The Case of
The Alien World
|reveals how many of our assumptions built into our models of the natural world break down when we move into the alien world of undersea life. In this world, we see the complications in the apparently simple acts of naming organisms or of distinguishing plants from animals.|
And out of the ground the LORD formed every beast of the field, and every fowl of the air; and brought them unto Adam to see what he would call them: and whatsoever Adam called every living creature, that was the name thereof.
One of the most amazing natural phenomena is the tropical coral reef, sitting not far below the waves, burgeoning with life and activity. Whether you've had the gift of swimming above or through such reefs or only shared the spectacle through the camera of Jacques Cousteau or his colleagues, it resonates with beautiful and lively vitality.
This chapter is a set of stories about human models of parts of the
natural world in general and of life on the coral reef in particular.
The reason to focus on the coral reef is that it is at the same time
both a natural and an alien world, where the processes of evolution
have shaped the stuff of life into unfamiliar and surprising forms.
Because of this mismatch of reality to expectation, the coral reef
provides striking examples of the assumptions made by
reference in both everyday and scientific models.
How To Point at Organisms
The first part of reference is pointing. Being able to identify the elements of the system to which a model refers is the starting point for thinking with any model. The simplest case of such reference is simply picking the object out of the environment. But that is not as simple as it might seem.
What constitutes an animal or plant on the coral reef? How do we draw bounds around an organism and say "see that!" in order to introduce our next statements? How do we separate organisms that are "living together" from one another? This must be the starting point of description of the natural world, because our description of kinds of organisms, of ecologies of interaction, of growth and change, all depend on being able to identify and separate the organisms of the natural world.
In the terrestrial environment where we evolved and matured, we use several cues to identify living organisms. Two of these cues are that organisms have an identifiable and persistent form and do not survive division well. I'll briefly introduce both of these and then look at some of the problems which these apparently simple criteria introduce in the not-too-distant realm of the reef.
Organisms have forms --- including shape, texture, and color --- which persist over time. This is also true of objects in general: couches, flowers, and dogs each have a relatively fixed form which helps us identify them as both they and we move about in our shared world. Part of our ability to understand and predict events and behaviours comes from our division of the world into such reliable pieces which generally (though not always) have reliable properties.
One such reliable property is that the form of an organism tends to fill some (possibly moving) region of space. This allows us to track organisms in motion, either of their own volition or moved by some outside force. Motion reinforces our ability to tell the beetle from the leaf or the snail from the coral. In cases where motion is not visible, as in a vine-wrapped tree, we must rely on form or appearance itself (vine-bark looks different from oak-bark and sponge-surface looks different from coral-surface) to help us tease apart organisms which sit and sway together.
The problem with form as a criterion of identity is that sometimes, by growth or aging or other processes, the form of an organism changes. However, this change is usually so gradual or uniform (for instance, just growing in size) that we can easily adapt to it and keep track of the organisms despite these gradual changes.
Organisms do not survive division well. If we split a stone down the middle, both pieces are still stones. If we split a creature (like the oft-suffering laboratory mouse) down the middle, we do not get two mice, but rather a bi-sected corpse. Organisms require a certain "wholeness" to function and violating the wholeness may kill the organism. Organisms, of course, can survive some loss of components (such as the snake shedding its skin or the tree its leaves), but this tends to leave the organism intact and its former components lifeless. The nineteenth-century biologist Cuvier was particularly struck by this holistic interdependence of parts on one another. He argued against theories of evolution because he could not imagine that changes in the nature of species could keep this remarkable holism intact.
The importance of form and the sensitivity to division are connected. Organisms have forms because they need to satisfy certain functions. Breaking the form --- splitting the organism --- will normally destroy the organism by disabling or separating the functions. Two disconnected halves of a human digestive system, for instance, are decidedly not a digestive system.
The form of organisms changes. This is true even on land, where organisms grow and age, changing size (nearly always), color (sometimes), or even form (more rarely). Perhaps the most familiar land-based example is the catepillar, whose formation of a chrysalis is followed by its emergence as a winged butterfly of radically different form.
