Image courtesy of JPL/NASA
About 2.5 billion years ago (BYA), after the iron in the ocean was gone to form iron ore deposits
, oxygen began collecting in the atmosphere. Life took advantage of this change and developed eukaryotic cells. These are cells in which the organelle are outside of the nucleus, and the nucleus has a separate and distinct place within the cell. Both heterotrophs (early animals) and autotrophs (early plants) adapted to the new cells.
In cells with a nucleus respiration and photosynthesis are more efficient, so the production of oxygen in the environment of the early Earth speeded up. That made it possible for respiration (for animals which breathe oxygen) to begin in simple organisms. The new life forms which came to be were single-celled organisms like protozoans, amoeba, etc, animals which we now classify as part of the, Protista Kingdom .
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