Maarg Manthan · Topic 1.1

Origin of Life Forms and Life on Earth

Environment & Ecology › Basic Concepts of Ecology · Topic 1.1

The origin of life on Earth refers to the processes by which non-living matter gave rise to the first living cells. The Earth formed about 4.54 billion years ago, and the oldest accepted evidence of life, in the form of microfossils and layered structures called stromatolites, is about 3.5 billion years old. The Oparin-Haldane hypothesis of the 1920s proposed that life arose from simple organic molecules in a warm primordial soup under an atmosphere without free oxygen. In 1953 Stanley Miller and Harold Urey passed electric sparks through methane, ammonia, hydrogen and water vapour and produced amino acids. Photosynthesis by cyanobacteria released oxygen in the Great Oxidation Event about 2.4 billion years ago, and complex animals appeared in the Cambrian explosion about 541 million years ago.

5 min read · UPSC Prelims · Environment & Ecology · Basic Concepts of Ecology

At a Glance

Age of Earth About 4.54 billion years
Oldest life About 3.5 billion years
Chemical theory Oparin-Haldane
Lab proof Miller-Urey, 1953
Oxygen rise About 2.4 billion years ago
Exam link UPSC Prelims, Environment & Ecology: Basic Concepts of Ecology

Where Does It Fit?

This is the first topic of the macro on basic concepts of ecology. It sets the long history of life on Earth, in which the living world and the physical environment shaped each other. Next topic: Types and Scope of Ecology.

Which Theories Explain How Life Began?

Theory	Main idea
Special creation	An old belief that life was created by a divine act. It is not a scientific theory
Spontaneous generation	The idea that living things arise from non-living matter every day. Louis Pasteur disproved it in 1862 with his swan-neck flask experiment
Panspermia	Life came to Earth from space on meteorites or comets. Proposed by Svante Arrhenius. It moves the question elsewhere
Oparin-Haldane hypothesis	A I Oparin (1924) and J B S Haldane (1929) suggested that simple organic molecules formed in the reducing atmosphere of early Earth and slowly became complex, forming the first cells
Miller-Urey experiment	In 1953, a closed apparatus with methane, ammonia, hydrogen and water vapour and electric sparks produced amino acids in a week, supporting chemical evolution
RNA world	RNA can store information and also act as an enzyme, so it may have come before DNA and proteins
Hydrothermal vents	Life may have begun near deep sea vents with heat and chemical energy

What Were the Major Steps in the History of Life?

Chemical evolution: formation of organic molecules, then self-copying molecules
First cells: anaerobic prokaryotes, simple cells without a nucleus, probably heterotrophs
Cyanobacteria: began oxygen-producing photosynthesis; stromatolites are layers built by them
Great Oxidation Event: rise of oxygen about 2.4 billion years ago changed the atmosphere and killed many anaerobes
Eukaryotes: cells with a nucleus about 2 billion years ago, formed by endosymbiosis, in which mitochondria and chloroplasts were once free bacteria, as proposed by Lynn Margulis
Multicellular life: appeared about 1 billion years ago, with the Ediacaran fauna near 600 million years ago
Cambrian explosion: rapid rise of animal groups about 541 million years ago
Land colonisation: plants about 470 million years ago, then insects, amphibians, reptiles, mammals and birds

What Else Should You Remember?

Geological time scale: eons, eras, periods and epochs; the Phanerozoic eon has the Palaeozoic, Mesozoic and Cenozoic eras
Hadean and Archaean: earliest eons of the Earth
Age of Reptiles: the Mesozoic era, ending about 66 million years ago with the dinosaur extinction
Age of Mammals: the Cenozoic era
Anthropocene: a proposed new epoch of human impact; geologists voted against making it a formal epoch in 2024
Big Five mass extinctions: Ordovician, Devonian, Permian, Triassic and Cretaceous
Permian extinction: about 252 million years ago, the worst, killing about 90 percent of marine species
Cretaceous extinction: about 66 million years ago, linked to an asteroid impact at Chicxulub and Deccan volcanism
Sixth extinction: a view that the present loss of species is the sixth mass extinction
Evolution: Darwin’s theory of natural selection, published in 1859
Fossil record: evidence from fossils shows the sequence of life
Chemical evidence: isotope ratios in rocks show early life
Stromatolites in India: occur in places like Rajasthan’s Aravalli rocks and Andhra Pradesh
Early atmosphere: rich in carbon dioxide, nitrogen, methane and water vapour, with little oxygen
Ozone layer: formed as oxygen rose, protecting land life from ultraviolet
Ecology link: life changed the atmosphere and the atmosphere shaped life, a key idea of ecology
Link to hierarchy: levels of organisation are in Ecological Hierarchy

Why Does It Matter for the Exam?

