Chapter 9: Evolution

Early Earth and biopoiesis

• Universe: About 13.8 billion years old; it contains galaxies made of stars, gas and dust.
• Big Bang theory: An immense explosion began the universe. Expansion caused cooling; hydrogen and helium formed, and gravity gathered gases into galaxies.
• Earth: Formed about 4.5 billion years ago in the Milky Way's solar system; it was initially hot and barren. Earth is the only known place that supports life.
• Biopoiesis: The origin of life on Earth from non-living matter; it occurred about 4,000–4,200 million years ago.

Pasteur's swan-neck flask experiment

• Finding: Pre-sterilised flasks did not develop life from killed yeast; microorganisms appeared in a flask open to air.
• Conclusion: Air carried microorganisms that could cause fermentation or putrefaction; life did not arise spontaneously from nutrient media.
• Principle: New life comes from pre-existing life, supporting biogenesis and rejecting spontaneous generation.

Modern chemical theory

• Proponents: Oparin of Russia and Haldane of England proposed that chemical evolution preceded life.
• Proposal: The first life could have arisen from non-living organic molecules, such as RNA and proteins, under early-Earth conditions radically different from today's.
• Present conditions: Spontaneous generation of life is not possible under present environmental conditions.
• Other names: Oparin's theory is also called the chemical theory or naturalistic theory; he published it in *The Origin of Life*.

Miller's experiment and the early organic mixture

• Miller (1953): S. L. Miller, a student of Harold C. Urey, simulated primitive Earth in a closed flask containing \(CH_4\), \(H_2\), \(NH_3\) and water vapour at 800°C, with an electric discharge.
• Result: Amino acids formed; similar experiments produced sugars, nitrogen bases, pigments and fats.
• Hot dilute soup: Haldane described the early sea, where organic substances probably accumulated, as a “hot dilute soup.”
• Significance: The formation of protein molecules is considered a landmark in the origin of life.

From macromolecules to early cells

• Early non-cellular forms: Around 3 billion years ago, giant molecules such as RNA, proteins and polysaccharides may have formed the earliest non-cellular life-like structures.
• Protobionts: Abiotic macromolecules in primitive oceans assembled into large colloidal, drop-like clusters of proteins, polysaccharides, lipids and nucleic acids. They could grow by absorbing surrounding molecules and show simple metabolism, but could not reproduce.
• Coacervates: Oparin made protobionts without a lipid membrane in the laboratory and called them coacervates.
• Microspheres: Sydney Fox made microscopic protein-like bodies with a lipid coat and called them microspheres.
• Nucleoproteins: After mutation, nucleic acids gained self-duplication ability and combined with proteins; these nucleoproteins were the first sign of life.
• Protocells: Lipid-coated clusters of nucleoproteins formed protocells, considered the first form of life.
• Prokaryotes: Mutations made protocells more complex and efficient at using their surroundings, leading to prokaryotic cells.

Nutrition, photosynthesis and eukaryotic cells

• First organisms: Single-celled prokaryotes, like bacteria with naked DNA, were probably anaerobic chemoheterotrophs.
• Chemoautotrophs: Some chemoheterotrophs evolved to make organic food from inorganic materials without oxygen, using chemosynthesis; examples include iron and nitrifying bacteria.
• Photosynthesis: The development of bacteriochlorophyll enabled some chemoautotrophs to convert light energy into chemical energy.
• Oxygenic photosynthesis: Changes in bacteriochlorophyll produced true chlorophyll. These organisms used water as the hydrogen source and released oxygen; cyanobacteria (blue-green algae) are examples.
• Oxygen release: Free oxygen released by cyanobacteria was a revolutionary change in Earth's history.
• Eukaryote-like cells: Nuclei, mitochondria and other organelles developed, making cells more metabolically active. Free-living eukaryote-like cells appeared in primitive oceans about 1.5 billion years ago.

Darwin’s observations

• Charles Darwin developed his conclusions from observations during a world voyage on the ship H.M.S. Beagle.
• Similarity: Living forms share similarities with one another and with organisms that lived millions of years ago.
• Change over time: Some life forms became extinct while new forms arose; life has evolved gradually.
• Common ancestry: Existing life forms share common ancestors, which lived in different periods of Earth’s history.

