Evolution

Evolutionary Biology: The Story of Life

This chapter deals with the origin, evolution and diversity of life forms, explaining major theories, evidences and mechanisms involved in the history of life on Earth, including the evolution of humans.

What is Evolution?

• Evolution means orderly change from one condition to another over long periods of time.
• Evolutionary biology studies the history of life on Earth and explains how flora and fauna have changed over millions of years.
• It helps us understand:
  • Origin of life
  • Gradual change in organisms
  • Present-day biodiversity
  • Diversity of life forms on Earth

ORIGIN OF LIFE – LOOKING BACK IN TIME

• The universe is extremely vast and about 20 billion years old
• Stars are far away; when we observe them, we see light emitted millions of years ago
• The universe consists of galaxies containing stars, gas and dust
• Earth is just a tiny part of this vast universe

BIG BANG THEORY

The Big Bang theory explains the origin of the universe as a result of a massive explosion.

• Post-Explosion Events
  • Universe expanded after the explosion
  • Temperature gradually decreased
  • Hydrogen and helium were formed
  • Galaxies, stars, and clouds of gas and dust formed
  • Gases condensed under gravity to form galaxies
• Formation of Earth
  • Earth was formed about 4.5 billion years ago
  • Formed in the solar system of the Milky Way galaxy
  • Initially, Earth was hot and barren

EARLY EARTH CONDITIONS

• Primitive Earth lacked present-day atmosphere
• There was no atmosphere initially
• Gases like methane, ammonia, hydrogen, carbon dioxide and water vapour were present
• These gases were released from molten Earth
• UV rays from the Sun broke water into hydrogen and oxygen
• As Earth cooled, water vapour condensed into rain
• Rain filled depressions forming oceans

• Life originated from non-living matter (biopoiesis) about 4–4.2 billion years ago
• Life appeared about 4 billion years ago

Today, Earth is the only known planet supporting life.

THEORIES ON THE ORIGIN OF LIFE

1. Theory of Special Creation
   • Life was created by a supernatural power (God).
   • Organisms were formed suddenly
   • Species have not changed since creation
   • Later this theory was challenged
2. Cosmic Theory (Panspermia)
   • Life came from outer space (panspermia).
   • In the form of spores or living units
   • Known as panspermia
3. Spontaneous Generation (Abiogenesis)
   • Life originated from decaying or non-living matter such as mud or straw
   • Widely believed in ancient times
   • Disproof
     • Louis Pasteur demonstrated that life did not arise from sterilised killed yeast in a closed flask
     • Microorganisms appeared only when exposed to air
     • Swan-neck flask experiment disproved spontaneous generation
4. Theory of Biogenesis
   • Proposed by Francesco Redi, Lazzaro Spallanzani and Louis Pasteur
   • Life arises only from pre-existing life
   • Does not explain the origin of the first life

CHEMICAL EVOLUTION (OPARIN–HALDANE THEORY)

• Proposed by Alexander Oparin and J. B. S. Haldane.
• This theory suggests:
  • Early Earth had a reducing atmosphere
  • Conditions included high temperature and volcanic activity
  • Life originated from non-living organic molecules such as RNA and proteins
  • Chemical reactions in primitive oceans led to formation of complex organic compounds
• Gradual formation of organic molecules is called chemical evolution.

MILLER’S EXPERIMENT (1953)

• Experimental Setup [conditions similar to primitive (early) Earth}
  • Closed flask containing methane (CH₄), hydrogen (H₂), ammonia (NH₃) and water vapour
  • Electric discharge at high temperature (about 800°C) simulated lightning
• Results
  • Amino acids were formed
  • Similar experiments (by other scientists) produced sugars, nitrogen bases, pigments and fats

Chemical evolution is widely accepted, though the exact process of origin of life is still not completely known.

