Imagine boarding a time machine and being transported back four billion years. What will await you when you step out? No green hills or sandy shores. No white cliffs or dense forests. The young planet bears little resemblance to its equable appearance today. Indeed, the name "Earth" seems a serious misnomer. "Ocean" would suit better, for the whole world is almost completely submerged beneath a deep layer of hot water. No continents divide the scalding seas. Here and there the peak of a mighty volcano thrusts above the surface of the water and belches forth immense clouds of noxious gas. The atmosphere is crushingly dense and completely unbreathable. The sky, when free of cloud, is lit by a sun as deadly as a nuclear reactor, drenching the planet in ultraviolet rays. At night, bright meteors flash across the heavens. Occasionally a large meteorite penetrates the atmosphere and plunges into the ocean, raising gigantic tsunamis, kilometers high, which crash around the globe.
The seabed at the base of the global ocean is unlike the familiar rock of today. A Hadean furnace lies just beneath, still aglow with primeval heat. In places the thin crust ruptures, producing vast fissures from which molten lava erupts to invade the ocean depths. The seawater, prevented from boiling by the enormous pressure of the overlying layers, infuses the labyrinthine fumaroles, creating a tumultuous chemical imbroglio that reaches deep into the heaving crust. And somewhere in those torrid depths, in the dark recesses of the seabed, something extraordinary is happening, something that is destined to reshape the planet and, eventually perhaps, the universe. Life is being born.
The foregoing description is undeniably a speculative reconstruction. It is but one of many possible scenarios offered by scientists for the origin of life, but increasingly it seems the most plausible. Twenty years ago, it would have been heresy to suggest that life on Earth began in the torrid volcanic depths, far from air and sunlight. Yet the evidence is mounting that our oldest ancestors did not crawl out of the slime so much as ascend from the sulfurous underworld. It may even be that we surface dwellers are something of an aberration, an eccentric adaptation that arose only because of the rather special circumstances of Earth. If there is life elsewhere in the universe, it may well be almost entirely subterranean, and only rarely manifested on a planetary surface.
Although there is now a measure of agreement that Earth's earliest bioforms were deep-living microbes, opinion remains divided over whether life actually began way down in the Earth's crust, or merely took up residence there early on. For, in spite of spectacular progress over the past few decades in molecular biology and biochemistry, scientists still don't know for sure how life began. The outline of a theory is available, but we are a long way from having a blow-by-blow account of the processes that transformed matter into life. Even the exact location of the incubator remains a frustrating mystery. It could be that life didn't originate on Earth at all; it may have come here from space.
The challenge facing scientists struggling to explain the origin of life is the need to piece together a narrative of events that happened billions of years ago and have left little or no trace. The task is a daunting one. Fortunately, during the last few years some remarkable discoveries have been made about the likely nature of Earth's most primitive organisms. There have also been great strides in laboratory procedures, and a growing understanding of conditions in the early solar system. The recent revival of interest in the possibility of life on Mars has also served to broaden the thinking about the conditions necessary for life. Together, these developments have elevated the subject from a speculative backwater of science to a mainstream research project.
The problem of how and where life began is one of the great outstanding mysteries of science. But it is more than that. The story of life's origin has ramifications for philosophy and even religion. Answers to such profound questions as whether we are the only sentient beings in the universe, whether life is the product of random accident or deeply rooted law, and whether there may be some sort of ultimate meaning to our existence, hinge on what science can reveal about the formation of life.
In a subject supercharged with such significance, lack of agreement is unsurprising. Some scientists regard life as a bizarre chemical freak, unique in the universe, whereas others insist that it is the expected product of felicitous natural laws. If the magnificent edifice of life is the consequence of a random and purely incidental quirk of fate, as the French biologist Jacques Monod claimed, we must surely find common cause with his bleak atheism, so eloquently expressed in these words: "The ancient covenant is in pieces: man at last knows that he is alone in the unfeeling immensity of the universe, out of which he has emerged only by chance. Neither his destiny nor his duty have been written down." But if it transpires that life emerged more or less on cue as part of the deep lawfulness of the cosmos -- if it is scripted into the great cosmic drama in a basic manner -- it hints at a universe with a purpose. In short, the origin of life is the key to the meaning of life.
In the coming chapters I shall carefully examine the latest scientific evidence in an attempt to confront these contentious philosophical issues. Just how bio-friendly is the universe? Is life unique to Planet Earth? How can something as complex as even the simplest organism be the product of straightforward physical processes?
