i) In the early time earth had environment, which was totally devoid of oxygen. The aerobic organism cannot exist in anaerobic environment. ii) Some existing organism like bacteria and parasites have anaerobic respiration. Many useful bacteria and yeasts are anaerobic. iii) Aerobic respiration is also earlier stage of anaerobic respiration. Aerobic organisms have evolved from anaerobic organisms. iv) In tissues of skeletal muscles aerobic respiration takes place. But when oxygen supply cannot keep pace with energy demand then anaerobic respiration supplies energy by break down of glucose. Some bacteria and fungi have same method to derive energy. v) Yeast is used to produce ‘khameer’ in the flour to make bread.