Protoctists (the smallest called protists) are all living beings other than plants, animals, fungi and bacteria. Ubiquitous in damp, wet, watery places, these amazingly diverse beings reside everywhere from ocean abysses to ephemeral dew drops, from moist plant tissues to dark deeps of animal bodies. Some 250,000 different species of protoctists are estimated to exist today!
The earlier joint venture of sluggish fermenting bacteria with microbial speedster spirochetes may have been a prime mover in the development of protoctists. Partners for some time, this hardy combination forms permanent attachments to a new larger cell and gets it moving. Some slide inside the cell and eventually become little organs of motility for that cell.
Some mergers also take oxygen-respiring bacteria as partners. These consortia set the stage for two more great Kingdoms of life: Animals and Fungi. Not about to be left out, cyanobacteria join the fray and convert many protists to photosynthesis. This distinguished clan, the algae, expands, eventually giving rise to the great Kingdom of Plants.