EVOLUTION

 

ORIGIN OF LIFE:

• Stellar distances are measured in light years.
• The universe is very old – almost 20 billion years old.
• The Big Bang theory attempts to explain to us the origin of universe.

The Big Bang theory:

• A singular huge explosion unimaginable in physical term.
• The universe expanded and hence the temperature came down.
• Hydrogen and Helium 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.

Condition of early earth:

• Earth formed 4.5 billion years back.
• There was no atmosphere on early earth.
• Water vapor, methane, carbon dioxide and ammonia released from molten mass covered the surface.
• The UV rays from the sun broke up water into Hydrogen and oxygen and lighter H₂ escaped.
• Oxygen combined with ammonia and methane to form water, CO₂ and others.
• The ozone layer was formed.
• 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.

Origin of life:

• Early Greek thinkers thought units of life called spores were transferred to different planets including earth.
• ‘Panspermia’ is still a favorite idea for some astronomers.
• For a long time it was also believed that life came out of decaying and rotting matter like straw, mud etc.This was the theory of spontaneous generation.

Louis Pasteur experiment:

• Careful experimentation demonstrated that life comes only from pre-existing life.
• He showed that in pre-sterilized flasks, life did not come from killed yeast while in another flask open to air, new living organism arose from ‘killed yeast’.
• This disproved the theory of spontaneous generation.

Oparin – Haldane theory of origin of life:

• Oparin of Russia and Haldane of England proposed that the first form of life could have come from pre- existing non-living organic molecule (e.g. RNA, protein etc.).
• Formation of life was preceded by chemical evolution i.e. formation of diverse organic molecule from inorganic constituents.

 Urey and Miller experiment:

• The conditions on earth were –
  • High temperature.
  • Volcanic storms.
  • Reducing atmosphere containing CH₄, NH₃ etc.
  
  
• In 1953, S.L.  Miller an American Scientist created similar conditions in a laboratory scale.
  • He created electric discharge in a closed flask to raise temperature upto 800oC as it was in primitive earth.
  • Used CH₄ H₂, NH₃ and water vapor inside the flask.
  • He observed the formation of amino acids.

Acceptance of chemical evolution theory: (evidences)

• Miller observed the synthesis of amino acids from simple inorganic chemicals in simulated condition in the laboratory.
• 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.

Theory of biogenesis:

• The first non-cellular forms of life could have originated 3 billion years back.
• They would have been giant molecules (RNA, proteins, Polysaccharides, etc).
• These capsules reproduced their molecules perhaps, named as coaservates.
• The first cellular form of life did not possibly originate till about 2000 millions years ago.
• The first cellular forms of life were probably unicellular.
• All life forms were in water environment only.
• This theory of biogenesis from non-living molecules was accepted by majority.

EVOLUTION OF LIFE FORMS – A THEORY:

• Conventional religious literature tells us about the theory of special creation.
• The theory of special creation has three connotations:-
  • All the living organisms (species types) that we see today were created as such.
  • The diversity was always the same since creation and will be same in future.
  • Earth is about 4000 years old.

Challenge to special creation theory:

• Observation made during a sea voyage in a sail ship called H.M.S. Beagle round the world. Charles Darwin concluded that existing life forms share similarities to varying degrees not only among themselves but also with life forms that millions of years ago.
• Many such life forms exist any more. There had been extinctions of different life forms in the years gone by just as new forms of life arose at different periods of history of earth.
• There has been gradual evolution of life forms.
• Any population has built in variation in characteristics.
• Those characteristics which enable some to survive better in natural conditions (climate, food, physical factors, etc) would outbreed others that are less-endowed to survive under such natural condition.
• Survival of the fittest. The fitness according to Darwin refers ultimately and only leaves more progeny than others.
• These, therefore, will survive more and hence are selected by nature. He called it as natural selection.
• Alfred Wallace, a naturalist who worked in Malay Archipelago had also come to similar conclusions around the same time.
• The geological history of earth closely correlates with the biological history of earth.

 

CBSE Biology (Chapter Wise) Class XII ( By Mr. Hare Krushna Giri )
Email Id : [email protected]