Revision: 12th Std >> Origin and Evolution of Life MAH-MHT CET (PCM/PCB) Origin and Evolution of Life

Biogenesis is the idea that life arises only from pre-existing life.

Autogenesis is another term for spontaneous generation, meaning self-origin of life from non-living matter.

Abiogenesis is the concept that living organisms can arise naturally from non-living substances.

Evolution is a slow and continuous process whereby complex forms of life have emerged from simpler forms over millions of years.

The total genetic information encoded in the sum total of genes in a Mendelian population is called gene pool.

Organic evolution can be defined as slow, gradual, continuous and irreversible changes through which the present-day complex forms of the life developed (or evolved) from their simple pre-existing forms.

The proportion of an allele in the gene pool, to the total number of alleles at a given locus, is called gene frequency.

Mutation breeding, sometimes referred to as “variation breeding”, is the process of exposing seeds to chemicals or radiation in order to generate mutants with desirable traits to be bred with other cultivars.

The modern version of Darwin’s theory of natural selection, which incorporates genetics as the source of variations, is called Neo-Darwinism.

Differences between individuals of the same species that arise naturally and can be passed to offspring are called variations.

The origin of new species by gradual modification is called speciation.

The fittest individuals in nature are most likely to reproduce and pass on their good qualities to their offspring. It is called natural selection.

The structural, morphological change in chromosome due to rearrangement is called Chromosomal aberrations.

Any random fluctuation (alteration) in allele frequency, occurring in the natural population by pure chance, is called genetic drift.

Insertion of a vector into the target bacterial cell is called transformation

Gene flow is the movement of genes into or out of a population. Gene movement may be in the form of migration of organism, or gametes or segments of DNA Gene flow also alters gene frequency, causing evolutionary change.

The evolution occurs by the sudden large differences in the population called mutation.

The process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats) is called adaptive radiation. 

Fossils are preserved remains or impressions of past organisms found mainly in sedimentary rocks, and their study is called palaeontology.

The preserved remains, impressions, or traces of ancient organisms found in Earth's crust, which provide evidence of past life and evolution, is called a fossil.

According to A.E. Emerson, speciation is the evolutionary process by which new species arise due to genetic divergence and isolation.

The process of formation of a new species from the pre-existing species is called speciation.

Bipedalism is a major type of locomotion that involves movement on two feet instead of four.

• Hardy–Weinberg’s principle states that allele frequencies in a population remain constant from generation to generation in the absence of evolutionary forces.
• The total collection of all alleles in a population is called the gene pool.
• Genetic equilibrium means no change in allele frequencies over time.
• If p is the frequency of dominant allele and q is the frequency of recessive allele, then
  p + q = 1.
• Genotype frequencies are expressed as:
  p² (AA) + 2pq (Aa) + q² (aa) = 1.
• Any deviation from Hardy–Weinberg equilibrium indicates that evolution is occurring.
• The principle helps in detecting the role of natural selection and other evolutionary forces.

• The origin of life is a unique event; Earth formed about 4.5 billion years ago, and life appeared about 4 billion years ago.
• Special Creation Theory: all organisms were created by a supernatural power; religion-based, no scientific proof.
• Cosmozoic Theory (Panspermia): life came from other planets as spores; not accepted, as it doesn't explain life's origin elsewhere.
• Spontaneous Generation (Abiogenesis): life arose from non-living matter; later disproved.
• Biogenesis: life arises only from pre-existing life; explains continuity but not the first origin.
• Redi (1668): maggots appeared only in the uncovered meat jar, showing they came from fly eggs.
• Spallanzani (1767): sealed boiled broth showed no growth, proving microbes come from air.
• Pasteur (1861): swan-neck flask trapped airborne microbes, conclusively disproving spontaneous generation.

• Chemical Evolution - Life originated by chemical reactions over a long period; proposed by Oparin (1924) and Haldane (1929).
• Primitive Atmosphere - Earth formed ~4.6 bya from Nebula; atmosphere was reducing type (no free oxygen), rich in H₂, C, N, S.
• Simple Molecules - H₂ reacted with other elements forming CH₄, NH₃, H₂O, H₂S; energy from UV rays and lightning formed amino acids, sugars, fatty acids, etc.
• Primitive Broth - Simple organic molecules collected in water bodies forming "hot dilute soup" (Haldane); stable due to the absence of oxygen.
• Complex Molecules & Nucleic Acids - Polymerisation formed proteins, fats; nucleotides joined to form RNA/DNA with self-replicating ability.
• First Cell - Protobionts (coacervates by Oparin; microspheres by Fox) formed from organic molecules; developed into the first cell — anaerobic and heterotrophic.

