Answer and Explanation:
The Endosymbiotic Theory states that mitochondria and chloroplasts were, at some point, individual free-living cells. Throughout time, they were either consumed or parasites that ended up inside a host cell. Then, this became a beneficial - symbiotic - relationship, hence the name "endosymbiotic" theory.
There are pieces of evidence that support this theory, for example:
- Eukaryotic organelles are far more similar to prokaryotes more than previously thought. For instance, plant plastid organelles divide in an extremely similar way in comparison to free-living bacteria.
- The genes that are present in organelles (eukaryotes) are also extremely similar to the genes of prokaryotes.
- Eukaryotic cells NEED its organelles because some of them contain their 'own' DNA. Therefore, a eukaryotic cell does not contain its own genetic information. This indicates that organelles were once free-living cells before 'living' inside the eukaryotic cell.