Life's mysterious origin
The origin of life appears...to be almost a miracle, so many are the conditions which would have had to be satisfied to get it going.
According to the Australian Aborigines of the Kimberley, in the Creation Time of Lalai, Wallanganda, the sovereign of the galaxy and maker of the Earth, let fresh water fall from space upon Wunggud, the giant Earth Snake. Wunggud, whose very body is made of the primeval material, was coiled into a ball of jellylike substance, ngallalla yawun. On receiving the invigorating water, Wunggud stirred. She formed depressions in the ground, garagi, to collect the water. Then she made the rain, and initiated the rhythmic processes of life: the seasons, the reproductive cycles, menstruation. Her creative powers shaped the landscape and brought forth all creatures and growing things, over which she still holds dominion.
All cultures have their creation myths, some more colorful than others. For centuries, Western civilization looked to the Bible for enlightenment on the subject. The biblical text seems disappointingly bland when set beside the Australian story: God created life in more or less its present form ab initio, as the fifth miracle.
Not far from the Kimberley -- across the Great Sandy Desert, in the mountains of the Pilbara -- lie the oldest known fossils on Earth. These extraordinary remains form part of the scientific account of creation. Science takes as its starting point the assumption that life wasn't made by a god or a supernatural being: it happened unaided and spontaneously, as a natural process.
Over the past two centuries, scientists have painstakingly pieced together the history of life. The fossil record shows clearly that ancient life was very different from extant life. Generally speaking, the farther back in time you go, the simpler were the living things that inhabited Earth. The great proliferation of complex life forms occurred only within the last billion years. The oldest well-documented true animal fossils, also to be found in Australia (in the Flinders Ranges, north of Adelaide), are dated at 560 million years. Known as Ediacara, they include creatures resembling jellyfish. Shortly after this epoch, about 545 million years ago, there began a veritable explosion of species, culminating in the colonization of the land by large plants and animals. But before about one billion years ago, life was restricted to single-celled organisms. This record of complexification and diversification is broadly explained by Darwin's theory of evolution, which paints a picture of species continually branching and rebranching to form more and more distinct lineages. Conversely, in the past these lineages converge. The evidence strongly affirms that all life on Earth descended via this branching process from a common ancestor. That is, every person, every animal and plant, every invisible bacterium can be traced back to the same tiny microbe that lived billions of years ago, and thence back to the first living thing. What remains to be explained -- what stands out as the central unsolved puzzle in the scientific account of life -- is how the first microbe came to exist.
Peering into life's innermost workings serves only to deepen the mystery. The living cell is the most complex system of its size known to mankind. Its host of specialized molecules, many found nowhere else but within living material, are themselves already enormously complex. They execute a dance of exquisite fidelity, orchestrated with breathtaking precision. Vastly more elaborate than the most complicated ballet, the dance of life encompasses countless molecular performers in synergetic coordination. Yet this is a dance with no sign of a choreographer. No intelligent supervisor, no mystic force, no conscious controlling agency swings the molecules into place at the right time, chooses the appropriate players, closes the links, uncouples the partners, moves them on. The dance of life is spontaneous, self-sustaining, and self-creating.
How did something so immensely complicated, so finessed, so exquisitely clever, come into being all on its own? How can mindless molecules, capable only of pushing and pulling their immediate neighbors, cooperate to form and sustain something as ingenious as a living organism?
Solving this riddle is an exercise in many disciplines -- biology foremost, but chemistry, geology, astronomy, mathematics, computing, and physics contribute too. It is also an exercise in history. Few scientists believe that life began in a single monumental leap. No physical process abruptly "breathed life" into inert matter. There must have been a long and complicated transitional stage between the nonliving and the first truly living thing, an extended chronology of events unlikely to be preordained in its myriad details. A law of nature could not alone explain how life began, because no conceivable law would compel a legion of atoms to follow precisely a prescribed sequence of assemblage. So, although complying with the laws of nature, the actual route to life must have owed much to chance and circumstance -- or contingency, as philosophers call it. Because of this, and because of our ignorance about the conditions that prevailed in the remote past, we will never know exactly which particular sequence of events produced the first life form.
