The oxygen that arose in early Earth's atmosphere was likely produced by organisms that were very similar to modern:

Question

Quizplus · Accepted Answer

Cyanobacteria, also known as blue-green algae, are the only organisms known to have the ability to conduct oxygenic photosynthesis, the process by which they produce oxygen as a byproduct. Therefore, the oxygen that arose in early Earth's atmosphere was likely produced by cyanobacteria.

The Oxygen That Arose in Early Earth's Atmosphere Was Likely