today mitochondria and chloroplasts would not be able to survive outside a cell but according to the endosymbiotic theory they were once independently living cells the first cells on earth are thought to have appeared around 3.8 billion years ago 750 million years after the Earth's formation these cells were prokaryotes cells lacking organelles or other internal membrane-bound structures it wasn't until 2.7 billion years ago that eukaryotes cells with a nucleus enclosed in membranes appeared in the fossil record eukaryotes evolved from prokaryotes here is how that is thought to have happened a prokaryote grew in size and as it did its surface area to volume ratio decreased this is a problem because it now needs to feed more bulk but it has less surface area to gather nutrients so to increase the ratio again the cell developed in foldings in its membrane eventually these in foldings pinched off from the cell membrane to form an early endomembrane system surrounding the nucleoid this was the first membrane-bound nucleus and hence this was the first eukaryotic cell this eukaryotic cell endo phagocytosed and aerobic or oxygen using prokaryotic cell which may have been prey or a parasite genomic sequencing of cells today indicates that this prokaryote was from a group of bacteria called the alpha Proteobacteria in any case this cell avoids digestion and becomes an endosymbiotic in other words a cell living within another cell funnily enough this endosymbiont became useful to the eukaryote the aerobic prokaryote was able to use oxygen to make energy a process called respiration which was nice because the Earth's oxygen concentrations were increasing at this time due to the activity of cyanobacteria the aerobic prokaryote benefited from its host because the cytoplasm was full of half digested food molecules digesting these molecules with oxygen the prokaryote produced so much energy that some of the ATP leaked into the cell's cytoplasm array so as other eukaryotes went extinct from the rising oxygen levels the Eukarya and the endosymbiont became best pals with the endosymbiont becoming a mitochondrion at this point the endosymbiotic became an obligate in December meaning it cannot survive on its own outside the cell sometime later the same process occurred with a cyanobacterium which became the chloroplast this you carry out was the ancestor of plants and algae we know that chloroplasts evolved later because plant cells have both mitochondria and chloroplasts while animal cells have only mitochondria but what's the proof that endosymbiosis happened first mitochondria and chloroplasts replicate on their own through something similar to binary fission and cells cannot create new ones otherwise the genome of these organelles is also remarkably similar to those of prokaryotes mitochondria chloroplasts in bacteria all have a single circular DNA molecule in addition porins and cardiolipin are only found in mitochondria chloroplasts and bacteria so if mitochondria can only come from other mitochondria how do you get your mitochondria in the first place well your mitochondria come from your mom so next time someone says your genetic info is 50-50 you can tell them that if you liked this video please like and subscribe it would help me make more videos and make sure to comment with any topics you'd like me to cover in future videos also it would be really nice if you could support me on patreon thank you