What does the endosymbiotic theory suggest about the origin of mitochondria?

The endosymbiotic theory posits that mitochondria originated from free-living bacteria that entered into a symbiotic relationship with a host cell. This theory is supported by several lines of evidence, including the presence of double membranes surrounding mitochondria, similarities between mitochondrial DNA and bacterial DNA, and the fact that mitochondria replicate independently of the host cell through a process similar to binary fission, which is characteristic of bacteria.

This evolutionary model suggests that instead of mitochondria evolving from existing internal cellular structures or from hybridization, they were originally independent organisms that established a beneficial relationship with early eukaryotic cells. Over time, these bacteria became integral to the host cells, contributing to the cell's energy production through aerobic respiration, which would have provided a significant evolutionary advantage. This connection highlights the importance of symbiosis in the evolution of complex life forms.