Theories: Oparin-Haldane, Miller-Urey
Dates: 4.54 billion, 3.5 billion, 2.4 billion, 541 million years
Events: Great Oxidation, Cambrian explosion
Current affairs link: Anthropocene debate and sixth extinction
Common errors: crediting Pasteur for chemical evolution or mixing eon order

As an analytical point, the history of life shows that living things have changed the Earth as much as the Earth has changed them. Understanding this long record helps explain why the present speed of extinction is seen as a crisis.

Why Is This Topic Important?

Theories Chemical evolution
Timeline Key dates
Extinction Big Five

What Came Next?

The next topic covers the types and scope of ecology. Next topic: Types and Scope of Ecology.

Key People and Terms

Primordial soup Early ocean rich in organic molecules
Stromatolite Layered structure built by microbes
Endosymbiosis Cells living inside cells
Prokaryote Cell without nucleus
Eukaryote Cell with nucleus
Panspermia Life from space

Exam Corner

Points to Remember

Earth: 4.54 billion years old.
Miller-Urey: 1953.
Great Oxidation Event: 2.4 billion years ago.
Cambrian explosion: 541 million years ago.
Dinosaur extinction: 66 million years ago.

Do Not Confuse With

Oparin-Haldane and Miller-Urey Hypothesis versus experiment
Prokaryote and eukaryote No nucleus versus nucleus
Panspermia and abiogenesis Life from space versus life from chemistry on Earth

Memory Hook

Oparin guessed, Miller sparked, cyanobacteria breathed out oxygen, Cambrian blew up.

Mains Angle

Questions ask for theories, dates and events. These points help.

Theories Chemical evolution
Milestones Oxygen, cells
Extinctions Big Five

A Question You May Face

An original practice question, not a past paper question.

“Discuss the major theories on the origin of life and the key events in the history of life on Earth.”

How to Answer

Theories Oparin, Miller.
Events Oxygen, eukaryotes.
Extinctions Big Five.
Link Present crisis.

GS Relevance

Prelims: theories of origin of life, Miller-Urey, geological events, mass extinctions. GS Paper 3: environment and biodiversity.

Frequently Asked Questions

What is the Oparin-Haldane hypothesis?

The Oparin-Haldane hypothesis, put forward by A I Oparin in 1924 and J B S Haldane in 1929, says that life arose gradually from simple organic molecules that formed in the oxygen-free atmosphere and oceans of early Earth, using energy from lightning and ultraviolet, and combined into complex molecules and the first cells.

What did the Miller-Urey experiment show?

In 1953 Stanley Miller and Harold Urey passed electric sparks through a mixture of methane, ammonia, hydrogen and water vapour to imitate early Earth. After a week, the water contained amino acids, the building blocks of proteins, showing that organic molecules can form from simple chemicals.

What was the Great Oxidation Event?

The Great Oxidation Event was the rise of free oxygen in the atmosphere about 2.4 billion years ago, caused by photosynthesis of cyanobacteria. It changed the chemistry of the Earth, wiped out many anaerobic organisms and made possible oxygen-using life and later the ozone layer.

What is the Cambrian explosion?

The Cambrian explosion was a rapid diversification of animal life that began about 541 million years ago, when most major animal groups appear in the fossil record within a few tens of millions of years. It marks the start of the Phanerozoic eon of obvious animal life.

What are the Big Five mass extinctions?

The five largest mass extinctions were at the end of the Ordovician, Devonian, Permian, Triassic and Cretaceous periods. The Permian extinction about 252 million years ago was the worst, killing most marine species, while the Cretaceous one about 66 million years ago ended the dinosaurs.

PYQ Practice — Statement Analysis

1 Miller and Urey produced amino acids in a laboratory in 1953.

True

Using simple gases and sparks.

2 The Great Oxidation Event was caused by animals.

False

Cyanobacteria caused it.

3 The dinosaurs became extinct about 66 million years ago.