Question

What is endocymbiotic theory?

Answer 1

according to this theory certain organelles originated as free-living bacteria that were taken inside another cell as endosymbionts. Mitochondria developed from proteobacteria (in particular, Rickettsiales or close relatives) and chloroplasts from cyanobacteria.

Answer 2

Endosymbiotic theory try to explain just a little bit about the nature of mitochondrion and chloroplast. According to this theory the fact is that mitochondrion originated from aerobacteria and chloroplast originated from cynobacteria

Answer 3

Just to add: Some evidence for this is the fact that those 2 organelles (mitochondria and chloroplasts) have their own distinct DNA separate from the nuclear DNA of the cell.