FIRST LIFE FORMS

• Non-Cellular Forms
  • First non-cellular life forms appeared about 3 billion years ago
  • Giant molecules such as RNA and proteins
• Protobionts
  • Macromolecules combined to form colloidal structures called protobionts
  • Could grow but could not reproduce
• Protocells
  • Clusters of nucleoproteins surrounded by a lipid coat
  • Considered earliest cellular forms
• First Cellular Life
  • Appeared about 2 billion years ago
  • Single-celled forms in water

• Characteristics of Earliest Organisms
  • Prokaryotic
  • Anaerobic
  • Likely chemoheterotrophic
  • Aquatic
• Later Developments
  • Evolution of photosynthetic organisms
  • Release of oxygen by cyanobacteria
  • Gradual origin of more complex cells
  • Emergence of eukaryotic cells about 1.5 billion years ago

All early life forms originated and evolved in aquatic environments.

Key Points

• Evolution explains gradual changes in life over millions of years
• The Big Bang theory explains the origin of the universe
• Life on Earth originated through chemical evolution from non-living matter
• First life forms were simple, single-celled organisms in water
• Complex biodiversity evolved from primitive forms over time

Evolution of Life Forms – A Theory

SPECIAL CREATION THEORY

• Religious literature states that:
  • All living organisms were created in their present form.
  • The diversity of life has always remained the same and does not change.
  • The Earth is only about 4000 years old.
• These ideas were strongly challenged during the nineteenth century.

CHALLENGES TO SPECIAL CREATION

• Charles Darwin, during his voyage on the H.M.S. Beagle (ship), observed:
  • Existing life forms show similarities among themselves.
  • Living forms share similarities with extinct species.
  • Many species had disappeared over time (extinction).
  • New species appeared during different periods of Earth’s history.
  • Life forms have changed gradually over long periods.
• From these observations, Darwin concluded that evolution is a gradual process.

CHARLES DARWIN’S OBSERVATIONS

• Every population shows variation in characteristics.
• Traits that help organisms survive better under natural conditions (climate, food, physical factors, etc.) increase their chances of survival.
• Individuals better adapted to the environment produce more offspring.

• Darwin used the term “fitness” to mean reproductive fitness:
  • Fitness means the ability to leave/produce more progeny.
  • It indicates how well an individual can reproduce.
• Those individuals who are more fit survive and reproduce more successfully.

NATURAL SELECTION

• Darwin proposed natural selection as the mechanism of evolution.
• According to this idea:
  • Individuals with favourable traits survive better.
  • They leave more offspring.
  • Over time, these traits become more common in the population.
• This process was termed “natural selection.”

Alfred Russel Wallace, working in the Malay Archipelago, independently arrived at similar conclusions.

EVOLUTION AND COMMON ANCESTORS

• Over long periods, new types of organisms become recognisable.
• All present-day life forms share similarities.
• These similarities indicate common ancestry.
• Ancestors existed during different geological periods (eras, periods and epochs).

Geological and Biological History

• The geological history of Earth is closely linked with the biological history of life.
• Earth is billions of years old, not just thousands.

Key Points:

• Evolution is gradual change in life forms over millions of years.
• Darwin’s observations challenged the special creation theory.
• Natural selection explains survival based on reproductive fitness.
• All organisms share common ancestors and evolved over geological time.

Evidence for Evolution

Fossils (Paleontological evidences)

Paleontology = the study of fossils.

• What are Fossils?
  • Preserved remains or impressions of hard parts of organisms found in rocks.
  • Remains of hard parts of life forms found in rocks.
• Formation of Fossils
  • Rocks are formed from layers of sediments deposited over long periods.
  • Organisms that died during a particular period became preserved in those layers.
• What Fossils Indicate
  • Different rock layers contain fossils of different organisms.
  • Life forms have changed over time.
  • Certain organisms are restricted to particular geological periods.
  • There is chronological succession of fossils from older to recent layers.

• For example:
  • Fossil records show fishes appeared before amphibians.
  • Amphibians appeared before reptiles.
  • Later birds and mammals evolved.
• This is referred to as palaeontological evidence.

SIGNIFICANCE OF FOSSILS

• Help in studying phylogeny (evolutionary history), e.g., evolution of humans.
• Provide connecting links between groups (e.g., transitional forms such as Archaeopteryx).
• Give information about extinct organisms (e.g., mammoths).
• Help in understanding geological periods and mass extinctions by analysing fossils in sedimentary rock layers.
• Fossil evidence strongly supports that life forms have evolved gradually over billions of years.

