Maarg Manthan · Topic 1.2

History and Origin of Life on Earth

Science & Technology › Cell Biology and Genetics · Topic 1.2

The origin of life is the question of how the first living cells arose from non-living matter on the early Earth, which formed about 4.5 billion years ago. The widely accepted scientific view is chemical evolution: simple molecules in the atmosphere and oceans reacted to form organic compounds, which then organised into self-copying systems and finally into cells. The Oparin-Haldane hypothesis of the 1920s proposed a primordial soup, and the Miller-Urey experiment of 1953 showed that amino acids can form from simple gases and electric sparks. The earliest evidence of life is in rocks that are about 3.5 billion years old. Early life was single-celled and lived without oxygen, and oxygen-producing cyanobacteria later changed the atmosphere.

5 min read · UPSC Prelims · Science & Technology · Cell Biology and Genetics

At a Glance

Earth’s age About 4.5 billion years
Hypothesis Oparin-Haldane, 1920s
Experiment Miller-Urey, 1953
Earliest evidence About 3.5 billion years ago
Oxygen rise Great Oxidation Event
Exam link UPSC Prelims, Science & Technology: Cell Biology and Genetics

Where Does It Fit?

This is the second topic of the macro on cell biology and genetics. It places the chemical building blocks of the previous topic in the story of how life began and changed the planet. Previous topic: Chemical Building Blocks of Life (Biomolecules). Next topic: Cell Structure: Prokaryotic and Eukaryotic Cells.

How Might Life Have Begun?

Early Earth: the young planet was hot, with volcanic activity and an atmosphere that was probably rich in water vapour, carbon dioxide, nitrogen, methane and ammonia, with little or no free oxygen.
Oparin-Haldane hypothesis: the Russian scientist Alexander Oparin and the British scientist J.B.S. Haldane, working separately in the 1920s, suggested that organic molecules formed from simple gases under energy from lightning and ultraviolet light, and collected in the oceans as a primordial soup.
Miller-Urey experiment: in 1953 Stanley Miller and Harold Urey passed electric sparks through a mixture of methane, ammonia, hydrogen and water vapour and obtained amino acids, which supported chemical evolution.
Self-copying molecules: the RNA world hypothesis says that RNA, which can store information and also act as a catalyst, came before DNA and proteins.
Protocells: droplets with membranes, called coacervates or protobionts, may have been the step between chemistry and the first cells.
Other ideas: panspermia suggests that life or its building blocks arrived from space on meteorites, and deep-sea vents are also proposed as places where life began.

What Was the Sequence After the First Cells?

Stage	Detail
First cells	Simple prokaryotes that fed on organic matter in the sea and respired without oxygen
Photosynthesis	Cyanobacteria began to make food using sunlight and released oxygen, from about 2.7 to 2.4 billion years ago
Great Oxidation Event	Oxygen began to build up in the atmosphere about 2.4 billion years ago, which changed the oceans and led to the spread of oxygen-using life
Eukaryotes	Cells with a nucleus appeared about 2 billion years ago
Endosymbiosis	Lynn Margulis proposed that mitochondria and chloroplasts came from bacteria that were taken into larger cells
Multicellular life	Appeared about a billion years ago, and animals diversified in the Cambrian explosion about 540 million years ago

What Else Should You Remember?

Oldest evidence: stromatolites, which are layered structures built by microbes, and microfossils in ancient rocks show life by about 3.5 billion years ago. Dates differ between studies, so figures are given as approximate.
Spontaneous generation: the old idea that life arises from non-living matter repeatedly was disproved by Louis Pasteur’s experiments in the 1860s.
Ozone layer: oxygen led to ozone, which shields the surface from ultraviolet rays and let life move onto land.
Evidence for endosymbiosis: mitochondria and chloroplasts have their own circular DNA and ribosomes and divide by a process like binary fission, which resembles bacteria.
Unanswered questions: scientists do not know exactly how the first self-copying system arose, so the origin of life remains an active research field.
Search for life elsewhere: missions to Mars and the moons of Jupiter and Saturn look for signs of past or present life, as in Space Technology and ISRO.
Link to cells: the types of cells that arose are described in Cell Structure: Prokaryotic and Eukaryotic Cells.

Why Does It Matter for the Exam?

