Figure by P. Halasz, Wikimedia Commons
(Click on the link to listen to an audio version of this blog ….Kingdoms
Over the past two weeks I have written about the human drive to name and organize the living things around us. Giving an organism a “true” (i.e. “scientific”) name involves recognizing its genus and species uniqueness. Taking that organism and then relating it to similar organisms via taxonomic family, order, class and phylum categorizations is an attempt to reconstruct the evolutionary “tree” of that particular species.
The logic behind a phylum designation of an animal, fungus, protist or bacterium (or “division” designation in plants) is the recognition of some basic body plan that defines the entire group. The next organizational step after phylum, though, must consider features that are even more fundamental than the phyla body composition. Putting these phyla into the next hierarchical level, kingdom, is a process fraught with uncertainty and, often, very slippery logic.
The idea of biological kingdoms have been around for as long as humans have thought about living things. Aristotle (384 to 322 BCE) in his History of Animals and his colleague and fellow student of Plato, Theophrastus (371 to 287 BCE), in his Enquiry into Plants described the two kingdoms of life: animals and plants. They were so convincing in their definitions and descriptions that all subsequent observers of biology for the next two and quarter millennia similarly defined living things as either plants or animals. Common sense and logic and, maybe, habit dictated that these two types of living things made up all of life.
Bust of Aristotle. Public Domain
Animals, according to Aristotle, took in food, were able to generate self-motion and had a natural source of heat. They also possessed both a vegetative soul (which accomplished reproduction and growth) and a sensitive soul (which accomplished sensation and mobility). Plants, according to Aristotle, only possessed a vegetative soul. They could grow and reproduce but not feel or move.
Theophrastus defined plants a bit more enigmatically: a “plant is a thing various and manifold, and so it is difficult to describe in general terms.” I interpret this to mean that one knows a plant when one sees one in spite of all of its great variety of growth forms and shapes.
The organisms in Kingdom Animalia move and eat. The organisms in Kingdom Plantae do neither (but they all “look like” plants!).
Carl von Linne. Painting by A. Roslin, PUblic Domain
In 1735 Carolus Linnaeus (to use his Latinized name) retained the two biological kingdoms of Aristotle and Theophrastus and added one of his own invention that included non-living minerals. His three “kingdoms,” then were “Regnum Animale,” “Regnum Vegetabile,” and “Regnum Lapideum.”
Revolutions in science often come about because new ways to observe Nature are invented. Sometimes these revolutions occur very rapidly and sometimes they take a remarkably long time. Antonie van Leeuwenhoek began exploring the world of the very small in the late 1600’s and sent detailed drawings of his observations to the Royal Society of London starting in 1674. Many of these microscopic life forms were included with the animals and the plants, but it wasn’t until 1866 that Ernst Haeckel gathered these microscopic life forms in their own kingdom-level classification of life. Haeckel considered these tiny entities to be neither plant nor animal and placed them in his new “Kingdom Protista.”
Dropping the minerals from the kingdom classification, Haeckel then constructed a Three Kingdom system with Kingdom Animalia, Kingdom Plantae and Kingdom Protista. Kingdom Protista, though, was steadily eviscerated since the many of the “protozoa” were increasingly recognized for their animal-like qualities and were removed to a phylum (Phylum Protozoa) in Animalia, and many of the algae were increasingly recognized for their plant-like qualities and subsumed into various phyla (“divisions”) within Kingdom Plantae. Haeckel’s kingdom of Protista would have to wait until the late 20th Century to be fully resurrected!
Anton von Leeuwenhoek. Painting by J. Verkolje. Public Domain.
Bacteria were also discovered by Antonie van Leeuwenhoek in 1674 along with the tiny “animalcules” that briefly populated Haeckel’s Kingdom Protista. Almost 200 years, though, had to pass before the work of Louis Pasteur and Robert Koch put bacteria into their recognized place in biology. A phylum called “Monera” was established within the Kingdom Protista to make a taxonomic space for bacteria, but the heterogeneity of the Kingdom Protista defied all pretense at an evolutionary based classification system.
In the late 1930’s, the Phylum Monera was removed from Protista and elevated to Kingdom status. This left Kingdom Protista so empty that most taxonomists simply omitted it from their scheme of Kingdoms. This left a Three Kingdom system with Kingdom Animalia, Kingdom Plantae and Kingdom Monera. These three kingdoms stood (especially in the Unite States) for the next 40 years or so.
These kingdoms, though were an evolutionary mess. Each kingdom was packed with distantly related phyla, and the stated goal of constructing the evolutionary tree of life via these taxonomic designations was in shambles.
