Origin of Universe

• The universe is approximately 20 billion years old (as mentioned in NCERT).
• Distances of stars are measured in light years.
• Looking at distant stars means looking into the past, because their light takes years or millions of years to reach Earth.

Big Bang Theory

• Explains the origin of the universe.
• Universe originated from a singular massive explosion.
• After expansion, the temperature decreased.
• Hydrogen and helium were the first elements formed.
• Gases condensed due to gravity, forming galaxies and stars.

Formation of Earth

• Earth formed about 4.5 billion years ago.
• Early Earth had no atmosphere.
• Primitive atmosphere contained:
  • Water vapour (H₂O)
  • Methane (CH₄)
  • Ammonia (NH₃)
  • Carbon dioxide (CO₂)
• Free hydrogen escaped into space.
• Oxygen combined with methane and ammonia.
• Ozone layer formed later.
• Rain formed oceans as Earth cooled.

Origin of Life

• Life appeared about 4 billion years ago.
• Life originated nearly 500 million years after Earth’s formation.

Panspermia Theory

• Suggests that life came from outer space.
• Early Greek philosophers proposed that spores reached Earth from space.

Spontaneous Generation Theory

• Claimed life originated from non-living decaying matter.
• Examples: mud, straw, rotting organic matter.
• This theory was rejected.

Louis Pasteur Experiment

• Demonstrated that life arises only from pre-existing life.
• Sterilized broth remained free of life unless exposed to microorganisms.
• Established the principle of Biogenesis.

Oparin-Haldane Theory

• Proposed by:
  • A.I. Oparin (Russia)
  • J.B.S. Haldane (England)
• Life originated through chemical evolution.
• Organic molecules formed from inorganic substances under primitive Earth conditions.

Primitive Earth Conditions

• High temperature
• Volcanic activity
• Lightning/electric discharges
• Reducing atmosphere
• Gases present:
  • CH₄
  • NH₃
  • H₂
  • Water vapour

Miller-Urey Experiment (1953)

• Performed by S.L. Miller.
• Simulated primitive Earth conditions.
• Used:
  • CH₄
  • NH₃
  • H₂
  • Water vapour
• Temperature around 800°C.
• Electric sparks simulated lightning.
• Produced amino acids.
• Supported the theory of chemical evolution.

Other Laboratory Findings

Scientists also produced:

• Sugars
• Nitrogen bases
• Pigments
• Fats

Meteorite Evidence

• Meteorites contain organic compounds similar to those produced experimentally.
• Suggests that organic molecules can form naturally in space.

First Living Organisms

• First self-replicating molecules probably appeared around 3 billion years ago.
• Likely molecules:
  • RNA
  • Proteins
  • Polysaccharides

First Cells

• First cellular organisms appeared about 2 billion years ago.
• They were:
  • Single-celled
  • Aquatic

Biogenesis

• Means life arises from pre-existing life.
• Accepted by most scientists today.

---

Important Years to Remember

Event	Time
Age of Universe (NCERT)	20 billion years
Formation of Earth	4.5 billion years ago
First Life	4 billion years ago
First Self-replicating Molecules	3 billion years ago
First Cellular Life	2 billion years ago
Miller Experiment	1953

---

Board Exam Keywords

• Big Bang Theory
• Light Year
• Chemical Evolution
• Biogenesis
• Panspermia
• Spontaneous Generation
• Oparin-Haldane Theory
• Miller-Urey Experiment
• Primitive Atmosphere
• Amino Acids
• Reducing Atmosphere
• First Cellular Life
• RNA World (first self-replicating molecules)

These are the highest-yield NCERT facts and frequently appear in CBSE Class 12 Board examinations.