Image courtesy of JPL/NASA
About 2.5 billion years ago (BYA) oxygen began to build up in the atmosphere. Life used this change to developed eukaryotic cells. These are cells which include a nucleus. Both heterotrophs (early animals) and autotrophs (early plants) began to use the new cells.
In cells with a nucleus respiration and photosynthesis are better, so the production of oxygen in the environment of the early Earth speeded up. This made it possible for new organisms which depended on oxygen for survival to develop.
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