Mass Extinction – Refers to the disappearance of large groups of organisms over a short geological time span.

Embryological Support

Embryology provides evidence for evolution through similarities observed in early developmental stages.

• Ernst Haeckel’s Observation
  • Vertebrate embryos show common features during early stages of development.
  • Embryos possess structures such as gill slits which disappear in adult forms.
  • These similarities suggest common ancestry.
• Karl Ernst von Baer’s View
  • Disagreed with Haeckel’s proposal that embryos pass through adult stages of other animals.
  • Demonstrated that embryos do not represent adult stages of other organisms.
  • Early embryos share similarities but follow their own developmental pathway.

Early embryos of fish, frog, lizard, chick and human appear very similar, supporting common ancestry.

Comparative Anatomy and Morphology

Comparative anatomy studies similarities and differences in body structures to understand evolutionary relationships.

HOMOLOGOUS STRUCTURES

• Homologous organs have:
  1. Similar basic structure
  2. Common embryonic origin
  3. Different functions
• Evolutionary Basis
  • Arise due to divergent evolution
  • Related species adapt to different environments
• Homology indicates common ancestry.

• Examples (Animals)
  • Forelimbs of whales, bats, cheetahs and humans
  • Share common bone pattern: humerus, radius, ulna, carpals, metacarpals and phalanges
  • Perform different functions
• Examples (Plants)
  • Thorns of Bougainvillea
  • Tendrils of Cucurbita

ANALOGOUS STRUCTURES

• Analogous organs:
  • Perform similar functions
  • Have different structural design and origin
• Evolutionary Basis
  • Arise due to convergent evolution
  • Unrelated species adapt similarly in similar environments

• Examples
  • Wings of insects (chitinous) and birds (modified forelimbs)
  • Eyes of octopus (skin retina) and mammals (brain retina)
  • Flippers of penguin and dolphin
  • Insect trachea (ectodermal origin) and vertebrate lungs (endodermal origin)
  • Sweet potato (root modification) and potato (stem modification)

Biochemical Evidence

• Similarities in proteins and genes performing the same functions among diverse organisms indicate common ancestry.
• Molecular similarities support structural and anatomical evidence of evolution.

Artificial Selection

Artificial selection refers to human-directed breeding of plants and animals to obtain desired traits.

• Applications
  • Agriculture
  • Horticulture
  • Animal domestication
• Example
  • Different breeds of dogs produced by selective breeding

Artificial selection demonstrates how selection can change populations over generations and mirrors natural selection.

INDUSTRIAL MELANISM IN MOTHS

(Natural Selection by Human Activities / Anthropogenic Action)

Industrial melanism is a clear example of natural selection influenced by human activities.

Natural selection: Organisms with favorable, heritable traits survive and reproduce.

A. Peppered Moth Example

• Two forms of peppered moth:
  1. Light-coloured form – Biston betularia
  2. Dark-coloured (melanic) form – Biston betularia carbonaria

1. Before Industrialisation (around 1850)
   • Tree trunks were covered with lichens.
   • Light-coloured moths were camouflaged and survived.
   • Dark moths were easily detected and eaten by birds.
2. After Industrialisation (around 1920)
   • Tree trunks became dark due to soot and smoke.
   • Dark moths were better camouflaged and survived.
   • Light moths were easily spotted and eaten.

• Lichens are indicators of pollution as they do not grow in polluted areas.
• This shift in moth population demonstrates natural selection.

B. Resistance in Organisms

Excessive use of chemicals leads to selection of resistant varieties.

1. Pesticide Resistance
   • Overuse of insecticides such as DDT led to resistant mosquito populations.
   • Resistant forms may appear within months or years.
2. Antibiotic Resistance
   • Misuse of antibiotics leads to evolution of resistant microbes.
   • Resistant varieties can develop over short time scales under strong selection pressure.

