Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as a biological kingdom formally called the Protista and included mostly unicellular organisms that did not fit into the other kingdoms. Molecular information has been used to redefine this group in modern taxonomy as diverse and often distantly related phyla. The group of protists is now considered to mean diverse phyla that are not closely related through evolution and have different life cycles, trophic levels, modes of locomotion and cellular structures. Besides their relatively simple levels of organization, the protists do not have much in common. They are unicellular, or they are multicellular without specialized tissues, and this simple cellular organization distinguishes the protists from other eukaryotes, such as fungi, animals and plants.