It has been hypothesized that the organelles in protists descend evolutionarily from specialized symbiotic bacteria living within the cells of other bacteria, contributing at least in part to the transition from prokaryotic (bacterial) cells (the earliest form of life on the planet, dating back at least 3.5 billion years) to early eukaryotic cells (the cells that define protists, dating back 1.5 billion years) and the more complex life forms of later plants and animals.
The protists comprise a very diverse group of organisms. They include some algae, the protozoans, and multicellular or multinucleate autotrophs, such as the water molds. Many have flagella that enable them to move about. Before the advent of modern biochemistry and the electron microscope, these organisms were fit into the plant and animal kingdoms. It is now thought that, although green plants probably evolved from the green algae and animals from some other early forms, most modern protists have followed independent evolutionary lines. There are approximately 60,000 living species of protists.
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