Which organisms were the first photosynthesizers around 2.4 billion years ago?

• A. Cyanobacteria
• B. Archaea
• C. Diatoms
• D. Proteobacteria

Answer: (A)

The key idea is when oxygen-producing photosynthesis first appeared. Cyanobacteria were the earliest organisms known to perform oxygenic photosynthesis, using water as the electron donor and releasing oxygen as a byproduct. This ability began around 2.4 billion years ago and had a huge impact, gradually increasing atmospheric oxygen and enabling aerobic life.

Archaea include many diverse microbes, but they are not the group that first carried out oxygenic photosynthesis. Some archaea are phototrophic in other ways, but they don’t produce oxygen as a byproduct in the way cyanobacteria do. Diatoms are photosynthetic eukaryotes that came later and rely on plastids derived from ancient cyanobacteria; they did not initiate photosynthesis. Proteobacteria include various bacteria, some capable of photosynthesis, but many perform anoxygenic photosynthesis (not releasing oxygen) and they appear after the rise of cyanobacteria.

So the earliest photosynthesizers around 2.4 billion years ago are cyanobacteria.