Names and years: questions ask who did the 1953 experiment, who proposed the primordial soup and what gases were used.
Sequence of events: questions ask which came first, such as prokaryotes before eukaryotes and anaerobic before aerobic life.
Link to current affairs: discoveries of organic molecules on Mars or on asteroid samples make the topic newsworthy.
Concept link: endosymbiosis explains why mitochondria have their own DNA, which also helps in genetics questions.
Caution: the topic is about scientific hypotheses, so answers should say that the explanation is supported by evidence but not fully proved.

As an analytical point, the story of life links chemistry, geology and biology. The rise of oxygen shows that living things can change the planet as well as being shaped by it.

Why Is This Topic Important?

Facts Experiments and people asked
Sequence Order of early life
Concept Endosymbiosis

What Came Next?

The next topic covers the structure of prokaryotic and eukaryotic cells. Next topic: Cell Structure: Prokaryotic and Eukaryotic Cells.

Key People and Terms

Chemical evolution Formation of organic molecules before life
Primordial soup Early ocean rich in organic molecules
RNA world Idea that RNA preceded DNA and proteins
Stromatolite Layered structure made by microbes
Endosymbiosis Bacteria living inside a host cell
Panspermia Life carried from space

Exam Corner

Points to Remember

Earth formed about 4.5 billion years ago.
Oparin and Haldane: primordial soup.
Miller and Urey: amino acids in 1953.
Cyanobacteria released oxygen.
Margulis: endosymbiosis.

Do Not Confuse With

Miller-Urey and Pasteur Chemical origin experiment versus disproof of spontaneous generation
Prokaryote and eukaryote No nucleus versus nucleus
Panspermia and chemical evolution Life from space versus chemistry on Earth

Memory Hook

Oparin-Haldane soup, Miller-Urey spark; fifty-three amino acids in the dark; cyanobacteria blew oxygen for the mark.

Mains Angle

Questions ask about theories of the origin of life. These points add depth.

Theories Chemical evolution, RNA world
Evidence Experiments and fossils
Impact Oxygen and eukaryotes

A Question You May Face

An original practice question, not a past paper question.

“Discuss the scientific theories on the origin of life on Earth and the major steps in early evolution.”

How to Answer

Theories Soup, RNA world.
Evidence Miller-Urey.
Steps Cells, oxygen.
Limits Open questions.

GS Relevance

Prelims: experiments, hypotheses, timeline, early organisms. GS Paper 3: science and technology, basic biology.

Frequently Asked Questions

What is the Oparin-Haldane hypothesis?

The Oparin-Haldane hypothesis, proposed separately in the 1920s by Alexander Oparin and J.B.S. Haldane, says that simple gases on the early Earth formed organic molecules under energy from lightning and ultraviolet light. These collected in the oceans as a primordial soup from which the first living cells gradually arose.

What did the Miller-Urey experiment show?

In 1953, Stanley Miller and Harold Urey passed electric sparks through methane, ammonia, hydrogen and water vapour in a closed apparatus and found amino acids in the water. The experiment showed that building blocks of proteins can form from simple gases, which supported the idea of chemical evolution.

What is the RNA world hypothesis?

The RNA world hypothesis says that RNA, which can store genetic information and also act as a catalyst, was the first self-copying molecule and came before DNA and proteins. It helps to explain the chicken-and-egg problem of whether genes or enzymes came first, though it is still a hypothesis.

How did cyanobacteria change the Earth?

Cyanobacteria began photosynthesis and released oxygen from about 2.7 billion years ago, and oxygen built up in the atmosphere about 2.4 billion years ago in the Great Oxidation Event. This led to the ozone layer, changed ocean chemistry and made it possible for oxygen-using organisms to evolve.

What is the endosymbiotic theory?

The endosymbiotic theory, developed by Lynn Margulis, says that mitochondria and chloroplasts were once free-living bacteria that were taken into larger cells and became permanent partners. Their own circular DNA, bacteria-like ribosomes and division by fission support the idea.

PYQ Practice — Statement Analysis

1 The Miller-Urey experiment produced amino acids from simple gases.

True

Done in 1953.

2 Louis Pasteur proved that life arises by spontaneous generation.

False

He disproved it.

3 Cyanobacteria released oxygen through photosynthesis.