Protists form one coherent natural kingdom of simple eukaryotes.

The kingdom Protista solved a practical classification problem.

Many microscopic organisms have cells with nuclei, making them eukaryotes, but they are not obviously animals, plants, or fungi. Putting them together as protists avoided forcing them into one of those familiar groups.

The difficulty was that “protist” mostly described what an organism was not.

A natural evolutionary group should contain organisms that share a common ancestor and, ideally, all of that ancestor’s descendants. Protista did not meet that test.

Microscopy began revealing major differences in cell structure among organisms grouped as protists. DNA and RNA comparisons made the evolutionary problem much clearer.

Some traditional protists are close relatives of animals and fungi. Others are related to plants. Still others belong to entirely different deep branches of the eukaryotic tree.

Modern classification therefore divides eukaryotes among several major evolutionary groups instead of treating all protists as one kingdom.

The word protist remains useful in ecology and teaching as an informal description of many mostly microscopic eukaryotes. What has been abandoned is the claim that they together form one coherent branch of the tree of life.