ANSWER:
Cyanobacteria or blue-green algae produce oxygen
EXPLANATION:
Most scientists consider that toward half of Earth's 4.6-billion-year antiquity, the atmosphere carried about no oxygen. Cyanobacteria or blue-green algae became the leading microbes to generate oxygen by photosynthesis, possibly as long ago as 3.5 billion years ago and certainly by 2.7 billion years ago
