PLSSS HELP WILL GIVE BRAINLIEST 15PTS! According to the endosymbiotic theory of the origin of eukaryotic cells, how did mitochondria originate? From engulfed, originally free-living bacteria When a protoeukaryote engaged in a symbiotic relationship with a protocell by secondary endosymbiosis From the nuclear envelope folding outward and forming mitochondrial membranes From infoldings of the plasma membrane, coupled with mutations of genes for proteins in energy-transfer reactions