by David Toomey
For instance, suppose we define a living organism as that which grows, consumes, converts matter into heat energy, maintains a metabolism (that is, perpetuates itself through chemical activity), and after a fashion dies. Candle flames and stars do all these things, and few would call them living. So we reconsider and define a living organism as that which, in addition to the previous, reproduces itself. Problem solved, we think. Until we recall that crystals reproduce, and most of us do not count them as living. And we remember that mules and worker ants cannot reproduce, yet they are to all appearances alive.
A PARTS LIST FOR LIFE
Attempts to define life by what it is or what it is made of have proved at least as difficult. The answers, in the broadest historical terms, have appeared as part of two suppositions. One was that any organism is animated by something that, like a spirit or soul, cannot be weighed, measured, or seen—something that came to be called a “vital force.” The idea may strike us as redolent of druids and tree spirits, but scientific arguments for such a force lasted into the early twentieth century. The other supposition—that life is fundamentally material—was well articulated in 1868 when Darwin’s staunch proponent T. H. Huxley declared that all organisms are composed of chemical compounds that are themselves lifeless.6
In the late nineteenth century and early twentieth, the materialist view gained much support. But since no one could be sure exactly which chemical compounds compose organisms, the case was never quite closed, and a strict definition of life remained elusive. In 1937 the British biologist Norman W. Pirie claimed that the terms “life” and “living,” especially as applied to cases for which the transition from nonliving is so gradual as to have no discernible boundary, were worthless.7 In a 1944 treatise called What Is Life?, physicist Erwin Schrödinger was more hopeful. He maintained that eventually life would be defined in some detail by physics and chemistry. But as years and decades passed, eventually was beginning to seem like a long time. As late as the 1960s, a much-respected textbook all but advised surrender, remarking that “attempts at an exact definition of life are not only fruitless, but meaningless.”8
A DARWINIAN DEFINITION
Well before the 1960s, it had become possible to add another item to a list of life’s functions, one that—conveniently for list makers—collapsed most of the others into itself. Life evolves, and any living organism is by definition a product of evolution. (In fact, by 1973 Darwin’s basic insight had been so thoroughly confirmed that Ukrainian-American geneticist Theodosius Dobzhansky could title an essay “Nothing in Biology Makes Sense Except in the Light of Evolution.”) Norman Horowitz, the geneticist and professor of biology at Caltech who in the 1950s had discovered that one gene governs one enzyme, elaborated. “Living organisms,” he wrote, “are systems that reproduce, mutate, and reproduce their mutations.”9 When those mutations are subject to natural selection over time, the organism adapts to its environment with ever-increasing precision.
If we define life as that which evolves, however, we are met immediately with another problem: viruses. Viruses are like nothing else in our category of the living. They are one-tenth to one-hundredth the size of the smallest bacterial cell, and nowhere near as complex as that cell, being little more than sets of genes held within a capsule of protein molecules. Viruses neither eat nor excrete, and as parasites they can do their reproducing only within the cell of an organism. It is this last behavioral limitation that compels authors of most biology textbooks to treat viruses as nonliving, and allows high school students reading those textbooks to discover, on a perennial basis, a scientifically grounded addition to their lexicon of insults.10 Yet it so happens that viruses evolve—which is to say they reproduce, mutate, and experience natural selection.
Suppose Horowitz is right that evolution is life’s defining feature, and that viruses are living. Or suppose that although evolution is an essential feature of life, it is not the defining feature, and that viruses (lacking that defining feature or set of features) are nonliving. In either case we still face a problem. There may be things that are nonliving yet evolve in the literal sense of that word. I’m writing this passage in a university library, a place housing journals, computer terminals, electronic databases, and of course, millions of books. A case might be made that all are as much products of evolution as the librarians who so kindly answer my reference questions. Consider the books. Some, like wildly successful species, enjoyed large press runs and many editions, producing, one might say, several generations of ever-larger populations. Others existed for a single edition of a few dozen copies, one of which happens to be preserved here like a rare fossil in a natural history museum. In marketplace competition, there were winners and losers. Some pushed into new markets entirely, and in so doing made a niche for their successors. A few, like a brave or unwitting microbe beginning a phylum, established what would come to be recognized as a new literary genre.
Biologist Richard Dawkins would demur a bit, maintaining that books are not themselves products of evolution. Rather, he would say, they are the vehicles that enable the evolution of ideas, stories, and language—these being examples of the units of cultural transmission and replicating entities that he calls “memes.” Arguing that evolution is “too big a theory to be confined to the narrow context of the gene,” Dawkins claims that memes evolve, and do so in the full sense of that word.11 Further, he maintains that since the ability to evolve is a property exclusive to living things, it follows that memes are living—not merely metaphorically, but literally.12 Many have made the same case for certain computer software programs, the first and most famous of which is British mathematician John Conway’s “Game of Life,” which used a few simple rules to generate complex self-organizing patterns.13
For all this, some claim that evolution may not be life’s defining feature, their evidence being certain organisms that are manifestly living but, in their view, did not evolve. They have argued, for instance, that the natural selection of many plants was suspended with the invention of agriculture, and the natural selection of humans was suspended with the invention of medicine.14 An evolutionary biologist might counter that no organism is beyond the reach of natural selection—that a highly cultivated plant like the tulip has insinuated itself into a relationship with another species (our own) that ensures the survival of its line, and that persons whose life spans are increased by medicine might possess genes that elicit sympathy or altruism in others (eventually producing medical students, physicians, and a good bedside manner), which in turn benefit other members of the species.
