ENDOSYMBIOSIS

Mitochondria and chloroplasts, though unable to survive independently today, were once free-living cells according to the endosymbiotic theory. Early cells, prokaryotes, appeared around 3.8 billion years ago. Eukaryotes, with membrane-bound nuclei, emerged much later, around 2.7 billion years ago, evolving from prokaryotes when an increase in cell size and a decrease in surface-area-to-volume ratio led to membrane infoldings that eventually formed an early endomembrane system, enclosing the nucleoid to create the first nucleus.

The first eukaryotic cell then engulfed an aerobic prokaryotic cell, likely an Alpha Proteobacteria. This prokaryote avoided digestion and became an endosymbiont, living within the host cell. The aerobic prokaryote provided a benefit by efficiently producing energy through respiration, especially as Earth's oxygen levels rose. This symbiotic relationship led to the aerobic prokaryote evolving into a mitochondrion, becoming an obligate endosymbiont that can no longer survive independently.

Later, a similar process occurred with a cyanobacterium, which was engulfed and became a chloroplast, giving rise to plant cells and algae. Evidence for endosymbiosis includes the fact that mitochondria and chloroplasts replicate independently via binary fission, and cells cannot create new ones. Their genomes are circular, similar to prokaryotes, and they share specific proteins like porins and cardiolipin exclusively with bacteria. Mitochondrial DNA is inherited maternally.