How selection works

• Variation: Populations naturally contain differences in characteristics.
• Environmental conditions: Climate, food availability and other physical factors affect survival.
• Differential reproduction: Better-adapted individuals leave more progeny than less well-adapted individuals.
• Evolutionary outcome: Favoured characteristics become more represented over generations.

Related conclusions and Earth’s history

• Alfred Wallace, a naturalist working in the Malay Archipelago, reached similar conclusions around the same time as Darwin.
• New forms: Over time, apparently new types of organisms become recognisable.
• Geological and biological history: The history of life closely corresponds with Earth’s geological history, which is divided into epochs, periods and eras.

What Fossils Reveal

• Sedimentary layers: Different-aged rock layers contain fossils of organisms that lived when those sediments formed.
• Geological age: Fossils in different layers help identify the geological period in which organisms existed.
• Change over time: Fossil records show that life forms varied through time, and some existed only during particular geological spans.
• Vertebrate succession: Fossil fishes appear earliest, followed by amphibians, reptiles, birds and mammals.

Possible Causes of Mass Extinction

• External impact: A meteor or comet may collide with Earth.
• Continental movement: Continents may drift, join together and later break apart.

• Comparative study: Similarities and differences in organisms living today and those from the past help infer whether they share common ancestors.
• Evidence: Structural, developmental and biochemical similarities can indicate common ancestry.

Homology and Divergent Evolution

• Mammalian forelimbs: Humans, cheetahs, whales and bats share the same bone pattern: humerus, radius, ulna, carpals, metacarpals and phalanges.
• Different functions: The shared plan is adapted for tasks such as grasping, running, swimming and flying.
• Evolutionary meaning: Modification of a common structure in different directions is divergent evolution (adaptive radiation); homology indicates common ancestry.

Heart as Evolutionary Evidence

• Shared plan: Vertebrate hearts occupy a similar position and share a general chambered organisation and cardiac muscle.
• Trend: Internal structure shows increasing complexity from fishes to birds and mammals, supporting evolutionary change across vertebrates.

Other Examples and Related Evidence

• Analogous organs: Octopus and mammalian eyes; penguin and dolphin flippers; sweet potato (modified root) and potato (modified stem).
• Biochemical evidence: Similarities in proteins and genes that perform the same function across diverse organisms support common ancestry.
• Connecting links: Organisms with characters of two taxonomic groups; *Protopterus* (lungfish) links bony fishes and amphibians.
• Sphenodon: A primitive reptile with amphibian characters.

Examples

• Vestigial organ: Nictitating membrane in humans.
• Atavism: Development of the ability to move the pinna in some people.

Peppered Moth Example

• Species: The best-studied example is the peppered moth, *Biston betularia*.
• Forms: The light, creamy-white *B. betularia typica* has dark blotches; the dark melanic form, *B. betularia carbonaria*, is described as arising from a mutation in a colour-determining gene.
• Before industrialisation: Whitish lichens covered tree bark, so light moths were camouflaged from predatory birds.
• After industrialisation: Soot darkened tree bark; birds more easily spotted and ate light moths, while dark moths were better camouflaged.
• Outcome: Dark moths survived and became more numerous, while light moths decreased.

Study and Report

• Researchers: R. A. Fischer, E. B. Ford and H. B. D. Kettlewell studied industrial melanism.
• Kettlewell (1958): Reported melanic forms in 70 moth species from Britain and Europe.

Four propositions of Lamarckism

• Internal vital force: Living organisms and their component parts continually increase due to an internal force.
• Environment and new needs: Environmental changes create new needs; these alter habits and may produce new structures. The doctrine of desires is called appetency.
• Use and disuse: Constant use makes an organ better developed, while disuse causes its degeneration.
• Inheritance of acquired characters: Characters acquired during life are transmitted to offspring; their accumulation over generations may lead to new species.

Example supporting Lamarckism

• Giraffe: Lamarck proposed that giraffes evolved from short-necked, short-forelimbed deer-like ancestors.
• Proposed explanation: Food scarcity on the barren ground encouraged stretching; repeated use gradually lengthened the neck and forelimbs, and these acquired changes were inherited.

Weismann’s evidence

• August Weismann opposed the inheritance of acquired characters and proposed the continuity of germplasm: only characters that influence germ cells are inherited.
• Tail-cutting experiment: He cut rats’ tails for 22 generations, but their offspring were never born tailless.

