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