Origin and Evolution of the Earth

How old is the Earth? How did continents form? Why is Earth the only planet with life? These questions from the origin and evolution of the Earth underpin UPSC questions on geological time scales, continental drift, and the uniqueness of Earth's environment. The chapter establishes deep time — the 4.6-billion-year history of the planet — and explains how the layered structure, the oceans, and the atmosphere came to be.

UPSC Prelims frequently tests the sequence of events in Earth's formation, the geological time scale, and the conditions that led to the emergence of life. Mains questions on climate change and biodiversity often require a long-term evolutionary perspective rooted in this chapter.

PART 1 — Quick Reference Tables

Table 1: Theories of Origin of the Universe and Solar System

Table 2: Geological Time Scale (Simplified)

(mya = million years ago)

Table 3: Stages in Earth's Early Evolution

Table 4: Composition of Early vs Present Atmosphere

Table 5: Key Numbers to Remember

PART 2 — Detailed Notes

The Big Bang Theory

The Big Bang is the prevailing cosmological model for the origin of the universe. Approximately 13.8 billion years ago, all matter and energy were concentrated in an infinitely dense, infinitely hot singularity. This exploded outward — not into pre-existing space, but creating space itself.

As the universe expanded, it cooled. Within the first few minutes, protons and neutrons formed. After about 380,000 years, atoms formed (hydrogen and helium). Over hundreds of millions of years, gravity clumped these atoms into galaxies, stars, and eventually planetary systems.

Evidence for the Big Bang:

• Hubble's observation (1929): Galaxies are moving away from each other (red-shift), implying expansion
• Cosmic Microwave Background Radiation (CMBR): Relic radiation from the early hot universe, detected by Penzias & Wilson (1965)
• Abundance of light elements: Universe is ~75% hydrogen and ~25% helium — consistent with Big Bang nucleosynthesis

💡 Explainer: The Nebular Hypothesis

The solar system formed from a nebula — a vast cloud of gas (mostly hydrogen and helium) and dust. About 4.6 billion years ago, this nebula began to collapse under gravity, perhaps triggered by a nearby supernova shockwave.

As it collapsed:

1. The cloud rotated faster (conservation of angular momentum — like a spinning skater pulling in arms)
2. It flattened into a disc (the solar nebula)
3. The centre became hot and dense enough for nuclear fusion → the Sun ignited
4. Remaining material in the disc clumped into planetesimals (small rocky bodies), which collided and merged to form planets

The inner solar system (close to the Sun) got rocky planets (Mercury, Venus, Earth, Mars) because only high-melting-point materials survived the heat. The outer solar system formed gas giants from the abundant lighter materials.

Earth's Internal Differentiation

Early Earth was largely homogeneous — a random mixture of silicates, metals, and other materials. The Great Differentiation occurred as the planet heated up from:

• Energy released by gravitational compression
• Radioactive decay of elements (uranium, thorium, potassium)
• Heat from meteorite bombardment

The interior melted. Denser materials (iron, nickel) sank to the centre forming the core. Lighter silicate materials rose to form the mantle and crust. This is why Earth has a layered structure — not a uniform composition.

Formation of the Atmosphere

Earth's first atmosphere was mostly hydrogen and helium — similar to the nebula. These light gases were blown away by solar winds. Earth's second atmosphere formed from volcanic outgassing — the release of gases trapped in the interior:

• Water vapour (H₂O)
• Carbon dioxide (CO₂)
• Nitrogen (N₂)
• Ammonia (NH₃)
• Methane (CH₄)

Critically, there was no free oxygen. The early atmosphere was reducing (chemically), which is significant because life began under these conditions.

Formation of Oceans

As Earth cooled below 100°C, water vapour condensed to form liquid water, filling the basins of the crust. The oceans are ~3.8 billion years old. Some water may have been delivered by comets and asteroids — this is still debated by scientists.

🎯 UPSC Connect: Origin of Life

Life on Earth began in the oceans approximately 3.8–3.5 billion years ago. The chemical evolution of life involved:

1. Miller-Urey experiment (1953): Demonstrated that organic molecules (amino acids) can form from inorganic gases (methane, ammonia, hydrogen, water) when energy (lightning/UV) is applied — simulating early Earth conditions.
2. First life: Single-celled prokaryotes (bacteria-like organisms) without a membrane-bound nucleus
3. Cyanobacteria: Evolved photosynthesis, releasing oxygen as a by-product (~2.4 billion years ago — the Great Oxidation Event)
4. The increase in atmospheric oxygen allowed the formation of the ozone layer, which filtered UV radiation and allowed life to colonise land

The Geological Time Scale

Geologists divide Earth's 4.6-billion-year history into eons, eras, periods, and epochs based on rock strata and fossil evidence.

Key divisions:

• Precambrian (4600–541 mya): Most of Earth's history; simple life only
• Palaeozoic (541–252 mya): "Ancient life" — Cambrian explosion, first vertebrates, first land organisms, Permian extinction
• Mesozoic (252–66 mya): "Middle life" — age of dinosaurs; Tethys Sea; Gondwanaland breakup
• Cenozoic (66 mya–present): "Recent life" — mammals dominant; Himalayan uplift; human evolution

📌 Key Fact: Gondwanaland and the Geological Time Scale

The supercontinent Gondwanaland (comprising present-day India, Antarctica, Australia, South America, and Africa) broke up during the Mesozoic Era (~200–130 mya). The Indian subcontinent drifted northward and collided with the Eurasian plate ~50 mya, forming the Himalayas. This directly shapes India's physical geography.

PART 3 — Frameworks & Analysis

Sequence of Events: Earth's First Billion Years

Compare: Theories of Earth Formation

Exam Strategy

Prelims Traps:

• The Big Bang explains the origin of the universe, not just the solar system.
• The Nebular Hypothesis (Kant-Laplace) is the accepted theory for the solar system's formation — not the Big Bang.
• Earth's second atmosphere (from outgassing) had no free oxygen — remember this for questions on early life.
• The Great Oxidation Event (~2400 mya) caused mass extinction of anaerobic organisms but enabled aerobic life to flourish.

Mains Frameworks:

• For "discuss the origin of the Earth" type questions, follow the sequence: Big Bang → solar nebula → accretion → differentiation → degassing → ocean formation → life.
• Link the geological time scale to contemporary issues: Gondwanaland breakup → India's drift → Himalayan formation → river systems → monsoon.

Previous Year Questions

1. UPSC Prelims 2017: With reference to the evolution of life on Earth, which of the following is the correct chronological order? (Tests knowledge of geological time scale)
2. UPSC Mains GS1 2013: What do you understand by the theory of continental drift? Discuss the evidences in its support. (Origin of continents from Gondwanaland links to this chapter)
3. UPSC Mains GS3 2019: Assess the impact of global warming on the coral life system with examples. (Long-term climate evolution perspective)
4. UPSC Prelims 2015: The atmosphere of which of the following planets is mostly composed of nitrogen? (Tests comparative planetology — links to Earth's atmospheric evolution)