Section 1: Origin of Life — Chemical Evolution
The chemical evolution hypothesis (Oparin, 1924; Haldane, 1929) proposes that life originated from simple inorganic molecules through sequential chemical reactions in a primordial reducing atmosphere lacking free O2. The Miller-Urey experiment (1953) provided experimental validation: CH4 + NH3 + H2 + H2O under electrical discharge (simulating lightning) produced amino acids (glycine, alanine) and other organic molecules within one week. The experiment's significance is that it demonstrated abiogenic synthesis of amino acids — not that it created life itself. NEET frequently asks about the specific gases, energy source, and products.
Section 2: Evidence for Biological Evolution
Paleontological evidence: The fossil record provides transitional forms (e.g., Archaeopteryx bridging reptiles and birds) and demonstrates progressive complexity over geological time.
Comparative anatomy: Homologous organs (same embryonic origin, different function — forelimbs of whale, bat, horse, human) indicate divergent evolution from a common ancestor. Analogous organs (different origin, same function — bat wings vs. butterfly wings; octopus eyes vs. mammalian eyes) indicate convergent evolution from different ancestors facing similar environmental pressures. Thorns (Bougainvillea) and tendrils (Cucurbita) are homologous — both are stem modifications.
Molecular evidence: Cytochrome c amino acid sequence comparisons: more similar sequences = more recent common ancestor.
Section 3: Darwin's Natural Selection and Its Modes
Darwin's four principles: variation exists → variations are heritable → overproduction of offspring → differential reproduction of better-adapted individuals. Three modes of selection: stabilizing (favours intermediate, reduces variation); directional (shifts mean toward one extreme); disruptive (favours both extremes, can lead to speciation).
Section 4: Hardy-Weinberg Principle
Mathematical null hypothesis for evolution. Equilibrium equations: p + q = 1; + 2pq + = 1. Five conditions: no mutation, no migration, no natural selection, large population, random mating. Violation of ANY condition drives evolution. Standard NEET calculation: → q → p → 2pq → multiply by population size.
Section 5: Speciation
Two pathways: allopatric (geographic isolation → gene flow stops → divergence → reproductive isolation → new species) and sympatric (same area → polyploidy or other mechanism → new species). Adaptive radiation: one ancestor → many species filling different niches. Examples: Darwin's finches (Galapagos, different beak shapes) and Australian marsupials (different body forms).
Section 6: Human Evolution (D-R-A-H-E-S)
Timeline: Dryopithecus (15 mya, ape-like) → Ramapithecus (14 mya) → Australopithecus (5 mya, FIRST BIPED) → Homo habilis (2 mya, FIRST TOOLS) → Homo erectus (1.5 mya, FIRE + out of Africa) → Homo sapiens (0.2 mya, language, art, agriculture). Brain volume increases progressively: ~300-400 cc (Dryopithecus) → 1300-1400 cc (Homo sapiens).