The mystery of biogenesis runs far deeper than ignorance over details, however. There is also a profound conceptual problem concerning the very nature of life. I have on my desk one of those lamps, popular in the 1960s, containing two differently colored fluids that don't mix. Blobs of one fluid slowly rise and fall through the other. People often comment that the behavior of the blobs is "lifelike." The lamp is not alone in this respect. Many inanimate systems have lifelike qualities -- flickering flames, snowflakes, cloud patterns, swirling eddies in a river. What is it that distinguishes genuine living organisms from merely lifelike systems? It is not simply a matter of degree; there is a real difference between the nature of the living and the merely lifelike. If a chicken lays an egg, it is a fair bet that the hatched fledgling will also be a chicken; but try predicting the precise shape of the next snowflake. The crucial difference is that the chicken is made according to specific genetic instructions, whereas lamp blobs, snowflakes, and eddies form willy-nilly. There is no gene for a snowflake. Biological complexity is instructed complexity or, to use modern parlance, it is information-based complexity. In the coming chapters I shall argue that it is not enough to know how life's immense structural complexity arose; we must also account for the origin of biological information. As we shall see, scientists are still very far from solving this fundamental conceptual puzzle. Some people rejoice in such ignorance, imagining that it leaves room for a miraculous creation. However, it is the job of science to solve mysteries without recourse to divine intervention. Just because scientists are still uncertain how life began does not mean life cannot have had a natural origin.
How does one go about assembling a scientific account of the genesis of life? At first sight the task seems hopeless. The traditional method of seeking rock fossils offers few clues. Most of the delicate prebiotic molecules that gave rise to life will long ago have been eradicated. The best we can hope for is some degraded chemical residue of the ancestral organisms from which familiar cellular life evolved.
If we had to rely on rock fossils alone, the task of understanding the origin and early evolution of life would indeed be formidable. Fortunately, there is another line of evidence altogether. It too stretches back into the dim and distant past, but it exists right here and now, inside extant life forms. Biologists are convinced that relics of ancient organisms live on in the structures and biochemical processes of their descendants -- including human beings. By studying how the modern cell operates, we can glimpse remnants of ancestral life at work -- a peculiar molecule here, an odd chemical reaction there -- in the same way that out-of-place coins, rusty tools, or suspicious mounds of earth alert the archaeologist. So, amid the intricate processes going on inside modern organisms, traces of primeval life survive, forming a bridge with our distant past. Analyzing these obscure traces, scientists have made a start on reconstructing the physical and chemical pathways that may have brought the first living cell into existence.
Even with such biochemical clues, the task of reconstruction would still be largely guesswork were it not for the recent discovery of certain "living fossils" -- microbes that inhabit weird and extreme environments. These so-called superbugs are being intensively investigated, and look set to revolutionize microbiology. It could be that we are glimpsing in these offbeat microbes something close to the primitive organisms that spawned all life on Earth. More clues may come from the search for life on Mars and other planets, and the study of comets and meteorites. By piecing together all these strands of evidence, we may yet be able to deduce, in broad outline at least, the way in which life first emerged in the universe.
What is life?
Before we tackle the problem of its origin, it is important to have a clear idea of what life is. Fifty years ago, many scientists were convinced the mystery of life was about to be solved. Biologists recognized that the key lay among the molecular components within the cell. Physicists had by then made impressive strides elucidating the structure of matter at the atomic level, and it looked as if they would soon clear up the problem of life too. The agenda was set by the publication of Erwin Schrödinger's book What Is Life? in 1944. Living organisms, it seemed at the time, would turn out to be nothing more than elaborate machines with microscopic parts that could be studied using the techniques of experimental physics. Careful investigation lent support to this view. The living cell is indeed crammed with miniature machines. All it required was an assembly manual and the problem would be solved. Today, however, the picture of the cell as nothing but a very complicated mechanism seems rather naïve. To be sure, molecular biology has scored some dazzling successes, but scientists still can't quite put their finger on exactly what it is that separates a living organism from other types of physical objects. Though treating organisms as mechanisms has undoubtedly proved very fruitful, it is important not to be mesmerized by its simplistic charm. Mechanistic explanation is an important part of understanding life, not the whole story.
Let me give a striking example of where the problem lies. Imagine throwing a dead bird and a live bird into the air. The dead bird will land with a thud, predictably, a few meters away. The live bird may well end up perched improbably on a television aerial across town, on the branch of a tree, on a rooftop, in a hedgerow, or in a nest. It would be hard to guess in advance exactly where.
