How Have the Atmosphere and Living Things Interacted Over ...

3. What gas increased dramatically over the history of the Earth? 4. What gas did not appear in any measureable amount until about 2.0 billion years ago? 5. What gas has been almost nonexistent in the atmosphere since about 3.0 billion years ago? 6. Which gas appeared in the atmosphere about the time that limestone deposits became common? 7. ................
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