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Endosymbiosis is a theory proposed by Lynn Margulis in the 1960s, which suggests that certain organelles within eukaryotic cells were once free-living prokaryotic organisms. This theory has significantly influenced our understanding of the evolution of eukaryotic cells. The most widely accepted example of endosymbiosis is the origin of mitochondria and chloroplasts.

Mitochondria are often referred to as the "powerhouses" of the cell, responsible for producing energy in the form of ATP through cellular respiration. According to the endosymbiotic theory, mitochondria were once independent bacteria that were engulfed by a larger cell. Over time, these bacteria evolved to become mitochondria, maintaining their own DNA and ribosomes, and eventually losing their ability to survive independently.

Chloroplasts, on the other hand, are the sites of photosynthesis in plant cells. The endosymbiotic theory proposes that chloroplasts were once cyanobacteria, which were engulfed by a host cell. These cyanobacteria evolved to become chloroplasts, retaining their own DNA and ribosomes, and eventually losing their ability to survive independently.

The endosymbiotic theory has been supported by various lines of evidence, including the presence of circular DNA in chloroplasts and mitochondria, similar to that found in prokaryotes. Additionally, the theory explains the presence of seven-celled structures called "mitochondrial contact sites" in the inner mitochondrial membrane, which are thought to be remnants of the engulfing event.

While the endosymbiotic theory is widely accepted, it is not without controversy. Some scientists argue that the evidence is not conclusive, and that alternative explanations for the origin of eukaryotic cells should be considered. Nonetheless, the endosymbiotic theory has revolutionized our understanding of the evolution of eukaryotic cells and continues to be a subject of active research.