Whittaker’s Five Kingdoms. Figure by P. Brieux. Wikimedia Commons
In 1969, Robert Whittaker published a paper in which he described a new system of biological kingdoms that would reduce some of these evolutionary discrepancies. He proposed that Kingdom Protista be re-established and all eukaryotic (i.e. cells with nuclei), unicellular life forms be included. The protozoa would be “returned” from Animalia and the algae, from Plantae. Whittaker also proposed moving the fungi out of Plantae and erecting an entirely new kingdom, Kingdom Fungi.
Whittaker’s proposal, then, was a Five Kingdom system: Animalia, Plantae, Protista, Fungi, and Monera. Many influential textbooks used Whittaker’s Five Kingdom system, and it became increasingly, although not universally, accepted. In particular many older botanists and zoologists felt quite put upon by the loss of the algae and fungi from the Plantae and the protists from Animalia! I knew one botanist in particular, who, to his dying day argued forcefully and a bit irrationally for the validity and quality of the Three Kingdom system that he had been taught in high school and college. The Five Kingdom system, though, steadily gained broad acceptance.
In Fall 1985, I was in my third year as an assistant professor of biology at Penn State-New Kensington. One of my courses that fall was Biology 101 (“Zoology”), and I had just received the new edition of my text book for the course (Zoology by Hickman). I hadn’t noticed that Hickman finally changed his taxonomic orientation to the Five Kingdom system (probably under pressure from his younger co-authors!). The old animal phylum Protozoa was now the Kingdom Protista. This meant that the hierarchical taxonomy of the protozoa, which I had painstakingly learned over the past two years of teaching, was now completely changed! I saw this about an hour before I was going in to my zoology class to do my introductory lecture on the protozoa! Needless to say, the lecture was very “fresh” that day!
As more and more precise tools were developed to explore and characterize cellular structures and compositions, the uniqueness and diversity of living cells and also their patterns of similarity became more and more apparent. The bacteria (the “prokaryotes,” living cells without nuclei), for example, increasing were seen as living entities with a greater range of biological diversity than all of the eukaryotic organisms combined! Kingdom Monera, then, was split into a kingdom of “modern” bacteria (the “Eubacteria” later called the “Bacteria”) and a kingdom of “ancient” bacteria (the “Archaebacteria” later called the “Archaea”). This split generated a Six Kingdoms of life system that was widely used in major biology text books.
These Six Kingdoms, though, were still polyphyletic and not accurate representations of the evolution of the included life forms! Many researchers, looking at their specific organisms of specialization, proposed major reconstructions and splits of these Six Kingdoms. Eight Kingdoms, Ten Kingdoms, Twelve Kingdoms and more were proposed based on very valid, but increasingly esoteric features of the classified organisms!
Thomas Cavalier-Smith. Drawing by D. de Lucioni. Wikimedia Commons
A major voice in some of these new kingdom classifications was Thomas Cavalier-Smith, a Professor of Evolutionary Biology at Oxford. Over a period of 35 years (from the early 1980’s to 2015) Cavalier-Smith and his numerous colleagues used the detailed structures of the membranes inside of cells and the cilia on the surfaces of cells to propose a logical split in the Kingdom Protista. All of the protists that lacked four-layered membranes around their plastid organelles and did not have the very specialized tubulin structure of their cilia were put into a Kingdom called “Protozoa.” All of the protists that did have four-layered plastid membranes and the specialized cilia structure were put into a Kingdom called “Chromista.” Further, Cavalier-Smith noted that some protists lack mitochondria! Assuming that this feature is an evolutionarily primitive state, he erected another kingdom out of Protista called the “Archezoa.”
Cavalier-Smith, then, with some back and forth and relabeling and reconsiderations, had an Eight Kingdom system of life!
Carl Woese. Photo by D. Hammerman. Wikimedia Commons
In 1990, Carl Woese, who had proposed the splitting of the bacteria kingdom, Monera, into two kingdoms (the “modern” bacteria and the “ancient” bacteria), suggested that all of these kingdoms be organized into even higher taxonomic levels. His idea of “Domains” of life (also termed by various authors as “Dominions,” “Super-kingdoms,” “Realms”, or “Empires”) would organize “similar” kingdoms and separate “dissimilar” ones. Woese’s system consisted of three domains: Domain Bacteria (the modern bacteria), Domain Archaea (the ancient bacteria) and Domain Eukaryota (all of the eukaryotic kingdoms). Notably, this system recognized that the greatest diversity of living organisms is found in the prokaryotes (the bacteria)!
Interestingly, none of the biological kingdoms or domains include viruses or prions. These “life forms” are waiting for some brave taxonomist to place them in the array of life! Just think of the controversy and buzz that will accompany that!