This sort of transformation is the exception among everyday terrestrial life: generally, children look like little people, puppies look like little dogs, and so forth. However, in the marine environment, it is the exception: fish, molluscs, and jellyfish (to just start a list) often look very different in different stages of their life and look particularly different in larval forms. In many creatures, the colors or distribution of colors changes as the fish matures through various stages. More striking is the change in the hydra: (placeholder images from BIODIDAC)
where the form goes from a sessile and plant-like hydra to a free-swimming jellyfish and back again.
"Pointing at" an organism throughout its life span may be complicated by these changes, but pointing at an organism at any particular moment is relatively unaffected. Change in the organism over longer spans of time do not generally confuse our everyday thought and reasoning because organisms in different stages of life often differ in properties besides appearance. Many of the things we learn about an organism of a particular appearance are likely to change with the radical changes in appearance. For example, catepillars cannot fly, but butterflies can and the change in function follows the change in form.
In our discussion of reference, we described the importance of systematicity to reference. In order to say things about the system we are describing, we must be able to connect to it reliably. The marine world, because we did not evolve or mature in it, provides examples where our typical ways of guaranteeing systematicity break down. If we only identified organisms by their distinct and separable forms, we would have a hard time in an environment (like the coral reef) where many creatures effectively hide themselves among others.
But one of the advantages of using multiple criteria for identifying organisms is that when one criterion fails, others may succeed. So when persistence of form fails, other criteria may succeed.
Bisecting organisms is a risky business. Though we can take a cutting from a plant and produce two plants, splitting a plant in two may be less likely to produce two robust plants than we would like it to be (though it might be safer than leaving it/them with me for a week). And the plant will not be functioning as a plant during its regeneration. In the animal world, successful bisections are even less likely, especially among the animals we interact with everyday.
On the reef, things are a little different. Many more animals can survive the sort of abuse which plants can normally take. For some marine animals, this is a natural part of their life cycle. Starfish or octopi can lose an arm and then grow a new one. Octopi, in fact, regularly sacrifice single arms to escape a predator, so that a genuinely eight-armed octopus is a rarity on some coral reefs. Taking this to a somewhat cruel extreme, one can (under carefully controlled circumstances and at some distress to the creature, so don't try this yourselves) split a starfish into several pieces and have each piece regenerate into a complete starfish.
This goes further in the case of the natural sponges, which are among the simplest and most robust of animals. One can sometimes divide a sponge in two or more pieces and have several complete, functioning, organisms. In time, each will grow into something that looks less artificially divided than it started out. And unlike plant cuttings or wounded starfish, the the bisected sponge will continue feeding and living remarkably undiminished by its bisection.
The explanation for this robustness is that sponges are filter feeders whose individual cells do the feeding. Sponges do not have multi-cell organs like more complex animals, but implement the functions of feeding directly by single individual cells along the wall of the sponge: (placeholder images from UC Museum of Paleontology)
When a sponge is split into two (or more) parts, most of the feeding cells are still intact and complete, so that the sponge can continue to function. This reflects the connection between form and function we mentioned above. The function of the sponge depends on the form of the individual sponge cells and not on the form of the organism as a whole. Indeed, on the reef, it is not uncommon to see intact and living sponges which have been half-chewed by sea turtles who enjoy nothing more than a good munch on a sponge.
Even more striking is the ability of sponges to be broken apart into individual cells and reform. Simpler sponges can even be pushed through a sieve and broken into a fluid of cells which will reorganize itself into an intact sponge. If "cell fluids" of different species are mixed, they will sort themselves out into distinct sponges of each original species based on complex molecules adorning the surface of each individual cell.
The point of these examples is that the way we determine what counts as an organism includes many assumptions. In particular, assumptions equating an organism's wholeness and its functioning are part of what makes criteria like wholeness useful for picking out organisms. But for certain sorts of organisms, these assumptions --- which have to do with how most animal organisms work --- clearly break down.
