The origin of the eukaryotic cell represents one of the most fundamental mysteries in the evolution of cellular life on Earth. Already in the beginning of the 20th century, several researchers and cell biologists noticed the resemblance of mitochondria and chloroplasts to free-living bacteria, an observation that shaped the basis for various subsequently formulated endosymbiotic theories, which suggest that eukaryotic organelles are derived from bacterial symbionts. Throughout the years, various detailed models have been proposed to account for the partners involved as well as the processes underlying this symbiosis (Lopez-Garcia and Moreira, 2015; Lopez-Garcia et al., 2017; Martin et al., 2017). Yet, while the identity of the ancestors of mitochondria and chloroplasts could be ascribed to alpha-proteobacteria and cyanobacteria, respectively, the nature of the host cell remained an enigma for several decades.