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A Poorly Illustrated Guide to the Tree of Life

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Hello everybody. My name is James, and I’ve been working on a little something. I want to explore some fundamental questions about life, and I want you, Dear Reader, to accompany me on this journey. To facilitate this desire, I’ve written some words and drew some pictures, but I didn’t do either particularly well. In fact, my creation has caused me no small quantity of shame. So much shame…

So, without further ado, I present the first part of what I can only hope will be a short series of pieces answering the most important questions ever. Dear Reader, kindly ask yourself:


Part 1: What the hell is an animal?


I made all of these with a broken laptop touchpad. This snail took me 7 hours.


Animals: what are they? This seems like an easy question: After all, we see animals all the time at zoos, in backyards, on the street, in adult movies, and every time we look in the mirror. Animals are all around us; scheming, searching, squirming, and doing things so degenerate I hardly dare speak of them.

Ask a child (or an adult religious fundamentalist) what an animal is, and they’ll probably tell you an animal is a creature that moves, breathes, thinks, eats, reproduces, and reacts to stimuli (or some variation of those traits). This sounds like a pretty good non-technical definition; certainly almost every animal encountered by most people on a regular basis fits these criteria. Indeed, the Latin root of the word animal, animalis, means the quality of having breath (it’s also the root of the words animate and, for you Jungians, anima). From a scientific perspective, however, this definition is totally and unequivocally wrong. We need something a bit more robust before a proper treatment of the animal kingdom can begin. Time to unleash the Semantic Cracken.


"No wait, I meant 'Whom!'


Much like the words human, fish, bird, plant, and fungus, the word animal is an attempt to define a category. A good definition of a category is precise and inclusive, defining every quality of the category while excluding every quality outside of it. For example, take the following definition of the category people who are your mother: any unattractive person willing to anonymously dispense sexual favors for a reasonable price.

This is a poor definition of the category your mother, as there are many such individuals in the world who fit those criteria, such as your sister and grandmother. And perhaps your mother charges a hefty premium during the holidays to earn a little extra money to buy you Christmas presents. This definition is neither precise nor inclusive. A better definition would be “the female person who gave birth to you.” This too has potential complications, as it clashes with the colloquial definitions of the word when considering adopted children and surrogate mothers, but it gets the job done (much like your aforementioned parent). It is both precise and exclusive: Anybody who did not give birth to you is not your mother, and only your mother gave birth to you.


I got a couple of these every month; I made a scrapbook.


We can extrapolate these general criteria to any categorical definition, with some special provisions for categories that describe groups of organisms, like the word animal. While we can create an endless number of arbitrary categories for living creatures (e.g. creatures that swim, creatures that are fuzzy, creatures that are delicious), the evolutionary history of all life enables us to group organisms based on shared ancestry. These special groups, known as clades or monophyletic taxa, contain a common ancestor of a group of organisms and all of that ancestor’s descendents (there are several other flavors of evolutionary groupings, such as grades, paraphyletic and polyphyletic taxa, but we’ll ignore those for now). If we want our idea of an animal to have any sort of scientific heft, it should define a monophyletic clade. As it happens, there is good evidence supporting this notion.


The tree on the left is monophyletic, while the tree on the right is delinquent on it's student loans.

Our proper definition of an animal must now satisfy three conditions: It must (1) describe all animals, (2) describe only animals, and (3) consist of a monophyletic grouping of creatures united by common ancestry. Condition (3) is its own field of study and a bit beyond the scope of this exercise, but I’ll be addressing it sometime in the future. For now, let’s address (1) and (2): what are the necessary and sufficient conditions for an organism to qualify as an animal? In other words, what defines all animals, and just animals? Let’s return to our erroneous colloquial definition of a creature that moves, breathes, thinks, eats, reproduces, and reacts to stimuli. All 6 of these characteristics fail to adequately satisfy conditions (1) and (2) as they are not precise and/or inclusive. To illustrate, let’s dissect each one.



