Lesson 23: Earth’s History & Diversification of Life

1. Conceptualizing the Geological Timescale

  • Geological timescale divides Earth’s 4.6-billion-year history into eons → eras → periods → epochs.

  • Major eons:

    • Hadean – formation of Earth (no life).

    • Archean – first life (single-celled).

    • Proterozoic – oxygen increases; first eukaryotes.

    • Phanerozoic – explosion of visible life.

  • Real-life example:

    • Humans exist in the Quaternary period, only 0.2% of Earth’s history.

2. Applying the Timescale to Early Earth & Origins of Life

  • Earth formed 4.6 BYA.

  • First life likely appeared 3.8–3.5 BYA.

  • Earliest life evidence = stromatolites (layered microbial mounds in rocks; still seen in Shark Bay, Australia).

  • Absolute dating = done using isotope decay (e.g., carbon-14, uranium-lead).

  • Before this, scientists used relative dating (ordering fossils in rock layers).

3. How Earth’s Early Environment Shaped Living Systems

  • Early Earth: hot, volcanic, no oxygen, high lightning + UV radiation.

  • Miller–Urey experiment showed early Earth conditions could form organic molecules (amino acids) from simple chemicals.

  • Early cells were:

    • Anaerobic (no oxygen).

    • Prokaryotic (simple, no nucleus).

    • Heterotrophic or autotrophic depending on chemical availability.

  • Real-life example:

    • Hydrothermal vent communities today resemble ancient ecosystems—life powered by chemicals instead of sunlight.

4. Evidence for the Order of Evolution of Early Life

  • Single-celled prokaryotes → first life.

  • Photosynthetic bacteria evolved next → produced oxygen (the “oxygen revolution”).

  • Rising O₂ allowed evolution of:

    • Aerobic respiration (more energy).

    • Eukaryotes (~2 BYA).

  • Endosymbiosis: mitochondria & chloroplasts originated as engulfed bacteria.

  • Multicellular organisms appeared much later (~1 BYA).

  • Real-life example:

    • Mitochondria still have their own DNA, evidence of bacterial origin.

5. Earth as a Dynamic (Changing) System

  • Climate, continents, oceans, and atmosphere constantly shift.

  • Supercontinents formed and broke apart:

    • Rodinia → Pannotia → Pangaea → present continents.

  • Climate cycles include:

    • Ice ages

    • Warm greenhouse periods

  • Real-life example:

    • Himalayas still rising as India collides with Asia.

6. How Living Systems Have Changed Earth

  • Cyanobacteria introduced oxygen → transformed atmosphere.

  • Oxygen allowed formation of the ozone layer, enabling life on land.

  • Plants colonizing land altered climate by removing CO₂.

  • Real-life example:

    • Massive CO₂ drawdown by ancient plants may have triggered global glaciations (“Snowball Earth”).

7. Mechanisms of Evolution in an Ever-Changing Earth

  • Evolution shaped by:

    • Natural selection (organisms adapt to changing environments).

    • Genetic drift (especially during extinctions).

    • Mutation (new traits).

    • Gene flow.

  • Environmental changes (climate shifts, oxygen changes, continental drift) constantly create new selective pressures.

  • Real-life example:

    • Continental drift separated populations—e.g., marsupials radiated in Australia due to geographic isolation.

8. Diversification of Life & the Need for Consistent Nomenclature

  • Cambrian Explosion (~540 MYA): sudden rise in complex multicellular animals.

  • Colonization of land led to huge radiations:

    • Plants → insects → amphibians → reptiles → mammals → birds.

  • With millions of species, scientists use binomial nomenclature (Genus species) for clarity.

  • Taxonomy fits organisms into hierarchical ranks:
    domain → kingdom → phylum → class → order → family → genus → species

  • Real-life example:

    • Humans = Homo sapiens everywhere in the world regardless of language.

9. Recognizing That Earth Is Still Changing

  • Climate change is accelerating due to human activity.

  • Biodiversity is rapidly shifting (extinctions, invasive species, habitat change).

  • Some scientists argue we’ve entered a new epoch: The Anthropocene.

  • Real-life example:

    • Coral bleaching, melting glaciers, rising sea levels, rapid species declines.