The Maverick Science of Astrobiology
THE INTIMATE MYSTERY
Nothing could be more familiar than life. But what exactly is it? On a practical level, how can we tell life from non-life wherever it occurs in the universe?
Defining life hasnít traditionally been the biologistís favorite pursuit. The English geneticist J. B. S. Haldane began his 1947 essay "What is Life?" with the statement: "I am not going to answer this question." Scientists donít need a dictionary to tell them that a field of daffodils or a colony of bacteria is alive and a tailorís dummy isnít. Biology has gotten along quite nicely without specifically saying what itís studying. But astrobiology doesn't have that luxury. How can we hope to find life on other worlds if we donít know what weíre looking for?
Maybe weíll be lucky. When future probes melt their way through the icy coating of Jupiterís moon Europa, they may send back glimpses of giant luminous creatures patrolling a Stygian sea. When the first manned expedition to Mars samples the bed of the ancient ocean that once sprawled across the northern hemisphere, it may unearth the perfectly preserved fossil of a Martian trilobite. The late Carl Sagan was among those who suggested that something big might lumber before the watching cameras of Viking on Mars or float visibly in the cloud-tops of Jupiter as the Voyager probes flew by.
Recognizing such large and obvious extraterrestrial life (or its remains) would be child's play. But the universe isn't likely to be so accommodating. Life may only rarely crop up on a grand scale. It could also be utterly bizarre, unlike anything we've previously met or imagined. And even if it follows a more familiar pattern, confirming its presence from far away will hinge on our ability to distinguish, clearly and unambiguously, the true signatures of biological activity.
So what is life exactly? "Something that can make copies of itself" according to a familiar textbook definition. That would certainly include every organism on Earth. Even in special cases, like those of mules, celibates, and men who have undergone vasectomies, where the individual can't or chooses not to engage in procreation, the Xeroxing of DNA goes on all the time at the cellular level.
For many scientists, however, while self-replication is a necessary feature of living things, it isn't the most fundamental. Stanley Miller, a biochemist at the University of California San Diego who did some of the pioneering experiments on the chemical origin of life, makes no bones about his dislike of definitions: "[They] are what you impose on your thoughts. There are so many more important things to discover that to engage in an extended discussion over definitions, I think, is a waste of time." Having said this, his money is firmly on evolution as the sine qua non of life. "My definition of life, viewed from the perspective of origins, is that the origin of life is the origin of evolution." Evolution in turn involves three key factors: replication, selection, and mutation. "Replication is the hard part. Selection is where nature selects out the ones that would replicate the fastest, and mutation means that you make a small number of errors. It is important that those mutations, or errors, be propagated onto the progeny, so the organism improves. Reproduction is simply making an accurate copy of genetic material."
Another origin-of-life researcher, Antonio Lazcano at the National Autonomous University of Mexico, holds a similar view: "Some people would say that as long as you have a single molecule that is able to replicate and evolve, that is enough. My own tendency is to define life as a system that is able to undergo Darwinian evolution. By this, I mean a chemical system that can actually undergo a process of mutations and rearrangement of the genetic material, and can adapt to the environment." For Lazcano, as for Miller, "the questions of defining life and the origins of life are connected."
Mark Bedau, a philosopher of biology at Reed College in Portland, Oregon, goes a step further. He regards evolution as "the thing which explains why all the other properties are there the essence, the root cause." Whatís really alive is the whole system: an ensemble of countless individual organisms of many species, all interacting, reproducing, and displaying unpredictable, open-ended evolution.
This is an idea with far-reaching implications. If life can be anything that shows open-ended evolution, then it isnít locked into a particular material form. It doesnít have to be carbon-based. In principle, it doesnít have to be chemically based at all. And if that sounds too far-fetched to take seriously, then watch out. Wildly unfamiliar creatures are already lurking in laboratories in the United States, Japan, Italy, Germany, Britain and elsewhere, and have even gained access to the Internet. They donít look like us. Their origin is completely different from that of any natural organism on Earth. In essence, they inhabit an alternative stratum of reality. Yet there they are, breeding, growing, competing, dying, evolving, just like the rest of us. They are artificial life-forms "a-life"and their home is the digital landscape of computers.
