Where Physiology Comes From

I have just described the apparently spontaneous production of large-scale patterns of fluid flow in a culture of swimming microorganisms. The process is not spontaneous, however: work is still being done on the system, just as it is in a room with baseboard heaters. It is a b C Figure 4.8 The progressive development of an

"anti-bubble" of dense microorganisms in a cul-0 ^ ^ ^ ç ture medium. a: A local grouping of cells, delimit % * ^ * g * ited by the dotted circle, may develop through

—-------------- -------------___________________________^ * $ random movements of the microorganisms. b:

(J^i j ' The incipient anti-bubble will begin to sink, be-

^ _ ^ —____________•[ cause of the slightly higher density of the group.

f JL 3 et ¿^ ^ £ c: As the anti-bubble sinks, it sets up a shear field

^^ ¿¡i —— ^ ^—-^.jy-i^--'----------------- * around it, which orients organisms outside the

just that the work is coming from a different source— namely, stored energy in the culture broth—and the energy is traveling through the system in a different way, through the metabolism of the microorganisms (Fig. 4.9). At first, the work is done by the microorganisms altering the distribution of oxygen in the culture, which is followed by work done on the fluid by the microorganisms themselves. All steps in the process are perfectly rational, explainable, and consistent with the Second Law. Yet order, not disorder, is the result.

To the microorganisms themselves, bioconvection has a more important consequence than the generation of pretty patterns of light and dark bands in a petri dish. The parcels of surface fluid that are dragged down with the plume carry with them the rich load of oxygen picked up while the parcel was at the surface. Their sinking transports oxygen deep into the culture at a rate much faster than diffusion: the sinking rate of a plume can be as fast as two or three millimeters per second, roughly ten times as fast as the microorganisms can swim. At this speed, oxygen and carbon dioxide are transported in a way similar to the transport of these gases by blood circulating through our bodies. Voila, we have a circulatory-respiratory system!

The phenomenon of bioconvection also raises some interesting questions about the evolutionary origins of physiology. As I have said earlier, we usually associate physiology with a high degree of coordination and control. In an organ like the intestine, for example, the muscle cells that drive material through the tube must contract in a highly coordinated manner, so much so that the intestine has embedded in it an accessory nervous system that controls the activity of the organ's muscles and multitudinous secretions. Yet, the Chlamydomonas culture is a collection of relatively independent and autonomous cells that, as far as we know, do not coordinate their activities with other inhabitants of the culture. Nevertheless, they become organized into structures many times larger than themselves: the typical diameter of a bioconvection cell is a few millimeters, the cells themselves about a thousand times smaller. Furthermore, these structures do physi-

gravitational potential energy metabolic energy metabolic work

Figure 4.9 The flows of energy in the development of large-scale order in cultures of swimming microorganisms. Metabolic energy, through the agency of powering locomotion, controls work done by the large-scale potential energy in gravity.

ological work at a large scale, transporting oxygen and carbon dioxide throughout the culture.

Could this kind of self-organization represent the evolutionary origin of physiology, the ephemeral superstructure upon which the more sophisticated physiology of multicellular organisms has been built? Certainly, it is not much of a stretch to see how such "superorganismal" physiology would benefit the microorganisms that are associated with it. But then how do we reconcile this diffuse physiological function with the conventional Darwinian model of adaptation described earlier? Certainly it is easy to posit that a particular gene governs the shape or distribution of mass within a Chlamydomonas or the efficiency and responsiveness of the proteins that make up its flagella. It is also easy to imagine that natural selection would favor those genes that give a Chlamydomonas just the right distribution of mass or flagellar responsiveness that promotes bioconvection. Genes could result in the "superorganismal physiology" phenotype, however, only if (a) they act in concert with numerous copies of the same gene scattered among different individuals and (b) if their action results in the

modification of the physical environment and the large-scale distributions of potential energy in it. The first condition is not hard to reconcile with neo-Dar-winism—we will come back to this in Chapter 11—but the second is rather more difficult, for it forces us to ask the uncomfortable question of what is adapting to what? Is the organism adapting to its environment, as a good Darwinian would insist it must, or is the environment being adapted to it? Certainly, in the genesis of bioconvection, such environmental properties as fluid density, viscosity and the partial pressure of oxygen are as much a part of the phenotype as are the Chlamydomonas themselves.

