The Origin and Evolution of the Earth, Class 11 Geography

What The Origin and Evolution of the Earth is about

Weightage: This chapter is part of Unit II, The Earth, of Fundamentals of Physical Geography, which carries 9 of the 70 theory marks in the school annual exam together with Interior of the Earth and Distribution of Oceans and Continents. Map work carries a further 5 marks for each book.Source: CBSE Geography (029) syllabus 2026-27, Class XI course structure

Key concepts

Early theories: nebular hypothesis and its revision

NCERT section Early Theories

Nebular hypothesis

• Proposed by the German philosopher Immanuel Kant; revised by the mathematician Laplace in 1796.
• Idea: planets formed from a cloud of material associated with a youthful sun that was rotating slowly.

Revision in 1950

• Otto Schmidt (Russia) and Carl Weizsacker (Germany), differing in details.
• The sun was surrounded by a solar nebula of mainly hydrogen and helium plus dust.
• Friction and collision of particles produced a disc-shaped cloud; planets formed through accretion (gradual sticking together of material).

Shift in focus: later scientists turned from the origin of the earth or planets to the bigger problem: the origin of the universe.

Worked example

What did Schmidt and Weizsacker add to the nebular hypothesis? (2 marks)

Try it first, then show the solutionSolution

1. They described a solar nebula of mostly hydrogen, helium and dust around the sun.
2. Friction and collision formed a disc-shaped cloud, and planets grew from it by accretion.

Answer: A solar nebula of hydrogen, helium and dust, a disc-shaped cloud from friction and collision, and planet formation by accretion.

Big Bang theory and the expanding universe

NCERT section Origin of the Universe

Also called the expanding universe hypothesis. Edwin Hubble (1920) gave evidence that the universe is expanding: with time, galaxies move further apart.

Balloon model: dots on a balloon move apart as it is inflated, like galaxies. But the dots themselves also grow, whereas observations show the space between galaxies increases, not the galaxies. So the balloon analogy is only partly correct.

Stages:
1. All matter existed in one place as a 'tiny ball' (singular atom): unimaginably small volume, infinite temperature and infinite density.
2. The tiny ball exploded violently, about 13.7 billion years ago, causing huge expansion that continues today. Some energy became matter. Expansion was very rapid in fractions of a second, then slowed. The first atom began to form within the first three minutes.
3. Within 300,000 years, temperature fell to 4,500 K, atomic matter formed and the universe became transparent.

Alternative: Hoyle's steady state concept, which held that the universe is roughly the same at any time. Growing evidence for expansion means scientists now favour the expanding universe.

Worked example

Why is the balloon experiment only partially correct as a model of the expanding universe? (2 marks)

Try it first, then show the solutionSolution

1. As the balloon inflates, the marked points move apart, like galaxies moving apart.
2. But the points themselves also get bigger, while observations show only the space between galaxies increases, not the galaxies.

Answer: It shows galaxies separating, but wrongly suggests galaxies themselves expand.

Star formation and the light year

NCERT section The Star Formation

How stars and galaxies formed:

• Matter and energy were unevenly distributed in the early universe.
• Initial density differences caused differences in gravitational force, so matter was drawn together: the basis of galaxies.
• A galaxy begins as a huge cloud of hydrogen gas called a nebula. The growing nebula develops localised clumps that become denser and form stars.
• Stars are believed to have formed about 5-6 billion years ago.
• A galaxy contains a very large number of stars; galaxies are thousands of light years apart and individual galaxies are 80,000-150,000 light years across.

Light year (a distance, not a time):

• Light travels at 300,000 km per second.
• One light year = distance light travels in one year = 9.461 × 10^12 km.
• The mean sun-earth distance of 149,598,000 km is covered by light in about 8.311 minutes.

Worked example

Show that sunlight takes about 8.3 minutes to reach the earth. (2 marks)

Try it first, then show the solutionSolution

1. Time = distance / speed = 149,598,000 km / 300,000 km per second = 498.66 seconds.
2. 498.66 / 60 = 8.311 minutes.

Answer: About 8.31 minutes.

