How does the fraction of oxygen in Earth's atmosphere today compare to what it was 3 billion years ago?
A) It has significantly declined.
B) It has significantly increased.
C) It increased up to 2 billion years ago but has been declining ever since.
D) It declined up to 2 billion years ago but has been increasing ever since.
E) It hasn't changed.

Question

Quizplus · Accepted Answer

The fraction of oxygen in Earth's atmosphere has significantly increased due to the Great Oxygenation Event, which began around 2.4 billion years ago, primarily due to photosynthetic organisms.

How Does the Fraction of Oxygen in Earth's Atmosphere Today