alleryAbout when did free oxygen begin to appear in Earth's atmosphere?
A4.5 billion years ago
B2.4 billion years ago
C539 million years ago
D60 million years ago
Answer & Solution
Correct answer: B. 2.4 billion years ago
1. Early oxygen was used up by reacting with surface materials.
2. Free oxygen appeared about 2.4 billion years ago, in the Great Oxygenation Event.
3. 539 million years ago marks the start of the Phanerozoic, much later.
_Source: Wikipedia, Atmosphere of Earth, Third atmosphere_
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