Question

When did cyanobacteria start producing pure oxygen?
Question 5 options:

A)

During the Phanerozoic eon

B)

After the Proterozoic eon

C)

During the Hadean eon

D)

After the Hadean eon

Answer 1

Cyanobacteria began producing oxygen through photosynthesis around 2.4 billion years ago, a period known as the Great Oxidation Event, which signifies a significant increase in Earth's atmospheric oxygen. This event occurred well into the geological time scale after the Hadean and prior to the Phanerozoic eon, and certainly within the Proterozoic eon.

Given the options, the best answer is:

**B) After the Proterozoic eon** (it began during the Proterozoic eon, but the other options are also incorrect).