Even if award-winning tulips and children of parents with excellent health care are products of evolution, it is certainly possible to imagine organisms made of chemical compounds that do not evolve and yet—because they might metabolize and reproduce themselves—would justifiably be called living.15 It is also possible, incidentally, to imagine evolution by different means—for instance, evolution whose mutations are provided not by RNA and DNA, but by random errors in the synthesis of chemicals used to replicate and metabolize.16 We would likely call them “alive” as well.
If anything is clear from all this, it is that any category of things called “living” will have exceptions, and any reasonable definition of life is likely to be provisional. So it isn’t surprising that the NRC report hedges on the matter. It lists the characteristics of known life and allows that the better definitions are those along the lines of a “chemical system capable of Darwinian evolution.”17 But it does not offer a new definition and, in so many words, admits that any such attempt seems doomed to fail. Where does the matter stand now? In some ways, about where it stood three-quarters of a century ago, when Schrödinger posed the question.18 It seems that natural philosophers and scientists have again and again made provisional definitions that seemed at first satisfactory but, on further examination, were found to have boundaries that were shifting and indistinct. Meanwhile, the thing they were trying to define—amoeba-like—slid, slipped, and wri
ggled free.
A census taker, standing in a dimly lit hallway, knocks on a door. It is opened by a man in a bathrobe, a bit disheveled. The census taker introduces himself, and the man agrees to answer questions. The census taker begins: “Do you receive mail at this address?” “Do you pay utility bills for this apartment?” “Do you pay rent to the owner of this building?” The man answers “Yes” to all of these. Finally, the census taker asks, “Including yourself, how many people are living here?” The man says, “None.” At this the census taker says, “I don’t understand. You receive mail here, pay utilities here, and pay rent here—but you aren’t living here?” The man opens the door wider, allowing a view of worn furniture and a threadbare carpet, and shrugs. “This you call living?”
It’s usually poor form to explain a joke, but I’ll risk impropriety in the service of a point and observe that some of its humor derives from the census taker’s assumption that a definition of living could be met with a list of criteria. Perhaps we’ve been acting a little like him. The problem is not merely that life resists our efforts to define it; it is that even a good definition (if we had one) would be nothing but a list of features, lacking an underlying principle explaining how those features are related or why they should appear together. A definition, in other words, is of little help in understanding life.
If we are to understand life, we need a theory—that is, a set of self-consistent hypotheses for defining all life, familiar and weird—that makes predictions and can be tested. Recently, Carol Cleland made the same point, noting that we have been defining life by enumerating features, imagined or real, and—like medieval philosophers—bickering over which are significant. She notes that without a molecular theory we would still be defining water as that which is wet, or as a colorless, odorless liquid.19 We were able to agree on a universally applicable definition of water (two hydrogen atoms covalently bonded to a single oxygen atom) only when we had a theory of matter at the molecular scale.
A THEORY OF LIFE
To construct a theory of life we will need to know which features in living organisms are necessary and which are merely contingent. Of course we can make guesses, but to be certain as to which is which we will need a second example of life. The realization is not new. Half a century ago, a panel of leading biologists commissioned by the US National Academy of Sciences opined, “The existence and accessibility of Martian life would mark the beginning of a true general biology, of which the terrestrial is a special case.”20 The panel argued that the chance to develop a theory of life was in itself a reason to explore Mars. Of course, fifty years later we still do not know whether Mars holds or once held life. We have no second example and no theory.
Can we lower our sights further, and at least draw up a set of criteria that scientists can use to point to something and say, “That’s alive”? In fact, NASA scientists asked themselves the same question—again, a half a century ago.
In 1960, America’s newly formed space agency was as ambitious as it was busy—especially with regard to questions relating to life elsewhere. In that single year it created an Office of Life Sciences to consider biological issues related to the exploration of neighboring worlds (including, interestingly enough, the danger of contaminating those worlds by Earth microbes); it established a life sciences laboratory at its Ames Research Center to study the conditions under which life might survive; and it authorized the Jet Propulsion Laboratory (JPL) in Pasadena, California, to consider the type of spacecraft that might be employed to search for life on Mars.
The early NASA was nothing if not forward-thinking. With unmanned flybys of Mars still years away, the agency had already contracted designs for instruments intended to detect life on its surface. Those instruments would be carried by two unmanned spacecraft. Their first iteration, a program for an especially ambitious remote-controlled laboratory called Voyager, was canceled in 1967.* The second, a scaled-back version approved by Congress a year later, was called Viking. For both programs, the clearinghouse for life-detecting proposals was the office of project scientist Gerald A. Soffen at JPL.
