The Origin and Evolution of the Earth

• Early Theories

  • Nebular Hypothesis
    • Proposed by: Immanuel Kant (revised by Laplace in 1796)
    • Core Idea: Planets formed from a slow-spinning nebula (cloud of material associated with a young sun)
  • Binary Theories
    • Proposed by: Sir James Jeans and Harold Jeffreys
    • Core Idea: Interaction of the sun with another approaching intruding star created cigar-shaped material that formed planets

• Modern Theories

  • Big Bang Theory (Expanding Universe Hypothesis)
    • Proposed by: Edwin Hubble (1920)
    • Core Idea: Universe is expanding; galaxies moving away from each other
    • Stages of Evolution:
      • Singularity: Tiny, dense, hot point containing all matter
      • The Big Bang: Violent explosion (~13.8 billion years ago)
      • Cooling & Expansion: Rapid expansion followed by cooling; atoms formed
      • Galaxy Formation: Condensation of matter due to gravitational pull

• Formation of Planets

  • Stage 1: Stars formed out of gas clouds (nebulae) through gravitational accretion
  • Stage 2: Core of the nebula develops localized lumps, forming planetesimals
  • Stage 3: Collision and merging of planetesimals to form large bodies (planets)

• Evolution of Earth

  • Layered Structure (Differentiation)
    • High temperature caused melting and redistribution of material
    • Heavier elements (iron, nickel) sank to the center (Core)
    • Lighter elements rose to the surface, cooling to form the Crust and Mantle
  • Evolution of Atmosphere and Hydrosphere
    • Stage 1 (Stripping): Early light primordial atmosphere stripped by solar winds
    • Stage 2 (Outgassing): Volcanic eruptions released water vapor, nitrogen, carbon dioxide, methane, and ammonia
    • Stage 3 (Modification):
      • Water vapor condensed to form rains, filling depressions to form oceans (approx. 4000 million years ago)
      • Photosynthesis by primitive plant life (blue-green algae) added oxygen to the atmosphere

• Origin of Life

  • Chemical reaction synthesized complex organic molecules
  • First single-celled microscopic organisms appeared around 3.8 billion years ago
  • Evolution from unicellular to multicellular organisms and complex life forms over geological time scales