Cases like the octopus or the sponge are counterexamples to our usual assumptions about identity. Imagine how complicated our everday life would be if human beings were capable of the same dissolution and reformation as sponges. These science-fiction scenarios --- radical changes of reference and identity --- are common place in parts of the natural world. And hidden in the way our models refer to the everyday world are exactly these assumptions about the world around us. These assumptions are only manifest when we carry them to a "new world" where the assumptions no longer hold.
The fact that organisms have a particular form and that this form
moves together is belied by the creatures from which the coral reef
gets its name. Each of the forms pictured below is not actually a
single organism, but is actually a colony of simpler organisms:
(placeholder images from BIODIDAC)
The colonies themselves have characteristic shapes (plates, wires,
branches, fans, etc) but the simpler organisms, called coral polyps,
also have a quite complex and distinctive form of their own. Each is
a sort of miniature jellyfish with its own complex structure as we can
see in this comparison of the polyp and the hydra's polyp form:
(placeholder images from BIODIDAC)
Each coral colony consists of hundreds or thousands of these
jellyfish-like animals bound together. Clusters of polyps share
common shells: (placeholder image from BIODIDAC)
and together, these shells form the intricate and sometimes flexible shapes which dot the reef.
For example, in this fan coral (placeholder image from BIODIDAC)
each small dark spot along the branches of the coral houses a tiny coral polyp, which live together as a single colony in the flexible fan-like shell they have constructed.
Why do we call the sponge an organism but call the coral a colony? The sponges consist of many different chambers and tubes which filter food from the water pumped through it by both its own actions and the natural motion of the water. The coral colonies consist of individual polyps which do the same. But there are three important differences.
First, the sponge consists of different components --- skeleton (which is what might become the sponge used or abused for human cleaning chores), digestive apparati (the filtering chambers and tubes), and a "foot" or "holdfast" connecting the sponge to its foundation. Though sponges do not have organs, they do have different kinds of cells. The coral colony, on the other hand, consists of many essentially identical polyps without the sort of specialization of structure we would see in a single organism.
Second, the individual coral polyps are much more complex than either the individual feeding cell or (up one level) a chamber or tube within the sponge. In the polyp, we see the same diversification of functions --- feeding, motion, waste handling --- which we note only throughout the entire sponge. This argument --- the level of functional diversity --- is another criteria of organism.
Finally, the polyp can survive on its own. Some coral have a free floating larval stage which precedes their settling into colonial existence. An individual cell of a sponge cannot survive for long in this condition, since it relies on other kinds of cells for its own survival.
This is why biology identifies both polyps and sponges as organisms, even though the most striking visual similarity is between sponges and colonies of polyps. But the level at which references are made depends on purpose. If we are navigating above a coral reef or thinking about the "synthetic landscape" created by sponges and corals for other creatures (like fish or crabs), it makes sense to think about the coral colony as the organism, whose spreading "leaves" or intricate skeletons create homes for larger organisms. But if we are trying to understand the physiology by which creatures feed and digest, identifying the individual polyp as an organism makes the most sense.
After we've picked the organisms out of our environment, the next distinction we are likely to make is between animals and plants. Even if we do not have a sophisticated system of organizing animals into species and genera and clades and taxa (some terms which describe our modern system of organizing animals), it is still useful (but not always straightfoward) to divide to divide the plants and animals from one another.
Aristotle, whose detailed accounts of marine biology remain striking in their precision and clarity, identified sponges as animals because they were responsive. If one grasps a sponge attached to a rock or coral head, it contracts to hold tighter to its foundation. This responsiveness to sensory stimulus was Aristotle's criterion for distinguishing plants from animals. It is much like our common sense understanding of animals and plants, where we expect animals to move by themselves while plants stay "rooted" in one place.
However, if we change our time scale, we can see that what we call plants demonstrate a peculiar form of motion called phototropism. If we set a lamp to one side of a plant, the plant will move --- ever so slowly --- to a better position for absorbing the light which the lamp emits. If we photograph the plant every few minutes, it actually seems to seek out the source of light we have introduced.
