Chapter 6 — Evolution

Chapter 6 of the Class 12 Biology NCERT textbook, "Evolution", explains the origin of life through chemical evolution, Darwin's theory of natural selection, evidences for evolution, adaptive radiation, the Hardy-Weinberg principle and the evolutionary history of life forms including the origin of man.

• Origin of life — The chapter traces life's beginnings from the early universe through the Oparin-Haldane chemical-evolution hypothesis, using Miller's experiment to argue that life's building blocks could arise abiotically.
• The mechanism of natural selection — It centres on Darwin's theory that heritable variations improving fitness let individuals leave more progeny, driving gradual change, with Wallace independently reaching the same core idea.
• Evidence and adaptive radiation — The chapter marshals evidence from fossils, comparative anatomy and biochemistry, and uses adaptive radiation — like Darwin's finches — to show how one lineage diversifies to fill many niches.
• Population genetics and human origins — It introduces the Hardy-Weinberg principle to define genetic equilibrium and its disturbances as evolution, then applies these ideas to the evolutionary timeline leading to modern Homo sapiens.

1. 01The universe is approximately 13.8 billion years old; Earth formed about 4.5 billion years ago, and life first appeared nearly 4 billion years ago.
2. 02Miller's 1953 experiment demonstrated that amino acids can form abiotically by passing electric discharge through CH₄, H₂, NH₃, and water vapour at 800°C, supporting the chemical evolution hypothesis.
3. 03Darwin's theory of natural selection holds that heritable variations conferring better fitness enable individuals to leave more progeny, driving gradual evolution; Alfred Wallace independently reached similar conclusions.
4. 04Homologous structures (e.g., forelimbs of whales, bats, cheetahs, and humans) indicate divergent evolution and common ancestry, while analogous structures (e.g., wings of birds and butterflies) indicate convergent evolution.
5. 05Hardy-Weinberg principle states that allele frequencies in a population remain constant across generations (genetic equilibrium); disturbances caused by gene flow, genetic drift, mutation, recombination, or natural selection result in evolution.
6. 06Homo sapiens arose in Africa; Neanderthal man (brain size ~1400cc) lived 1,00,000–40,000 years ago, and modern Homo sapiens emerged during the ice age between 75,000–10,000 years ago.