In-Depth Notes on the History of Life on Earth

Overview of Life's History on Earth

• Earth is approximately 4.6 billion years old.

Origin of Early Cells

• Chemical and Physical Processes: Early Earth conditions facilitated the formation of simple cells.
  1. Abiotic Synthesis: Formation of small organic molecules (e.g., amino acids).
  2. Macromolecule Formation: Small molecules combine to form larger macromolecules (e.g., proteins, nucleic acids).
  3. Protocells: These macromolecules aggregated into primitive cellular structures.
  4. Self-Replicating Molecules: Emergence of molecules capable of replication led to heredity.

RNA vs. DNA

• DNA (Deoxyribonucleic Acid)

  • Structure: Double-stranded.
  • Sugar: Deoxyribose.
  • Base pairs include Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).

• RNA (Ribonucleic Acid)

  • Structure: Usually single-stranded.
  • Sugar: Ribose.
  • Base pairs include Adenine (A), Uracil (U), Cytosine (C), and Guanine (G).

Geological Time Scale

• Precambrian: Earliest period, encompasses roughly 88% of Earth’s history.

• Eras:

  • Hadean: Formation of Earth (4.6 billion years ago).
  • Archean: First one-celled organisms - origin of life.
  • Proterozoic: Development of multicellular organisms.
  • Phanerozoic Eon: Divided into Paleozoic, Mesozoic, and Cenozoic eras.

• Major Events:

  • Cambrian Explosion: Rapid diversification of life forms.
  • Extinction Events: Notable mass extinctions that deeply affected biodiversity:
    • Ordovician: 445 million years ago, 50% of species extinct.
    • Devonian: 360-375 million years ago, 30% extinction.
    • Permian: 252 million years ago, the largest known extinction event, 95% extinction rate.
    • Triassic: 200 million years ago, 76% extinction.
    • Cretaceous (K-T): 65 million years ago, 80% extinction including dinosaurs.

Evidence of Early Life

• Fossil Evidence: Early cells like cyanobacteria can be traced through stromatolites and banded iron formations.
• Index Fossils: Help in dating rock layers and relative age assessments of new fossil finds.

Miller-Urey Experiment**

• Demonstrated that organic molecules could form from inorganic precursors under early Earth conditions, simulating lightning. Steps include:
  • Heating water to create vapor.
  • Introducing gas mixture (CH₄, NH₃, H₂, H₂O).
  • Electric sparks to simulate lightning.
  • Organic compounds formed over several days, suggesting a pathway for the origin of life.

Conclusion

• The history of life is a complex interplay of chemical processes, geological occurrences, and biological evolution. Understanding these aspects is crucial for grasping how life developed on Earth during billions of years.