Oxygen damage to DNA may explain why complex cells, and sex, evolved
When cyanobacteria began filling Earth's air with oxygen about 2.45 billion years ago, the gas was probably poisonous to much surrounding life. One hypothesis holds that the pressure to survive oxygen's damage helped turn an archaeal lineage into the first complex cells, and that sex itself evolved partly as a way to repair oxidised DNA.
Oxidative stress is an imbalance: reactive oxygen species are produced faster than a cell can neutralise them or fix the harm they cause. These molecules, including the superoxide radical, the hydroxyl radical and hydrogen peroxide, attack proteins, fats and DNA, damaging DNA bases and snapping strands. Most come from ordinary aerobic life, such as oxygen leaking from mitochondria as they make energy. Yet they are not purely villains; cells also use them as messengers in redox signalling, and plants need brief bursts of oxidative stress to launch their stress responses.
Repair is relentless. Rat urine carries about 74,000 oxidised DNA fragments per cell each day, the debris of constant fixing. Some damage still persists: young rats carry around 24,000 oxidative lesions per cell, old rats about 66,000. Mild stress is simply absorbed, moderate stress can trigger orderly cell suicide, and severe stress drains a cell's energy so completely that it cannot even die in a controlled way and simply falls apart. Free iron and copper make matters worse, converting hydrogen peroxide into the far more aggressive hydroxyl radical.
Cells defend themselves with enzymes such as superoxide dismutase, catalase and glutathione peroxidase. Polyunsaturated fats like linoleic acid are prime targets, and their oxidation products serve as telltale markers of damage, for example in the fatty plaques that clog arteries. In the brains of people with Alzheimer's disease, peroxidised fats and proteins accumulate inside cells.
Swallowing antioxidants has not proved a simple fix. In smokers, high doses of beta carotene increased lung cancer rates, apparently acting as a pro-oxidant under high oxygen levels. Vitamin E from food has been linked to lower heart disease risk, but vitamin E supplements have been associated with higher overall mortality, heart failure and haemorrhagic stroke. The US Department of Agriculture even withdrew its table of food antioxidant scores, citing no evidence that those levels translate into effects inside the body.
Source: Oxidative stress
