For well over a century, biologists had a very simple way of organizing the eukaryotes (i.e., the organisms that were not bacteria, whose cells contained a nucleus) (1). Basically, the one-celled organisms were all lumped into one biological class, the protozoans (also called protists). With the exception of animals and plants, and some of the fungi (e.g., mushrooms), life on earth is unicellular. The idea of lumping every type of unicellular organism into one class, having shared properties, shared ancestry, and shared descendants, made no sense. What's more, the leading taxonomists of the nineteenth century, such as Ernst Haeckel (1834 - 1919), understood the class Protozoa was at best, a temporary grab-bag holding unrelated organisms that would eventually be split into their own classes. Well, a century passed, and complacent taxonomists preserved the Protozoan class. In the 1950s, Robert Whittaker elevated Class Protozoa as a kingdom in his broad new "Five Kingdom" classification of living organisms (2). This classification (more accurately, misclassification) persisted through the last five decades of the twentieth century.
Modern classifications, based on genetics, metabolic pathways, shared morphologic features, and evolutionary lineage, have dispensed with Class Protozoa, assigning each individual class of eukaryotes to its own hierarchical position. A simple schema demonstrates the modern classification of eukaryotes (3). Many modern taxonomists are busy improving this fluid list (vida infra), but, most significantly, Class Protozoa is nowhere to be found.
Eukaryota (organisms that have nucleated cells) Bikonta (2-flagella) Excavata Metamonada Discoba Euglenozoa Percolozoa Archaeplastida, from which Kingdom Plantae derives Chromalveolata Alveolata Apicomplexa Ciliophora Heterokontophyta Unikonta Amoebozoa Opisthokonta Choanozoa Animalia FungiWhy is it important to expunge Class Protozoa from modern classifications of living organisms? Every class of living organism contains members that are pathogenic to other classes of organisms. To the point, most classes of organisms contain members that are pathogenic to humans, or to the organisms that humans depend on for their existence (e.g., other animals, food plants, beneficial organisms). There are way too many species of pathogens for us to develop specific drugs and techniques to control the growth of each disease-causing organism. Our only hope is to develop general treatments for classes of organisms, that share the same properties; hence the same weaknesses. For example, in theory, it's much easier to develop drugs that work on Apicomplexans that it is to develop separate drugs that work on each pathogenic species of Apicomplexan (3).
By lumping every single-celled organisms into one blended class, we have missed the opportunity to develop true class-based remedies for the most elusive disease-causing organisms on our planet. The past two decades have seen enormous progress in reclassifying the former protozoans. Unfortunately, the errors of the past are repeated in textbooks and dictionaries.
Here are three definitions of protozoa that I found on the web. Notice that these definitions don't even agree with one another. Notice that the first definition includes single celled organisms that may be free-living or parasitic. The second definition indicates that protozoans are obligate intracellular organisms. The third definition indicates that some protozoans are pathogenic in animals but omits mention of pathogenicity for other types of organisms. None of the definitions tell us that modern taxonomists have abandoned "protozoa" as a bona fide class of organisms.
Protozoan: Any of a large group of one-celled organisms (called protists) that live in water or as parasites. Many protozoans move about by means of appendages known as cilia or flagella. Protozoans include the amoebas, flagellates, foraminiferans, and ciliates.
Protozoa: A parasitic single-celled organism that can divide only within a host organism. For example, malaria is caused by the protozoa Plasmodium.
Protozoan: any of a phylum or subkingdom (Protozoa) of chiefly motile and heterotrophic unicellular protists (as amoebas, trypanosomes, sporozoans, and paramecia) that are represented in almost every kind of habitat and include some pathogenic parasites of humans and domestic animals.
 Scamardella JM. Not plants or animals: a brief history of the origin of Kingdoms Protozoa, Protista and Protoctista. Internatl Microbiol 2:207-216, 1999.
 Hagen JB. Five kingdoms, more or less: Robert Whittaker and the broad classification of organisms. BioScience 62:67-74, 2012.
 Berman JJ. Taxonomic Guide to Infectious Diseases: Understanding the Biologic Classes of Pathogenic Organisms. Academic Press, Waltham, 2012.
- Jules Berman (copyrighted material)
key words: data science, irreproducible results, complexity, classification, ontology, ontologies, protozoa, Apicomplexa, protists, protoctista,jules j berman