Just before the Cambrian explosion, the atmosphere became oxygenated, leading to an explosion of new life in the form of primative blue green algae called cyanobacteria and oxygen dependent organisms in the Proteozoic Eon (2500-541 million years ago). Later, multicellular organisms followed, along with higher levels organization and complex life. Oxygen levels rose from low levels, steadily over this period of time.

Further Explanation:

The rise in O2 gas within the aquatic habitat and atmosphere is attributed to photosynthesis in cyanobacteria which allowed respiring eukaryotes to thrive.

Photosynthesizing cells in blue green algae contain chlorophyll, a specialized compound which facilitates the conversion of light energy to energy stored in carbohydrates in the process photosynthesis. This is a chemical pathway that’s integral to producing energy in primary producers. Energy in the form of molecules of glucose is produced from light, water and carbon dioxide while oxygen is released. This occurs in several complex steps, photosynthesis is a rate limited reaction, depends on several factors including carbon dioxide concentration, ambient temperature and light intensity; the energy is retrieved from photons, I.e. particles of light, and water is used as a reducing agent.

With the evolution of oxygen dependent organisms like Ediacara biota, came respiration in the mitochondria. Eukaryotes utilize glucose along with oxygen for the production of ATP in the Krebs’s cycle via the oxidization of pyruvate( through the process of glycolysis). Oxidative phosphorylation describes a process in which the NADH and FADH2 made in previous steps of respiration process give up electrons in the electron transport chain these are converted it to their previous forms, NADH+ and FAD. Electrons continue to move down the chain and the energy they release is used in pumping H+ to produce ATP from ATP synthase. At the end of the ETC molecules of oxygen accept electrons and protons to form molecules of water.