At the beginning of this time period, some 2 billion years ago, some of the single-celled bacteria that filled the Earth's oceans developed the ability to use oxygen to help convert food into energy. This process is called respiration and turned out to be a life process which drastically changed the course of events on Earth. Nearly all subsequent forms of life would use and refine this process.
Sometime during this period, perhaps about 1.8 billion years ago, another major step in the progression of life occured. Until this point all organisms were bacteria that were prokaryotic (without an organized nucleus containing genetic material). There must have been an amazing variety of different types of bacteria throughout the Earth's water environments. It is thought that there were some types of these prokaryotic bacteria living together, one inside of the other, helping each other survive (this is called symbiosis). Eventually, they lived together so long that they lost the ability to live apart. The ones that lived inside the others became mitochondria. Mitochondria are the parts inside of all complex cells where respiration takes place (respiration is using oxygen and food to make energy). It is thought that other cell parts evolved in the same way, from partnerships between large and small bacteria. Complex cells that have these smaller parts inside of them are called eukaryotic. These first eukaryotic cells were the basis for all plant cells, animal cells, fungi cells, and protoctista (protist) cells. Animals would never have evolved at all without eukaryotic cells. Before long, single-celled eukaryotic organisms of all types evolved and filled up the oceans, competing with, and perhaps feeding on, bacteria.
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