For thousands of years people did not know why we breathe air. Plato and Aristotle believed that nutrients from food were burned in the heart, making the flame that brought warmth and life to the body. They thought that the air that we breathe helped to keep the fire controlled.
Oxygen was discovered to be a chemical element, and vital in breathing, by 18th century scientists such as Joseph Priestley, Carl Wilhelm Scheele and Antoine-Laurent Lavoisier. It was Lavoisier, who is regarded as the founder of modern chemistry, who named oxygen.
Oxygen is necessary for the chemical reactions inside a cell. (The word oxygen is from the Greek meaning acid burning.) These cellular inspirations break up nutrients from food and set free the energy for driving the cell’s life process. A pretty important process considering you breathe up to 23,000 times a day.