What was needed to evolve before there was a significant amount of #O_2# in the atmosphere?

Answer 1

Photosynthesis

The increase in atmospheric oxygen was due to the evolution of organisms that were capable of photosynthesis. These were Cyanobacterica, what we use to classify as "blue-green algae". After about 200 million years of photosynthesis, there was enough oxygen in the atmosphere to cause the Great Oxygenation Event which was a mass extinction of organism that were obligate anaerobes (organisms that were obligated to live in an oxygen free environment).

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Answer 2

Aaron Anderson

The evolution of oxygen-producing photosynthesis was necessary before a significant amount of O2 could accumulate in the atmosphere.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.