Question

What important change in the atmosphere occurred during the Proterozoic Eon?
Question 12 options:

A)

Carbon dioxide levels increased, causing glaciation that cooled the planet.

B)

Nitrogen and carbon dioxide levels decreased, fueling bacterial growth.

C)

Methane levels decreased and oxygen levels increased, promoting aerobic life forms.

D)

Helium escaped into space and hydrogen was bound into water molecules.

Answer 1

C) Methane levels decreased and oxygen levels increased, promoting aerobic life forms.

During the Proterozoic (after the Great Oxidation Event), photosynthetic microbes (cyanobacteria) produced large amounts of O2, which reduced methane (a potent greenhouse gas) and allowed aerobic organisms and more complex life to evolve.