Oxygen, essential for life, can be toxic. Oxygen-centered free radicals are routinely synthesized and recycled as a normal part of cellular metabolism. Oxidative stress—when these highly reactive molecules tax a cell’s antioxidant systems—is a fundamental featureof many cancers. Evolutionary theory states that, as oxygen became more abundant on primordial Earth, respiring single-celled organisms developed antioxidant systems to defend their DNA against free radicals formed from oxygen.
More at Progressnotes, MUSC’s Medical Magazine