Origin and Evolution of the Earth

Why this chapter matters for UPSC: This is Chapter 2 of Fundamentals of Physical Geography in the current rationalised NCERT (Reprint 2026-27), the first chapter of Unit II "The Earth". It covers how the universe, the stars and the planets formed, and how the earth acquired its layers, atmosphere, oceans and life. Prelims tests the numbers (13.7 billion years, 4.6 billion years, 4,000 and 3,800 million years) and the order of events; Mains GS1 has asked how a space mission helps explain the origin and evolution of the earth (2017).

Contemporary hook: The European Space Agency's Planck mission, in its final 2018 results, put the age of the universe at 13.787 ± 0.020 billion years. NCERT's rounded figure is 13.7 billion. Both appear in exam material, so know which source a question is following.

🧠 First Principles — Read This First

This chapter tells one story in two parts. The first part is about the universe. The most widely accepted explanation, the Big Bang theory, says that all matter was once packed into a single, extremely small and dense point. It expanded suddenly, and it is still expanding: the space between galaxies keeps growing. As it expanded and cooled, matter clumped into galaxies, and inside galaxies, clouds of gas called nebulae collapsed into stars.

The second part is about the earth. Around a young star, our sun, a disc of gas and dust formed. Small grains stuck together into larger bodies called planetesimals, and these combined into planets. This joining-together is called accretion.

The young earth was hot, rocky and barren, with a thin atmosphere of hydrogen and helium. Three changes made it the planet we know. Heavy material such as iron sank to the centre and lighter material rose, giving the earth its layers; this sorting is called differentiation. Gases and water vapour escaped from the hot interior, a process called degassing, and built a new atmosphere; the vapour condensed into rain and filled the oceans. Finally, life appeared in the oceans and, through photosynthesis, slowly added oxygen to the water and then to the air.

A simple way to hold the sequence: dust becomes planet, planet becomes layered, layers release air and water, water hosts life, and life changes the air. UPSC asks both for the numbers attached to these stages and for the order in which they happened.

PART 1 — Quick Reference

Table 1: Theories in NCERT

Source: NCERT, Fundamentals of Physical Geography, Class XI, Ch.2, Reprint 2026-27, pp.13–14 — kegy202.pdf. NCERT spells the second name "Weizascar".

Table 2: Stages of the Big Bang (NCERT)

Table 3: Numbers NCERT gives for stars and distances

Table 4: Evolution of the earth — NCERT timeline

Source: kegy202.pdf pp.15–16.

Table 5: Three stages in the evolution of the present atmosphere

The present atmosphere is chiefly nitrogen and oxygen (NCERT; composition is covered in the Atmosphere chapter).

Table 6: Geological time scale — major divisions (beyond NCERT)

Source: International Commission on Stratigraphy, International Chronostratigraphic Chart v2024/12 — ChronostratChart2024-12.pdf. The geological time scale is not part of the rationalised NCERT chapter. Everything before 538.8 million years is informally called the Precambrian.

PART 2 — Concepts & Narrative

Early theories: the earth and the planets

The early theories tried to explain only the origin of the earth and the planets. Kant's argument, revised by Laplace in 1796, held that the planets formed from a cloud of material around a young, slowly rotating sun. In 1950 Otto Schmidt and Carl Weizsäcker revised it: the sun was surrounded by a solar nebula of mostly hydrogen and helium with dust; friction and collisions between particles produced a disc-shaped cloud, and the planets grew within it by accretion. NCERT then notes that later scientists turned to the larger problem of the origin of the universe itself.

The Big Bang and the expanding universe

The Big Bang theory is also called the expanding universe hypothesis. Its key observation is that galaxies are moving farther apart as time passes.

The alternative NCERT mentions is Hoyle's steady state concept, which held that the universe is roughly the same at any point of time. As evidence for expansion accumulated, the scientific community came to favour the expanding universe.

Stars and planets

The early universe did not have matter and energy spread evenly. These density differences produced differences in gravity, which drew matter together into galaxies. A galaxy begins as a large cloud of hydrogen called a nebula. The growing nebula develops localised clumps of gas, which become denser gaseous bodies and then stars.

