Final answer:
Photosynthesis probably originated in prokaryotes, specifically bacterial prokaryotes, and led to the production of an oxygenated atmosphere.
Step-by-step explanation:
The process of photosynthesis probably originated in prokaryotes. The timelines of divergence suggest that prokaryotes likely evolved first, followed by the branching of protists into fungi, plants, and animals. In fact, there is evidence that bacterial prokaryotes were responsible for the first accumulation of oxygen in our atmosphere through photosynthesis, which led to the production of an oxygenated atmosphere.