The endosymbiotic hypothesis is developed by Margulis; this hypothesis proposes an explanation for the evolutionary origin of organelles (mitochondria and chloroplast) in eukaryotic cells. This hypothesis tells us that these organelles were once free living prokaryotes that got engulfed by anaerobic bacteria that could not live in an aerobic environment, due to the oxygen that was release by these photosynthetic bacteria.
The prokaryotes carried out their metabolic functions where in some cases; it was helpful for the host cell and at the same time gave the prokaryote a stable place to live in. Therefore this arrangement was a ‘‘gain-gain’’ situation which meant a beneficial relationship for both cells. A very important piece of evidence is that there is evidence of anaerobic bacteria 3. 8 billion years ago when there was no oxygen on earth. But after a relatively short period of time which is 3. billion years ago photosynthetic bacteria appeared, after using the suns energy to make sugar these bacteria released oxygen which is bad for cells thus anaerobic bacteria started to die with the accumulation of oxygen in the atmosphere which was a problem. After this stage of life, aerobic cells appeared (2. 5 billion years ago) this meant that the free living prokaryotes were ingested inside a larger ancestor bacterial cell (anaerobic) but were not digested which happens sometimes when bacteria absorb smaller bacteria that do not get fully digested.
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As they worked together they were able to survive in the oxygen containing atmosphere and help one another (Endosymbiotic Theory 2002). One of the biggest pieces of evidence that proves this theory correct is that the DNA of these organelles is similar to the DNA of prokaryotes: it is circular not like the DNA of the eukaryotic cells which is linear. Also the first amino acid of mitochondrial transcripts is the same as the one of the eubacteria and different from the one of eukaryotes.
The DNA of these organelles evolves independently and at different rate from the DNA of eukaryotic cells, also mitochondria comes from pre-existing mitochondria so does chloroplasts which is clear proof that they were once free living bacteria(Endosymbionts 2008). I think that the endosymbiotic hypothesis is the correct explanation for the origins of these organelles, even if there is some evidence against the hypothesis it is minor evidence and cannot be fully proved. Most of the evidence for this hypothesis is clear and proven which means that this is most likely the best hypothesis for the origin of these organelles.