Thomas Ray, a professor of zoology at the University of Oklahoma, is one of the pioneering investigators of these new, non-organic organisms and author of the Tierra a-life software system. Genesis inside Tierra dawns with a single, minuscule progenitor, the "Ancestor." It's a tiny string of machine code, just 80 bytes long, brought into existence with the capacity to make copies of itself inside the computer's working memory. The Ancestor spawns a daughter program. Then Ancestor and daughter each replicate again, as do their offspring, and so it goes on, multiplication upon multiplication. The little programs, with their self-copying ability, are simple analogues of the nucleic-acid-based genetic code of biological life. And crucially, just like that DNA-mediated system, Rayís self-replicators are slightly less than perfect. They donít always result in exact copies of the original because the Tierra environment is set up so as to occasionally reach in and randomly flip one of the bits the binary digits in a daughter program, making it genetically distinct from its parent. Usually the switch is bad news, rendering a program unable to copy itself as well as before, if at all. But sometimes turning a zero into a one or vice versa works to the creature's advantage, enabling it to multiply a little faster than its rivals. In this way, mutation, the master key to novelty and adaptation, is introduced into the proceedings. By the time the computer's memory is chockfull of Tierrans, there are all manner of variations on the original theme a host of genetically distinct self-copiers battling for survival in their overcrowded electronic domain. At this point the real fun begins. In accordance with certain "fitness" criteria built into the system at the outset, the little programs begin competing for memory space. The success of a particular species, or byte-string, depends on how effectively it can replicate and transmit its genes to the next generation, or even usurp its rivals' private memory space.
Ray believes that systems like his provide the first experimental basis for a comparative biology:
Life on Earth is the product of evolution by natural selection operating in the medium of carbon chemistry. However, in theory, the process of evolution is neither limited to occurring on the Earth, nor in carbon chemistry. Just as it may occur on other planets, it may also operate in other media, such as the medium of digital computation. And just as evolution on other planets is not a model of life on Earth, nor is natural evolution in the digital medium.Like others in his field, Ray is adamant that his creations are "not models of life but independent instances of life." Forget semantics. Forget metaphysical musings on the meaning or nature of life. If it acts like a duck, it's a duck; if it evolves, it's alive. This is proof-of-pudding empiricism like that employed by an old chestnut in the field of artificial intelligence, the Turing Test. If by questioning alone, says the Turing Test, you can't tell which of two interviewees is human and which is machine, then the machine should be considered to be genuinely intelligent. But whereas the Turing Test is a long way from being applied in practice, the "strong" a-life claim is with us here and now and it is extraordinarily radical. Life, it suggests, can be defined without reference to a material medium. Its fundamental essence isn't solid, liquid, or gas, or any kind of chemistry, or even a digital dance of electrons. The form of matter is irrelevant. What distinguishes life, at its most basic level, is information.
Thatís a lot for flesh-and-blood bipeds to swallow. We may live and pass on life courtesy of the encyclopedic database enshrined within our DNA, but like every other terrestrial organism, we depend upon our carbonaceous bodies. Weíre material beings. The point the strong a-life claim makes, however, is not that life can exist in the absence of a medium (chemical or otherwise), but that the medium isnít what matters. What does is that there are general principles of the living state that are independent of a particular implementation. These principles, the idea goes, operate purely at the level of the informational and organizational substructure of life. Consequently, they apply anywhere in the universe.
