Final answer:
Before the appearance of oxygen in Earth's atmosphere, primary metabolic pathways included anoxygenic photosynthesis and glycolysis. These processes did not rely on oxygen, aligning with the anoxic conditions of early Earth. The evolution of cyanobacteria and subsequent oxygenic photosynthesis led to the rise of atmospheric oxygen and more efficient metabolic pathways.
Step-by-step explanation:
The primary metabolic pathway(s) before the appearance of O2 in the atmosphere were processes that didn't rely on oxygen since the ancient atmosphere was anoxic. The evidence indicates that early life forms were anaerobic, carrying out metabolic functions without oxygen. Before oxygenic photosynthesis, anoxygenic photosynthesis used compounds such as hydrogen sulfide to harvest solar energy; this form of photosynthesis did not produce oxygen.
Another important pathway from this era is glycolysis, capable of breaking down simple sugars without the need for oxygen. Glycolysis likely developed before even anoxygenic photosynthesis. Only later did cyanobacteria evolve to use water in photosynthesis, leading to the production of oxygen and the eventual oxygenation of Earth's atmosphere.
Following the rise of atmospheric oxygen, more efficient oxygen-utilizing pathways, such as the Krebs cycle and oxidative phosphorylation, began to evolve, which were critical in the development of aerobic life forms that populate the Earth today.