Answer
Simple anaerobic metabolisms (fermentation & chemolithotrophy) → 2. Anoxygenic photosynthesis (using H₂S or other donors) → 3. Oxygenic photosynthesis (cyanobacteria producing O₂) → 4. Aerobic respiration (using oxygen, higher ATP yield).
Why more than one type evolved? Because Earth environments changed and different metabolisms used different energy and electron sources; having multiple types let organisms exploit different niches and increased overall efficiency and complexity.
Work Step by Step
Start with early Earth conditions
Early Earth had very little free oxygen; energy > needed from chemical reactions and sunlight but no oxygen to use.
First life forms therefore used anaerobic ways to get energy — simple fermentation (breaking molecules without oxygen) or using inorganic molecules (chemolithotrophy).
Chemolithotrophy & fermentation first
These reactions work without oxygen and can use available compounds (H₂, Fe²⁺, H₂S). They are ancient and likely simple to evolve.
Photosynthesis appears
Anoxygenic photosynthesis (doesn’t produce oxygen) likely evolved next — organisms that used light but used molecules like H₂S as electron donors.
Later oxygenic photosynthesis (by cyanobacteria) evolved, which splits water and releases O₂. This was a major turning point (Great Oxygenation Event).
Oxygen rises → aerobic respiration evolves
With free O₂ present, organisms that evolved aerobic respiration could extract far more energy (ATP) from food — a big advantage.
Aerobic respiration likely evolved after oxygenic photosynthesis increased oxygen in the atmosphere.
Why not just one metabolism?
Different resources in different places: some environments have no oxygen, others have sunlight, others have particular inorganic molecules — different metabolisms exploit different resources.
Selective advantage: aerobic respiration yields more energy but only where oxygen exists. Fermentation and chemolithotrophy still work where oxygen is absent.
Stepwise evolution: complex pathways often evolve from simpler ones; multiple types persisted because they suit different niches and conditions.