• Discovery of Ribozymes - Sidney Altman and Thomas Cech independently discovered that RNAs can act as biocatalysts.
• RNA World hypothesis - The RNA World hypothesis suggests that early life was based exclusively on nucleic acids, most probably RNA, and was first proposed by Carl Woese, Francis Crick, and Leslie Orgel in 1960.
• Evidence for RNA World - RNA is found abundantly in all living cells, structurally related to DNA, and can evolve, replicate, and catalyse reactions.
• Formation of primitive cells - RNA molecules underwent replication, mutation, and developed their own machinery to form primitive cells.
• Formation of DNA - Double-stranded DNA formed eventually, resulting in rich biodiversity.

• Purpose - Stanley Miller and Harold Urey provided the first experimental evidence in support of Oparin's chemical evolution theory.
• Apparatus - A sterilised and evacuated spark-discharge glass apparatus was used; CH₄, NH₃, and H₂ gases were pumped in the ratio 1:2:2 along with water vapour.
• Lightning Effect - Electric discharge carbon arc spark was used to mimic lightning; heating mantle and condenser simulated evaporation and precipitation.
• Result - After several days of continuous electric discharge, the gases interacted and condensed; the collected liquid turned brown and contained simple organic compounds like urea, amino acids, and lactic acid.
• Significance - The experiment strongly proved that simple molecules present in Earth's early atmosphere can combine to form organic building blocks of life.

• Organic Evolution - Slow, gradual, continuous and irreversible changes through which present-day complex life forms developed from simple pre-existing ones.
• Darwin's Theory - Charles Darwin defined evolution as "descent with modification" — descendants become different from their ancestors over time.
• Lamarck's Theory - Traits are acquired due to internal force, changes in environment, new needs, and use and disuse of organs; after several generations, new species arise.
• Weismann's Disproof - August Weismann cut tails of rats for 21 generations but found no change in tail size, disproving Lamarckism.
• Theory of Continuity of Germplasm - Weismann concluded that variations in somatic cells (somatoplasm) are NOT inherited, while variations in germ cells (germplasm) ARE inherited to the next generation.

1. Darwinism - Proposed by Charles Darwin after extensive travels; published in the book "On the Origin of Species by Means of Natural Selection"; the essence of the theory is natural selection.

2. Galapagos Islands - Darwin observed variations in tortoises and finches across the islands of the Archipelago, which became key evidence for his theory.

3. Natural Selection - Nature selects only those organisms that are fit to survive; the rest perish — this is the concept of "Survival of the Fittest".

4. Competition & Modification - All organisms reproduce prolifically and compete for survival; only those with useful modifications win the competition and sustain.

5. Evidences for Darwinism -

• Long-necked Giraffe - Giraffes developed long necks to reach leaves; this adaptation became fixed and was passed to offspring.
• Black Peppered Moths - Gradually became more common due to industrial melanism.
• DDT Resistance in Mosquitoes - Some mosquitoes developed resistance to DDT, survived, and produced resistant offspring.

• Mutation Theory - Proposed by Hugo de Vries (1901), based on observations on seven generations of evening primrose (Oenothera lamarckiana); sudden, spontaneous variations clearly different from parents were called mutations.
• Nature of Mutations - Mutations are large, sudden, random and discontinuous variations; they are heritable and may not be directional.
• Raw Material for Evolution - Mutations provide the raw material for organic evolution; variant offspring produced more variants, not normal plants.
• Useful vs. Harmful - Useful mutations are selected by nature and persist; harmful mutations may persist or get eliminated by nature.
• New Species - Accumulation of mutations over a period of time leads to the origin and establishment of new species.

• Modern synthetic theory of evolution is a combination of different biological fields (genetics, ecology, palaeontology, etc.), explaining evolution using mutation and natural selection.
• It was developed by scientists like Fisher, Haldane, Dobzhansky, Huxley, Mayr, and others.
• Evolution occurs in populations (Mendelian populations), and the total genetic material of a population is called the gene pool.
• Changes in gene pool and gene frequency (due to mutation, recombination, and migration) lead to genetic variation in a population.
• Natural selection and reproductive isolation act on these variations, leading to the formation of new species over time.

