The Unicellular Fallacy

by Richard L. Howey, Wyoming, USA


Imagine isolating an epithelial cell from the inside of your cheek and a neuron from a frog’s brain and then claiming: Here are two distinct and complete organisms. Anyone who knew the least bit of biology would emphatically reply: “NONSENSE!” However, now let’s reverse things and take a Paramecium and an Amoeba and I’m going to claim that these are not single-celled creatures, but two distinct complete organisms and I think the way we conceptualize and talk about these critters is a matter of considerable importance. They are not at all like cheek cells or neurons. Very early on, even in Leeuwenhoek’s time, such organisms were referred to an “animalcules” or “little animals”. A bit later on we’ll return to the issue of whether they’re animals or not. Another early designation for them was “Infusoria” which category did include rotifers and other assorted invertebrates. It was applied primarily to ciliates, but quite a number of other organisms got included at one time or another, some of which are clearly multicellular. When the Schwann Schleiden cell theory was proposed in 1838, it stated that all life forms are made up of one or more cells and this had enormous impact on the way biological thought developed subsequently. (As an aside, it is worth noting that, if my argument is convincing then the terms “unicellular” and “multicellular” are redundant and unnecessary.) It’s clear that the unicellular view dominated the later half of the 19th Century and a large portion of the 20th Century.

So, what’s wrong with saying that Paramecia and Amoebae are one-celled? They are small, self-contained packages surrounded by a membrane. However, the same can be said for Trichoplax, rotifers, gastrotrichs, etc. and some species of rotifers and gastrotrichs can be smaller than some Paramecia and Amoebae, so size is not a critical determinant. The heart of the problem lies in the fact that protozoa can carry out all of the basic functions for survival: reproduction, locomotion, defense, feeding, excretion, osmotic regulation, etc. Neither an isolated epithelial cheek cell nor a neuron can survive independently and function as a complete organism.

There is a tendency to think of protozoa (and other protists) as “primitive” and “simple” and furthermore we are inclined to regard them as insignificant in the world of the grand projects of the human being. Or that was the case until we discovered that protozoa were responsible for some things that drastically affect the human situation: malaria, giardiasis, amoebic dysentery, gingivitis, trichomoniasis, leishmaniasis, balantidiasis, toxoplasmosis, and several forms of trypanosomiasis or sleeping sickness. Even in the 21st Century, after all of our advances in medicine and the development of more powerful insecticides, malaria is still a scourge in many parts of the world and the estimates are that 300 to 500 million people are infected each year resulting in 1.5 to 2.7 million deaths annually. Those biting, barbed, baleful, barbarous, belligerent, beasts, the mosquitoes, are, of course, the transmitters of the protozoan Plasmodium which causes malaria.

How we conceptualize a particular organism may affect the way in which we develop methodologies to study it or try to develop strategies to combat it, if it is a pathogen or pest. Just think about the AIDS retrovirus. After nearly 25 years of research, only very recently has a strategy developed that revealed a portion of the protein casing that does not mutate and is therefore susceptible to attack by essentially closing down the activity of the virus. With malaria, it would seem that we would have an advantage in having 2 organisms to attack. However, Mother Nature is harsh and both Plasmodium and the mosquitoes also mutate–certainly at a slower rate than viruses, but nonetheless with devastating consequences.

The argument against the concept of unicellular organisms does have extensive implications, especially taxonomically with the development during the last half of the 20th Century of the Five Kingdom system, and more recently the Six Kingdom system. Shall we try for 7? Somehow the world seemed more nicely arranged when we just had the 2 kingdoms of Plants and Animals. However, this model did lead to some rather heated disputes between botanists and zoologists regarding which had the authentic territorial rights to claim organisms such as Volvox, Euglena, etc. The botanists made their case on the basis of the fact that these organisms possess chlorophyll-bearing structures called chloroplasts. The zoologists countered by pointing out that they also have flagella and are very active swimmers and who ever heard of “swimming plants”? Furthermore, they observed, there are some flagellates that appear very like the green ones, but don’t have any chloroplasts! At this point, there were 2 basic courses of action: 1) make things simpler by creating just one giant kingdom which we call “Bio-things” or 2) add some additional kingdoms and make some finer distinctions and that is, of course, exactly what happened.

