Evolution - Notes | Class 12 | Part 1: Origin of Life
Evolution: Origin of Life
Origin of Life
Evolution is an orderly change from one form to another.
Evolutionary Biology is the study of the evolutionary history of life forms.
Big Bang Theory states that the 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
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.
Biogenesis: Proposed by Francisco Redi, Spallanzani, & Louis Pasteur. It states that life originates from pre-existing life. But it does not explain the origin of first life.
Cosmic Theory (Theory of Panspermia): It states that the units of life (spores) were transferred to different planets including Earth.
Theory of Special Creation: It states that living things were created by some supernatural power (God).
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
Harold Urey & Stanley Miller experimentally proved the theory of chemical evolution. They created a condition like that of primitive Earth (i.e., high temperature, volcanic storms, reducing atmosphere with CH4, NH3, H2O, H2, etc.).
They made electric discharge in a closed flask containing CH4, NH3, H2, and water vapour at 800o C. As a result, some amino acids were formed.
In similar experiments, others observed the formation of sugars, nitrogen bases, pigments, and fats.
First non-cellular forms of life originated 3 billion years ago. They were self-replicating metabolic capsules containing RNA, proteins, polysaccharides, etc.
