Fortunately, most minerals on the primitive earth was highly reduced state and adsorbed almost all of the molecular oxygen so that the oxygen concentration in the atmosphere of the earth remained very low. During these 2 billion years, bacteria elaborated efficient photosynthetic machinery and gradually developed into unicellular and multicellular plants. They produced more and more molecular oxygen. That means respiration consuming the molecular oxygen can produce energy much more efficiently than anaerobic fermentation. Nineteen times more ATP can be produced from 1 Mol glucose than in fermentive process.
Animals can utilize molecular oxygen to create a great amount of energy for their life.
Extremely high rate of genomic changes were produced so that all the possible taxonomic forms of animals and plants radiated. This was a huge scale experiment in biotechnology of the nature. After selection of well-adapted forms, our ancestors of vertebrate were created. These large scale changes were possible, probably because asexual reproduction could produce offspring that did not survive zygote production. From the drastic biotechnological experiments, set about by the Nature in the Cambrian era, we can learn some important lessons.