The figure above shows the geological times scale. The Earth's atmosphere has been oxygen rich for
A)about 500 million years.
B)its entire history.
C)about 2 billion years.
D)a few million years.

Question

Quizplus · Accepted Answer

The Earth's atmosphere has been oxygen rich for about 500 million years. This is because the first photosynthetic organisms evolved around 2.7 billion years ago, and they began to release oxygen into the atmosphere. Over time, the amount of oxygen in the atmosphere increased, and it eventually reached a level that was high enough to support the evolution of complex life forms.

The Figure Above Shows the Geological Times Scale