1. Movement: At first, this character seems fairly solid: Animals are wily critters with important shit to take care of, and it seems to draw a clear distinction from plants and fungi. Movement, however, is a fairly subjective and vague quality. Does growth count as movement? It entails a change in position, however slight. What about tropism, the direct growth of a plant or fungus towards some kind of stimulus or gradient? Non-animal organisms are capable of orienting themselves in relation to a variety of environmental factors such as light, chemicals, or the pull of gravity. Even if we don’t classify tropism as true movement, there are plenty of animals with mobility comparable to that of a plant. Corals, sponges, brachiopods (lampshells), many mollusks, some echinoderms (the group containing sea-stars and its relatives), a few insects, and a variety of internal parasites simply stand (or encrust) idly as the world passes them by. Motility fails both conditions, as (1) not all animals are capable of movement, and (2) movement is found in many non-animals.


They're lungs... or really gross angel wings.

2. Breathing (respiration): Depending on what is meant by breathing, this character is either bad or awful at defining animals. If you mean the process of using lungs to pump air into and out of the body, it’s worth noting that the vast majority of animals (fish, insects, marine mollusks, and practically every other invertebrate) do not have lungs in the proper sense, thus failing condition (1) (gills, swim bladders, and tracheal tubes don’t count). Breathing is, at its most basic level, the act of gas exchange. Specifically, it is the process of utilizing Oxygen to facilitate metabolism while removing Carbon Dioxide from the body. But even this definition sucks and violates condition (2), as all know forms of life on this planet use some form of respiration (some bacteria swap out Oxygen for other electron-accepting atoms, but I digress).


This is what thinking looks like, I promise.

3. Thinking: For a quick and entertaining treatment of this idea, simply look at any discussion about the ethics of eating meat to get a taste (wink wink) of the popular sentiment. Although some non-experts may debate where exactly to draw the line between thinking and non-thinking animals (if eating a chicken is unethical, than killing a cuttlefish is tantamount to cold-blooded murder), scientific opinion squarely puts animals like sponges, cnidarians (jellyfish, coral, and sea anemones), and other creatures lacking much of a nervous system in the non-thinking camp, thus violating condition (1).


Yes, the bird goes into the cat. It's science.

4. Eating: Surely eating is a worthy qualifier for defining an animal. If watching the world’s fattest married couple devour 2000 kilograms of hotdog on the 4th of July doesn’t strike you as the most animalistic activity imaginably, what hope do we have of finding a predicate worthy of the kingdom Animalia? Well, I’m not here to pass judgment on how your parents spend their holidays, but I will tell you that eating is about as good a character as thinking or breathing (i.e. bad). Again, depending on our choice of definition, eating is either a poor or terrible feature. If you consider a stomach necessary equipment for eating, then we need only look towards the innumerable examples of digestively-impaired animals, including sponges, several kinds of parasitic worm (e.g. tapeworms and flukes), and cnidarians (provided your idea of a stomach isn’t a gastrovascular cavity) to disqualify eating on condition (1). If eating means heterotrophy, deriving substance consumption of other living organisms (as opposed to autotrophy, deriving chemical energy via photo- or chemosynthesis), then it fails on condition (2), as many bacteria, all fungi, and even some plants must obtain organic Carbon from other living creatures.


I made 8 different drawings symbolizing reproduction. This was the only one without a penis.

5. Reproduction: I’ll cut your mother some slack and simply say that there is reproduction going on all around us and in every kind of organism, invalidating this feature on the grounds of condition (2)…



Your mom’s a whore.

"Bad Kitty! Ethnic slurs aren't allowed at the dinner table."

6. Reaction to stimuli: See the dissection of Character 1 and replace the words “movement” with “reaction to stimuli”; “plants or fungi” with “mom and dad”; and “towards some kind of stimulus or gradient” with “towards some kind of snack cake or bacon fat.”


So, what the hell is an animal? I assure you, there are several traits that do actually meet conditions (1) and (2). What are they? You could look in a zoology textbook, which is boring and difficult and risk revealing your crippling illiteracy, or you can just wait and I’ll tell you; not with stupid words, but with a series of bitchin’ pictures. Huzzah!


James is a graduate of the University of Missouri, Columbia. He is a research biologist specializing in the molecular evolution of invertebrates. Feel free to visit his subpar research blog.


2 comments on “A Poorly Illustrated Guide to the Tree of Life

  1. Pingback: A Poorly Illustrated Guide to the Tree of Life: Part 2 | The Heathen Kings

  2. Pingback: A Poorly Illustrated Guide to the Tree of Life: Part 3 « The Official MU SASHA Blog, Updated Daily

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This entry was posted on November 15, 2011 by in Author: James Pflug, Uncategorized.
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