The Kingdom Protista is a convenient taxonomic group but it is not a true phylogenetic group. It consists of all the eukaryotes that aren’t plants, animals, or fungi. They share some general features in common, but aren’t more closely related to one another than they are to any other group.

Protists may be one-celled or multicellular, sexual or asexual, autotrophic or heterotrophic. Because the Protista is such an odd assemblage of often poorly-known creatures, the group is traditionally subdivided not by close relationships but by passing resemblances to better-known organisms. There are three main groups: plant-like protists, called algae; animal-like protists, or protozoa; and fungi-like protists, such as slime molds and water molds.

Protist Characteristics
Members of the Protista share the following characteristics:

Eukaryote Origins
The protists show up in the fossil record beginning about 2.0 billion years ago. They include the oldest known eukaryotes, and are distinguished from the more ancient bacteria and archaea by the possession of organelles and internal membranes. The most widely accepted explanation for their evolution is endosymbiosis.

The endosymbiotic theory proposes that mitochondria and chloroplasts originated as aerobic bacteria that entered other single-celled prokaryotes, either as prey or as internal parasites. Instead of being digested, the engulfed cells continued to function. Thus organelles were formed.

Evolution of Multicellularity
The protists exhibit a range of lifestyles that provide insight into the evolution of multicellularity, a condition that characterizes all of the other eukaryotic kingdoms.

Unicellular beings are compact and constrained. All of the functions and structures of such organisms must be accommodated within the small confines of one cell. This limits the complexity they can achieve. In contrast, multi-cellular organisms employ a division of labor. As the name suggests, multi-cellular individuals are composed of many cells, which are associated with one another and integrate their activities. This allows specialization – distinct types of cells can be differentiated within an individual, each having a different function.

The protists exhibit both the primitive single-celled condition and also the derived multi-cellular state. The group also contains members that exhibit the likely intermediate step, colonialism.

A protist colony is a collection of cells that associate together and can act with some degree of coordination. Each cell can survive on its own, but living as a colony provides some benefits and allows for specialization to develop.

The green algae called Volvox sp.is one example of a colonial protist. Individuals form a hollow ball of cells, mostly undifferentiated, with some specialized for reproduction. All the cells use their flagella in concert to propel the colony.