In physics, this kind of abstraction is routine. Physicists make a living out of searching for and finding relationships that underpin otherwise seemingly diverse phenomena. Since at least the time of Galileo, the inorganic world has been well known to have a mathematical infrastructure. But weíre not used to thinking about life in such terms. The theoretical physicist may be happy to work in a world of equations, a Platonic universe that stands behind the reality we perceive. But it comes as a shock to be told that a similar, intangible domain of symbols and logical relationships may form the backdrop to the very phenomenon of life. Obviously weíre more than mere dust-devils of data. Still, as one of the originators of the a-life field, Christopher Langton, put it:
There's nothing implicit about the material of anything if you can capture its logical organization in some other medium you can have that same "machine," because it's the organization that constitutes the machine, not the stuff it's made of . . .A-life researchers aren't the first to make this argument. The naturalist D'Arcy Wentworth Thompson was drawing attention to the common mathematical architecture of organisms in 1917 in his magnum opus On Growth and Form. More recently, the Chilean biologists Humberto Maturana and Francisco Varela have sought to build a theoretical foundation for life in its broadest sense in terms of autopoiesis ("self-creation") the ability of a system to invent and define itself by virtue of having a circular organization. The cells of terrestrial organisms, for instance, are autopoietic because they're made up of a physically bounded network of chemicals that, through an intricate series of reactions, actually generates the very network, together with the boundary the membrane that sets the system apart from its surroundings.
Autopoiesis is an all or nothing affair, because unless a system has a closed organization (albeit it may have an open structure to allow the inflow and outflow of energy and materials), it can't manufacture itself from within. Which prompts the question: How could such a completely self-reflexive entity get off the ground in the first place? How could life originate if it had to, as it were, pull itself up by its own bootstraps? As Stuart Kauffman, at the Santa Fe Institute in New Mexico, sees it:
Life emerged . . . not simple, but complex and whole, and has remained complex and whole ever since not because of a mysterious ťlan vital, but thanks to the simple, profound transformation of dead molecules into an organization by which each moleculeís formation is catalyzed by some other molecule in the organization.Kauffman doesn't go as far as those in the a-life field, who'd like to ground the definition of life in purely abstract and logical terms. He's orthodox to the extent that he regards every living thing as having both a chemical aspect (a body) and an informational aspect (a genome), and that it's meaningless to talk about one without the other. But he believes a new concept, that of an "autonomous agent," is needed as the unifying factor of life. "It's not matter, it's not energy, it's not information," he says. "Itís something else."
Such sweeping theories of life promise to do for biology what Newtonís theory of gravity did for physics form the basis of a science whose principles apply everywhere in the cosmos. But right now astrobiologists have a more pressing concern. They simply want to detect extraterrestrial life. A single example any example will do. It might be a beleaguered underground colony of Martian microbes, a battered, desiccated cell scraped from some frozen ice-field on Callisto, or a radiation-seared viroid particle blown in on the stellar wind. The overwhelming priority for this raw young science is to find something that is biogenic and didn't come from Earth. That means focusing on a property of life that will give a reading on an instrument that will actually generate a signal indicating that a living process is (or has been) at work. "When you think about how you recognize life," says Michael Mayer, head of NASA's astrobiology program in Washington, D.C., "you think about what life does, not what it is."
One of the things life does that underpins its very existence is metabolize. Biologists and astrobiologists are unanimous that metabolism has to be a linchpin of life everywhere. The term comes from the Greek metabole, meaning "change," and it refers to the functions and effects of the suite of interlocking chemical reactions found within an organism. A pivotal aspect of metabolism is the harnessing of energy. Living things, whatever their nature, must be able to capture energy from their surroundings, turn it into a storable form, and then release it when needed in precisely controlled amounts. Energy must be available on demand for essential biological tasks such as building complex substances from simpler starting materials, effecting repairs to living structures, and reproducing.
These processes require that certain chemicals be taken in from the surroundings and certain others given out. The particular chemicals involved and the specific way in which the chemical balance of the environment is affected are signs that a biological process is at work. And that fact is the crux as far as astrobiology is concerned. Metabolism causes changes that, given the knowledge and the right equipment, can be detected and used to diagnose the presence of life.
Even extinct organisms may leave clues to their metabolic activity chemical and mineralogical traces in rocks that speak strongly or uniquely of a biogenic origin. Such rocks may arrive as meteorites splashed from the surface of neighboring planets following collisions with asteroids or, in the future, they may be brought back by sample-return probes. Close examination in the lab may reveal another familiar and possibly universal characteristic of life.
