Fermentation is a process under which organisms can continue the production of ATP without requiring oxygen. The process is much quicker, but much less efficient than cellular respiration. Often times organisms undergo fermentation solely anaerobic conditions, however some organisms, like yeast, fermentation can occur in aerobic conditions. Fermentation occurs in mammalian muscle, fruits, bacteria, yeasts, etc. When using fermentation, an organism will only undergo the first part of cellular respiration (glycolosis), in order to obtain 2 ATP molecules through substrate-level phosphorylation and converting the excess pyruvate molecules into some other form (such as lactic acid). Being so simple, fermentation was probably a process developed in the most primitive forms of life, during a time when oxygen was scarce on Earth. Cellular Respiration probably developed further down the road from this adaptation. The bread in the image above is given its light, airy texture from fermentation.