A philosopher, one Bishop Berkeley,

Remarked metaphysically, darkly,

That what we don't see

Cannot possibly be,

And the rest is altogether unlarkly.

—FRANCIS AND VERA MEYNELL (1938)

chapter five

Then a Miracle Occurs ...

In the last chapter, I argued that energy flowing through organisms, when it interacts with large-scale gradients of potential energy in the environment, can impose orderliness on the environment at a scale many times larger than the organisms themselves. I asked you to believe that the generation of this orderliness somehow relates to the external physiology I posited in Chapter 1.

Skeptics confronted with that argument probably will be reminded of the famous Sydney Harris cartoon showing a blackboard covered by complex equations, in the center of which is a straightforward note: "then a miracle occurs" (Fig. 5.1). The weak point of the argument I tried to develop in Chapter 4—the "miracle," if you will—was my assertion that bioconvection cells in protozoan cultures represent a primitive type of structure that does physiological work. The implication, of course, was that such orderly structures should arise any time there is an interaction of the metabolic and physical energy streams, and that the structures should always do physiological work. Ergo, structures built by animals are physiological organs. Right?

Well, no. As tantalizing as the phenomenon of bioconvection is, it admittedly is not a firm foundation upon which to build such an assertion. Bioconvection cells undoubtedly are orderly things, they undoubtedly do physiological work, and they undoubtedly arise from the interaction between two flows of energy, one metabolic and one physical. But are they structures? If they cannot justifiably be called that, or at least the precursors to the more substantial structures built by animals, then the whole argument is a

"I think you should be more explicit here in step two."

Figure 5.1 The problem of orderliness . .. solved! [From Harris (1977)]

dead end. What is needed is a way to bridge the gap between the tangible structures built by animals and the orderly, albeit ephemeral, "structure" of a bio-convection cell.

In this chapter I will try to bridge this gap by considering structures built by the most primitive animals, sponges and coelenterates (which include the jellyfish, corals, and hydras). Because it is crucial that we understand the difference between an animal's body (which, by convention, handles internally driven physiological work) and the structure an animal builds (which purportedly handles externally driven physiological work), we must start with a digression into just what animals are, and how they come to be. Sponges and coelenterates, particularly corals, provide an interesting venue for exploring this question, because the distinction between body and external structure among these animals is a fuzzy one. We will also have to delve into some fairly deep questions of animal form: how it is measured, how it develops, and how it is influenced by both the environment the animal lives in and by its evolutionary history. We will then be ready to bridge the gap between the quasi-structures represented by bioconvection cells and the more substantial structures built by animals. By then, I hope to reduce the "miracle" to a reasonable possibility, or at least to a mere improbability.

What Animals Are

At present, biologists more or less agree that the living world is properly divided into six large categories, designated kingdoms. Two of them are prokaryotes (literally "before the seed"), also known as bacteria, single-celled organisms that do not enclose their genetic material (invariably DNA) within a distinct nucleus. The prokaryotic kingdoms are:

• The Archaea, primitive bacteria that probably are the most closely related organisms to the first living cells. Archaea cannot live in the presence of oxygen, and they often inhabit extreme environments, like hot springs or strongly acidic or briny environments.

• The Eubacteria, derived from the Archaea, which inhabit a diversity of environments and carry out a diversity of life styles.

I will have much more to say about the prokaryotes in Chapter 6.

Derived from the prokaryotes are four kingdoms comprising the eukaryotes (literally "true seed"), organisms that confine their genetic material (again, DNA) within a membrane-bounded nucleus. The eukaryotic kingdoms are:

• The Protista, an extraordinarily diverse assemblage of single-celled organisms, divided roughly into the "animal-like protists," which do not synthesize their own food, and the "plant-like protists," which manufacture their own food photosynthetically. Usually, protists exist as solitary cells, but some species form colonial assemblages. These probably gave rise to the three remaining kingdoms.

• The Plantae, which include all the green plants. Plants are autotrophic ("self-feeding"), that is, they produce their own food through photosynthesis.

• The Fungi, which include the mushrooms and yeasts. Fungi are heterotrophic ("other feeding"), that is, they steal food from other organisms. The Fungi, of all things, are probably the most closely related to our own kingdom.

• The Animalia, which include all the animals. Animals are invariably heterotrophic, although some enter into symbioses with autotrophs.

