The Great Oxygenation

• The diverity of [[bacteria]] exploded during the [[Archaean]], leading to the evolution of [[Cyanobacteria]]
  • This does not mean that [[Spirulina]] existed during the [[Archaean]]: it is a modern organism
• There was almost no O₂ in the atmosphere before the [[Archaean]]-[[Proterozoic]] break
  • No O₂ means no formation of peroxides
  • Peroxides are dangerous to many cells as they rob electrons from proteins, fatally altering them
• Cells had to develop anti-oxidation defenses, including [[catalase]]
• Cells had two ways to adapt to O₂
  1. Become a strict anaerobe, and avoid oxygen-containing habitats
  2. Tolerate O₂
     • O₂ toleration can easily lead to aerobic metabolism