Life is thriving on planet Earth thanks to oxygen. Oxygen is a highly reactive element; it can form compounds with nearly every other element on the periodic table, releasing energy in the process. In a process known as cellular respiration, organisms use oxygen to oxidize substrates (for example sugars and fats) and generate energy.
The Earth formed approximately 4.6 billion years ago. Intense volcanic activity released gases that formed a toxic atmosphere, likely a mixture of carbon dioxide, methane, ammonia, and water vapor. Traces found in ancient rocks suggest that about 2.7 billion to 2.8 billion ago for the first time oxygen was released into Earth’s atmosphere, forming new minerals like iron-oxide. Scientists think that early photosynthetic microorganisms, able to use sunlight to chemically break carbon dioxide molecules into carbon and oxygen, caused the amount of carbon dioxide to decrease and oxygen to increase. But only around 2.45 billion to 1.5 billion years ago oxygen was becoming a significant component of Earth’s atmosphere. Today Earth’s atmosphere contains roughly 21 percent of oxygen.