Formation of planets

NCERT section Formation of Planets

Three stages:
1. Core and disc: stars are localised lumps of gas in a nebula. Gravity inside a lump forms a core, and a huge rotating disc of gas and dust develops around it.
2. Planetesimals: the gas cloud condenses; matter around the core forms small rounded objects which, by cohesion, become planetesimals. Collisions and gravitational attraction make material stick together into larger bodies. (Planetesimals are a large number of smaller bodies.)
3. Planets: the many small planetesimals accrete into a few large bodies, the planets.

Keywords to use: core, rotating disc, condensation, cohesion, planetesimals, collision, gravitational attraction, accretion.

Worked example

Arrange in order: accretion into planets; formation of a rotating disc around a gas core; cohesion of small rounded objects into planetesimals. (1 mark)

Try it first, then show the solutionSolution

1. First the core and rotating disc of gas and dust form.
2. Then small rounded objects cohere into planetesimals; finally planetesimals accrete into planets.

Answer: Rotating disc around core → planetesimals → planets.

Evolution of the lithosphere: differentiation

NCERT section Evolution of the Earth

Starting point: the early earth was barren, rocky and hot with a thin atmosphere of hydrogen and helium, very unlike today.

Layered structure: from the top of the atmosphere to the centre, material is not uniform; the atmosphere has the least density, and density increases inward.

How the layers formed:

• In its primordial stage the earth was mostly volatile.
• As density gradually increased, the temperature inside rose.
• Material began to separate by density: heavier material (like iron) sank to the centre, lighter material rose to the surface.
• The earth cooled, solidified and condensed into a smaller size; the outer surface became the crust.
• During the formation of the moon, a giant impact heated the earth further.
• This separation of earth material into layers is called differentiation.

Result: crust → mantle → outer core → inner core, with density increasing from crust to core.

Worked example

What is differentiation? Why did iron end up at the centre of the earth? (2 marks)

Try it first, then show the solutionSolution

1. Differentiation is the process by which earth-forming material separated into layers according to density.
2. Iron is heavy (dense), so it sank towards the centre while lighter materials moved up.

Answer: Separation of materials by density; iron sank because it is heavier.

Evolution of atmosphere and hydrosphere

NCERT section Evolution of the Earth

The present atmosphere is chiefly nitrogen and oxygen. It evolved in three stages:
1. Loss of the primordial atmosphere: the early hydrogen-helium atmosphere was stripped away by solar winds, as happened to all the terrestrial planets.
2. Contribution of the hot interior: as the earth cooled, gases and water vapour were released from the solid interior. This outpouring is called degassing; continuous volcanic eruptions added water vapour and gases. The early atmosphere had water vapour, nitrogen, carbon dioxide, methane, ammonia and very little free oxygen.
3. Modification by life: photosynthesis changed the composition.

Formation of oceans:

• As the earth cooled, water vapour condensed; carbon dioxide dissolved in rainwater; temperature fell further, causing more condensation and rain.
• Rainwater collected in depressions to form oceans within 500 million years of the earth's formation, so oceans are about 4,000 million years old.

Oxygen timeline: life began to evolve about 3,800 million years ago; photosynthesis evolved about 2,500-3,000 million years ago; life stayed in the oceans for a long time; oceans became saturated with oxygen, and about 2,000 million years ago oxygen began to flood the atmosphere.

Worked example

Mars and Venus also lost their first hydrogen-helium atmospheres. Which stage of atmospheric evolution explains this, and which stage made the earth's air different from theirs today? (2 marks)

Try it first, then show the solutionSolution

1. The primordial hydrogen-helium atmosphere of all terrestrial planets was stripped away by solar winds: the first stage.
2. The earth's air was later modified by life through photosynthesis, which, after the oceans were saturated, let oxygen flood the atmosphere about 2,000 million years ago.

Answer: Solar winds removed the first atmosphere; photosynthesis by living organisms later gave the earth its oxygen.

Origin of life

NCERT section Evolution of the Earth

Last phase of the earth's evolution. The early earth and its atmosphere were not conducive to life.

