The accumulation of free oxygen in Earth's atmosphere began
A) with the origin of life and respiratory metabolism.
B) with the origin of photosynthetic bacteria that had photosystem I.
C) with the origin of cyanobacteria that had both photosystem I and photosystem II.
D) with the origin of chloroplasts in photosynthetic eukaryotic algae.
E) with the origin of land plants.

Question

Quizplus · Accepted Answer

The accumulation of free oxygen in Earth's atmosphere began with the origin of cyanobacteria that had both photosystem I and photosystem II. These were the first photosynthetic organisms that could produce oxygen as a byproduct of photosynthesis. This oxygen gradually accumulated in the atmosphere over time, leading to the oxygen-rich atmosphere we have today.

The Accumulation of Free Oxygen in Earth's Atmosphere Began