Final answer:
Photosynthesis first evolved in prokaryotes, with cyanobacteria being one of the earliest organisms performing this process and contributing to Earth's oxygenated atmosphere. Therefore, the correct option is A
Step-by-step explanation:
Photosynthesis first evolved in prokaryotes. This process was responsible for the production of an oxygenated atmosphere, fundamentally changing Earth's environment and setting the stage for the evolution of complex life. Some of the oldest fossilized organisms, known as stromatolites, provide evidence of early prokaryotic life performing photosynthesis around 3.5 billion years ago. These prokaryotic pathways laid the foundation for the evolution of photosynthesis in eukaryotic cells, as suggested by the similarities between certain photosynthetic prokaryotes and eukaryotic chloroplasts. Cyanobacteria, one form of these prokaryotic photosynthesizers, appeared around 2.5 billion years ago and are credited with the initial high levels of oxygen in the atmosphere due to their oxygen-producing photosynthesis.