificial synthetic theory. 
• Louis Pasteur also proposed the 'Germ theory of diseases' and he is famous for his pasteurization technique.  
  

• From protocells or eobionts, few cores of nucleoproteins get separated free in oceans and became inactive but when they enter in another eobiont they became active so virus-like structures were formed. This is an example of retrogressive evolution.  
• The universe originated about 20 bya.  
• The solar system and earth were formed about 4.5 bya. l Life appeared about 4 bya.  
• Non-cellular form of life appeared – 3 bya. 
• First cellular form of life – 2 bya.