Evolution

The origin of life on Earth is explained by the chemical evolution theory, proposed independently by Oparin and Haldane, which states that life arose from simple inorganic molecules through a series of chemical reactions in a primordial reducing atmosphere. Stanley Miller and Harold Urey experimentally demonstrated this in 1953 by simulating early Earth conditions: a mixture of methane (CH₄), ammonia (NH₃), hydrogen (H₂), and water vapour was subjected to electric sparks (simulating lightning) in a closed apparatus. After one week, amino acids (including glycine and alanine) and other simple organic molecules were found in the collected liquid, supporting abiogenic origin. Critically, the experiment used a reducing atmosphere with no free oxygen.

Multiple lines of evidence support biological evolution. Paleontological evidence from the fossil record shows transitional forms and increasing complexity over geological time. Comparative anatomy reveals homologous organs -- structures with the same embryonic origin but different functions, such as the forelimbs of a whale (swimming), bat (flying), horse (running), and human (grasping). Homologous organs indicate divergent evolution from a common ancestor. Analogous organs have different embryonic origins but similar functions, such as the wings of a bat (bone-supported, mammalian) and a butterfly (chitinous, insect), representing convergent evolution. Molecular evidence, including amino acid sequence comparisons of cytochrome c across species, provides quantitative measures of evolutionary relatedness.

Charles Darwin's theory of natural selection operates through four principles: variation exists within populations, these variations are heritable, organisms produce more offspring than can survive, and individuals with advantageous traits reproduce more successfully. Over generations, beneficial traits increase in frequency.

The Hardy-Weinberg principle provides a mathematical baseline for detecting evolution. In a population at equilibrium, allele frequencies remain constant across generations: p + q = 1 (allele frequencies) and $p^{2}$ + 2pq + $q^{2}$ = 1 (genotype frequencies), where p is the frequency of the dominant allele and q of the recessive allele. This equilibrium holds only when five conditions are met: no mutation, no migration (gene flow), no natural selection, large population size (no genetic drift), and random mating. Any departure from these conditions drives evolution.

Natural selection operates in three modes: stabilizing selection (favours intermediate phenotypes, reduces variation), directional selection (shifts the mean toward one extreme), and disruptive selection (favours both extremes, may lead to speciation).

Speciation occurs through two main pathways. Allopatric speciation results from geographical isolation that prevents gene flow, allowing populations to diverge over time. Sympatric speciation occurs within the same geographical area through reproductive isolation mechanisms. Adaptive radiation, exemplified by Darwin's finches on the Galapagos Islands and Australian marsupials, occurs when a single ancestral species diversifies into many forms occupying different ecological niches.

Human evolution progressed from Dryopithecus (ape-like, ~15 mya) through Ramapithecus (~14 mya), Australopithecus (~5 mya, first bipedal), Homo habilis (~2 mya, first tool-maker), Homo erectus (~1.5 mya, used fire), to Homo sapiens (~2 lakh years ago, modern humans with ~1400 cc brain volume). The key testable concept is distinguishing homologous organs (same origin, different function -- divergent evolution) from analogous organs (different origin, same function -- convergent evolution) and applying the Hardy-Weinberg equation with its five equilibrium conditions.