1. Origin of Earth and Early Conditions
- Earth is approximately 4.5 billion years old; life first appeared about 3.8–4 billion years ago.
- Early Earth atmosphere: reducing atmosphere — rich in CH₄, NH₃, H₂O vapour, H₂; no free oxygen (O₂).
- The reducing atmosphere was critical: it allowed organic molecules to form without being immediately oxidised.
- Energy for early chemical reactions came from lightning, UV radiation, volcanic activity, and heat.
2. Chemical Evolution — Abiogenesis
Chemical evolution is the step-by-step progression from simple inorganic molecules to complex organic molecules, and eventually to the first living cells.
Oparin-Haldane Hypothesis (Primordial Soup Theory)
- Proposed independently by A.I. Oparin (Russia, 1923) and J.B.S. Haldane (England, 1929).
- Inorganic molecules (CH₄, NH₃, H₂O, H₂) in the early oceans reacted under energy input → first organic molecules.
- The early ocean became a "primordial soup" — a dilute solution of organic compounds.
- Over time, organic molecules aggregated into complex coacervates (Oparin) — droplet-like colloid aggregates that showed some properties of living systems.
Miller-Urey Experiment (1953)
| Aspect | Detail |
|---|---|
| Scientists | Stanley Miller and Harold Urey |
| Setup | Sealed apparatus with CH₄, NH₃, H₂, H₂O vapour; electric sparks simulated lightning |
| Result | After one week: amino acids detected (glycine, alanine, aspartic acid); also urea, sugars, HCN |
| Significance | Experimental proof that organic molecules can form abiotically from inorganic precursors |
Limitations: Produced only a few amino acids; did not produce proteins, nucleic acids, or cells. But it validated the Oparin-Haldane hypothesis experimentally.
Steps in Chemical Evolution
- Inorganic molecules → Simple organic molecules (sugars, amino acids, purines, pyrimidines)
- Simple organics → Polymers (proteins, nucleic acids) — by condensation reactions
- Polymers → Protobionts (coacervates, microspheres) — droplets with membrane-like boundary
- Protobionts → First cells — capable of self-replication (RNA first — "RNA World" hypothesis)
RNA World Hypothesis: RNA was the first genetic material — it could both carry information (like DNA) AND catalyse reactions (like enzymes = ribozymes). Later, DNA took over as the stable genetic material.
3. Theories of Biological Evolution
Lamarck's Theory of Evolution (1809) — Theory of Inheritance of Acquired Characters
| Principle | Explanation |
|---|---|
| Internal vital force | All living organisms have a natural tendency toward perfection and complexity |
| Use and Disuse | Organs used extensively become stronger/larger; organs not used degenerate |
| Inheritance of acquired characters | Characters acquired during lifetime are passed on to offspring |
Classic example: Giraffe neck elongation — giraffes stretched their necks to reach higher leaves; this acquired elongation was passed to offspring over generations.
Disproved by: Weismann's experiment — he cut tails of mice for 22 generations; offspring always had normal tails. Acquired somatic characters are NOT inherited (only changes in germline DNA are heritable).
Darwin's Theory of Natural Selection (1859) — "On the Origin of Species"
| Postulate | Explanation |
|---|---|
| Overproduction | Organisms produce far more offspring than can survive (geometric increase) |
| Variation | Individuals in a population vary in inherited traits (heritable variation exists) |
| Struggle for Existence | Resources are limited → competition between organisms for survival |
| Survival of the Fittest | Organisms best adapted to their environment survive and reproduce (coined by Herbert Spencer) |
| Natural Selection | Nature selects organisms with favourable variations → they reproduce more → traits become more common |
| Speciation | Over long time, accumulated changes lead to new species (descent with modification) |
Darwin's observations: Voyage on HMS Beagle (1831–1836); Galápagos finches with different beak shapes adapted to different food sources — a key inspiration.
Darwin's limitation: He could not explain the source of variation (genetics was unknown in his time). Mendel's work was concurrent but unrecognised until 1900.
De Vries and Mutation Theory
Hugo de Vries (1901) proposed that evolution occurs by sudden, large heritable changes called mutations, not by gradual accumulation of small variations as Darwin suggested. He studied Oenothera lamarckiana (evening primrose).
Modern view: Mutations provide the raw material for evolution; natural selection then acts on mutant variants.
4. Modern Synthetic Theory of Evolution (Neo-Darwinism)
The Modern Synthesis combines Darwin's natural selection with Mendelian genetics, population genetics, and the study of mutations.
Key contributors: Theodosius Dobzhansky, Ernst Mayr, J.B.S. Haldane, Ronald Fisher, Sewall Wright.
Five factors of evolution according to Modern Synthesis:
- Gene mutations — changes in DNA sequences; source of new alleles
- Chromosomal aberrations — structural or numerical changes in chromosomes
- Genetic recombination — crossing over creates new combinations of alleles
- Natural selection — differential survival and reproduction based on fitness
- Reproductive isolation — prevents gene flow between populations, leading to speciation