All the so-called higher kingdoms (the Fungi, Plantae, and Animalia) evolved from the Protista. This involved a fundamental shift in the ways cells associate with one another. Specifically, there had to be a shift from the relatively loose association of the colonial protozoa to the complex multicellular organization that characterizes the higher kingdoms.

Body Plans of the Animals

Among the animals, zoologists recognize roughly thirty-two phyla1 (the actual number is inexact because the status of some of the phyla is in dispute).

1. The category of phylum is the most inclusive of the "official" system of Linnaean classification. Each category in the Linnaean system forms part of a nested hierarchy, where each category is divided into ever more exclusive categories. By agreement, these categories are, from most inclusive to most exclusive: phylum, class, order, family, genus, species. Biology students learn various mnemonic devices to keep the order of these categories straight. Unfortunately, every one I know is rude and vulgar, so I cannot share them with you here.

Phyla can be differentiated from one another by the body plans of the organisms they include. For example, most of the animal phyla, known as the triploblasts, have organs derived from three so-called germ layers: an endoderm ("inside skin"), lining the interior of the gut;an ectoderm ("outside skin"), forming the outside skin;and, sandwiched in between, a mesoderm ("middle skin"), which gives rise to most everything else. At least one of the animal phyla, the coelenterates, has only two layers, an ectoderm and an endoderm. These animals, called diploblasts, obviously differ fundamentally from the triploblasts in their body plans.

A body plan is a phenomenon of embryological development, and different body plans represent different sequences of growth from a single-celled fertilized egg, or zygote, to the adult. For example, among the triploblasts, two major body plans are recognized, each based upon a fundamental difference in the development of the digestive tract. The digestive tract of most adult animals has two openings to the outside: a mouth, which takes food in, and, at the opposite end, an anus, from which the remnants of digestion leave. During the early stages of embryonic life, though, the digestive tract starts out as a closed sac, or archenteron (literally "ancient gut"), which opens to the outside through a single opening, the blastopore. To become a proper tube, a second opening must form between the body wall and archenteron, opposite the blastopore. These two openings arise sequentially during development, and the order of appearance offers two possibilities for the digestive tract's further development. The two great lines of evolutionary descent among the animals are distinguished by which sequence was followed. In the primitive condition, known as protostomy (literally "first mouth"), the blastopore becomes the mouth and the second becomes the anus. Protostomy is characteristic of most of the so-called higher invertebrate phyla, like the annelid worms, molluscs, and the group that is arguably the pinnacle of animal evolution, the arthropods (crustaceans, spi ders and their allies, and the insects). The more recent condition, derived probably from a protostomous ancestor, turns this arrangement on its head, so to speak. The blastopore becomes the anus and the second opening becomes the mouth in the condition known as deuterostomy (literally "second mouth"). The deuterostomes include the chordates, the echinoderms, and one or two other minor invertebrate phyla.

Body plans arise from a sequence of developmental events controlled by a genetic developmental program. These programs ensure a body plan's integrity as it is passed to future generations, and it is through modification of these programs that body plans evolve. Let us briefly explore how.

First, a genetic developmental program modifies lines of descent of the many generations of cell division that are required to build a multicellular adult from a zygote. A line of descent, in this case, is similar in principle to the line of descent that gave rise to you: each cell in a line of descent has a parent cell, and if the cell itself reproduces, it will produce one or more daughter cells. A particular line of descent is traced sequentially through the parent and daughter cells of several generations, and all lines of descent form a sort of "family tree" of development. During embryonic growth, lines of descent are modified in three basic ways. These are: growth, or rates of cell division; differentiation, essentially the activation of particular genes so that cells in a particular line of descent specialize to do one or a few things well; and apoptosis, a programmed degeneration and death of particular cells and, by implication, whatever lines of descent that might have arisen from them.

Second, development operates by a sequence of contingent events. To become an adult, for example, each of us must successfully negotiate our way through childbirth, childhood, puberty, and adolescence. If our passage through any stage is unsuccessful or abnormal, the aberration will affect all subsequent stages of our lives. Suppose we designate, rather arbi trarily, the sequence of developmental events as follows:

Zygote ^ A ^ B ^ C ^ D ^ E ^ F ^ . . . ^ Adult [5.1]

The successful completion of each step is contingent upon the successful completion of all the previous steps. The development of the vertebrate spinal cord, for example, requires the embryo to fold in on itself in a particular way. If this folding process does not occur, or does not occur successfully, normal spinal cord development will not occur, nor will all the subsequent events that depend upon normal spinal cord development. The folding of the embryo that precedes spinal cord development in turn relies on a prior step involving migration of particular cells to particular locales in the embryo.

