In the origin of eukaryotes, endosymbiosis: a. describes the engulfing of a photosynthetic bacterium that ultimately gave rise to mitochondria b. most likely began with the endosymbiont entering the host cell through the endomembrane system c. most likely began with a heterotrophic endosymbiont entering the cell as undigested prey or as an internal parasite d. is thought to explain the origin of the nucleus