Protistas’ ancestors

The most ancient fossil traces of the first eukaryote cells date back to approximately 1 and a half billion years ago. Prokaryotes had appeared two billion years earlier, and up to then they had been the only inhabitants: in the intervening time, the bacteria had had the opportunity to differentiate into two different lines of evolution. One of these originated the first eukaryotes, i.e. Protista, following the spread of bacteria that could perform the photosynthesis (cyanobacteria), enriching the atmosphere with oxygen. Since all Protista have an aerobic metabolism, they could not have developed before that time. It seems some bacteria may have started to form symbiotic associations with several bacteria, some of which could have evolved into cellular organelles (e.g. mitochondria and chloroplasts).

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