Entamoeba invadens is not the secondarily reduced and simplified eukaryote unrelated to a cenancestor as thought by some protist researchers. On the contrary, E. invadens is a single-celled eukaryote with a complex life cycle comprised of stem cells and stem cell lines. It contains hypoxic and oxygenic life cycle stages revealing to us details about the living conditions of LECA at the time of Unikonts’ and Excavata’s divergence. At the end of the super group evolution are protists such as Entamoeba and Giardia and metazoans such as Hydra and humans. Stem cells in Entamoeba and humans are controlled by the same basal mechanisms. At the time of super group divergence LECA had an anaerobic metabolism similar to that of Entamoeba; it contains non-aerobic mitochondria, different metabolic pathways using oxygen or not, anti-oxidative defense mechanisms against oxygen and a life cycle organized with a stem cell lineage. LECA evolved in the Proterozoic age from a unique vegetative cell type living in strict hypoxic environments (temporarily protected by a cyst wall) to a more and more complex cell system capable of living and proliferating in a wide range of hypoxic and moderate oxygenic niches. In conditions of alternating oxygen-, nitrogen- and nutrient-cycles LECA developed the characteristics of a true stem cell protolineage and transferred them to protist divergents and early animals. Without this ancestral heritage, protists such as E. invadens would not be able to become pathogens. They would live in a commensal relationship with their host. The metabolic anaerobe LECA is the common cenancestor of all eukaryotic stem cell lineages.
Keywords: Entamoebainvadens, ancient stem cells (AnSC), stem cell lineage, induced terminal differentiation, autonomous terminal differentiation, LECA, life cycle, stem cell protolineage