Ch 5Origin and Evolution of Life — Class 12 Biology, concept-first.

Living matter can be recognised by a set of shared attributes — responsiveness, growth, metabolism, energy transformation and reproduction — that non-living matter does not show.

Unlike the theories in the previous section, the self-assembly (or biochemical) theory of the origin of life offers a testable, chemistry-based explanation: it proposes that life originated on Earth t…

The universe itself is thought to have originated through the Big Bang described by Georges Lemaitre (1931) — a single, immense, titanic explosion after which the universe expanded, cooled, and its sc…

The newly formed atmosphere was intensely hot at first. As it cooled only gradually, its lighter constituent elements — hydrogen, carbon, nitrogen and sulphur — began reacting with one another.

As Earth's temperature fell further, the atmosphere's steam condensed into liquid water, triggering heavy and prolonged rainfall that gradually cooled the planet's surface.

The primitive broth was neutral in character and completely free of oxygen — conditions that allowed further polymerisation rather than degradation.

Nucleotides are thought to have formed through reactions between phosphoric acid, sugar and nitrogenous bases (the purines and pyrimidines that had already appeared in the primitive broth).

With nucleic acids now present alongside the surrounding inorganic and organic molecules, the first proper form of life-like matter could assemble: the protobiont.

Once a self-replicating RNA or DNA system had developed inside a protocell, the resulting structure began to resemble a simple bacterium or virus, and — crucially — it could now regulate its own metab…

Urey and Miller's Experiment : The chemical-evolution theory proposed by Oparin remained a hypothesis until Stanley L. Miller, working together with his teacher Harold C.

RNA World Hypothesis : The Oparin-Haldane theory and the Miller-Urey experiment together make the case that the pathway to life on Earth ran from non-living matter to living matter — but the principle…

The word evolution comes from the Latin e (from) and volvere (to roll), literally meaning the act of unrolling or unfolding — an apt image for a process that brings about slow, orderly changes carryin…

Several theories had already been proposed to explain organic evolution before Darwin's own work — including Lamarck's 1809 theory of the inheritance of acquired characters, which was ultimately ruled…

The mutation theory was proposed by the Dutch botanist Hugo de Vries in 1901, shortly after the rediscovery of Gregor Mendel's work in 1900.

The modern synthetic theory of evolution represents a genuine synthesis of essentially every relevant biological discipline — genetics, ecology, comparative anatomy, biogeography and palaeontology wer…

Genetic variation refers to any change in the genes present in a population, or in the relative frequency of those genes (alleles), and it is generated by several distinct mechanisms:

According to Darwin, natural selection is the principal driving force behind evolution. It works because genetic variation is constantly arising within a population; individuals that are 'fittest' for…

Isolation is the separation of the population of a particular species into smaller units in a way that prevents interbreeding between those units.

When thinking about the mechanism of evolution, it is essential to focus on the population rather than the individual organism, because it is the population as a whole that evolves over time — an indi…

Type of Natural selection :

The Hardy-Weinberg principle, also known as the Hardy-Weinberg equilibrium law, states that at equilibrium, both the gene (allele) frequency and the genotypic frequency of a population remain constant…

Adaptive radiation is the process of evolution through which a single original species gradually transforms into a number of different varieties, each suited to a different ecological niche or set of…

The theory of organic evolution holds that today's complex organisms originated, over long periods of time, from earlier and simpler forms of life.

A. Palaeontology : Palaeontology is the study of ancient life through the fossils that ancient organisms have left behind — the preserved dead remains of plants and animals that lived at various times…

B. Morphology : Morphology deals with the study of external structures, while anatomy deals with the study of internal structures.

Speciation is the process by which a new species is formed from a pre-existing one. A species, in this context, is defined as a group of similar organisms that are capable of interbreeding with one an…

The Earth's biodiversity today looks nothing like it did when the planet was first formed, and the fossil record makes clear that the life forms present millions of years ago (mya) were very different…

The evolutionary history of human beings has been traced back to a tree-dwelling, shrew-like ancestral animal, with the process beginning during the Palaeocene epoch, when shrinking forests forced arb…

Q.1 of the textbook exercise (p117): ten multiple-choice questions.

Q.2 of the textbook exercise (pp117-118): define the five terms (1a-1e) and answer questions 2 to 10 briefly.

Q.3 of the textbook exercise (p118): six short-answer questions.

Q.4 of the textbook exercise (p118): match the four scientists of Column-I with their theories in Column-II.

Q.5 of the textbook exercise (p118): six long-answer questions.

Q.6 of the textbook exercise (p118): complete the era / dominating-animal-group chart.

The textbook's end-of-chapter Project (p118): prepare a diagrammatic chart of the chemical evolution of life. A guided approach is given.

Sample papers and previous-year board questions for this subject.