Every living thing on Earth exists inside some sort of bag. Those of us fortunate enough to be multicellular have a general outer wrapping skin, scales, an exoskeleton, a waxy cuticle. But each individual cell, whether part of a larger organism or not, has its own bag, a cell membrane, that serves a variety of purposes. The metabolism of life-as-we-know-it requires that a rich cocktail of exotic chemicals be closely confined under conditions radically different from those in the immediate nonliving neighborhood.
On Earth, every organism, from the lowliest bacterium to a human being, stores energy in exactly the same way in the form of the chemical bonds that link together the phosphate groups in the molecule known as adenosine triphosphate, or ATP. Tacking on phosphate groups to make ATP stores energy, splitting them away sets energy free. This released energy drives all the other aspects of metabolism, including assembling new molecules such as proteins from simpler components (anabolism), breaking down existing large molecules (digestion, or catabolism), extracting chemical energy (respiration), and, if you happen to be a green plant, directly intercepting and making use of the energy in sunlight (photosynthesis).
A chart of the main metabolic pathways of terrestrial life reveals an interlocking network of reaction cycles and chains, marvelous in complexity and organization. But none of this chemical wizardry is self-starting. If all the reactants involved in metabolism were simply thrown together and kept roughly at room temperature, nothing would happen. The biochemical reactions upon which all known life depends just aren't energetically favorable; they don't take place merely by the various reactant molecules bumping into one another. They need to be helped along. The substances that do this, biological catalysts or enzymes, are mostly protein molecules (there are a few notable exceptions) and each one is specific to a particular reaction or small set of similar reactions, owing to its unique three-dimensional shape. This specificity is crucial because it means that by regulating the production of different enzymes, an organism can run a system of highly ordered, interlinked reaction chains, rather than just a muddled-up chemical jamboree.
The result is a repeatable pattern of reactions that, without this continuous intervention, wouldn't occur or would quickly grind to a stop. In thermodynamic terms, it's a system far out of chemical equilibrium. If this state doesn't quite qualify as a definition of life, it's certainly one of life's defining characteristics.
But enzymes, like many proteins, are delicate. If they get too hot, their shape changes, they start to fall apart, and they lose their catalytic ability. The same thing happens if their environment becomes too acid or alkaline. For these and other reasons, an important part of metabolism is homeostasis the maintenance of a relatively stable ambience inside an organism. Whatever happens outside, it's crucial that an organism's internal state stay pretty much the same. The only way that can happen is if it exists within a kind of protective bubble. If life as we can reasonably imagine it and physicochemical states that are far from equilibrium go hand in hand, then a means of containment and segregation is absolutely essential. And this is where cell membranes come in. The cell membrane holds the contents of an organism together and separates the region within which metabolism takes place from the outside world.
At the same time, the membrane isn't an impenetrable barrier. It's a subtly constructed interface, itself a product of metabolism, that allows, with the expenditure of some energy by its owner, the controlled two-way passage of certain substances that the organism needs both to acquire and dispose of. Could life exist without such an interface? Not according to Lynn Margulis, a biologist at the University of Massachusetts at Amherst who has written much about life's nature and evolution. "Life," she says, "is a self-bounded system where the boundary is made by the material in the system. It's not a thing, it's a process, and these processes involve the production and maintenance of identity."
Jeffrey Bada, who directs NASA's astrobiology research at the Scripps Institution of Oceanography in La Jolla, California, disagrees. He's theorized that very primitive forms of life might exist as a sort of boundary-less broth. This may be how life started out on Earth, and it could be that the genesis of living things always involves an early pre-cellular phase. If so, Bada argues, on worlds where there's little evolutionary pressure to force further change, life might never progress beyond its primordial soupy state. One such place, he suggests, might be the supposed underground ocean on Europa.
But if something like Bada's broth exists, would most scientists call it alive? Its discovery would certainly cause plenty of excitement. Not-quite-life, iffy-life, life-of-sorts astrobiologists would gladly take whatever comes along, because anything remotely biological found on another world (assuming it had developed independently) would be powerful evidence that life is common throughout the universe. Still, the question strikes to the heart of how life is defined, especially at the lower end of the scale: Would an essentially unchanging sea of membrane-less, self-copying chemicals qualify as an instance of life? Probably not, in the judgment of most researchers. It would more likely be called prebiological. And the reason goes back to that key criterion of evolution. "The ability to evolve," insists Jack Szostak, a molecular biologist at the Massachusetts General Hospital in Boston, "is what distinguishes systems that are alive biologically from prebiotic chemical systems."
