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 accumulating in the atmosphere. Soon, enough oxygen accumulated in the early atmosphere for a change in the genetic code which resulted in the formation of 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 sophisticated cells with a nucleus, not only respiration, but even photosynthesis is more efficient, so the production of oxygen in the environment of the early Earth accelerated. 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 .
By the end of the Proterozoic, new life forms evolved which were multi-cellular and colonial.
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