NCERT gives three stages in the formation of planets:

1. Stars are localised lumps of gas within a nebula. Gravity inside a lump forms a core, and a large rotating disc of gas and dust develops around it.
2. The gas cloud condenses, and matter around the core forms small rounded objects. By cohesion these become planetesimals; collisions and gravitational attraction make them stick together into larger bodies.
3. The many small planetesimals accrete into a few large bodies, the planets.

Evolution of the lithosphere: differentiation

The earth was mostly in a volatile state in its primordial stage. As its density increased, the temperature inside rose, and the material began to separate according to density. Heavier materials such as iron sank towards the centre and lighter ones moved towards the surface. The earth cooled, solidified, condensed into a smaller size and developed an outer crust.

Evolution of the atmosphere and hydrosphere

The first atmosphere, of hydrogen and helium, was stripped away by solar winds, as happened with all the terrestrial planets. The second was released from inside the earth as it cooled. Degassing and continuous volcanic eruptions supplied water vapour, nitrogen, carbon dioxide, methane and ammonia, with very little free oxygen. As the earth cooled further, the vapour condensed. Carbon dioxide dissolved in the rainwater, which lowered the temperature further and caused more condensation and more rain. The rainwater collected in depressions and formed the oceans, within 500 million years of the earth's formation.

Life then changed the atmosphere. Photosynthesis evolved about 2,500–3,000 million years ago. Oxygen first accumulated in the oceans; once they were saturated, about 2,000 million years ago, oxygen began to flood the atmosphere.

Origin of life

NCERT describes the origin of life as a chemical process. Chemical reactions first produced complex organic molecules, which assembled in a way that let them duplicate themselves, turning inanimate matter into living substance. The record of past life is preserved as fossils in rocks. Microscopic structures closely related to present-day blue algae occur in formations older than about 3,000 million years, and NCERT concludes that life began to evolve about 3,800 million years ago.

A classic experiment tested the chemical idea. In 1953 Stanley Miller passed electric discharges through a mixture of methane, ammonia, water and hydrogen, chosen to model the primitive atmosphere, and obtained amino acids ("A production of amino acids under possible primitive earth conditions", Science 117, 1953). This experiment is not in NCERT.

PART 3 — UPSC Integration

PYQ pattern

• Mains GS1 2017 (gs1-pyq-2017-05, 10 marks): "How does the Juno Mission of NASA help to understand the origin and evolution of the Earth? Explain." NASA's Juno spacecraft reached Jupiter on 4 July 2016. The answer frame is this chapter: Jupiter, the largest planet, preserves information about the solar nebula from which all the planets accreted.
• Prelims 2012 (bank ID st-055): evidence for the continued expansion of the universe (microwaves in space; red-shift).
• Adjacent: Mains GS1 2013 (gs1-pyq-2013-19a), "What do you understand by the theory of continental drift? Discuss the prominent evidences in its support." It belongs to the chapter on the distribution of oceans and continents.

Mains frameworks

1. Sequence answer. For "stages in the evolution of the earth" (NCERT Ex. 3(ii)): accretion → differentiation (lithosphere) → degassing and condensation (atmosphere and oceans) → origin of life → photosynthesis and oxygenation.
2. Theory comparison. Nebular (origin of the solar system) vs Big Bang (origin of the universe) vs steady state (rejected).
3. Evidence-based answer. Name the evidence for each claim: red-shift, CMB, zircons, fossils, laboratory synthesis.