• Modern scientists see the origin of life as a kind of chemical reaction: first complex organic molecules were generated and assembled; the assemblage could duplicate itself, turning inanimate matter into living substance.
• The record of past life is preserved in rocks as fossils.
• Microscopic structures closely related to present-day blue algae have been found in rock formations much older than about 3,000 million years.
• So life is assumed to have begun about 3,800 million years ago.

Worked example

What evidence suggests that life existed more than 3,000 million years ago? (1 mark)

Try it first, then show the solutionSolution

1. Fossils in old rocks record past life.
2. Microscopic structures like present-day blue algae occur in formations older than about 3,000 million years.

Answer: Fossil structures resembling blue algae in rocks older than about 3,000 million years.

The Origin and Evolution of the Earth in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Big Bang, Formation of Planets and Evolution of the Earth's Layers, Atmosphere and Life

The Origin and Evolution of the Earth: key facts

Light year

$1lightyear=300,000km/s\times secondsinayear\approx 9.461\times 10^{1}2km$

A unit of distance, not time.

Sun-earth light time

$149,598,000km÷300,000km/s\approx 498.7s\approx 8.311min$

NCERT's figure is 8.311 minutes.

Big Bang timeline

1. 1.Bang 13.7 billion years ago
2. 2.first atom within 3 minutes
3. 3.300,000 years: 4,500 K, atomic matter, transparent universe

Expansion was fastest in fractions of a second, then slowed.

Earth timeline (million years before present)

1. 1.Earth 4,600
2. 2.oceans by about 4,000
3. 3.life about 3,800
4. 4.photosynthesis 2,500-3,000
5. 5.oxygen floods air about 2,000

Stars formed 5-6 billion years ago.

Planet formation

1. 1.Core + rotating disc
2. 2.small rounded objects
3. 3.planetesimals (cohesion)
4. 4.planets (accretion)

Three stages.

Atmosphere evolution

1. 1.Solar winds strip H-He
2. 2.degassing (and volcanoes) adds gases
3. 3.photosynthesis adds oxygen

Early atmosphere: water vapour, N2, CO2, CH4, NH3, little free O2.

Layering

1. 1.Crust
2. 2.mantle
3. 3.outer core
4. 4.inner core; density increases inward

Produced by differentiation.

Galaxy size

Diameter 80,000-150,000 light years

Galaxies form from hydrogen nebulae.

Names and dates

1. 1.Kant (nebular)
2. 2.Laplace 1796
3. 3.Schmidt and Weizsacker 1950
4. 4.Hubble 1920 (expansion)
5. 5.Hoyle (steady state)

Hubble's evidence came before the 1950 revision of the nebular idea.

The Origin and Evolution of the Earth: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks The Origin and Evolution of the Earth

The question patterns to expect, and the method that works for each.

Tricks to remember The Origin and Evolution of the Earth

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Nebular chain

  'Kant Let Schmidt Wait': Kant → Laplace (1796) → Schmidt and Weizsacker (1950).

  Use it when: Order of the early theories.

• Big Bang numbers

  '13.7, 3, 300k, 4.5k': 13.7 billion years ago, first atom in 3 minutes, 300,000 years to transparency at 4,500 K.

  Use it when: Big Bang stages.

• Atmosphere stages

  'Sun Steals, Earth Exhales, Life Breathes': solar winds, degassing, photosynthesis.

  Use it when: Three stages of atmospheric evolution.

• Earth timeline

  '46-40-38-25/30-20' (hundreds of millions of years): earth, oceans, life, photosynthesis, oxygen in air.

  Use it when: Timeline MCQs and gap calculations.

• Planet stages

  'Disc, Dots, Planets': core with disc, small rounded objects into planetesimals, accretion into planets.

  Use it when: Planet formation answers.

The Origin and Evolution of the Earth: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in The Origin and Evolution of the Earth

The slips examiners see most often, and what to write instead.

• ✗Mistake: Confusing the age of the universe with the age of the earth

  Both numbers appear in the chapter and in MCQ options.

