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Evidence for simple, single-celled life can be found in fossils dating back about 3.5 billion years. These are bacteria and archaea, two prokaryote life forms that differ in their genes and cell membranes. We have to wait another billion years or so before we see complex eukaryote cells, of the kind that makes up larger life forms (including us). Why?
Over time, photosynthesizing bacteria slowly released their waste oxygen into the atmosphere and oceans, leading eventually to the Great Oxidation Event. Other organisms had to adapt to survive and some, such as α-proteobacteria, evolved mechanisms that allowed them to use oxygen to derive energy from dead organic matter. It is believed that, in a fateful encounter, an anaerobic archaeon desperate for a source of energy absorbed a few α-proteobacteria, swelling in size. The absorbed bacteria became mitochondria, the ‘power plants’ of eukaryote cells. A similar encounter involving the absorption of photosynthesizing bacteria led to chloroplasts, the power generators of complex plant life.
This wasn’t plain sailing. The archaeon host DNA had no way of dealing with the ‘jumping genes’ imported from the bacteria. It’s possible that the larger cell evolved a nucleus to reduce the damage (eukaryote means ‘true nucleus’).
It’s all a bit make-do-and-mend. But once eukaryotes figured out how to survive, there was no going back.