Origin and Evolution of the Earth

Why this chapter matters for UPSC: This is Chapter 2 of Fundamentals of Physical Geography (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. Learn its numbers and their order (13.7 billion, 4.6 billion, 4,000 and 3,800 million years) and the evidence for an expanding universe, and be ready to connect space missions to what they reveal about the origin of the planets.

Contemporary hook: The European Space Agency's Planck mission, in its final 2018 analysis, put the age of the universe at 13.787 ± 0.020 billion years. NCERT's rounded figure is 13.7 billion. Learn both, with their sources.

🧠 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 (differentiation). Gases and water vapour escaped from the hot interior (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 way to hold the sequence: dust becomes planet, planet becomes layered, layers release air and water, water hosts life, and life changes the air.

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 and current science

Table 3: Numbers NCERT gives for stars and distances

Table 4: Evolution of the earth: NCERT's timeline

Source: kegy202.pdf pp. 15–16. The USGS gives the earth's age as 4.54 billion years, "with an uncertainty of less than 1 percent".

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

The present atmosphere is chiefly nitrogen and oxygen. NCERT's text still says the composition is dealt with "in Chapter 8"; in the 2026-27 book that is Chapter 7, Composition and Structure of Atmosphere.

Table 6: Geological time scale, major divisions (removed from NCERT in the 2023-24 rationalisation)

Source: International Commission on Stratigraphy, International Chronostratigraphic Chart v2024/12 (ChronostratChart2024-12.pdf). The pre-2023 NCERT chapter printed a geological time scale with older boundary dates (for example, Cenozoic from 65 million years); it was dropped in the rationalisation. Everything before 538.8 million years is informally called the Precambrian.

PART 2 — Concepts & Narrative

NCERT opens with the nursery rhyme "Twinkle, twinkle little star" and the questions it raises: how many stars are there, how did they come into existence, can one reach the end of the sky? The chapter answers in two steps, the origin of the universe and then the evolution of the earth.

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. Otto Schmidt and Carl Weizsäcker revised it (NCERT dates the revision to 1950): 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. 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 move farther apart as time passes.

NCERT's alternative 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 grow into denser gaseous bodies and then stars. NCERT dates star formation to "some 5–6 billion years ago"; read that as the era of our own sun's nebula, not the age of the first stars (Table 3).

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 the 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 by 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, the temperature fell further, and there was more condensation and more rain. The rainwater collected in depressions and formed the oceans within 500 million years of the earth's formation, so the oceans are about 4,000 million years old.

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 (cyanobacteria) 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, though NCERT does not mention it. 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).

PART 3 — UPSC Integration

Frames that score.

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) against Big Bang (origin of the universe) against steady state (rejected).
3. Evidence-based answer. Name the evidence for each claim: red-shift, the microwave background, zircons, fossils, laboratory synthesis.
4. Missions and origins. A planetary mission helps here because the outer planets keep material close to the composition of the solar nebula from which all the planets accreted.

Exam Strategy

Prelims fact-traps from this chapter

• Age of the earth: 4.6 billion years, not 4.6 million (NCERT Ex. 1(i) offers both).
• 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 (about 4,000 million years) are older than life (about 3,800 million years).
• 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.
• Transparency within 300,000 years at 4,500 K is NCERT's statement; current cosmology gives later and cooler figures (Table 2).
• A light year is a unit of distance.
• The early atmosphere had very little free oxygen (NCERT's wording), not none.

Mains question patterns

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

Practice Questions

Practice (UPSC-pattern, not past papers). Questions 1 to 3 are NCERT's own exercise MCQs.

Prelims:

1. Which one of the following figures represents the age of the earth?
   (a) 4.6 million years
   (b) 13.7 billion years
   (c) 4.6 billion years
   (d) 13.7 trillion years

2. 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

3. Life on the earth appeared around how many years before the present?
   (a) 13.7 billion
   (b) 3.8 million
   (c) 4.6 billion
   (d) 3.8 billion

4. 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

5. Consider the following observations: 1. The red-shift of light from distant galaxies 2. The cosmic microwave background 3. The steady state concept of Hoyle. Which of these support the expanding-universe (Big Bang) view?
   (a) 1 and 2 only
   (b) 2 and 3 only
   (c) 1 and 3 only
   (d) 1, 2 and 3

6. In NCERT's account of planet formation, planetesimals form by:
   (a) degassing of the gas core
   (b) cohesion of small rounded objects, then collision and gravitational attraction
   (c) differentiation of the earth's layers
   (d) the explosion of a supernova

Mains (practice):

1. Write an explanatory note on the Big Bang theory. (NCERT Ex. 3(i), 150 words) Approach: the three stages with NCERT's figures, the evidence (red-shift, microwave background), the balloon caveat, and Hoyle's alternative.
2. List the stages in the evolution of the earth and explain how the atmosphere was modified at each stage. (NCERT Ex. 3(ii), 150 words)

Short answers (NCERT Ex. 2, about 30 words): differentiation is the separation of earth-forming material into layers by density; the early surface was barren, rocky and hot with a thin hydrogen-helium atmosphere; the early degassed atmosphere held water vapour, nitrogen, carbon dioxide, methane and ammonia with very little free oxygen.

📦 Revision Capsule

Sources

• NCERT, Fundamentals of Physical Geography, Textbook for Class XI, ch 2 "The Origin and Evolution of the Earth", Reprint 2026-27: kegy202.pdf; the 2019-20 edition of the chapter (solar system section, geological time scale) via the Wayback copy of the book: kegy2dd.zip, 4 May 2019.
• Planck Collaboration, "Planck 2018 results. VI. Cosmological parameters", Astronomy & Astrophysics 641, A6 (2020): arXiv: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): 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.
• NASA Science, "Cosmic History": science.nasa.gov/universe/overview; "What Were the First Stars Like?": science.nasa.gov; University of Cambridge, Centre for Theoretical Cosmology, "The Cosmic Microwave Background": ctc.cam.ac.uk.
• IAU 2012 Resolution B2 (astronomical unit): Res_IAU2012_B2.pdf; IAU, "Result of the IAU Resolution votes", 24 August 2006: iau.org; IAU press release iau1812, 29 October 2018: iauarchive.eso.org.
• H. Kragh, "Hubble Law or Hubble-Lemaître Law? The IAU Resolution" (2018): arXiv:1809.02557.
• USGS, "Age of the Earth": pubs.usgs.gov/gip/geotime/age.html.
• A. Bekker et al., "Dating the rise of atmospheric oxygen", Nature 427 (2004): 117–120: doi:10.1038/nature02260; G. Luo et al., "Rapid oxygenation of Earth's atmosphere 2.33 billion years ago", Science Advances 2 (2016): e1600134: doi:10.1126/sciadv.1600134.
• S.L. Miller, "A production of amino acids under possible primitive earth conditions", Science 117 (1953): 528–529: PubMed 13056598.