In the minds of most biologists, Darwinian-style evolution outranks replication, metabolism, or individuality as the chief definer of life. But these properties aren't mutually independent quite the opposite. Evolution in the Darwinian sense implies that replication and natural selection are going on within a genetically diverse population of individuals. Replication implies metabolism. It's hard to see how these factors could be disentangled. As Oxford biologist John Maynard Smith put it: "entities with the properties of multiplication, variation, and heredity are alive, and entities lacking one or more of those properties are not."
If Darwinian evolution is what fundamentally marks out life from non-life or pre-life then all living things must also, because of the way natural selection works, be capable of making copies of themselves. That implies the need for a genome a complete set of instructions for self-replication. In theory, the genome could exist in any form. In Ray's Tierra system, itís a string of zeros and ones held in the computer's memory. But in nature, not only does the genome itself have to have a foot in the physical world (as a set of chromosomes, for example) but it also has to be encapsulated within a larger structure a living organism. Some see this latter fact as almost incidental. The Oxford biologist Richard Dawkins has long championed the view that the gene what he calls the "replicator" is the central fact of life. In his view, cells and multicellular creatures evolved as mere vehicles to ensure the survival and transmission of their genetic cargo. That's pretty close to the a-life position, in which the emphasis is all on self-propagating patterns of information. But it's too extreme for most biologists, who tend to regard organisms as more than just housings for their genetic database. A more conventional view of life was suggested by the philosopher David Hull. In his scheme, biological evolution involves not only replicators (things that pass on their structure directly by replication), but also interactors (things that produce differential replication as a result of interacting as cohesive wholes with their environment) and lineages of these interactors. As he saw it:
A process is a selection process because of the interplay between replication and interaction. The structure of replicators is differentially perpetuated because of the relative success of the interactors of which the replicators are part. In order to perform the functions they do, both replicators and interactors must be discrete individuals which come into existence and cease to exist. In this process they produce lineages which change indefinitely through time.For terrestrial life, the replicators are genes, made from DNA, and the interactors are the many organisms found on Earth. On other worlds, the details of implementation may differ, but there seems every reason to suppose that the same overall arrangement applies to living things everywhere. According to Carl Emmeche, a philosopher who studies the nature of life at the Niels Bohr Institute in Copenhagen:
It is highly conceivable that all life in the universe evolves by a kind of Darwinian selection of interactors, whose properties are in part specified by an informational storage that can be replicated. . . [T]he very notion of natural selection and replication . . . seems to be specific for biological entities. . . This definition is simple, elegant, general, and crystallizes our ideas of the general mechanism of the creation of living systems within an evolutionary perspective.Again, it's possible to work with a paradigm of life like this on a very abstract level. Replicators, interactors, and lineages could all be set up inside a computer, for instance, or played with as patterns of little squares on gridded paper that follow certain rules. But astrobiologists, although they sometimes use computers for biological simulations, aren't so interested in such intangibilities. Their quest is for life in the real universe, life that has assembled itself from the raw materials on other worlds. And that brings us back to the other key factor of life metabolism. Replicators and interactors can function in nature only if they have the means to manipulate energy and matter to their own ends. In the case of life that has evolved naturally, the informational and material aspects of life can't be divorced from one another. To both retain and act upon its onboard genome, for self-construction, self-maintenance, and reproduction, any living thing must harbor a network of component metabolites. That network, requiring conditions far out of equilibrium with the surroundings, can function only within some kind of boundary. And that boundary, in turn, defines an individual.
