For the first 1 billion years of Earth's evolution, the fraction of oxygen in its atmosphere was approximately
A) zero.
B) half of what it is today.
C) 2 times what it is today.
D) 10 times what it is today.
E) the same as it is today.

Question

Quizplus · Accepted Answer

During the first billion years of Earth's evolution, the atmosphere had virtually no oxygen. Oxygen levels began to rise significantly only after the Great Oxidation Event, which occurred around 2.4 billion years ago.

For the First 1 Billion Years of Earth's Evolution, the Fraction