The atmosphere of Earth wasn't always full of oxygen. At one point, the world was void of multi-cellular organisms, and only single-celled organisms existed. An accidental mutation would some day change the world forever as these organisms would become able to feed off of the sunlight around them, and produce oxygen in a process known as photosynthesis.
This oxygen, which none of these organisms could use, was toxic. Because of this, single-cell organisms that weren't killed off by the abundance of the toxic byproduct evolved into multi-cell organisms, which would adapt to use oxygen to their advantage.
What we have today are single-cell and multi-cell organisms living together in one environment to create a symbiant circle. We humans, for example, depend on the oxygen that plants create, while the opposite is also true in that plants depend on the carbon dioxide that we create.
It's dreary to think that the air we all depend on to live one day almost killed all forms of life from the face of the Earth because of an accidental mutation.