The nature of individuality might seem obvious. We think of ourselves as individuals. But in reality, each of us is a city home to vast armies of bacteria encamped on the surface of our skin and mucous membranes, as well as within us most of them, luckily for us, benign. More disturbingly, every cell of our bodies is inhabited profusely by beings from another place and time. Mitochondria are at the heart of energy production in the cell the sites of cellular respiration. We literally can't lift a finger without them. Yet they have their own inner membranes and DNA, strikingly similar to those of bacteria. This is the essential clue to their probable origin. According to the endosymbiotic theory (first proposed in 1885, cast in modern form by Lynn Margulis, and now widely accepted), mitochondria, together with the light-harvesting chloroplasts found in green plant cells, are descendants of ancient, free-living microbes. At some point, more than a billion years ago, they became incorporated within larger cells as part of a symbiotic liaison, and there they have remained ever since. Are the mitochondria the "real" individuals and each of us a kind of hive in which they collectively dwell? And are we, in turn, mere elements of a much larger superorganism?
The web of life leaves no creature in isolation. Most obviously, social insects, like ants and bees, simply die if cut off from their swarm. But on a wider scale it's true of us all. From Staphylococcus to Homo sapiens, we're minuscule parts of a stupendously complex, planet-wide system of interconnected organic and inorganic components the biosphere. Self-regulating through a myriad of feedback loops, endlessly cycling biological staples such as carbon, nitrogen, and water, is this the true quantum of life? Some supporters of the controversial Gaia theory think so. The whole biosphere is the primary life-form, the collective product of a host of lesser beings. Perhaps CŤzanne conveyed it best with his depiction of an apple as part fruit, part Earth. Anyway, it's a sobering thought that we who habitually think of ourselves as free and self-sufficient may be more like cells in a giant planetary super-organism.
Not all Gaia theorists go so far as to say that the Earth is a living individual. In any case, what matters from the perspective of astrobiology is that biospheres offer another opportunity for detecting life. Just as the environment within an organism is well out of equilibrium with its surroundings, so biospheres are expected to give themselves away by their markedly "unnatural" appearance. The originator of the Gaia theory, British chemist James Lovelock, wrote of Earth's atmosphere:
Almost everything about its composition seems to violate the laws of chemistry. . . The air we breathe . . . can only be an artifact maintained in a steady state far from chemical equilibrium by biological processes.On other worlds, too, life may have altered conditions on a planet-wide scale. So a key strategy in astrobiology will be to look for any combination of constituents in an atmosphere that is well out of normal chemical balance a suspiciously unstable mixture that only living metabolisms could maintain.
The more radical Gaia interpretation of an entire planet as a single life-form makes one wonder how unusual life might be elsewhere. Over the years, scientists and science-fiction writers have dreamed up an extraordinary cosmic menagerie. In The Black Cloud cosmologist Fred Hoyle imagined an intelligent, self-propelled interstellar cloud that arrives in the solar system and wreaks havoc by blocking out the Sunís light. Its "brain," a complex network of widely-spaced molecules, can be expanded and reconfigured at will, giving the creature stupendous mental powers. Aerospace engineer Robert Forward, building on an idea by SETI pioneer Frank Drake, wrote about the diminutive, high-density inhabitants of a neutron star. His "cheelans," made of nuclear matter, live out their lives a million times faster than human beings, see in the far ultraviolet, and communicate by strumming the crust of their unusual stellar home with their abdomens.
Could such wildly flamboyant creatures actually exist? As Fred Hoyle wrote in the preface to The Black Cloud, "there is very little here that could not conceivably happen." Life elsewhere could be so strange that if we base our expectations too rigidly on terrestrial standards we might even have trouble recognizing it. Astrobiologists are well aware they have no way yet of putting constraints on the outer limits of life. Having only one data point to work with, they're compelled to be open-minded. Maybe there are star-dwelling communities, interstellar behemoths, energy-based life-forms, and other exotica that would put the Star Trek universe to shame. But while such speculation is entertaining and the subject of many an informal discussion between scientists, it isn't a central issue in professional circles. The dominant question is where the search for extraterrestrial life should be focused here and now. And the answer is evident from every astrobiological program, underway or planned, in which there is a significant investment of funds and other resources. It's evident in the "Roadmap" drawn up by the Astrobiology Institute to help guide NASA's activities in this field. It's evident in the overwhelming majority of papers published on the subject of extraterrestrial life in leading scientific journals and in the proceedings of relevant conferences, such as the first annual science conference on astrobiology held at the Ames Research Center in April 2000. Most tellingly, it's evident in the design and implementation of the multi-million-dollar instruments that have been built, or are being built, to test for the presence of biological activity on other worlds. The approach adopted by the scientific community is simple, straightforward, and practical: to look for the kind of life we know, allowing for possible adaptations to different environments.