KEY POINTS

• Embryological similarities support common ancestry.
• Homologous organs indicate divergent evolution.
• Analogous organs indicate convergent evolution.
• Biochemical similarities strengthen evolutionary relationships.
• Artificial selection mirrors natural selection.
• Industrial melanism and resistance in organisms are modern examples of natural selection.
• Evolution is a gradual process occurring through variation and selection, not by directed purpose.

Adaptive Radiation

Darwin’s Discovery

• During his voyage, Charles Darwin observed a variety of small black birds, later known as Darwin’s finches.
• Location:
  • Galapagos Islands.
• Observation:
  • Finches on different islands had different types of beaks.
  • They were adapted to different feeding habits.
  • Many types evolved from an original seed-eating ancestor.
  • Some became insect-eaters, others adapted to vegetarian diets.

• Definition:
  • Adaptive radiation is the evolution of different species in a given geographical area from a common ancestor, spreading into different habitats.
  • It represents:
    • Divergent evolution
    • Biogeographical evidence of evolution

Examples

1. Darwin’s Finches
   • Different beak shapes adapted to different food sources.
2. Australian Marsupials
   • Various marsupials evolved from a common ancestor within Australia.
   • Each adapted to distinct environments.

CONVERGENT EVOLUTION

• Definition:
  • When more than one adaptive radiation occurs in an isolated geographical region and unrelated species develop similar adaptations, it is called convergent evolution.
  • It results from similar environmental pressures acting on unrelated species.
• Example:
  • Placental Mammals and Australian Marsupials
  • Similar body forms evolved independently.

Placental Mammals	Australian Marsupials
Mole	Marsupial mole
Ant eater	Numbat (Ant eater)
Mouse	Marsupial mouse
Lemur	Spotted cuscus
Flying squirrel	Flying phalanger
Bobcat	Tasmanian tiger cat
Wolf	Tasmanian wolf

These similarities arose due to adaptation to similar environments, not due to close ancestry.

A Brief Account of Evolution

The Geological Time Scale divides Earth’s history into eras, periods and epochs based on major geological and biological events.

1. PROTEROZOIC ERA (2500 – 541 mya) – Early Life Forms

• Around 2000 million years ago (mya), the first cellular forms of life appeared.
• Single-celled organisms gradually evolved into multicellular forms.
• Some early cells developed the ability to release oxygen (similar to photosynthesis).
• Life existed only in aquatic environments.

2. PALAEOZOIC ERA (540 – 252 mya)

Periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian

• Major Milestones
  • About 500 mya – Invertebrates became abundant and active.
  • Around 450 mya – First land plants appeared and spread widely.
  • About 400 mya – Arthropods invaded land.
  • Around 350 mya – Jawless fishes evolved.
• Lobe-finned Fishes and Transition
  • Fish with strong fins were able to move on land and return to water.
  • Lobefins evolved into the first amphibians (ancestors of modern frogs and salamanders).
• Coelacanth
  • In 1938, a living specimen of Coelacanth was discovered in South Africa.
  • It was earlier thought to be extinct.
• Amphibians to Reptiles
  • Amphibians later evolved into reptiles.
  • Reptiles developed thick-shelled eggs that did not dry up on land.
• Around 320 mya – Seaweeds and early plants existed.
• Giant ferns later formed coal deposits.

3. MESOZOIC ERA (252 – 66 mya) – Age of Reptiles

Periods: Triassic, Jurassic, Cretaceous

• Reptiles dominated the Earth for nearly 200 million years.
• Dinosaurs were the dominant land reptiles.
• Largest dinosaurs (e.g., Tyrannosaurus rex) were massive and powerful.

• Aquatic Reptiles
  • Some land reptiles returned to water.
  • Evolved into fish-like reptiles such as Ichthyosaurs.
• Plants
  • Gymnosperms flourished.
• Extinction Event (around 65 mya)
  • Dinosaurs suddenly disappeared.
  • Possible reasons: climatic changes or evolutionary transition into birds.
  • Small reptiles continued to survive.
  • Some reptiles evolved into birds.

