The Lives of a Cell: Notes of a Biology Watcher

Book Preface

The Lives of a Cell
We are told that the trouble with Modern Man is that he has been trying to detach himself from nature. He sits in the topmost tiers of polymer, glass, and steel, dangling his pulsing legs, surveying at a distance the writhing life of the planet. In this scenario, Man comes on as a stupendous lethal force, and the earth is pictured as something delicate, like rising bubbles at the surface of a country pond, or flights of fragile birds.
But it is illusion to think that there is anything fragile about the life of the earth; surely this is the toughest membrane imaginable in the uni-verse, opaque to probability, impermeable to death. We are the delicate part, transient and vulnerable as cilia. Nor is it a new thing for man to invent an existence that he imagines to be above the rest of life; this has been his most consistent intellectual exertion down the millennia. As illu-sion, it has never worked out to his satisfaction in the past, any more than it does today. Man is embedded in nature.
The biologic science of recent years has been making this a more ur-gent fact of life. The new, hard problem will be to cope with the dawning, intensifying realization of lust how interlocked we are. The old, clung-to notions most of us have held about our special lordship are being deeply undermined.
Item. A good case can be made for our nonexistence as entities. We are not made up, as we had always supposed, of successively enriched pack-ets of our own parts. We are shared, rented, occupied. At the interior of our cells, driving them, providing the oxidative energy that sends us out for the improvement of each shining day, are the mitochondria, and in a strict sense they are not ours. They turn out to be little separate creatures, the colonial posterity of migrant prokaryocytes, probably primitive bacte-ria that swam into ancestral precursors of our eukaryotic cells and stayed there. Ever since, they have maintained themselves and their ways, repli-cating in their own fashion, privately, with their own DNA and RNA quite different from ours. They are as much symbionts as the rhizobial bacteria in the roots of beans. Without them, we would not move a muscle, drum a finger, think a thought. Mitochondria are stable and responsible lodgers, and I choose to trust them.
But what of the other little animals, similarly established in my cells, sorting and balancing me, clustering me together? My centrioles, basal bodies, and probably a good many other more obscure tiny beings at work inside my cells, each with its own special genome, are as foreign, and as essential, as aphids in anthills. My cells are no longer the pure line entities I was raised with; they are ecosystems more complex than Tamaica Bay.
I like to think that they work in my interest, that each breath they draw for me, but perhaps it is they who walk through the local park in the early morning, sensing my senses, listening to my music, thinking my thoughts. I am consoled, somewhat, by the thought that the green plants are in the same fix. They could not be plants, or green, without their chloroplasts, which run the photosynthetic enterprise and generate oxygen for the rest of us. As it turns out, chloroplasts are also separate creatures with their own genomes, speaking their own language.
We carry stores of DNA in our nuclei that may have come in, at one time or another, from the fusion of ancestral cells and the linking of ancestral organisms in symbiosis. Our genomes are catalogues of instructions from all kinds of sources in nature, filed for all kinds of contingencies. As for me, I am grateful for differentiation and speciation, but I cannot feel as separate an entity as I did a few years ago, before I was told these things, nor, I should think, can anyone else.
Item. The uniformity of the earth’s life, more astonishing than its di-versity, is accountable by the high probability that we derived, originally, from some single cell, fertilized in a bolt of lightning as the earth cooled. It is from the progeny of this parent cell that we take our looks; we still share genes around, and the resemblance of the enzymes of grasses to those of whales is a family resemblance.
The viruses, instead of being single-minded agents of disease and death, now begin to look more like mobile genes. Evolution is still an infinitely long and tedious biologic game, with only the winners staying at the ta-ble, but the rules are beginning to look more flexible. We live in a dancing matrix of viruses; they dart, rather like bees, from organism to organism, from plant to insect to mammal to me and back again, and into the sea, tugging along pieces of this genome, strings of genes from that, transplant-ing grafts of DNA, passing around heredity as though at a great party. They may be a mechanism for keeping new, mutant kinds of DNA in the widest circulation among us. If this is true, the odd virus disease, on which we must focus so much of our attention in medicine, may be looked on as an accident, something dropped.

