Evolution

Origin of Life

The Big Bang and Early Earth

• Big Bang Theory: Explains the origin of the universe via a singular, unimaginably huge explosion ~20 billion years ago.
• Earth formed ~4.5 billion years ago.
• Primitive Atmosphere: Earth initially had no atmosphere. Water vapour, CH₄, CO₂, and NH₃ were released from molten mass. UV rays broke water into H₂ and O₂.
• Crucial detail: The primitive atmosphere was highly reducing; free oxygen was strictly absent 1989 2004 2012.

Theories of Origin of Life

• Panspermia (Cosmozoic Theory): Life was transferred to Earth from other planets in the form of units called spores NEET 2022 Re.
• Spontaneous Generation: Belief that life came from decaying matter. Disproved unequivocally by Louis Pasteur using carefully sterilized swan-necked flasks 2008 NEET 2012.
• Chemical Evolution (Oparin-Haldane): First life arose from pre-existing non-living organic molecules driven by chemical evolution. Key Concept: Origin of life by combination of chemicals under suitable environmental conditions (high temp, volcanic storms, reducing atmosphere) 2007.
• Miller-Urey Experiment (1953): Simulated primitive earth conditions in a closed flask.
  • Mixed CH₄, CO₂, NH₃ and water vapour at 800^oC and passed an electric discharge NEET 2020.
  • Formed amino acids.
  • Exception/Crucial detail: Alanine, Glycine, and Aspartic acid were formed, but Glutamic acid was not synthesised in this experiment 2006.
• Protobionts / Coacervates: First non-cellular forms of life. Crucial detail: They could maintain an internal environment and partially isolate molecules, but they could not independently reproduce initially 2008.

Evidences for Evolution

Paleontological and Embryological Evidence

• Paleontology: Study of fossils in sedimentary rocks.
• Embryological Support: Proposed by Ernst Haeckel based on common embryonic features.
• Crucial Exception: Strongly rejected by Karl Ernst von Baer, who proved that embryos never pass through the adult stages of other animals NEET 2020.

Morphological and Anatomical Evidence

Feature	Homologous Organs	Analogous Organs
Origin & Anatomy	Same fundamental structure and origin.	Different structures and origin.
Function	Perform different functions.	Perform the same function.
Evolutionary Process	Indicates Divergent Evolution 2009.	Indicates Convergent Evolution 2013.
Genetic Basis	Points to Common Ancestry 1995 2009.	Does NOT indicate common ancestry.
Animal Examples	Forelimbs of whales, bats, cheetah, humans 2014 NEET 2024. Vertebrate hearts and brains.	Wings of butterfly and bird 2015 NEET 2020. Eye of octopus and mammal 2013 2014. Flippers of penguins and dolphins NEET 2020 2024 Re.
Plant Examples	Thorns of Bougainvillea and tendrils of Cucurbita 2008 NEET 2014 NEET 2018.	Sweet potato (root modification) and potato (stem modification) NEET 2018.

Evolution by Anthropogenic Action

• Industrial Melanism: Post-industrialization in England, dark-winged (melanised) Biston betularia moths survived better due to soot-covered trees, illustrating Directional Natural Selection 2015 NEET 2024. Detail: No variant was completely wiped out NEET 2014.
• Creation of antibiotic-resistant bacteria and herbicide-resistant weeds due to excessive chemical use are prime examples of natural selection by anthropogenic action NEET 2020.

Adaptive Radiation

Concept and Examples

• Key Term: Evolution of different species in a given geographical area starting from a point and radiating to other habitats 2010 NEET 2021.
• Darwin's Finches: Galapagos Islands. From original seed-eating ancestors, altered beaks evolved for insectivorous and vegetarian diets 2008 2010.
• Australian Marsupials: Multiple marsupials evolved from an ancestral stock within the isolated Australian continent NEET 2021.
• Convergent Evolution in Radiation: When more than one adaptive radiation occurs in an isolated geographical area. Example: Placental mammals (Placental Wolf) and Australian marsupials (Tasmanian Wolf) evolving to resemble each other 2011 NEET 2018 NEET 2019.

Mechanism of Evolution

Lamarckism vs Darwinism vs Mutation Theory

• Lamarck: Evolution is driven by the use and disuse of organs resulting in inheritance of acquired characters.
• Charles Darwin: Proposed Natural Selection and Branching Descent NEET 2020. Fitness strictly refers to reproductive fitness (individuals who leave more progeny survive) 2015.

Feature	Darwinian Evolution	de Vries' Mutation Theory
Nature of Change	Minor, continuous variations.	Sudden, large mutations.
Direction	Small and directional	Random and directionless NEET 2019.
Pace of Speciation	Gradual (Gradualism).	Single-step large mutation (Saltation) NEET 2019.

Hardy-Weinberg Principle

Genetic Equilibrium

• Key Term: Allele frequencies in a population are stable and constant from generation to generation. Sum total of allelic frequencies is 1 2010.
• Equation: p² + 2pq + q² = 1 (p² = homozygous dominant, 2pq = heterozygous, q² = homozygous recessive) NEET 2021.
• Factors affecting Equilibrium: Gene migration, Genetic drift, Mutation, Genetic recombination, Natural selection NEET 2019.
• Crucial detail: Genetic Drift is a random change in allele frequencies occurring by chance, operating strictly in small, isolated populations 2002 2011 NEET 2018 NEET 2021. Leads to the Founder Effect when the drifted population becomes founders of a new species 2011 NEET 2019.

Types of Natural Selection

• Stabilising Selection: More individuals acquire the mean character value. The peak gets higher and narrower NEET 2019 NEET 2024.
• Directional Selection: Individuals acquire a value other than the mean. The peak shifts in one direction 2014 2015.
• Disruptive Selection: Individuals acquire peripheral values at both ends of the curve, forming two peaks NEET 2019.

A Brief Account of Evolution

• ~2000 mya: First cellular life forms appeared.
• ~500 mya: Invertebrates formed and became active.
• ~350 mya: Jawless fish evolved.
• Lobe-finned fish (Coelacanth): Caught in South Africa (1938). Evolved into the first amphibians (ancestors of modern frogs/salamanders) 2014.
• Reptile Era: Amphibians evolved into reptiles. Dinosaurs dominated. Tyrannosaurus rex was the largest. Extinct abruptly ~65 mya.
• Mammalian Era: Australia's isolation allowed pouched mammals to survive due to a lack of placental competition after continental drift 2009.

Origin and Evolution of Man

Humans belong to the order Primates. Progression is marked by increased cranial capacity, bipedalism, and orthognathous facial structure.

Hominid Stage	Timeline	Brain Capacity	High-Yield Features & PYQ Matches
Dryopithecus	~15 mya	350–400 cc	Common ancestor. More ape-like, hairy, walked like gorillas.
Ramapithecus	~15 mya	350–400 cc	More man-like, but still walked like gorillas 2002.
Australopithecus	~2 mya	400–600 cc	First to show bipedal movement. Hunted with stone weapons but essentially ate fruit NEET 2021.
Homo habilis	~2 mya	650 - 800 cc	First human-like hominid. Did not eat meat NEET 2019 NEET 2021.
Homo erectus	~1.5 mya	900 cc	Java/Peking man. Probably ate meat NEET 2019 NEET 2021.
Neanderthal man	100k - 40k years ago	1400 cc	East/Central Asia. Used hides to protect bodies and buried their dead NEET 2019 NEET 2021.
Homo sapiens	75k - 10k years ago	~1450 cc	Arose in Africa. Pre-historic cave art developed ~18,000 years ago. Agriculture arose ~10,000 years ago 2015.