4. CENOZOIC ERA (66 – 0 mya) – Age of Mammals and Angiosperms

Periods: Tertiary, Quaternary

After the decline of reptiles, mammals became dominant.

• Early Mammals
  • First mammals were small, shrew-like and viviparous.
  • Mammals developed improved senses and intelligence.
• Continental Drift and Survival
  • In South America, mammals resembling horses, bears and rabbits evolved.
  • When South America joined North America, North American fauna dominated.
  • Australian marsupials survived due to geographical isolation and lack of competition.
• Aquatic Mammals
  • Whales
  • Dolphins
  • Seals
  • Sea cows
• Plants
  • Angiosperms became dominant plant forms.
• Humans
  • Humans evolved during the later part of this era.
  • Developed language skills and self-consciousness.

Key Points

• Life evolved from unicellular to complex multicellular organisms.
• Plants and animals gradually adapted from water to land.
• Reptiles dominated the Mesozoic era; mammals dominated the Cenozoic era.
• Continental drift influenced species distribution and survival.
• Human evolution forms an important part of evolutionary history.

Origin and Evolution of Man

Human evolution (anthropogenesis) traces the emergence of modern humans from earlier hominids.

Early Primates (About 15 mya)

1. Dryopithecus – more ape-like.
2. Ramapithecus – more man-like.

Hominid Fossils

• Around 3–4 mya – Man-like primates walked upright in Eastern Africa.
• About 4 feet tall.

• Around 2 mya – Australopithecus lived in East African grasslands.
• Used stone tools / weapons.
• Mainly fruit-eaters.

Evolution of Homo Species

• Homo habilis
  • First human-like being.
  • Brain capacity: 650–800 cc.
  • Probably did not eat meat.
• Homo erectus
  • Lived about 1.5 mya (fossils found in Java).
  • Brain size: around 900 cc.
  • Likely meat-eater.
• Homo neanderthalensis
  • Lived 100,000–40,000 years ago.
  • Found in Near East and Central Asia.
  • Brain size: about 1400 cc.
  • Used hides and advanced tools.
  • Buried their dead.

Homo sapiens (Modern Humans)

• Originated in Africa.
• Migrated across continents.
• Developed language and cultural skills.
• Developed into distinct races.

• Ice Age (75,000–10,000 years ago)
  • Modern Homo sapiens became widespread.
• Around 18,000 years ago
  • Prehistoric cave art developed.
  • Example: Bhimbetka rock shelters (Madhya Pradesh).
• Around 10,000 years ago
  • Agriculture began.
  • Led to permanent settlements and rise and fall of civilizations.

EVOLUTIONARY SEQUENCE

Dryopithecus → Ramapithecus → Australopithecus → Homo habilis → Homo erectus → Homo neanderthalensis → Homo sapiens

• Human evolution reflects gradual changes in:
  • Body structure
  • Brain capacity
  • Behaviour
• over millions of years.

Chapter Summary

• The origin of life on Earth is closely linked with the origin of the universe.
• After the formation of Earth, non-living organic molecules were formed through chemical evolution.
• The formation of biomolecules preceded the appearance of the first cellular life forms.

• Early life began as simple unicellular organisms, which gradually evolved into complex multicellular forms.

• Darwin’s concept of organic evolution through natural selection explains how life diversified after its origin.
• According to this idea, populations show variations, and individuals with advantageous heritable traits have greater reproductive fitness.
• Over generations, such favourable traits accumulate and lead to evolutionary change.

• The diversity of life on Earth has changed continuously over millions of years due to processes like mutation, recombination, natural selection, gene flow and genetic drift.
• Habitat fragmentation and genetic drift can alter allele frequencies and increase variation within populations.
• These changes may eventually result in the formation of new species.

• Homology supports the idea of branching descent, indicating that different organisms share common ancestors.

• Evidence for evolution is obtained from:
  • Comparative anatomy and morphology
  • Fossil records (palaeontological evidence)
  • Biochemical similarities among organisms

• Human evolution represents a significant part of evolutionary history.
• The evolution of modern humans is closely associated with increased brain capacity, development of language and cultural advancement.