7. EVOLUTION
Evolution
is an orderly change from one form to
another.
Evolutionary Biology is the study of evolutionary history of life forms.
ORIGIN OF LIFE
- Big
Bang Theory states that universe originated
about 20 billion years ago by a singular huge explosion.
- The earth was formed
about 4.5 billion years ago.
- There
was no atmosphere on early earth. Water vapour, CH4, CO2
& NH3 released from molten mass covered the surface.
- The UV rays from the sun
broke up water into H2 and O2.
- Oxygen combined with NH3
& CH4 to form water, CO2 etc.
- The ozone layer was
formed. As it cooled, the water vapour fell as rain to form oceans.
- Life appeared almost four
billion years ago.
THEORIES OF ORIGIN OF LIFE
1. Theory
of spontaneous generation (Abiogenesis): It
states that, life came out of decaying and rotting matter like straw, mud etc.
Louis
Pasteur disproved this theory. He demonstrated
that life comes only from pre-existing life.
He
showed that life did not come from killed yeast in a closed pre-sterilized
flask. But in an opened flask, life (microbes) appeared.
2. Biogenesis:
Proposed by Francisco Redi, Spallanzani
& Louis Pasteur. It states that,
life originates from pre-existing life. But it does not explain origin of first
life.
3. Cosmic theory (Theory of
Panspermia): It states that, the units of life
(spores) were transferred to different planets including earth.
4. Theory of special
creation: It states that, living things were created
by some supernatural power (God).
5. Theory of chemical
evolution: Proposed by Oparin & Haldane.
It states that, the first form of life was originated from non-living inorganic
& organic molecules such as CH4, NH3, H2O,
sugars, proteins, nucleic acids etc. i.e. “Abiogenesis
first, but biogenesis ever since”.
Urey-Miller experiment
-
- They made electric
discharge in a closed flask containing CH4, NH3, H2
and water vapour at 800o C. As a result, some amino acids are
formed.
- In similar experiments,
others observed formation of sugars, nitrogen bases, pigment and fats.
First non-cellular forms of life originated 3 billion years ago. They were self-replicating metabolic capsule containing RNA, proteins, Polysaccharides etc.