Background and key ideas

• Charles Darwin (1809–1882): An English naturalist who studied nature during a world voyage aboard the *H.M.S. Beagle*.
• Observations: Darwin explored the flora and fauna of many continents and islands, including the Galapagos Islands, where he observed great variation among organisms.
• Galapagos Islands: Darwin described them as a “living laboratory of evolution.”
• Key concepts: Darwin’s theory centres on branching descent and natural selection.

Evidence supporting Darwinism

• Variation: Individuals within a population show abundant variations.
• Adaptations: Mimicry and protective colouration in animals are examples.
• Flower–pollinator relation: The position of flower nectaries correlates with the proboscis length of pollinating insects.
• Lederberg’s replica experiment: Using penicillin, the experiment illustrates adaptation and helps explain Darwin’s theory.

Limitations identified in the lecture

• Variation: Darwin did not distinguish somatic from germinal variations and treated all variations as heritable.
• Unresolved patterns: His theory could not explain organisms remaining unchanged for millions of years or the occurrence of discontinuous variations.
• Other limitations: It could not explain survival of the fittest, effects of use and disuse, or the presence of vestigial organs.

Allele and genotype frequencies

• Population: The principle applies to a large, randomly mating (panmictic) sexual population.
• Alleles: For alleles \(D\) and \(d\), their frequencies are \(p\) and \(q\), respectively.
• Genotypes: \(DD\) occurs at frequency \(p^2\), \(Dd\) at \(2pq\), and \(dd\) at \(q^2\).
• Meaning: \(p^2\) is the homozygous-dominant frequency; \(2pq\) is the heterozygous frequency; \(q^2\) is the homozygous-recessive frequency.

• Five factors: Gene migration or gene flow, genetic drift, mutation, genetic recombination and natural selection can alter Hardy–Weinberg equilibrium.

• Gene flow: Repeated gene migration between populations produces gene flow.
• Founder effect: A founding population may have allele frequencies very different from the source because of chance. This divergence can sometimes contribute to speciation.

• Mutation: Microbial experiments show that selecting pre-existing advantageous mutations can produce new phenotypes; over generations, this may lead to speciation.
• Recombination: During gametogenesis, recombination creates new gene combinations that contribute to variation.

Early evolution

• First life: Cellular life appeared about 2000 million years ago (mya); single-celled organisms gradually gave rise to multicellular forms.
• Invertebrates: They had formed and were active by about 500 mya.
• Plants on land: Plants were the first organisms to invade land and were widespread before animals moved onto land.
• Early aquatic life: Jawless fish probably evolved around 350 mya; seaweeds and a few plants existed around 320 mya.

Lobefins and amphibians

• Coelacanth: A Coelacanth was caught in South Africa in 1938, although it had been thought extinct.
• Ancestry: The early amphibians have no surviving specimens; they were ancestors of modern frogs and salamanders.

Reptile dominance

• Dominance: Reptiles of many shapes and sizes dominated Earth for roughly the next 200 million years.
• Living reptiles: Small-sized reptiles from that era still exist today.
• Pteridophytes: Giant ferns grew during this period; their remains gradually formed coal deposits.
• Aquatic reptiles: Some land reptiles returned to water, probably around 200 mya; Ichthyosaurs are an example.
• Dinosaurs: Land reptiles included dinosaurs; Tyrannosaurus rex was about 20 feet tall and had huge, dagger-like teeth.

Dinosaur disappearance

• Timing: Dinosaurs disappeared from Earth about 65 mya.
• Cause: The exact reason is unknown; possible explanations are climatic change or that most dinosaurs evolved into birds.

Mammals

• Early mammals: The first mammals were shrew-like; their fossils are small.
• Viviparity: Mammals protect their unborn young inside the mother's body.
• Survival advantage: Mammals were more intelligent in sensing and avoiding danger.
• Expansion: As reptiles declined, mammals took over the Earth.

Mammals in water

• Aquatic mammals: Some mammals live wholly in water; examples include whales, dolphins, seals and sea cows.

Cultural development

• Modern Homo sapiens: Arose during the ice age, between 75,000 and 10,000 years ago.
• Cave art: Prehistoric cave paintings developed about 18,000 years ago. An example is at Bhimbetka rock shelter, Raisen district, Madhya Pradesh.
• Agriculture: Began about 10,000 years ago, leading to the start of human settlements.