CBSE Class 12 Biology
Revision Notes
CHAPTER- 07
EVOLUTION
Evolutionary biology is the study of history of life forms on earth. The evolution of life
earth, different changes in flora and fauna around earth that co-exist along with human
beings also forms parts of evolution.
Origin of Life
The origin of life is considered unique events in the history of universe. Huge cluster of
galaxies comprises the universe. Galaxies contain stars and clouds of dust and smoke.
Big Bang Theory attempts to explain the origin of universe. According to this theory, a
huge explosion occurs that forms the different galaxies.
In solar system of Milky Way galaxies, earth has been supposed to be formed about 4.5
billion years ago. There was no atmosphere in early earth. Water vapour, methane, carbon
dioxide and ammonia released from molten mass covered the earth surface.
UV rays from sun splits the water into hydrogen and oxygen. Life appeared 500 million
years after the formation of earth.
There are different theories regarding the origin of life on earth-
•Some scientist believes that life comes from other planets. Early Greek thinker
thoughts that unit of life is called spores transferred from other planets.
•According to other theory, life comes out of dead and decaying matters like straw and
mud. This theory is called theory of spontaneous origin.
•Louis Pasture experimentally proved that life arises only from pre-existing life.
Spontaneous theory of origin of life is dismissed after that.
•Oparin and Haldane proposed that the first form of life could have come from pre-
existing non-living organic molecules like RNA and protein etc. The formation of life
preceded by chemical evolution. At that time condition on earth were- high
temperature, volcanic eruption, reducing atmosphere containing CH4and NH3.
Miller experiment of Origin of Life- S.L. Miller in 1953, conducted an experiment to show
the origin of life on earth in the physical environment similar to condition prevails at that
time.
, In the figure X= Methane, Ammonia, Water vapour & Hydrogen
Fig.: 7.1 Diagrammatic Representation of Miller's Experiment
Miller created similar condition of temperature and pressure in laboratory scale. He
created electric discharge in a flask containing CH4, H2 and NH3 and water vapour at
8000C.
He observed formation of amino acids in flask after 15 days of electric discharge. Similar
experiment by other scientist found formation of sugars, nitrogen bases, pigments and
fats.
Analysis of meteorite content also reveals similar compounds that reveal that similar
process are occurring elsewhere in the space. This experimental evidence about the
origin of life is called chemical evolution of life.
Experimental representation of Miller’s experiment
The first non-cellular forms of life could have originated 3 billion years back. They could
have been giant molecules like RNA, Protein, and Polysaccharide etc.
The cellular form of life was probably single cell and originates in water medium. The
theory that first form of life arose slowly through evolutionary forces from non-living
molecules is called biogenesis.
Evidence of Evolution: Evidence that evolution of life forms has taken place on earth
have many proofs as mentioned below-
1. Paleontological evidence- different aged rock sediments contain fossils of different life
forms that probably died during the formation of particular sediment. Fossils are remains
Revision Notes
CHAPTER- 07
EVOLUTION
Evolutionary biology is the study of history of life forms on earth. The evolution of life
earth, different changes in flora and fauna around earth that co-exist along with human
beings also forms parts of evolution.
Origin of Life
The origin of life is considered unique events in the history of universe. Huge cluster of
galaxies comprises the universe. Galaxies contain stars and clouds of dust and smoke.
Big Bang Theory attempts to explain the origin of universe. According to this theory, a
huge explosion occurs that forms the different galaxies.
In solar system of Milky Way galaxies, earth has been supposed to be formed about 4.5
billion years ago. There was no atmosphere in early earth. Water vapour, methane, carbon
dioxide and ammonia released from molten mass covered the earth surface.
UV rays from sun splits the water into hydrogen and oxygen. Life appeared 500 million
years after the formation of earth.
There are different theories regarding the origin of life on earth-
•Some scientist believes that life comes from other planets. Early Greek thinker
thoughts that unit of life is called spores transferred from other planets.
•According to other theory, life comes out of dead and decaying matters like straw and
mud. This theory is called theory of spontaneous origin.
•Louis Pasture experimentally proved that life arises only from pre-existing life.
Spontaneous theory of origin of life is dismissed after that.
•Oparin and Haldane proposed that the first form of life could have come from pre-
existing non-living organic molecules like RNA and protein etc. The formation of life
preceded by chemical evolution. At that time condition on earth were- high
temperature, volcanic eruption, reducing atmosphere containing CH4and NH3.
Miller experiment of Origin of Life- S.L. Miller in 1953, conducted an experiment to show
the origin of life on earth in the physical environment similar to condition prevails at that
time.
, In the figure X= Methane, Ammonia, Water vapour & Hydrogen
Fig.: 7.1 Diagrammatic Representation of Miller's Experiment
Miller created similar condition of temperature and pressure in laboratory scale. He
created electric discharge in a flask containing CH4, H2 and NH3 and water vapour at
8000C.
He observed formation of amino acids in flask after 15 days of electric discharge. Similar
experiment by other scientist found formation of sugars, nitrogen bases, pigments and
fats.
Analysis of meteorite content also reveals similar compounds that reveal that similar
process are occurring elsewhere in the space. This experimental evidence about the
origin of life is called chemical evolution of life.
Experimental representation of Miller’s experiment
The first non-cellular forms of life could have originated 3 billion years back. They could
have been giant molecules like RNA, Protein, and Polysaccharide etc.
The cellular form of life was probably single cell and originates in water medium. The
theory that first form of life arose slowly through evolutionary forces from non-living
molecules is called biogenesis.
Evidence of Evolution: Evidence that evolution of life forms has taken place on earth
have many proofs as mentioned below-
1. Paleontological evidence- different aged rock sediments contain fossils of different life
forms that probably died during the formation of particular sediment. Fossils are remains
