Final answer:
Glycolysis is the first stage of cellular respiration that evolved, capable of functioning without oxygen and well-suited to the anoxic conditions of early Earth.
Step-by-step explanation:
The stage of cellular respiration that evolved first is glycolysis. This process is ancient and does not require oxygen, making it well-suited to the anoxic conditions of early Earth. Glycolysis involves the breakdown of glucose into pyruvate, producing ATP in the cytosol of the cell's cytoplasm. It precedes the evolution of the Krebs cycle and the electron transport chain, mechanisms that rely on the presence of oxygen, which only accumulated in the atmosphere due to the advent of oxygenic photosynthesis. Therefore, in the absence of oxygen, early life forms would have relied on glycolysis and fermentation to fulfill their energy requirements.
Anaerobic respiration, such as glycolysis and fermentation, provided energy before the oxygenic atmosphere allowed for the evolution of the more oxygen-dependent processes like the Krebs cycle and electron transport chain. The latter processes evolved as a response to the increasing levels of oxygen produced by photosynthetic organisms, enabling cells to extract more energy from glucose.