Answer and Explanation:
Before free oxygen became available on earth, the environment was somewhat inhospitable and impossible to house aerobic beings, that is, beings that need oxygen to survive. Does this mean that the planet did not have any living beings? No. Because there are many living beings that do not need oxygen to be alive and some only live with the complete absence of oxygen, such as some bacteria for example. In this case, we can say that the cells of living beings at the time when the earth's oxygen was not available used to perform anaerobic metabolic processes, in addition to being able to do processes such as glycolysis, pyruvate oxidation, citric acid cycle and oxidative phosphorylation, using an acceptor of electrons other than oxygen.