• Genetic Variation - Change in gene and gene frequencies; caused by mutation, recombination, gene flow, genetic drift, and chromosomal aberrations.
• Gene Mutation - Sudden, permanent, heritable change in gene/chromosome; leads to a change in phenotype.
• Genetic Recombination - Exchange of genes by crossing over during gamete formation; produces new genetic combinations.
• Gene Flow & Genetic Drift - Gene flow - movement of genes in/out of population via migration; Genetic drift - random change in allele frequency by chance, more common in smaller populations.
• Chromosomal Aberrations - Deletion (loss), Duplication (repetition), Inversion (180° reattachment), Translocation (transfer to non-homologous chromosome).
• Significance - All factors alter gene frequency and drive evolutionary change.

• Natural selection is the process where better adapted individuals survive and reproduce more, leading to evolution (“survival of the fittest”).
• It acts on genetic variations in a population and causes changes in gene frequency over generations.
• According to Darwin, natural selection is the main driving force of evolution and leads to the formation of new species over time.
• Example: Industrial melanism in Biston betularia, where black moths increased in polluted areas due to better camouflage, while white moths decreased.
• Types of natural selection include stabilising (favours the average), directional (favours one extreme), and disruptive (favours both extremes).
• Natural selection removes harmful traits and favours useful adaptations, helping organisms survive in changing environments.

• Isolation - Separation of a species population into smaller units, preventing interbreeding; barriers that prevent gene flow between isolated populations are called isolating mechanisms; this leads to divergence and speciation.
• Geographical Isolation - Physical barriers like rivers, mountains, glaciers divide populations; separated groups acquire new traits by mutations, develop distinct gene pools and eventually form new species; e.g. Darwin's Finches.
• Reproductive Isolation - Occurs due to changes in genetic material, gene pool, and structure of genital organs; prevents interbreeding between populations.
• Pre-mating (Pre-zygotic) Mechanisms - Prevent fertilisation and zygote formation; types: Habitat (separate habitats), Seasonal/Temporal (different mating seasons), Ethological (different mating behaviour), Mechanical (different reproductive organ structure).
• Post-mating (Post-zygotic) Barriers - Occur after mating; types: Gamete mortality (gametes die before union), Zygote mortality (zygote dies after fertilisation), Hybrid sterility (hybrids are sterile due to failed meiosis; e.g., mule).
• Significance - Isolating mechanisms promote divergence between separated populations, alter gene pools, and ultimately lead to the formation of new species (speciation).

• Basic Processes - Evolution occurs in populations, not individuals; driven by mutations, recombination, gene flow, genetic drift, natural selection, isolation, and speciation.
• Mutations & Recombination - Mutations - permanent heritable changes in genetic material; Recombination - new variations via crossing over, random gamete union, and anaphase separation.
• Gene Flow & Genetic Drift - Gene flow - gene transfer between populations via migration; Genetic drift - random change in allele frequency by chance (Sewall Wright effect); greater effect in small populations.
• Founder & Bottleneck Effect - Founder effect - a few individuals form a new population with different allele frequencies; Bottleneck effect - most of the population is wiped out by a disaster, and a few survivors rebuild.
• Natural Selection Types - Stabilizing (favours intermediate; e.g. birth weight), Directional (eliminates one extreme; e.g. industrial melanism), Disruptive (eliminates intermediate; e.g. African finch beak sizes).
• Isolation & Speciation - Isolation restricts gene flow via geographical barriers; over generations, isolated populations become so different that they form new species (speciation).

• Adaptive radiation is the evolution of different species from a common ancestor, radiating into different habitats and ecological niches in a given geographical area.
• Mechanism: common ancestor → spreads into new habitats → different environmental pressures → natural selection favours useful variations → distinct species form.
• Darwin's finches (Galapagos): an ancestral seed-eating finch evolved into many species with different beak shapes for different food sources.
• Australian marsupials: many marsupial species evolved from a common ancestor and adapted to different ecological roles.
• Convergent evolution: unrelated placental mammals and marsupials (e.g., the placental wolf and the Tasmanian wolf) evolved similar forms under similar environments.

• Organic evolution states that present-day complex organisms originated from earlier, simpler life forms, supported by multiple branches of biology.
• Evidence comes from Palaeontology (fossil records), Comparative Anatomy & Morphology, Embryology, and Molecular Biology.
• Biochemical evidence (similarities in DNA, proteins) and biogeographical evidence (distribution of species across regions) further support evolution.
• Together, all these evidences strongly support that life on Earth has undergone gradual evolution from simple to complex forms over millions of years.

