ORIGIN AND EVOLUTION OF LIFE - Notes : 12th Biology Chapter 5 : Maharashtra State Board New Syllabus

Notes, Imp Points, Definitions and Diagrams 

For Class 12th Biology Chapter 5 Origin and Evolution of Life Based on HSC Syllabus

5.1 Origin of Life  (Protobiogenesis)

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

Various theories and hypothesis have been proposed to find the probable answer to the question of origin of life.

A. Theory of special creation :

It states that all living organisms are created by super natural power.

B. Cosmozoic thoery/ Theory of Panspermia :

It states that life descended on planet earth from other planets in form of spores or micro- organisms, called cosmozoa/panspermia.

C. Theory of spontaneous generation (Abiogenesis) :

It states that life originated from non living material spontaneously.

Louis Pasteur disproved it.

D. Theory of biogenesis :

It states that living organisms are always produced from pre-existing living forms, by process called reproduction.

It could not explain origin of first life on earth.

5.2 Chemical Evolution of Life (Self assembly theory of origin of life)

It states that life originated on earth by combinations of several chemicals through constant chemical reactions over a long period of time.

Also called, self assembly theory of origin of life or biochemical origin of life.

Formulated by Haeckel, Developed by Alexander I.Oparin and J.B.S Haldane .

It is divided into four steps : 

A) Origin of Earth and Primitive Atmosphere :

Bigbang theory : by Georges Lemaitre Explained origin of universe

It states that universe originated about 20 billion years ago by a single huge titanic explosion. 

As Universe expanded , temperature decrease and various galaxies including milky way galaxy was formed.

Earth originated in our solar system 4.6 Billion years ago.

Rotating cloud of hot gases and cosmic dust called nebula condensed and cooled and heavier elements passed to core and lighter ones remained on the surface which formed earth's atmosphere. 

Primitive atmosphere of earth was devoid of free oxygen. 

B) Formation of ammonia, water and methane :

Primitive atmosphere was very hot and slowly cooled down.

It was rich in hydrogen, carbon, nitrogen and sulphur of which hydrogen being more active reacted with other elements to form chemicals like, Methane (CH4) , water (H2O) , NH3 and H2S