The chapter on Evolution covers two interrelated domains: the origin of life on Earth and the mechanisms by which living species diversify over geological time. Together, these topics explain both how life began and how it has transformed into the spectacular diversity seen today.

Origin of Life: Chemical Evolution

The prevailing scientific explanation for the origin of life is the chemical evolution theory, independently proposed by Alexander Oparin (1924) and J.B.S. Haldane (1929). They proposed that life arose from simple inorganic molecules through a series of progressively complex chemical reactions in the primordial reducing atmosphere of early Earth. The defining feature of this early atmosphere was the complete absence of free oxygen (O2), which is why it is called a reducing atmosphere — oxygen would have oxidized and destroyed organic molecules as they formed.

In 1953, Stanley Miller and Harold Urey provided the first experimental evidence supporting Oparin and Haldane. Their landmark experiment simulated early Earth conditions in a closed apparatus: a mixture of methane (CH4), ammonia (NH3), hydrogen (H2), and water vapour was subjected to electrical sparks simulating lightning. After one week, amino acids (including glycine and alanine) and other simple organic molecules were detected in the collected liquid. This demonstrated that the building blocks of proteins — amino acids — could form abiogenically (without living organisms) under conditions plausibly resembling early Earth.

Evidence for Evolution

Multiple independent lines of evidence support the theory of biological evolution. Paleontological evidence from the fossil record documents transitional forms — organisms with characteristics intermediate between ancestral and descendant species — and shows progressive increases in complexity over geological time. Comparative anatomy provides two key categories of evidence: homologous organs (same embryonic origin, different adult functions — e.g., the forelimbs of whales, bats, horses, and humans) demonstrate divergent evolution from a common ancestor; analogous organs (different embryonic origins, similar adult functions — e.g., the wings of bats and butterflies) demonstrate convergent evolution driven by similar environmental pressures. Molecular evidence, such as amino acid sequence comparisons of cytochrome c across species, provides quantitative measures of evolutionary relatedness: more similar sequences indicate a more recent shared ancestor.

Natural Selection and Hardy-Weinberg

Charles Darwin's theory of natural selection operates through four principles: heritable variation exists in populations; organisms produce more offspring than can survive; individuals with advantageous traits reproduce more successfully; and beneficial traits thus increase in frequency over generations. Natural selection operates in three distinct modes: stabilizing selection (favours intermediate phenotypes, reducing variation, e.g., birth weight in humans); directional selection (shifts the phenotypic mean toward one extreme, e.g., industrial melanism in moths); and disruptive selection (favours both extremes simultaneously, which can lead to sympatric speciation).

The Hardy-Weinberg principle provides the mathematical framework for detecting evolution in populations. It states that allele frequencies remain constant across generations under five conditions: no mutation, no migration (gene flow), no natural selection, large population size (eliminating genetic drift), and random mating. The equations are: p + q = 1 (allele frequencies) and $p^{2}$ + 2pq + $q^{2}$ = 1 (genotype frequencies), where $p^{2}$ = frequency of homozygous dominant (AA), 2pq = heterozygous carriers (Aa), and $q^{2}$ = homozygous recessive (aa). NEET problems typically provide $q^{2}$ (recessive phenotype frequency) and require calculating carrier frequency (2pq) or dominant allele frequency (p) through a sequential algorithm: identify $q^{2}$, take the square root to get q, subtract from 1 to get p, then calculate the required term.

Speciation and Adaptive Radiation

Species formation occurs through two main pathways. Allopatric speciation results from geographical isolation (a mountain range, river, or ocean) that prevents gene flow between populations; over thousands of generations, the isolated groups accumulate enough genetic divergence to become reproductively incompatible. Sympatric speciation occurs within the same geographic area through mechanisms like polyploidy (chromosome doubling, especially in plants), which instantly creates reproductive isolation. 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, typically facilitated by geographic isolation (different islands) combined with different selection pressures (different food resources).

Human Evolution

The evolutionary timeline of humans progresses from Dryopithecus (most ape-like, ~15 mya, 300-400 cc brain) through Ramapithecus (~14 mya), Australopithecus (~5 mya, first confirmed biped, 400-500 cc), Homo habilis (~2 mya, first tool-maker using Oldowan stone tools, 600-700 cc), and Homo erectus (~1.5 mya, first fire user, Acheulean tools, migrated out of Africa, 800-1100 cc), to modern Homo sapiens sapiens (~2 lakh years ago, 1300-1400 cc, art, language, agriculture). Neanderthals (Homo sapiens neanderthalensis, ~1400 cc) were cold-adapted, made sophisticated tools, and were the first to bury their dead. Two milestones are especially critical for NEET: Australopithecus was the first biped and Homo habilis was the first tool-maker — these two are frequently tested as traps.