Lecture Notes: Chapter 25 - Prokaryotes and Rise of Eukaryotes

Introduction

• Focus: Understanding prokaryotes and the rise of eukaryotes.
• Context: Prokaryotes dominated early Earth.
• Analogy: Life history is compared to a clock relative to Earth's age.

Key Historical Events

• Formation of Earth: 4.6 billion years ago.
• First Prokaryotes: Evidence from 3.9 billion years ago.
• Atmospheric Oxygen: Appeared around 2.5 billion years ago due to photosynthesis.
• First Single-Celled Eukaryotes: 2 billion years ago.
• Multicellular Eukaryotes: Appeared around 1.5 billion years ago.
• Modern Complex Life: Represents a very short period in Earth's history.

Prokaryotic Dominance

• Stromatolites: Oldest known fossils, 3.5 billion years old, consisting of layered bacteria and sediment.
• Electron Transport Chains: Critical for energy acquisition, evolved in prokaryotes.
• Diverse Photosynthesis: Existed, but oxygenic photosynthesis linked to cyanobacteria.

Oxygen Accumulation

• Evidence: Rust layers in rocks indicate oxygen presence from 2.7 billion years ago.
• Impact: Oxygen was both a challenge and an energy opportunity for life.

Evolution of Eukaryotes

• Earliest Fossils: 2.1 billion years ago, correlating with oxygen rise.
• Endosymbiosis Theory:
  • Concept: Eukaryotic cells evolved through symbiosis and genetic exchanges.
  • Mitochondria and Plastids: Originally free-living prokaryotes.
  • Possible Mechanisms: Entered host cells as prey or parasites.
  • Membrane Folding: Increased surface area, larger cells.
  • Nucleus Formation: Isolated DNA, more complex structure.
• Cellular Evolution:
  • Animal Lineage: Mitochondria only.
  • Plant Lineage: Mitochondria and chloroplasts.

Evidence for Endosymbiont Theory

• Membrane and Function: Similarities between mitochondria and free-living bacteria.
• DNA Structure: Mitochondria and chloroplasts have circular DNA, like bacteria.

Conclusion

• Significance: Evolution of eukaryotic cells marked a significant advance from prokaryotic dominance.
• Next Steps: Mass extinctions will be covered in the following lecture.