CBSE Class 12 Biology – Brief Account of Evolution (NCERT) Important Facts

These are the highest-yield NCERT facts for CBSE Board Exams (MCQs, 1-mark, 2-mark, Assertion-Reason, and Case-Based Questions).

Evolution Timeline (NCERT)

Time (Million Years Ago, mya)Evolutionary Event
2000 myaFirst cellular life appeared
500 myaInvertebrates became abundant
350 myaJawless fishes evolved
350 myaFish with strong fins moved onto land
320 myaSeaweeds and early land plants existed
200 myaFish-like reptiles (Ichthyosaurs) evolved
65 myaDinosaurs became extinct

Board Tip: Remember the sequence rather than only the dates.

1. First Cellular Life

  • Appeared about 2000 million years ago (mya).
  • Some early cells released oxygen (O₂) through a photosynthesis-like process.
  • Oxygen gradually accumulated in the atmosphere.

2. Multicellular Organisms

  • Single-celled organisms gradually evolved into multicellular organisms.

3. Invertebrates

  • Appeared around 500 mya.
  • They were the first major group of complex animals.

4. Jawless Fishes

  • Evolved around 350 mya.
  • Among the earliest vertebrates.

5. First Plants on Land

  • Plants were the first organisms to colonize land.
  • They spread on land before animals.

6. Transition from Water to Land

  • Some fishes developed strong, stout fins.
  • These enabled movement on land and back to water.
  • They gave rise to the first amphibians.

7. Coelacanth

  • A living fossil.
  • Thought to be extinct until rediscovered in 1938 off the coast of South Africa.
  • Belongs to the lobe-finned fishes.

Important Board Fact: Coelacanth provides evidence for the transition from fishes to amphibians.

8. Amphibians

  • First vertebrates to live on both land and water.
  • Ancestors of modern frogs and salamanders.

9. Reptiles

  • Evolved from amphibians.
  • Laid thick-shelled eggs, preventing drying on land.
  • Modern descendants include:
    • Turtles
    • Tortoises
    • Crocodiles

10. Dinosaurs

  • Reptiles dominated the Earth for about 200 million years.
  • Largest dinosaur mentioned in NCERT:
    • Tyrannosaurus rex
  • About 20 feet tall with large, dagger-like teeth.
  • Became extinct around 65 mya.

11. Ichthyosaurs

  • Reptiles that returned to water.
  • Evolved around 200 mya.
  • Fish-like in appearance.

12. Giant Ferns

  • Giant pteridophytes formed dense forests.
  • Their remains gradually formed coal deposits.

13. Birds

  • Some scientists suggest birds evolved from reptiles (including dinosaurs).
  • NCERT notes that the exact reason for dinosaur extinction is still uncertain.

14. Mammals

Characteristics:

  • Viviparous (give birth to live young).
  • Young develop inside the mother’s body.
  • More advanced nervous system and better parental care.

The earliest mammals resembled shrews.

15. Continental Drift

  • South America later joined North America.
  • South American mammals were largely replaced by North American species.
  • Australia’s isolation allowed marsupials (pouched mammals) to survive due to reduced competition.

16. Aquatic Mammals

Examples:

  • Whale
  • Dolphin
  • Seal
  • Sea cow

Despite living in water, they are mammals.

17. Evolution of Humans

NCERT identifies the evolution of humans as one of the most successful evolutionary stories because of:

  • Language
  • Intelligence
  • Self-consciousness

Evolution Sequence (Very Important)

First Cells
⬇
Multicellular Organisms
⬇
Invertebrates
⬇
Jawless Fishes
⬇
Lobe-finned Fishes (Coelacanth-type)
⬇
Amphibians
⬇
Reptiles
⬇
Birds and Mammals
⬇
Humans

One-Mark Questions

Q1. When did the first cellular life appear?
Answer: About 2000 million years ago (mya).

Q2. Which organisms first invaded land?
Answer: Plants.

Q3. What is Coelacanth?
Answer: A lobe-finned fish regarded as a living fossil, rediscovered in 1938.

Q4. Which vertebrates first lived on both land and water?
Answer: Amphibians.

Q5. Which reptiles returned to water?
Answer: Ichthyosaurs.

Q6. When did dinosaurs become extinct?
Answer: Around 65 million years ago.

Q7. Name two modern reptiles mentioned in NCERT.
Answer: Turtles and crocodiles (also tortoises).

Q8. Name two aquatic mammals.
Answer: Whale and dolphin (also seals and sea cows).

NCERT Keywords for Board Exams

  • First Cellular Life
  • Oxygen Evolution
  • Multicellular Organisms
  • Invertebrates
  • Jawless Fish
  • Coelacanth
  • Lobe-finned Fish
  • Amphibians
  • Reptiles
  • Thick-shelled Eggs
  • Dinosaurs
  • Tyrannosaurus rex
  • Ichthyosaurs
  • Pteridophytes
  • Coal Formation
  • Mammals
  • Viviparous
  • Continental Drift
  • Marsupials
  • Aquatic Mammals
  • Human Evolution

High-Yield NCERT Facts to Memorize

  • 2000 mya: First cellular life.
  • 500 mya: Invertebrates became abundant.
  • 350 mya: Jawless fishes and transition of vertebrates toward land.
  • Plants colonized land before animals.
  • Coelacanth is a living fossil rediscovered in 1938.
  • Reptiles evolved from amphibians and laid thick-shelled eggs.
  • Dinosaurs became extinct about 65 mya.
  • Earliest mammals resembled shrews.
  • Australia’s marsupials survived because of geographic isolation and reduced competition.

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