Evolution

Syllabus Unit: Genetics and Evolution

Why it matters

Chapter overview

Evolution explains how life's staggering diversity arose from common ancestors through natural, observable mechanisms rather than fixed design. This topic traces the story from chemical origins on early Earth, through the fossil, comparative-anatomy and molecular evidence that establishes common descent, to the actual mechanisms — natural selection, genetic drift and speciation — that drive change, closing with the fossil record of human evolution NEET tests almost every year.

Biology Syllabus Genetics and Evolution 18%–20% weightage

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1Inorganic GasesCH4, NH3, H2, H2O in reducing atmosphere

2Organic MonomersAmino acids, nucleotides form

3PolymersProteins & nucleic acids assemble

4ProtobiontsMembrane-bound precursors to cells

1Origin of Life & Its Theories

Before evolution could act on life, life itself had to arise from non-living chemistry — a process explained by chemical evolution, not spontaneous appearance.

Learning Objectives

State the Theory of Special Creation and explain why it is not a scientific/testable theory.
Explain Oparin-Haldane's theory of chemical evolution in early Earth's reducing atmosphere.
Describe the Miller-Urey experiment and what it demonstrated about abiogenesis.
Sequence the proposed stages: simple inorganic molecules → organic monomers → polymers → protobionts → first cells.

Key Terms

Chemical EvolutionOparin and Haldane's proposal that organic molecules formed spontaneously from inorganic gases under early Earth's reducing atmosphere, before biological life existed.

Reducing AtmosphereEarly Earth's oxygen-free atmosphere (rich in CH4, NH3, H2, water vapour) that favoured formation and survival of organic molecules.

Miller-Urey Experiment1953 lab simulation of early-Earth conditions (electric spark through CH4, NH3, H2, H2O) that produced amino acids from inorganic starting material.

ProtobiontA self-organised aggregate of organic molecules with a boundary membrane, considered a precursor to the first true living cell.

How did the first life arise from non-living matter? NEET tests two competing historical ideas and the experimental evidence that settled the question in favour of a chemical origin.

Special Creation vs. chemical evolution

The **Theory of Special Creation** holds that life was created in its present form by a supernatural act; it is not scientifically testable. In contrast, **Oparin (Russia)** and **Haldane (Britain)** independently proposed that life arose through a slow chemical process: simple inorganic molecules in early Earth's **reducing atmosphere** (rich in methane, ammonia, hydrogen, water vapour — no free oxygen) combined, using energy from UV radiation, lightning and heat, to form simple organic monomers.

The Miller-Urey experiment

In 1953, **Stanley Miller and Harold Urey** built a closed apparatus with CH4, NH3, H2 and water vapour, circulated near boiling point and exposed to a continuous electric spark (simulating lightning). Within a week, amino acids had formed — direct experimental support for Oparin-Haldane's chemical evolution hypothesis.

From monomers to the first cell

Simple inorganic molecules (CH4, NH3, H2O) combine using energy input to form organic monomers (amino acids, nucleotides, sugars).
Monomers polymerise into macromolecules (proteins, nucleic acids) in the primitive ocean's 'organic soup'.
Macromolecules aggregate into protobionts — membrane-bound droplets capable of maintaining an internal chemical environment.
Protobionts that could grow, divide and use RNA/DNA for heritable information became the first true living cells.

Common MistakeNEET often asks which gases Miller and Urey used — remember CH4, NH3, H2 and water vapour, sparked electrically, no free O2.