From First Cells to Modern Humans: Life’s Evolution

Posted on Mar 13, 2025 in Biology

1. The Origin of Life

Spontaneous Generation Theory

  • Aristotle (384–322 BC): Life can arise from nonliving matter (e.g., frogs from mud, mice from grain).
  • Theory persisted until the 17th century.

Refutation of Spontaneous Generation

  • Francesco Redi (1626–1697): Experiment with meat and maggots.
  • Louis Pasteur (1822–1895): Swan-neck flask experiment disproved spontaneous generation.

Panspermia Hypothesis

  • Anaxagoras (500–428 BC): Life exists throughout the universe and spreads via space.
  • Svante Arrhenius (1903): Microspores travel between planets via radiation pressure.
  • Francis Crick & Leslie Orgel: Suggested life on Earth may have been deliberately seeded by aliens.

Oparin-Haldane Hypothesis

  • Early Earth had an oxygen-poor atmosphere.
  • Simple inorganic molecules formed amino acids and nucleotides.
  • Energy sources: lightning, sunlight → “primordial soup”.
  • These molecules self-organized into proteins and nucleic acids, forming the first cells.

Miller & Urey Experiment (1953)

  • Simulated early Earth conditions.
  • Created amino acids from simple chemicals, supporting the Oparin-Haldane hypothesis.


2. Biological Evolution

Protobionts (Protocells)

  • Abiotic structures with characteristics of living cells:
    • Self-organization, lipid membranes, metabolism-like functions.

First Cells

  • Self-replicating RNA enclosed in phospholipid membranes.
  • First organisms were heterotrophs, consuming organic molecules.
  • First photosynthetic bacteria used H₂S to convert CO₂ into organic molecules (>3 billion years ago).

3. Theories of Evolution

Fixism (Pre-Evolution Theories)

  • Species were created by a divine force and never changed.
  • Catastrophism: Extinctions happened due to sudden catastrophes.

Lamarckism (Jean-Baptiste Lamarck)

  • Organisms change due to their needs.
  • Acquired traits are passed to offspring.
  • Example: Giraffes stretching necks → longer necks in next generations.
  • Refuted by modern genetics (no inheritance of acquired traits).


Darwinism (Charles Darwin & Alfred Wallace)

  • Key concepts:
    • Artificial selection (breeding).
    • Natural selection (survival of the fittest).
  • Hypotheses:
    • The world is not static, but in a process of transformation.
    • The process of change is gradual and continuous.
    • Similar organisms are related and have a common ancestor.
    • Evolutionary change is the result of natural selection.

Natural Selection Process

  1. Overproduction: More offspring than can survive.
  2. Variation: Differences exist in populations.
  3. Competition: Limited resources = survival struggle.
  4. Survival of the Fittest: Best-adapted individuals reproduce.
  5. Speciation: Over time, new species emerge.


4. Biodiversity & Speciation

Definition of Biodiversity

  • The variety of life forms: Genes → Species → Ecosystems.

Adaptation

  • Structural: Physical traits (e.g., camouflage).
  • Physiological: Internal body processes (e.g., venom production).
  • Behavioral: Learned behaviors (e.g., migration).

Speciation (Formation of New Species)

  • Allopatric: Geographic isolation (e.g., separated by mountains, rivers).
  • Sympatric: New species evolve in the same area (e.g., genetic mutations).
  • Artificial: Human-influenced (e.g., selective breeding).


5. Evidence for Evolution

Anatomical Proofs

  • Homologous structures: Same origin, different function (e.g., human arm & bat wing).
  • Analogous structures: Different origin, same function (e.g., wings of birds & insects).

Fossil Evidence (Paleontology)

  • Shows extinct species and gradual changes over time (phylogenetic sequences).

Biogeography

  • Similar species live in nearby regions.
  • Fossil similarities between continents (evidence of continental drift).

Embryological Evidence

  • Similar embryos in related species (e.g., vertebrates look alike in early development).

Molecular Evidence

  • Similar DNA, proteins, & metabolic pathways in related species.
  • Phylogenetic trees based on genetic comparisons.


6. Human Evolution

Hominization Process

  • Homo sapiens belong to the Hominidae family.
  • Australopithecus (~4 million years ago): First bipedal primates.
  • Homo habilis (2.3 – 1.6 million years ago): First tool-makers.
  • Homo erectus (1.6M – 100,000 years ago): Controlled fire, made advanced tools.
  • Homo neanderthalensis (166,000 years ago): Strong build, large brain, buried the dead.
  • Homo sapiens (300,000 years ago – present): Modern humans.

Human Adaptations

  • Bipedalism: S-shaped spine, stronger femur, efficient walking.
  • Encephalization: Larger brain = better thinking, tool use.
  • Language Development: Communication, culture.
  • Neoteny: Prolonged childhood for learning.