Protists are a diverse group of eukaryotic organisms that are not classified as plants, animals, fungi, or prokaryotes. They include single-celled and multicellular organisms that may be photosynthetic, heterotrophic, or parasitic. The endosymbiotic theory proposes that eukaryotic cells originated from prokaryotic cells engaging in endosymbiosis. Protists are classified into 5 candidate kingdoms based on genetic and cellular structure similarities. They display a wide variety of morphologies and behaviors.