Phototropism is an interesting example, because it only exists on time scales longer than normal human perception. Common sense informs us that plants do not move of their own accord and this is certainly true from "the evidence of our senses." We don't normally see trees turning to face the sun or bushes oozing across the meadow. But had Aristotle had access to the time-lapse photography which reveals phototropism on human time scales, he would have had to reconsider his criteria. But the observation need not have been entirely technology-bound: any competent gardener would have known that changes in sun and shade will yield a change in orientation and position for the plants in his care.
If Aristotle had known of phototropism, he would have had to revise his distinction between plants and animals, but it is quite close to the everyday common sense distinction which people make naturally. Looking at a coral reef, it looks like we are seeing a diverse field of plants, while in fact, we are seeing a free-for-all of slowly competing animals. However, most of the animals of the reef are sessile, moving only in response to outside intervention, and so our common sense classifies them as plants, just as Aristotle would have.
The modern distinction between plants and animals is primarily ecological. Animals consume other organisms, while plants consume sunlight and raw materials from the environment. This is why modern science classifies filter feeders like sponges and coral as animals. Carnivorous plants, which trap flies and other small insects, do not directly consume the creatures they catch. Instead, they consume the materials produced by the natural decay of their victims, which is driven by the actions of other animals harbored by the plant. Of course, such plants may specifically attract and kill insects and harbor the organisms responsible for the prey's decay, making them indirect beneficiaries of the deaths. (A court might call them un-indicted co-conspirators).
The distinction between plants and animals has played an important role in the evolution of biological thought. For most of history, biologists have imagined that the natural world reflected an ordering called the "chain of being" where creatures were naturally sorted by their complexity with people being near the top (possibly beneath angels and God) and natural materials (like stone and water) being at the bottom (possibly above basic elements like earth, air, fire, and water). In this hierarchy, plants and animals were strictly ordered. Common sense categories, such as dogs or oak trees, were also ordered, so that the primacy of dogs over oak trees was consistent with the primacy of animals over plants.
The chain of being was a model of natural order which reflected an organization of species by complexity. It was a useful way of describing nature because it related organisms to one another and also allowed naturalists to "make sense" of the diversity of species. Creatures could be understood, in part, by understanding their relationship to other creatures.
However, the chain of being eventually broke down as a model during the 1700s and 1800s as more and more species were discovered during Europe's scientific explorations, missionary endeavours and colonial expansions. Fitting all of these new creatures into the chain of being eventually became too complicated.
A notable case for coral reef life came during the 1700's. In 1723, the naturalist Peysonnel first noted that coral polyps moved their individual tentacles and argued that they were animals (in particular, coral "insects") rather than plants. However, Aristotle's distinction between animals and plants was already losing strength, and in the 1730s, Anton van Leeuwonhoek argued in the other direction, classifying the closely-related hydra as a plant despite the movement of its arms. The question was finally resolved by Abraham Trembley in the 1740s in a series of observations which convinced him that the hydra. Confused by the fact that hydras regenerated (like plants) but could move about an aquarium (like animals), Trembley finally decided that they were animals when he observed the hydra intentionally using its arms to grab prey and move them to its mouth. Since close examination of coral polyps showed structure like the hydra --- many times smaller --- scientists to believe that the individual polyps, like the hydra, were animals feeding on creatures too small to see. In fact, a patient observer can see the individual coral creatures feeding, especially at night.
Together with other causes, the recategorization of creatures from plants to animals broke down the chain of being which had organized human understanding of the natural world. This shift in the model of the natural world is part of what led to the understanding of species which eventually led to models of evolution in the middle of the nineteenth century. When the need to place species in a strict hierarchy went away, descriptions could attend to other kinds of relationships between species. The evolution of evolution, which starts from these rearrangements, is a fascinating story, but not one to which we will turn here. Instead, we will advance to the 1950's, when biologists studying coral reefs encountered and resolved a quiet mystery.