Exam Strategy

Prelims fact-traps from this chapter

• Age of the earth: 4.6 billion years, not 4.6 million (NCERT Ex. 1(i) includes both as options).
• Age of the universe: 13.7 billion (NCERT); 13.787 billion (Planck 2018). Not 13.7 trillion.
• Life began 3.8 billion years ago, not 3.8 million (NCERT Ex. 1(iii)).
• Oceans (~4,000 million years) are older than life (~3,800 million years). Do not give 3.8 billion years as the age of the oceans.
• Photosynthesis 2,500–3,000 million years ago; oxygen floods the atmosphere 2,000 million years ago (NCERT).
• Differentiation is not an atmospheric process; solar winds, degassing and photosynthesis are.
• First atom within the first three minutes; transparency within 300,000 years at 4,500 K (NCERT).
• Revised nebular hypothesis: Schmidt and Weizsäcker, 1950. Laplace's revision: 1796.
• A light year is a unit of distance.
• The early atmosphere had very little free oxygen (NCERT wording), not none.

Mains question patterns

• Explanatory note on the Big Bang theory (NCERT Ex. 3(i)): three stages, expansion, evidence, the steady-state alternative.
• Space missions and the origin of planets (2017): link the mission to the nebular hypothesis and accretion.

Practice Questions

1. Mains GS1 2017 (gs1-pyq-2017-05): "How does the Juno Mission of NASA help to understand the origin and evolution of the Earth? Explain."
2. Prelims 2012 (bank ID st-055): "Which of the following is/are cited by the scientists as evidence/evidences for the continued expansion of universe? 1. Detection of microwaves in space 2. Observation of redshift phenomenon in space 3. Movement of asteroids in space 4. Occurrence of supernova explosions in space. Select the correct answer using the codes given below: (a) 1 and 2 (b) 2 only (c) 1, 3 and 4 (d) None of the above can be cited as evidence." (Answer: a)
3. Practice (UPSC-pattern, not a past paper): Arrange in chronological order, earliest first, as given in NCERT: 1. Oxygen begins to flood the atmosphere 2. Formation of the oceans 3. Beginning of life 4. Evolution of photosynthesis. (a) 2-3-4-1 (b) 3-2-4-1 (c) 2-4-3-1 (d) 3-4-2-1. (Answer: a)
4. Practice (UPSC-pattern, not a past paper): Which one of the following is not related to the formation or modification of the present atmosphere? (a) Solar winds (b) Differentiation (c) Degassing (d) Photosynthesis. (NCERT Ex. 1(ii); answer: b)
5. Practice (UPSC-pattern, not a past paper): List the stages in the evolution of the earth and explain how the atmosphere was modified at each stage. (150 words)

📦 Revision Capsule

Sources

Sources

• NCERT, Fundamentals of Physical Geography, Textbook for Class XI, Chapter 2 "The Origin and Evolution of the Earth", rationalised edition, Reprint 2026-27 — kegy202.pdf.
• Planck Collaboration, "Planck 2018 results. VI. Cosmological parameters", Astronomy & Astrophysics (arXiv:1807.06209) — arxiv.org/abs/1807.06209.
• E. Hubble, "A relation between distance and radial velocity among extra-galactic nebulae", PNAS 15 (1929): 168–173 — PMC522427.
• NASA Goddard, "Cosmic Times 1965" (discovery of the cosmic microwave background by Penzias and Wilson) — imagine.gsfc.nasa.gov.
• International Commission on Stratigraphy, International Chronostratigraphic Chart v2024/12 — ChronostratChart2024-12.pdf.
• S.A. Wilde, J.W. Valley, W.H. Peck, C.M. Graham, "Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago", Nature 409 (2001): 175–178 — doi:10.1038/35051550.
• S.L. Miller, "A production of amino acids under possible primitive earth conditions", Science 117 (1953): 528–529 — PubMed 13056598.
• "Georges Lemaître and the Foundations of Big Bang Cosmology" (biographical review), arXiv:2007.09459 — arxiv.org/abs/2007.09459.
• OpenStax, Astronomy (2nd ed.), §29.4 "The Cosmic Microwave Background" — openstax.org.
• "Dynamics of the Great Oxidation Event from a 3D photochemical-climate model", arXiv:2212.01389 — arxiv.org/abs/2212.01389.
• NASA, "This Week in NASA History: Juno Arrives at Jupiter – July 4, 2016" — nasa.gov.
• PYQ texts: BharatNotes Mains GS1 bank (2013, 2017) and Prelims science-and-technology bank (2012, st-055).