Item. I have been trying to think of the earth as a kind of organism, but it is no go. I cannot think of it this way. It is too big, too complex, with too many working parts lacking visible connections. The other night, driving through a hilly, wooded part of southern New England, I wondered about this. If not like an organism, what is it like, what is it most like? Then, satisfactorily for that moment, it came to me: it is most like a single cell.
Thoughts for a Countdown
There is ambiguity, and some symbolism, in the elaborate ritual observed by each returning expedition of astronauts from the moon. They celebrate first of all the inviolability of the earth, and they re-enact, each time, in stereotyped choreography, our long anxiety about the nature of life. They do not, as one might expect, fall to their knees and kiss the carrier deck; this would violate, intrude upon, contaminate the deck, the vessel, the sea around, the whole earth. Instead, they wear surgical masks. They walk briskly, arms up, untouching, into a sterile box. They wave enigmat-ically, gnotobiotically, to the President from behind glass panes, so as not to breathe moondust on him. They are levitated to another sealed box in Houston, to wait out their days in quarantine, while inoculated animals and tissue cultures are squinted at for omens.
It is only after the long antiseptic ceremony has been completed that they are allowed out into the sun, for the ride up Broadway.
A visitor from another planet, or another century, would view the ex-ercise as precisely lunatic behavior, but no one from outside would under-stand it. We must do things this way, these days. If there should be life on the moon, we must begin by fearing it. We must guard against it, lest we catch something.
It might be a microbe, a strand of lost nucleic acid, a molecule of en-zyme, or a nameless hairless little being with sharp gray eyes. Whatever, once we have imagined it, foreign and therefore hostile, it is not to be pet-ted. It must be locked up. I imagine the debate would turn on how best to kill it.
It is remarkable that we have all accepted this, without hooting, as though it simply conformed to a law of nature. It says something about our century, our attitude toward life, our obsession with disease and death, our human chauvinism.

There are pieces of evidence that we have had it the wrong way round. Most of the associations between the living things we know about are es-sentially cooperative ones, symbiotic in one degree or another; when they have the look of adversaries, it is usually a standoff relation, with one party issuing signals, warnings, flagging the other off. It takes long inti-macy, long and familiar interliving, before one kind of creature can cause illness in another. If there were to be life on the moon, it would have a lonely time waiting for acceptance to membership here. We do not have solitary beings. Every creature is, in some sense, connected to and depen-dent on the rest.
It has been estimated that we probably have real knowledge of only a small proportion of the microbes of the earth, because most of them cannot be cultivated alone. They live together in dense, interdependent commu-nities, feeding and supporting the environment for each other, regulating the balance of populations between different species by a complex system of chemical signals. With our present technology, we can no more isolate one from the rest, and rear it alone, than we can keep a single bee from drying up like a desquamated cell when removed from his hive.
The bacteria are beginning to have the aspect of social animals; they should provide nice models for the study of interactions between forms of life at all levels. They live by collaboration, accommodation, exchange, and barter. They, and the fungi, probably with help from a communication system laid on by the viruses, comprise the parenchyma of the soil (some-one has suggested that humic acid, to which the microbes contribute, is a sort of counterpart for the ground substance of our own connective tis-sue). They live on each other. Sometimes they live inside each other; the Bdellovibrio penetrate the walls of other bacteria, tuck themselves up in-side, replicate, and burst out again as though they thought themselves phages. Some microbial communities extend so deeply into the affairs of higher forms of life as to seem like new kinds of tissue in plants and ani-mals. The rhizobial bacteria that swarm over the root hairs of leguminous plants have the look of voracious, invasive pathogens, but the root nodules that they then construct, in collaboration with the plant cells, become the earth’s chief organ for nitrogen fixation. The production of leghemoglobin in the membrane-lined space between plant and bacterial cells is an exam-ple of the high technology of symbiosis; the protein is synthesized by the plant, but only on instructions from the bacteria, and it is possible that the plant DNA for coding this substance came originally from the microbe, early in the evolution of the arrangement.
The bacteria that live in the tissues of insects, like those incorporated into the mycetocytes of cockroaches and termites, have the appearance of specialized organs in their hosts. It is not yet clear what they accomplish for the insect, but it is known that the species cannot survive long without them. They are transmitted, like mitochondria, from generation to gener-ation of eggs.
It has been proposed that symbiotic linkages between prokaryotic cells were the origin of eukaryotes, and that fusion between different sorts of eukaryotes (e.g., motile, ciliated cells joined to phagocytic ones) led to the construction of the communities that eventually turned out to be meta-zoan creatures. If this is true, the marks of identity, distinguishing self from non-self, have long since been blurred. Today, in the symbiotic as-sociations that dominate so much of the life of the sea, there is rarely a question of who is who, even when the combination functions like a sin-gle animal. The anemones that fasten themselves to the shells, even to the claws, of certain crabs are capable of recognizing precisely the molec-ular configurations that identify those surfaces: the crab, for his part, can recognize his own anemone, and will sometimes seek him out and attach him to the shell like an ornament. The damsel fish that have become, from their point of view, functioning parts of certain species of anemones adapt themselves when very young to life among the lethal tentacles of their host; they cannot just swim in forthwith—they must dart around the edges until labeled at their surfaces by markers acceptable to the anemone.
Sometimes, in the course of the modulation of relations between ani-mals, there are inventions that seem to have been thought up on the spur of the moment, like propositions to be submitted for possible evolution. Some are goodhumored, even witty. Certain Australian surf bathers, sev-eral years ago, were stung by tiny creatures that turned out to be nudi-branchs armed with the stingers of Portuguese men-of-war. Having fed on jellyfish, the Glaucus community had edited their meal and allowed the stinging cells to make their way to the surface of their new host, thus cre-ating, for the time, a sort of instant hybrid with, allowing for some asym-metry, the essential attributes of each partner.