Mitochondria and chloroplasts most likely arose through a process known as endosymbiosis, where these organelles originated from free-living prokaryotic cells that were engulfed by ancestral eukaryotic cells. Mitochondria are believed to have evolved from an ancestral alphaproteobacterium, while chloroplasts likely originated from ancestral cyanobacteria. These prokaryotes formed a symbiotic relationship inside the host cell, eventually becoming permanent organelles. This theory, known as the endosymbiotic theory or symbiogenesis, is supported by several lines of evidence:
- Both mitochondria and chloroplasts contain their own circular DNA, similar to bacteria.
- They reproduce independently within the cell by binary fission, like bacteria.
- Their ribosomes resemble bacterial ribosomes.
- Various membrane components and proteins are similar to those in bacteria.
- Many genes have been transferred from these organelles to the host cell's nucleus over time.
This evolutionary event occurred more than 1.4 billion years ago and was a key step in the origin of eukaryotic cells.
