Protists are defined by what they are not, and most remain unnamed
Biologists define protists by exclusion: any organism with a nucleus that is not an animal, land plant or fungus. That leftover category turns out to hold most of the diversity of complex life. Environmental DNA surveys suggest the majority of protists have never even been formally described.
Because the definition is negative, protists do not form a natural family tree branch. They span the whole eukaryote tree, and animals, plants and fungi all evolved from within them. Most are single cells, including amoebae, ciliates and, most commonly, flagellates, but several lineages independently became multicellular, producing giant slime moulds and seaweeds with distinct tissues.
Older schemes either gave them their own kingdom or split them between plants, as algae, and animals, as protozoa. Electron microscopes and molecular studies wrecked that tidy picture, revealing that some protists are closer to animals or plants than to each other. Today they are scattered across supergroups. One, Archaeplastida, holds the red and green algae that gave rise to land plants. Opisthokonta unites fungi and animals with their single-celled kin. Others include shell-building foraminifers and radiolarians, diatoms and brown algae, and the early-branching excavates, flagellates that preserve traits of the last common ancestor of all eukaryotes.
Their biology is astonishingly varied. Some photosynthesise, some engulf prey, and many do both. They carry specialised structures such as contractile vacuoles to pump out water and eyespots to sense light, and often host bacterial or archaeal partners. Though long assumed to be asexual, many reproduce sexually with complex life cycles.
Ecologically they matter enormously, producing a large share of the planet's primary production and carbon fixation and keeping bacterial and fungal populations in check. Some partner with corals and termites; others cause malaria, toxoplasmosis and potato blight, or form harmful algal blooms. Eukaryotes split from archaea around 3 billion years ago, and after their common ancestor appeared, its descendants diversified into the modern supergroups within about 300 million years, leaving a rich fossil trail of silica and chalk shells.
Source: Protist
