Maharashtra Board Class 12 Biology Chapter 5 Origin and Evolution of life PDF Download

Origin And Evolution Of Life

5.1 Origin Of Life: (Protobiogenesis)

The living matter shows attributes or characters like responsiveness, growth, metabolism, energy transformations and reproduction.

As far as origin of life is considered, it has remained an enigma for intellectuals at all times. Despite of advancements in various fields like biochemistry, astrobiology, geography, molecular biology, etc. scientists are unable to ascertain the truth. Various theories and hypotheses have been proposed to find the probable answer to this question.

a. Theory Of Special Creation:

It is the oldest theory and is based on religious belief without any scientific proof. It states that all living organisms are created by a super-natural power.

b. Cosmozoic Theory/Theory Of Panspermia:

This theory advocates that life did not arise on the planet Earth. It may have descended to the earth from other planets in the form of spores or micro-organisms, called cosmozoa/panspermia. Recently, NASA has reported fossils of bacteria-like organisms on a piece of Martian rock recovered from Antarctica.

c. Theory Of Spontaneous Generation (Abiogenesis):

According to this theory, life originated from non-living material spontaneously. This theory was disproved by Louis Pasteur.

d. Theory Of Biogenesis:

According to this theory, living organisms are always produced from pre-existing living forms, by process called reproduction.

Theory of biogenesis however could not explain origin of first life on earth but could explain only the continuity of life.

Teacher's Note

Living things always come from other living things. Just like babies come from mothers, all life comes from life.

Exam Trick

Remember: Biogenesis = life from life. Think of a baby born from parents. That is biogenesis.

Points to Remember

Living things come from pre-existing living things.
Reproduction is how new life is made.
Biogenesis does not explain the first life on Earth.
This theory only explains how life continues.

5.2 Chemical Evolution Of Life (Self Assembly Theory Of Origin Of Life):

According to this theory, life originated on earth by combinations of several chemicals through constant chemical reactions over a long period of time. This theory is also called self assembly theory of origin of life or biochemical origin of life.

This theory was first formulated by Haeckel but later developed by the Russian scientist Alexander I. Oparin (1924) and British biologist J. B. S. Haldane (1929). The process of chemical evolution can be divided into following steps:

a. Origin Of Earth And Primitive Atmosphere:

The origin of universe was explained by the Big-Bang theory of Georges Lemaitre (1931). According to this theory the Universe originated about 20 billion years ago by a single huge titanic explosion. As the universe expanded, the temperature decreased and various galaxies of solid objects were formed. Milky Way is one such galaxy of which our solar system is one small part. Earth is one of the planets of solar system and originated about 4.6 billion year ago. When formed, it was a rotating cloud of hot gases and cosmic dust called Nebula. The condensation and cooling resulted in stratification with heavier elements like nickel and iron passing to the core and lighter ones like helium, hydrogen, nitrogen, oxygen, carbon, etc. remaining on the surface. They formed the atmosphere of the earth. The primitive atmosphere of the earth was quite different from the present one and it was of a reducing type, devoid of free oxygen.

b. Formation Of Ammonia, Water And Methane:

Primitive atmosphere was very hot. As it slowly cooled, the lighter elements started to react with each other. The early atmosphere was rich in hydrogen, carbon, nitrogen and sulphur of which hydrogen being more active, it reacted with other elements to form chemicals on earth like CH₄, NH₃, H₂O and H₂S.

c. Formation Of Simple Organic Molecules:

As temperature of the earth decreased, steam condensed into water that resulted in heavy rain fall and the earth gradually cooled. Rain water got accumulated on the land to form rivers, streams, lakes, seas and oceans. The atmosphere then did not contain ozone layer and thus ultra-violet radiations reached the surface of earth directly. Under the influence of available energy sources such as ultra-violet rays, radiations, lightening and volcanic activities, the early molecules of hydrocarbons, ammonia, methane and water underwent reactions like condensation, polymerisation, oxidation and reduction. These reactions resulted in formation of simple molecules like monosaccharides, amino acids, purines, pyrimidines, fatty acids, glycerol, etc. All these simple organic molecules accumulated at the bottom of water bodies. Haldane described it as the "hot dilute soup" or "primitive broth". It did not show any degradation due to absence of free oxygen and enzymes.

Teacher's Note

The early Earth had no oxygen. Simple molecules formed in the oceans. This is like how sweets dissolve in water to make sweet juice.

Exam Trick

Remember: "Hot dilute soup" = primitive broth. Think of chicken soup cooling down - that is what Earth's oceans were like long ago.

Points to Remember

Early Earth was very hot with no oxygen.
Simple molecules formed from chemicals in the air.
These molecules collected in the ocean.
Haldane called this the "primitive broth".

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