The evolution of the various animal phyla is largely a matter of how these developmental programs have been modified through lines of evolutionary descent. Generally, modifications arise from mutations either in the genes controlling the fates of particular lines of descent of cells within the embryo or in the genes that control the contingent events.2 For example, suppose a mutation arises that controls the transition C ^ D in the developmental sequence in equation 5.1. The new developmental program will now look like this:

Zygote ^ A ^ B ^ C ^ D' ^ E' ^ F' ^ ... ^ Adult' [5.2]

with the mutation at one step affecting the development of all subsequent steps. The mutation has led to a branching of the evolutionary line of descent: the lin

2. The genes that control major developmental events are known as homeotic genes. Mutations in homeotic genes can sometimes result in radically different body plans. Animal body plans were thought to arise from the appearance of these homeotic genes, and indeed, homeotic genes of distantly related organisms are very similar, indicating common origins. For example, fruit flies and humans share five of these homeotic genes, indicating that our common ancestor, something a lot like a rotifer or a bryozoan, had them too.

eage of the common ancestor, Adult, continues through those individuals that lack the mutation, while the descendants of individuals with the mutation form the new lineage Adult'. Generally, the earlier in embryogenesis the developmental mutation occurs, the more dramatic the change of body plan will be (Fig. 5.2). The radically different body plans of the protostomes and deuterostomes, for example, arise from differences that appear very early in embryonic development. Modifications that occur later in the program result in less dramatic differences. Differences between some breeds of dogs, for example, result from modifications in the growth rates of the bones of the skull, when bone growth ceases, and so forth, and these events occur fairly late in development.

Body Plans of Sponges

The concept of the body plan becomes a little shifty, though, when it is applied to the organisms that presumably are at the roots of the animal kingdom. The sponges, for example, members of the phylum

Figure 5.2 Major differences in body plan, like those between the nautilus and dog, imply a divergence early during the development of the adult from the zygote. Animals with more similar body plans, like the fox and the dog, develop along the same pathway until late in development. The divergence of body plan frequently reflects an organism's evolutionary history.

zygote adult

Figure 5.2 Major differences in body plan, like those between the nautilus and dog, imply a divergence early during the development of the adult from the zygote. Animals with more similar body plans, like the fox and the dog, develop along the same pathway until late in development. The divergence of body plan frequently reflects an organism's evolutionary history.

Porifera, are considered the most primitive of the animals, just one step up from colonial Protozoa, perhaps even super-colonial Protozoa. Some zoologists actually put them into a unique grouping called the Parazoa (literally, "beside the animals") rather than with the animals proper, the Metazoa ("higher animals"). It is only through mutual agreement and long-standing convention rather than good evidence that we currently lump the sponges in with the animals.

One of the problems in deciding just where the sponges' affinities lie is identifying just what a sponge's body plan is. Compared with the more tightly constrained development of the Metazoa, the rules of development for sponges are, shall we say, flexible. Development in a sponge starts conventionally enough, with a single-celled zygote giving rise to multiple types of cells (Fig. 5.3). Unlike in more complex animals, where differentiation results in dozens or hundreds of specialized cell types, in sponges cells differentiate into just three or four: flat pinacocytes that form a protective sheet on the outside;archaeocytes that migrate around the body of the sponge like amoebas, laying down tendrils of fibrous spongin or secreting mineralized bodies known as spicules; and choanocytes, cells that use flagella to drive water currents through the sponge. These cells organize themselves, according to certain rules of association, into a hollow tube pierced by numerous pores that connect the outside of the animal with a hollow interior cavity, the spongocoel. Water flows into the spongocoel through the many openings in the side, called ostia, and out through a single opening, the osculum, at the top. As the water flows through the sponge, suspended particles of food are captured by the choanocytes.

It is questionable whether this organization strictly qualifies as a body plan. On the one hand, certain families of sponges have characteristic body forms, implying that developmental programs do shape their development. The asconoid sponges, for example, are organized into simple columnar tubes. In contrast, the leuconoid sponges typically are spherical, with the spongocoel reduced and the ostia arranged in ramify-

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