The kind of life we know is, first and foremost, based on carbon. "No other element comes close to forming such a diverse array of bonds," explains Jeff Bada. Carbon's closest analogue is silicon and there's been no shortage of speculation about the possibility of silicon-based life over the last century or so. In 1893, the chemist James Emerson Reynolds used his inaugural address to the British Association for the Advancement of Science to point out that the heat stability of silicon compounds might allow life to exist at very high temperatures. Picking up on this idea in an article published the following year, H. G. Wells wrote:
One is startled towards fantastic imaginings by such a suggestion: visions of silicon-aluminium organisms why not silicon-aluminium men at once? wandering through an atmosphere of gaseous sulphur . . .Thirty years later, J. B. S. Haldane proposed that life might be found deep inside a planet based on partly molten silicates.
At first sight, silicon does seem a promising alternative to carbon. Like carbon, it's common in the universe, and much of its basic chemistry is similar. Just as carbon combines with four hydrogen atoms to form methane, silicon yields silane; silicates are analogues of carbonates; both elements form long chains in which they alternate with oxygen; and so on. But on closer examination, silicon's biological credentials become less convincing. The biggest stumbling block seems to be the extreme ease with which silicon combines with oxygen. Wherever astronomers have looked in meteorites, in comets, in the interstellar medium, in the outer layers of cool stars they've found molecules of oxidized silicon (silicon dioxide and silicates) but no evidence at all of substances that might serve as the building-blocks of a silicon biochemistry. The silicon analogues of hydrocarbons long chains of hydrogen-silicon compounds are nowhere to be found. And there's a further problem with silicon dioxide. When carbon is oxidized during respiration, it becomes the gas carbon dioxide a waste material thatís easy for a creature to dispose of. But silicon dioxide turns into a solid a crystalline lattice the instant it forms. To put it mildly, that poses a respiratory challenge.
This difficulty didn't faze Stanley Weisbaum in his SF classic A Martian Odyssey. Observing the unusual behavior of one of the indigenous life-forms, a scientist in the novel notes:
Those bricks were its waste matter. . . We're carbon, and our waste matter is carbon dioxide, and this thing is silicon, and its waste is silicon dioxide silica. But silica is a solid, hence the bricks. And it builds itself in, and when it is covered, it moves over to a fresh place to start over.The door may still be ajar to the possibility of silicon-based biology and for other novel biologies for that matter. But the fact remains that carbon really has no serious rival in the minds of most researchers who are actively involved in seeking out extraterrestrial life. The major point of debate is how much the details of the carbon chemistry of life will vary from one world to the next. Do all living things, for example, use DNA as their genetic material? Are the chemical pathways of their metabolism essentially the same? The Harvard biologist and Nobel laureate George Wald had no doubts. He said: "I tell my students, learn your biochemistry here and you will be able to pass examinations on Arcturus." Harold Morowitz, a biologist at George Mason University near Washington, DC, points to the fact that "there are only four different kinds of one-carbon compounds." That severely limits the number of ways of building up and breaking down larger molecules. Others, like Christopher Chyba of the SETI Institute, urge caution in drawing too many conclusions about the small print of biology elsewhere. Again, it's the problem of one data point.
Details aside, astrobiologists agree that the most promising places to look for life will be those where carbon-based molecules have had a chance to collect and become concentrated. Two other ingredients have also been singled out, more or less unanimously, as key biological prerequisites: the availability of liquid water and a suitable energy source that can be tapped by the metabolism of living things. Intriguingly, all of these commodities organic matter, water, and metabolically useful energy sources are starting to look pretty common in the universe. But whether life proves to be plentiful or not beyond the Earth depends crucially, too, on a number of other factors. Most importantly, there's the question of abiogenesis. How easily, given the right raw ingredients, does life arise?