Life first appeared in our oceans during the Archean eon when the moon was far closer to Earth. The air was too toxic for life on land
From The Daily Mail:
Life existed in the oceans for hundreds of millions of years while the Earth's air was not fit to breathe, research suggested today.
Plant-like bacteria evolved at least 200 million years before oxygen began to build up in the atmosphere, a study has shown.
During this period in its history, known as the Archaean, the Earth was covered by a poisonous smog of methane, ammonia and other toxic gases.
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