What primary chemical reaction do scientists suggest for the evolution of an aerobic environment to an anaerobic one?

Oxygenic photosynthesis

Explanation:

The process of oxygenic photosynthesis in which organisms use atmospheric carbon dioxide and fix it in the presence of sunlight into the organic nutrients led to the transition of anaerobic conditions on the primitive earth into the aerobic ones.

The oxygenic photosynthesis uses a water molecule as an electron donor. The water molecule is oxidized and oxygen gas is released as a by-product. This process led to the accumulation of oxygen in the atmosphere of the primitive earth.  

hypoxemia refers to a below usual concentration of oxygen in the blood, mainly in the arteries. hypoxemia is an indication of an issue associated with circulation or breathing and may lead to various symptoms like shortness of breath.  

the condition is identified by determining the level of oxygen in the blood sample obtained from an artery. it can also be predicted by determining the oxygen saturation of the blood with the application of a pulse oximeter.  

the normal arterial oxygen is about 75 to 100 mm hg. the values underneath 60 mm hg is generally considered with the requirement for supplemental oxygen. the normal pulse oximeter readings generally range between 95 to 100 percent. the values under 90 percent are regarded as low.  

in the given case, the oxygen saturation is measured at 93 percent, hence, there is no need to administer oxygen, that is, one should withhold oxygen.