Question

How did oxygen become such a major component of the current atmosphere?

Answer 1

The first single cell organisms used photosynthesis for energy, and oxygen, one of the byproducts of photosynthesis, was released into the atmosphere in abundance.

Explanation:

Oxygen has always been around in our atmosphere, however, since it reacts so readily, it started out as mostly carbon dioxide, `CO_2`. When we talk about atmospheric oxygen, mostly we refer to diatomic oxygen, `O_2`. In order to increase the concentration of `O_2` we just need a way to pull the carbon atom out of the `CO_2` molecules.

It turns out that photosynthesis does exactly that. During photosynthesis, `CO_2` and water, `H_2O` react with light to form glucose and `O_2`. Here's a deeper explanation of photosynthesis.

Photosynthesis equation;
`6CO_2 + 6H_2O -> C_6H_12O_6 + 6O_2`

Looking at the chemical equation above, we can see that carbon atoms are pulled off the `CO_2` molecules and added to the water molecules to form glucose. The `O_2` is then released into the atmosphere.

Today, those `O_2` molecules are used by animals during respiration to release energy. The formula is the opposite of photosynthesis, and converts glocose and oxygen into `CO_2` and water.

Respiration;
`C_6H_12O_6 + 6O_2->6CO_2 + 6H_2O`

Before there were animals, however, the `O_2` was free to build up in the atmosphere, so as the amount of `CO_2` decreased, the `O_2` increased, and the atmosphere became oxygen rich.

During the Archaean Era, about 3.5 billion years ago, the first single celled organisms began photosynthesizing. These cyanobacteria pumped out tons of oxygen into the atmosphere.