EVOLUTION NEET

7.1 ORIGIN OF LIFE

1. The Big Bang theory:  The Big Bang theory attempts to explain to us the origin of universe.

• Proposed by Abbe Lemaitre 
• The universe is very old – almost 20 billion years old.
•  Huge clusters of galaxies comprise the universe. 
• Galaxies contain stars and clouds of gas and dust. 
• Considering the size of universe, earth is indeed a speck
•  Big Bang theory talks of a singular huge explosion unimaginable in physical terms. 
• The universe expanded and hence, the temperature came down. 
• Hydrogen [H] and Helium [He] formed sometime later. 
• The gases condensed under gravitation and formed the galaxies of the present day universe. 
• 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 as it is today (oxidizing). There was no oxygen too.
• Water vapour, methane, carbondioxide and ammonia released from molten mass covered the surface (reducing earth). 
• The UV rays from the sun brokeup water into Hydrogen and Oxygen and the lighter H2 escaped.
• (H₂O + uv rays  ----->  [O] + H₂ (escaped)
• Oxygen combined with ammonia and methane to form water, CO₂ and others.
•  (O₂ + NH₄ + CH₄  --------> H₂O + + CO₂ + other)
• The ozone layer was formed. (3[O] ----> O₃)
• As it cooled, the water vapor fell as rain, to fill all the depressions and form oceans. 
• Life appeared 500 million years after the formation of earth, i.e., almost four billion years back.

2. Panspermia: 

• Some scientists believe that it came from outside. 
• Early Greek thinkers (Svante August Arrhenius) thought units of life called spores were transferred to different planets including earth. 
• ‘Panspermia’ is still a favourite idea for some astronomers. 

3. Spontaneous generation (abiogenesis): For a long time, it was also believed that life (maggot) came out of decaying and rotting matter like straw, mud, etc. This was the theory of spontaneous generation. (Aristotle, William Harvey)

4. Biogenesis:  

• Francesco Redi, an Italian physician, proved as early as 1668 that higher forms of life did not originate spontaneously by demonstrating that maggots come from eggs of flies (omne vivum ex ovo).

• Lazzaro Spallanzani's experiment showed that it is not an inherent feature of matter, and that it can be destroyed by an hour of boiling. As the microbes did not re-appear as long as the material was hermetically sealed, he proposed that microbes move through the air and that they could be killed through boiling.

• Louis Pasteur by careful experimentation (swan neck flask) demonstrated that life comes only from pre-existing life (Omne vivum ex vivo). 
• He showed that in pre-sterilised flasks, life did not come from killed yeast while in another flask open to air, new living organisms arose from ‘killed yeast’. 
• Spontaneous generation theory was dismissed once and for all after Louis pasteur's experiment. 
• However, this did not answer how the first life form came on earth is the production of new living organisms. 
• Conceptually, biogenesis is primarily attributed to Louis Pasteur and encompasses the belief that complex living things come only from other living things, by means of reproduction. 

5. Catastrofism: 

• Georges Cuvier His motivation was to explain the patterns of extinction and faunal succession that he and others were observing in the fossil record. 
• While he did speculate that the catastrophe responsible for the most recent extinctions in Eurasia might have been the result of the inundation of low-lying areas by the sea.

6. Chemical abiogenesis Oparins theory: 

• Oparin of Russia and Haldane of England proposed that the first form of life (protobiont/protocell/precell/ebiont) 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 (coacervates) from inorganic constituents. 
• Coacervates = protein + polysachharides + some water, shows some metabolism but had no lipid membrane and could not reproduce (oparin).
• Microsphere: Sydny Fox obtained these bodies by mixing of few dry Amino Acids at 130⁰-180⁰ C, later cooling them in water, resemble in size and shape of cocci bacteria also resembles budding in bacteria.
• The conditions on earth were – high temperature, volcanic storms, reducing atmosphere containing CH₄ , NH₃, etc. (reducing earth).
• Urey-S.L. Miller experiment (electric discharge experiment): In 1953, S.L. Miller, an American scientist created similar conditions in a laboratory scale.
• He created electric discharge in a closed flask containing CH₄ , H₂ , NH₃  and water vapour at 800C. He observed formation of amino acids. In similar experiments others observed, formation of sugars, nitrogen bases, pigment and fats.

• Analysis of meteorite content also revealed similar compounds indicating that similar processes are occurring elsewhere in space.
• Chemical abiogenesis not possible today, because the atmosphere today is oxidizing rather reducing as on early earth.

Try to answer

1. according to big bang theory how old is our universe?

2. The universe is very old – almost 20 billion years old.

3.  what comprise the universe?

4. what does a Galaxies contain?

5. Big Bang theory talks of how many huge explosion?  

6. In the solar system of the milky way galaxy, earth was supposed to have been formed about how many billion years back. 

7. what do you mean by reducing earth?

8. Chemical abiogenesis not possible today. Why?

9. what are the difference between coaservates and microsphere?
10. Read each line carefully and think. It can be asked as statement (true/false) or (correct/incorrect).