The effect of this restructuring is that not only must we learn to stop talking about unicellular organisms, but we must also stop talking about protozoa and algae (and a whole bunch of other creatures) as animals or plants. Also, fungi are no longer plants, but have their own kingdom and bacteria which were placed under Monera in the 5 kingdom system have been split into 2 kingdoms in the 6 kingdom system–Eubacteria and Archaebacteria. And what happened to the protozoa and algae? Well, they have been moved to the kingdom of Protista (or Protoctista). There is a hot debate about the correct term for this kingdom. All of this reshuffling has generated a lot of heated discussion among some very prominent biologists. However, within this new system, one classical debate disappears; there is no longer an argument about whether Volvox is a plant or an animal–it is a protist. So, now things should be smooth sailing; we have things in order. Well, not quite. There are 2 very large difficulties with these new systems.

1) The biological concepts of plant, animal, and unicellular organisms have been around for almost 200 years and an enormous body of language has evolved which was derived from and reinforced those concepts. It is not at all easy to alter conceptual and terminological frameworks that have become deeply entrenched within the scientific culture and to a lesser, but no less critical degree, in the popular culture. It will likely take a generation or two for these changes to be embraced and, of course, by then there will be new changes.

2)The kingdom of Protists (Protoctista) by its very nature raises a foundational issue. This kingdom is a rather murky amalgam of many bizarre organisms that don’t seem to fit anywhere else, but which also don’t, in many cases, seem related to one another. This kingdom has numerous phyla containing some of the weirdest micro-beasties imaginable. The danger here is that as biologists investigate these organisms further, we may end up with the need for 20 or 30 or more new kingdoms, especially considering that it is extremely likely that there is a plethora of deep sea organisms that we have not yet even glimpsed. This increase in the number of kingdoms is not necessarily a bad thing (I wouldn’t mind having my own little kingdom–something like Monaco), but most scientists are driven by a passion to simplify. Perhaps the clearest example is modern physics. Over 100 elements (92 without the transuranic ones)–too many, so we look for smaller building blocks. It was much simpler in ancient Greece, 4 elements–earth, air, fire and water. So, protons, neutrons, electrons, but then come neutrinos, positrons, pi mesons, bosons, and soon there were around 100 subatomic particles–too many. So the search was on again–quarks–and so far there are only 4 or 5 quarks (or maybe a few more with anti-quarks) and then leptons, the photon, and a few bosons which don’t have quarks or anti-quarks. So, even at the quark level things are starting to get a bit messy and this may lead to yet new and simpler theoretical models.

There are two radical extremes; on the one hand, to search for one single ultimate principle that explains everything or to insist (as Aristotle did at times) that each particular “this here thing” was unique and thus a species. The problem with the first approach is that it very readily lapses into theology and is no longer science and the difficulty with the second approach is that it is simply impossible to have a science of the particular. We have to have classificatory systems with hierarchies of categories and biological diversity and complexity makes this a very challenging issue. We have everything from the phylum Placozoa with a single genus and a single species, namely, Trichoplax adhaerens to phyla that contain hundreds of thousands of species.

Nature utilizes every possible niche where a life form can “learn” to survive, often employing a series of bizarre adaptations. Some biologists have conjectured that there are probably more parasitic life forms than any other type and many of these creatures are utterly weird, have very strange life cycles, and live in unexpected places. Consider Sacculina which is a parasite of the blue crab and the green crab. The larval female enters the crab’s body at a joint and then migrates to the genitalia under the genital flap and begins to grow, forming an orangish sac. It feeds by producing a network of root-like structures that eventually extend through the body and even into the eye stalks. Remarkably, it doesn’t actually kill the crab, but it does neuter it so that it can no longer reproduce and it effectively turns the crab into a “zombie” allowing it to continue to feed and move so that the parasite can continue to survive. Here is a real-life instance of an Invasion of the Body Snatchers. This strange orange blob with an intricate network of feeding “rhizopodia” turns out, astonishingly, to be a barnacle. Biologists only discovered this after finding the larvae of this unnerving, but fascinating organism. Biologists are a busy, nosy breed, but just consider how many hundreds of thousands of species have never been examined for parasites. The point of this ramble is that there are almost certainly creatures out there which we have not yet discovered which will require the creation of new phyla and perhaps even a new kingdom or five. With each new organism, all kinds of new questions and puzzles arise which is why being a naturalist is never boring–there is no end of challenges and with each major taxonomic change comes a need for us to learn to think differently.

All comments to the author Richard Howey are welcomed.

Editor's note: Visit Richard Howey's new website at where he plans to share aspects of his wide interests.


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Published in the October 2008 edition of Micscape Magazine.

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