Final answer:
Mitochondria evolved through the process of endosymbiosis, in which an ancestral eukaryotic cell absorbed an aerobic bacterium. The theory is evidenced by mitochondria's double membrane and prokaryote-like DNA, with mitochondrial DNA solely inherited from the mother.
Step-by-step explanation:
Mitochondria most likely evolved by endosymbiosis. This process occurred during an ancient symbiotic relationship when an ancestral eukaryotic cell absorbed an aerobic bacterium, which over time became an integral part of the cell as the mitochondrion. This theory is widely accepted and is supported by the fact that mitochondria have prokaryote-like DNA, which indicates they originated from bacteria.
The endosymbiotic theory is further substantiated by the presence of a double membrane in mitochondria, which is characteristic of gram-negative bacteria that were engulfed by the ancestral eukaryotic cell. Additionally, mitochondrial DNA is unique as it is only inherited from the mother, suggesting a distinct evolutionary path separate from the nuclear genome.