A flow of air has rushed into your lungs, where the oxygen moves into your bloodstream, fueling metabolic fires in cells throughout your body. You, being an aerobic organism, use oxygen as the cellular spark that frees molecular energy from the food you eat. But not all organisms on the planet live or breathe this way. It gives aerobes like us life, but for many anaerobes, or creatures that respire without oxygen, oxygen is a toxin that reacts with and damages their specialized molecular machinery. Back then, the air and oceans were mostly devoid of oxygen, so life was almost entirely anaerobic, or non-oxygen-breathing.