  Wrong working

  The earth is 13.7 billion years old.

  Right working

  Universe: 13.7 billion years; earth: 4.6 billion (4,600 million) years.

  ✓Do this: Universe is the bigger number because it is older.

  Usually costs: 1 mark per MCQ.

• ✗Mistake: Calling a light year a unit of time

  The word 'year' misleads.

  Wrong working

  A light year is the time light takes to travel.

  Right working

  A light year is the distance light travels in one year, 9.461 × 10^12 km.

  ✓Do this: Light year = distance.

  Usually costs: 1-2 marks in a definition.

• ✗Mistake: Listing differentiation as a stage of the atmosphere

  All the process words sound similar.

  Wrong working

  Atmosphere evolved by differentiation, degassing and photosynthesis.

  Right working

  Atmosphere: solar winds, degassing, photosynthesis. Differentiation built the layered interior.

  ✓Do this: Differentiation = layers; degassing = gases.

  Usually costs: 1 mark in an MCQ, 1 in a short answer.

• ✗Mistake: Writing that oxygen entered the atmosphere when life began

  Students compress the timeline.

  Wrong working

  Life began 3,800 million years ago and filled the air with oxygen.

  Right working

  Photosynthesis evolved 2,500-3,000 million years ago; oxygen saturated the oceans first and flooded the air about 2,000 million years ago.

  ✓Do this: Life → photosynthesis → oceans saturated → air.

  Usually costs: 1-2 marks in a 3-5 mark answer.

• ✗Mistake: Mixing up million and billion

  NCERT uses both units.

  Wrong working

  Life appeared 3.8 million years ago.

  Right working

  3,800 million = 3.8 billion years ago.

  ✓Do this: 1 billion = 1,000 million; check zeros before ticking.

  Usually costs: 1 mark per MCQ.

NCERT exercise guide: The Origin and Evolution of the Earth

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on The Origin and Evolution of the Earth

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   The nebular hypothesis was revised in 1796 by:
   (a) Kant
   (b) Laplace
   (c) Hubble
   (d) Hoyle

   Hint

   A mathematician.

   Answer

   (b) Laplace.

2. Q2 · 1 mark · MCQExam level

   Within how many years of the Big Bang did the universe become transparent?
   (a) 3 minutes
   (b) 300,000 years
   (c) 5-6 billion years
   (d) 500 million years

   Hint

   The temperature had fallen to 4,500 K.

   Answer

   (b) 300,000 years.

3. Q3 · 1 mark · Assertion-reasonHarder exam level

   Assertion (A): The early earth lost its first atmosphere.
   Reason (R): Solar winds stripped off the hydrogen-helium atmosphere of all terrestrial planets.
   Choose (a)-(d) as usual.

   Hint

   Does R give the cause of A?

   Answer

   (a). Both true and R explains A.

4. Q4 · 2 marks · Short answerHarder exam level

   Galaxy X has a diameter of 120,000 light years. Express it in km using NCERT's value. Is it within the range given in the chapter? (2 marks)

   Hint

   Multiply by 9.461 × 10^12.

   Answer

   120,000 × 9.461 × 10^12 = 1.135 × 10^18 km. Yes, it lies within 80,000-150,000 light years.

5. Q5 · 3 marks · Short answerHarder exam level

   Distinguish between degassing and differentiation. (3 marks)

   Hint

   One builds the atmosphere, one builds the layers.

   Answer

   Degassing: release of gases and water vapour from the cooling interior (with volcanic eruptions) that built the early atmosphere. Differentiation: separation of earth material by density into crust, mantle and core, with heavy iron sinking. Degassing belongs to atmospheric evolution, differentiation to the lithosphere.

6. Q6 · 3 marks · Short answerHOTS / competency

   Iron is far more common in the earth's core than in its crust. Using the evolution of the lithosphere, explain how the early earth came to be sorted into layers, and name the event linked with the moon that added heat. (3 marks)

   Hint

   Volatile early earth, rising temperature, separation by density.

