Nearly all of the oxygen in the Earth's atmosphere was originally produced by photosynthesis in the leaves of trees that covered the continents 3.5 billion years ago.

Question

Quizplus · Accepted Answer

While it is true that a significant amount of oxygen in the atmosphere is produced through photosynthesis, the earliest trees did not appear until around 400 million years ago. The majority of oxygen in the atmosphere is believed to have come from cyanobacteria and other photosynthetic organisms in the oceans.

Nearly All of the Oxygen in the Earth's Atmosphere Was