A protist ( PROH-tist) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a paraphyletic group encompassing the entire eukaryote tree of life, from which land plants, animals, and fungi evolved. They are primarily single-celled, exhibiting a wide range of forms such as amoebae, ciliates, thick-walled microalgae and, more commonly, flagellates. Several transitions to multicellularity have occurred among protists, from colonies with alternating cell types to giant slime molds, fungus-like organisms, and seaweeds with differentiated tissues. Protists were historically regarded as a separate taxonomic kingdom known as Protista or Protoctista, or were lumped together as part of the traditional plant and animal kingdoms as algae and protozoa, respectively. With the advent of molecular phylogenetics and electron microscopy studies, some protists were shown to be more closely related to animals or plants than to other protists, and algae were found to be intermixed with protozoa. The classification suffered major revisions, as seemingly unrelated forms were found to be evolutionarily related, and vice versa. In modern classifications, protists are spread across several large clades known as supergroups, many of them containing disparate forms. For example, the Archaeplastida includes mostly phototrophs like red and green algae, from which land plants evolved. Opisthokonta groups fungi, animals, and their single-celled relatives. Amoebozoa and Rhizaria harbor the majority of amoeboid organisms, such as testate amoebae, foraminifers and radiolarians. Stramenopiles and Alveolata are diverse groups of flagellates, many of which have evolved into major parasites (e.g., oomycetes, apicomplexans) or phototrophs (diatoms, brown algae, dinoflagellates). The earliest diverging groups, collectively known as Excavata (e.g., euglenids, metamonads), are flagellates that represent the ancestral traits of the last eukaryotic common ancestor (LECA). Despite the comparatively low number of described species, protists compose the majority of eukaryotic diversity as indicated by environmental DNA studies. Most protists are yet undescribed. Protists encompass almost all of the biological traits seen in eukaryotes, and many exhibit unique adaptations. These include a range of nutritional modes through specialized feeding structures (phagotrophy, osmotrophy, myzocytosis) or chloroplasts (phototrophy), often mixing both as mixotrophy. Cellular respiration also varies due to modifications of their mitochondria. Almost all protists have a complex cytoskeleton composed of relatively conserved structures across evolution, namely a flagellar apparatus with basal bodies from which microtubules emerge and support the remaining cellular structures. Many protists have unique organelles that serve other functions, such as contractile vacuoles for homeostasis, or eyespots for light perception. Protist cells tend to host symbionts such as bacteria and archaea, usually to support their metabolism and nutrition. Although traditionally presumed to be asexual, protists are capable of sexual reproduction, and can exhibit diverse and complex life cycles with different generations and life stages. Protists are abundantly present in all ecosystems, including extreme habitats, as important components of the biogeochemical cycles and trophic webs. As producers, they are responsible for a large portion of global primary production and carbon fixation. As consumers and decomposers, they regulate fungal and bacterial populations, and release nutrients to other trophic levels. Some form mutualistic relationships with other protists or animals such as corals and termites. Others are important parasites. Pathogenic protists cause many well-known human and animal diseases such as malaria and toxoplasmosis, or significant plant diseases like clubroot and potato blight. Free-living protists can also negatively impact aquatic life as harmful algal blooms. The early evolution of protists corresponds with the evolution of eukaryotes, which split from archaea around 3 billion years ago and eventually gave rise to a common ancestor (LECA) with essential traits such as mitochondria and a complex endomembrane system, some time during the Paleo- or Mesoproterozoic eras. In the gap between these two events, fossils are often interpreted as stem-group eukaryotes, with intermediate traits. Following the appearance of LECA, its descendants (crown-group eukaryotes) experienced a rapid diversification in the span of 300 million years that originated the modern supergroups. Still, their abundance in the fossil record remained low until the Neoproterozoic, when the first fossils of opisthokonts, amoebae, and multicellular algae appear. Throughout the Phanerozoic, protists evolved into the forms that dominate ecosystems today, leaving an extensive fossil record of primarily siliceous and calcareous shells.
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