Among those attending Soffen’s brainstorming sessions was a forty-year-old British inventor of medical equipment named James Lovelock. Lovelock couldn’t help but observe that most of the proposals were of the “add water and stir over low heat” variety, betraying rather parochial assumptions about the nature of life. Some in the session challenged Lovelock to describe a way to cast a wider experimental net, and he said that he would look for “thermodynamic disequilibrium,” a condition in which otherwise inert matter channels energy into a form that counters entropy—that universal tendency of things to slide, slip, fall down, and fall apart. At that moment Lovelock had given no thought to exactly how one might detect thermodynamic disequilibrium on Mars, but in the days following he reread Schrödinger’s treatise and soon came up with a short list of approaches. The most promising, he thought, would be a chemical analysis of the planet’s atmosphere.21
For an atmosphere in thermodynamic equilibrium, the chemical transactions are over and the scores are settled. All the chemistry that can happen has happened. But an atmosphere in disequilibrium is unsettled, and chemical transactions continue. Lovelock’s point was that if you detected the presence of a highly reactive chemical, one absorbed quickly and efficiently by other chemicals in the environment, you would have reason to suspect that it was being produced just as quickly and efficiently—perhaps, as is the case with oxygen and methane on Earth, by living organisms.
While at JPL, Lovelock met a young planetary scientist named Carl Sagan. Sagan disagreed with Lovelock on several counts, but he was intrigued enough to publish Lovelock’s work in Icarus, a journal he edited, and somewhat later to introduce him to Lynn Margulis (Sagan’s former wife), a biologist who was interested, like Lovelock, in the unexplained stability of the chemistry of Earth’s atmosphere over geological time. In subsequent years and with Margulis’s help, Lovelock would develop the notion that thermodynamic disequilibrium signifies life into the Gaia hypothesis and theory. Both are propositions that the Earth and its life compose a single complex system analogous to a self-regulating organism, and that through various feedback loops the system regulates (and for several billion years has regulated) the chemical composition and temperature of its atmosphere and oceans so as to keep them suitable for life.
Soffen’s office considered fifty proposals for various life-detecting instruments. Because of size and weight constraints, only three were chosen. Lovelock’s was not among them.22
Each of the two Viking landers had a mechanical sampler arm with a scoop on its end. It would use this arm to dig soil and drop it into one of three hoppers. From these the soil would be dropped below decks into the “biology package,” this being a collection of pipes, hoses, and small tanks of gases and incubation chambers that rotated on carousels. The package, weighing all of twenty pounds and squeezed into a cubic foot, represented what one historian called “the most sophisticated thinking of the twentieth century on the subject of extraterrestrial life in the solar system.”23 Nonetheless, the three experiments it contained were designed around a simple and straightforward premise: that any organism would take in nutrients and discharge waste.
The “labeled release” experiment was developed from methods of detecting contaminants in municipal water supplies by Gilbert Levin of Biospherics Inc. In its Martian version, a dilute solution of radioactively labeled organic compounds would be added to an incubation chamber containing a few ounces of soil. If organisms ate the compounds and exhaled gases like carbon dioxide, hydrogen, or methane, those gases would be identified by an onboard carbon-14 detector.
The “gas exchange” experiment, designed by Vance Oyama of NASA’s Ames Research Center, would add water moisture (or, in another mode, a rich broth of organic compounds) to a soil sample, and also would test for waste gases—not with a carbon-14 detector as in Levin’s experiment, but with a chromatograph.
The “
pyrolytic release” experiment was developed by Caltech’s Norman Horowitz, mentioned earlier, in the discussion of the definition of life. It would use radioactive labeling as did Levin’s, but (at least in Horowitz’s view) it would make fewer suppositions about Martian life. The only things Horowitz’s experiment would add to the soil in the incubation chamber were carbon dioxide and carbon monoxide, gases known to exist in Mars’s atmosphere.24
Levin and Oyama suspected Horowitz was offering so little that he might fail to provoke a metabolism, but Horowitz was sure that one thing the Martians would not want was water, since on Mars’s surface, the environment in which they had presumably evolved, it would freeze or evaporate. Horowitz thought that his experiment was the only one that was purely Martian, and that Levin and Oyama were looking for Earthlike life on Mars. Ideally, the incubation chamber that his experiment used would mimic Martian conditions, and it particularly irked him that in order for Levin and Oyama to be able to use liquid water, the temperature of the whole biology package—including his incubation chamber—was kept at 10°C, some 60 degrees higher than the summer average for the places the Viking landers would touch down.25 In the end, the fact that the twin landers could not carry scientists may have been for the best. It would have been a very long trip.
SQUAMOUS PURPLE OVOIDS
The three Viking biology experiments were part of thirteen separate investigations conducted by as many teams of scientists—seventy-eight researchers in all. Among them was Sagan. In 1975, the year of the Viking launches, he was forty-one, the David Duncan Professor of Astronomy and Space Sciences at Cornell, and author of several popular books about astrobiology. Sagan was a frequent guest on TV news programs and talk shows, where he proved a refreshing antidote to stereotypes of the scientist as drone. He was witty, charming, and above all else, enthusiastic—visibly excited about what he was doing.
