Final answer:
Mitochondria were derived after ancestral eukaryotic cells engulfed aerobic bacteria in a symbiotic relationship, according to the endosymbiotic theory proposed by Lynn Margulis. Evidence includes the presence of mitochondrial DNA, a double membrane structure, and the resemblance of mitochondria to alpha-proteobacteria.
Step-by-step explanation:
Mitochondria are proposed to have arisen after host cells engulfed aerobic bacteria in endosymbiosis. This process occurred when an ancestral eukaryotic cell engulfed a free-living aerobic prokaryote, which then became an endosymbiont within the host cell's cytoplasm. The endosymbiotic theory, first proposed by Lynn Margulis, suggests that through this symbiotic relationship, the engulfed bacteria provided the host with the ability to utilize oxygen to release energy from nutrients. Over time, the once-independent aerobic bacteria evolved into mitochondria.
Support for this theory comes from the fact that mitochondria contain their own DNA and reproduce asexually, resembling their prokaryotic ancestors. The double membrane structure of mitochondria, with an inner bacterial-type membrane and an outer eukaryotic-type membrane, also supports this theory. The presence of infoldings, or cristae, on the inner membrane that resembles the texture of alpha-proteobacteria is another piece of evidence aligning with the endosymbiotic origin of mitochondria.
Through millions of years of evolution, these primitive relationships diversified and became more specialized, leading to the complex eukaryotic cells we observe today, where mitochondria play a vital role in cellular respiration and energy production.