Primitive photosynthetic bacteria, specifically cyanobacteria, produced oxygen as a byproduct of their photosynthesis. This was a revolutionary event in Earth's history, as the early atmosphere had very little oxygen.

Here's a breakdown:

* Early Photosynthesis: Primitive photosynthetic bacteria used a different type of photosynthesis than plants do today. They didn't use water as their source of electrons, but rather compounds like hydrogen sulfide. This process didn't produce oxygen.

* Cyanobacteria and Oxygenic Photosynthesis: Cyanobacteria emerged later and developed a more efficient form of photosynthesis. They used water as their electron source, and in this process, oxygen was released as a byproduct.

* The Great Oxidation Event: The accumulation of oxygen released by cyanobacteria led to a significant shift in Earth's atmosphere. This event, known as the Great Oxidation Event, caused massive changes in the planet's environment and the evolution of life.

So, in short, primitive photosynthetic bacteria, specifically cyanobacteria, were responsible for producing the oxygen that we breathe today.