Anaerobic respiration is believed to have evolved earlier than aerobic respiration because it does not require oxygen, which was not present in the early Earth's atmosphere. The Earth's atmosphere during its early stages was composed mainly of carbon dioxide, nitrogen, and small amounts of methane and ammonia. Therefore, the earliest forms of life would have had to rely on anaerobic respiration, which involves the breakdown of glucose without the use of oxygen to produce energy.
Anaerobic respiration is less efficient than aerobic respiration in terms of the amount of energy produced per glucose molecule, but it is still effective in producing ATP (adenosine triphosphate) for the cell. As life forms evolved and oxygen levels in the atmosphere increased, organisms that were capable of aerobic respiration were able to utilize oxygen to produce more ATP and became more energy-efficient. This gave them an evolutionary advantage over anaerobic organisms, which eventually led to the dominance of aerobic respiration in the evolution of complex life forms.
In summary, the evolution of anaerobic respiration predates that of aerobic respiration because the early Earth's atmosphere did not contain sufficient amounts of oxygen to support the latter. As oxygen levels increased over time, organisms capable of aerobic respiration gained an evolutionary advantage, leading to its eventual dominance in the evolution of complex life forms.