• Fossils are the preserved remains or impressions of organisms buried under the 
• Palaeontology is the study of ancient life using fossils, which are preserved remains of organisms found mainly in sedimentary rocks.
• Fossils provide direct and strong evidence for evolution and help in understanding the history of life on Earth.
• During fossilisation, older and more primitive organisms are found in lower layers, while more advanced forms occur in upper layers.
• Types of fossils include actual remains (e.g. mammoth in ice), moulds (impressions), casts (filled moulds), and compressions (carbon film outlines).
• Fossilisation occurs through processes like replacement and infiltration, where organic material is replaced or filled with minerals.
• Palaeontology helps in studying extinct organisms, reconstructing evolutionary history (phylogeny), and identifying connecting links between species.

• Homologous Organs - Same structure, different function; e.g. forelimbs of whale, bat, cheetah; indicate common ancestry and divergent evolution.
• Analogous Organs - Different structure, similar function; e.g. flippers of penguins and dolphins; do not indicate common ancestry; represent convergent evolution.
• Vestigial Organs - Non-functional remnants of ancestral organs; e.g. coccyx, nictitating membrane, appendix, wisdom teeth in humans.
• Molecular Evidence - Similarities in DNA, RNA, and proteins among diverse organisms indicate common ancestry.
• Homologous organs show divergence; analogous organs show convergence; together with molecular evidence, all strongly support organic evolution.

• Speciation - Process of formation of a new species from pre-existing species; a species is a group of similar organisms that can interbreed and produce fertile offspring.
• Intraspecific Speciation - Formation of new species within the same population; divided into allopatric and sympatric speciation.
• Allopatric Speciation - New species formed due to geographical isolation (mountains, glaciers, migration); mode of evolution is adaptive radiation; e.g. 14 species of finches in the Galapagos Islands, marsupials in Australia.
• Sympatric Speciation - New species formed within a single population without geographical isolation; occurs due to reproductive isolation and mutations; e.g. Cichlid fishes in Lake Victoria.
• Interspecific Speciation - New species formed by hybridisation of two different species; e.g. Triticum aestivum (hexaploid) and Gossypium herbaceum (tetraploid).

• The geological time-scale represents the entire history of the Earth and helps correlate geological and biological events in proper sequence.
• The Earth’s crust is made of layered rocks, and radioactive dating techniques estimate the age of the Earth to be about 4.5 billion years.
• Earth’s history is divided into five major eras: Archaeozoic, Proterozoic, Palaeozoic, Mesozoic, and Cenozoic.
• Each era is further divided into periods and epochs, marked by changes in climate, plant life, and animal life.
• The time-scale shows the gradual evolution of life, from simple unicellular organisms to complex plants, animals, and humans.
• Major events such as the origin of life, the rise and extinction of dinosaurs, and the evolution of mammals and humans are recorded in the geological time scale.
• Scientists believe Earth is currently entering a sixth mass extinction (Anthropocene), mainly due to human activities causing rapid loss of biodiversity.

• Origin: human evolution began in the Palaeocene epoch from a tree-dwelling, shrew-like ancestor.
• As forests declined, arboreal mammals were forced to adapt to life on land, an important driving force in human evolution.
• Humans are most closely related to gibbons, chimpanzees, and gorillas.
• Major changes: increase in brain size and cranial capacity, bipedal locomotion, opposable thumb, erect posture, and development of chin and forehead.
• Functional significance: free forelimbs and opposable thumbs aided tool use, while bipedal locomotion and stereoscopic vision improved movement and survival.
• Early ancestors: Dryopithecus (ape-like) and Ramapithecus (man-like) lived ~15 mya.
• Australopithecines (~2 mya) lived in East African grasslands, hunted with stone tools, but ate mainly fruits.
• Homo habilis - first hominid, brain ~650–800 cc, with little or no meat in diet; Homo erectus (~1.5 mya, Java, brain ~900 cc) probably ate meat.
• Neanderthal man (~1400 cc, 100,000–40,000 years ago) used hides for protection and buried the dead.
• Modern humans: Homo sapiens arose in Africa during the ice age, spread worldwide; cave art developed ~18,000 years ago (Bhimbetka, M.P.), and agriculture and settlements began ~10,000 years ago.