As a physicist, I am used to thinking of matter as passive, inert and clodlike, responding only when coerced by external forces -- as when the dead bird plunges to the ground under the tug of gravity. But living creatures literally have a life of their own. It is as if they contain some inner spark that gives them autonomy, so that they can (within limits) do as they please. Even bacteria do their own thing in a restricted way. Does this inner freedom, this spontaneity, imply that life defies the laws of physics, or do organisms merely harness those laws for their own ends? If so, how? And where do such "ends" come from in a world apparently ruled by blind and purposeless forces?
This property of autonomy, or self-determination, seems to touch on the most enigmatic aspect that distinguishes living from nonliving things, but it is hard to know where it comes from. What physical properties of living organisms confer autonomy upon them? Nobody knows.
Autonomy is one important characteristic of life. But there are many others, including the following:
Reproduction. A living organism should be able to reproduce. However, some nonliving things, like crystals and bush fires, can reproduce, whereas viruses, which many people would regard as living, are unable to multiply on their own. Mules are certainly living, even though, being sterile, they cannot reproduce. A successful offspring is more than a mere facsimile of the original; it also includes a copy of the replication apparatus. To propagate their genes beyond the next generation, organisms must replicate the means of replication, as well as replicating the genes themselves.
Metabolism. To be considered as properly alive, an organism has to do something. Every organism processes chemicals through complicated sequences of reactions, and as a result garners energy to enable it to carry out tasks, such as movement and reproduction. This chemical processing and energy liberation is called metabolism. However, metabolism cannot be equated with life. Some micro-organisms can become completely dormant for long periods of time, with their vital functions shut down. We would be reluctant to pronounce them dead if it is possible for them to be revived.
Nutrition. This is closely related to metabolism. Seal up a living organism in a box for long enough and in due course it will cease to function and eventually die. Crucial to life is a continual throughput of matter and energy. For example, animals eat, plants photosynthesize. But a flow of matter and energy alone fails to capture the real business of life. The Great Red Spot of Jupiter is a fluid vortex sustained by a flow of matter and energy. Nobody suggests it is alive. In addition, it is not energy as such that life needs, but something like useful, or free, energy. More on this later.
Complexity. All known forms of life are amazingly complex. Even single-celled organisms such as bacteria are veritable beehives of activity involving millions of components. In part, it is this complexity that guarantees the unpredictability of organisms. On the other hand, a hurricane and a galaxy are also very complex. Hurricanes are notoriously unpredictable. Many nonliving physical systems are what scientists call chaotic -- their behavior is too complicated to predict, and may even be random.
Organization. Maybe it is not complexity per se that is significant, but organized complexity. The components of an organism must cooperate with each other or the organism will cease to function as a coherent unity. For example, a set of arteries and veins are not much use without a heart to pump blood through them. A pair of legs will offer little locomotive advantage if each leg moves on its own, without reference to the other. Even within individual cells the degree of cooperation is astonishing. Molecules don't simply career about haphazardly, but show all the hallmarks of a factory assembly line, with a high degree of specialization, a division of labor, and a command-and-control structure.
Growth and development. Individual organisms grow and ecosystems tend to spread (if conditions are right). But many nonliving things grow too (crystals, rust, clouds). A subtler yet altogether more significant property of living things, treated as a class, is development. The remarkable story of life on Earth is one of gradual evolutionary adaptation, as a result of variety and novelty. Variation is the key. It is replication combined with variation that leads to Darwinian evolution. We might consider turning the problem upside down and say: if it evolves in the way Darwin described, it lives.
Information content. In recent years scientists have stressed the analogy between living organisms and computers. Crucially, the information needed to replicate an organism is passed on in the genes from parent to offspring. So life is information technology writ small. But, again, information as such is not enough. Though there is information aplenty in the positions of the fallen leaves in a forest, it doesn't mean anything. To qualify for the description of living, information must be meaningful to the system that receives it: there must be a "context." In other words, the information must be specified. But where does this context itself come from, and how does a meaningful specification arise spontaneously in nature?
Hardware/software entanglement. As we shall see, all life of the sort found on Earth stems from a deal struck between two very different classes of molecules: nucleic acids and proteins. These groups complement each other in terms of their chemical properties, but the contract goes much deeper than that, to the very heart of what is meant by life. Nucleic acids store life's software; the proteins are the real workers and constitute the hardware. The two chemical realms can support each other only because there is a highly specific and refined communication channel between them mediated by a code, the so-called genetic code. This code, and the communication channel -- both advanced products of evolution -- have the effect of entangling the hardware and software aspects of life in a baffling and almost paradoxical manner.