   Answer

   The primordial earth was mostly in a volatile state; as its density increased, its internal temperature rose, so materials began to separate by density. Heavier materials such as iron sank towards the centre and lighter ones moved towards the surface: this process is differentiation, which produced the crust, mantle, outer core and inner core. The giant impact during the formation of the moon heated the earth further. As it cooled and solidified, the outer surface developed into a crust.

7. Q7 · 3 marks · Short answerHOTS / competency

   Suppose a new planet is found with a thick atmosphere of carbon dioxide, methane and ammonia but almost no free oxygen. Which stage of the earth's atmospheric evolution does it resemble, and what would have to happen next for it to become earth-like? (3 marks)

   Hint

   Degassing stage; then condensation and life.

   Answer

   It resembles the second stage (degassing), when the earth's air had water vapour, nitrogen, carbon dioxide, methane, ammonia and little oxygen. It would need cooling and condensation to form oceans that absorb carbon dioxide, then life and photosynthesis to add oxygen.

8. Q8 · 5 marks · Long answerHarder exam level

   Explain the stages in the evolution of the earth. (5 marks)

   Hint

   Lithosphere, atmosphere, hydrosphere, life.

   Answer

   Start: barren, hot, rocky with thin H-He air. Lithosphere: differentiation by density, cooling, crust; layers crust-mantle-outer core-inner core; moon-forming impact added heat. Atmosphere: solar winds, degassing and volcanoes, photosynthesis. Hydrosphere: condensation, CO2 dissolved in rain, oceans within 500 million years. Life: organic molecules assembling and self-duplicating about 3,800 million years ago, recorded in fossils.

9. Q9 · 2 marks · Short answerExam level

   If a time machine took you to the earth shortly after its formation, what three things would you notice about the surface and the air, and why could you not breathe? (2 marks)

   Hint

   Barren, rocky, hot; a thin atmosphere of light gases.

   Answer

   A barren, rocky and hot surface under a thin atmosphere of hydrogen and helium; there was no free oxygen to breathe, as oxygen flooded the air only about 2,000 million years ago.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Who gave evidence in 1920 that the universe is expanding?Show answer

   Edwin Hubble.

2. 2.What is the 'tiny ball' in the Big Bang theory called?Show answer

   A singular atom (singularity).

3. 3.At what temperature did atomic matter form after the Big Bang?Show answer

   4,500 K, within 300,000 years.

4. 4.What is a nebula?Show answer

   A very large cloud of hydrogen gas from which a galaxy starts to form.

5. 5.What are planetesimals?Show answer

   A large number of small bodies formed by cohesion of small rounded objects, which later accrete into planets.

6. 6.Name the gases of the early (degassed) atmosphere.Show answer

   Water vapour, nitrogen, carbon dioxide, methane, ammonia, with very little free oxygen.

7. 7.How old are the oceans?Show answer

   About 4,000 million years; they formed within 500 million years of the earth.

8. 8.Who proposed the steady state concept?Show answer

   Hoyle.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Nebular hypothesis: Kant; revised by Laplace (1796); revised again by Schmidt and Weizsacker (1950).
• ☐Big Bang = expanding universe; Hubble (1920): galaxies moving apart.
• ☐Singular atom → explosion 13.7 billion years ago → first atom in 3 minutes → 4,500 K and transparency by 300,000 years.
• ☐Balloon analogy is only partly correct: galaxies do not expand, the space between them does.
• ☐Hoyle's steady state is the alternative; expansion is favoured.
• ☐Stars formed 5-6 billion years ago from clumps in hydrogen nebulae; galaxies are 80,000-150,000 light years across.
• ☐1 light year = 9.461 × 10^12 km; sunlight takes 8.311 minutes to reach us.
• ☐Planets: core and disc → planetesimals → planets by accretion.
• ☐Differentiation made the crust, mantle, outer core and inner core.
• ☐Atmosphere: solar winds → degassing → photosynthesis; oceans within 500 million years; life about 3,800 million years ago; oxygen in air about 2,000 million years ago.

The Origin and Evolution of the Earth: frequently asked questions