Permanence and change. A further paradox of life concerns the strange conjunction of permanence and change. This ancient puzzle is sometimes referred to by philosophers as the problem of being versus becoming. The job of genes is to replicate, to conserve the genetic message. But without variation, adaptation is impossible and the genes will eventually get snuffed out: adapt or die is the Darwinian imperative. How do conservation and change coexist in one system? This contradiction lies at the heart of biology. Life flourishes on Earth because of the creative tension that exists between these conflicting demands; we still do not fully understand how the game is played out.
It will be obvious that there is no easy answer to Schrödinger's question: what is life? No simple defining quality distinguishes the living from the nonliving. Perhaps that is just as well, because science presents the natural world as a unity. Anything that drives a wedge between the domains of the living and the nonliving risks biasing us towards the belief that life is magical or mystical, rather than something entirely natural. It is a mistake to seek a sharp dividing line between living and nonliving systems. You can't strip away the frills and identify some irreducible core of life, such as a particular molecule. There is no such thing as a living molecule, only a system of molecular processes that, taken collectively, may be considered alive.
I can summarize this list of qualities by stating that, broadly speaking, life seems to involve two crucial factors: metabolism and reproduction. We can see that in our own lives. The most basic things that human beings do are breathe, eat, drink, excrete, and have sex. The first four activities are necessary for metabolism; the last is necessary for reproduction. It is doubtful that we would consider a population of entities that have metabolism but no reproduction, or reproduction without metabolism, to be living in the full sense of the term.
The life force and other discredited notions
Given the elusive character of life, it is not surprising that some people have resorted to mystical interpretations. Perhaps organisms are infused with some sort of essence or soul that brings them alive? The belief that life requires an extra ingredient over and above ordinary matter obeying normal physical laws is known as vitalism. It is a beguiling idea with a long history. The Greek philosopher Aristotle proposed that a special quality which he called the life force, or psyche, bestowed upon living organisms their remarkable properties, especially that of autonomy or self-movement. Aristotle's psyche was different from the later Christian idea of the soul as a special and separate entity. Indeed, in Aristotle's scheme, everything in the universe was considered to possess intrinsic properties that determined its behavior. In effect, he regarded the whole cosmos as an organism.
Over the centuries, the notion of a life force reappeared in many different guises. From time to time attempts were made to link it with specific substances -- for example, air. Perhaps this was not unreasonable; after all, breathing stops on death, and artificial respiration can sometimes restore vital functions. Later, blood became the life-giving substance. These ancient myths live on in expressions like "breathing life" into something, or "draining away the lifeblood," as if there were more than one kind of blood.
As scientific understanding advanced, so the life force became associated with more sophisticated concepts. Claims were made that it was attributable to phlogiston, or the ether -- imaginary substances that themselves became discredited in due course. Another idea, popular in the eighteenth century, was to identify the life force with electricity. At that time electrical phenomena were sufficiently mysterious to serve such a purpose, and Volta's famous experiments demonstrated that electricity could make severed frog muscles twitch. The belief that electricity could revivify matter was dramatically exploited by Mary Shelley in her famous novel Frankenstein, in which the monster, assembled from dead human organs, is brought to life with a huge spark from a thunderstorm. In the late nineteenth century, radioactivity replaced electricity as the latest mysterious phenomenon; sure enough, claims were made that a solution of gelatine could be instilled with life by exposing it to emissions from radium crystals.
These early attempts to pin down the life force appear to us today as plain daft. Nevertheless, the assumption that life requires something in addition to normal physical forces survived well into the twentieth century. For a long time, chemicals made by organisms were regarded as somehow different from the rest. Even today, the subject of chemistry is divided into "organic" and "inorganic." The implication was that organic substances like alcohol, formaldehyde, and urea somehow retain the magical essence of life even when separated from any living organism. By contrast, inorganic substances such as common salt are well and truly dead.
It came as something of a shock to vitalists when, in 1828, Friedrich Wöhler managed to synthesize urea from ammonium cyanate, an inorganic substance. By breaching the invisible barrier between the inorganic and organic worlds, and demonstrating that life itself was not needed to make organic substances, Wöhler scotched the idea that organic chemicals are subtly different from the rest. No longer was it necessary to posit two distinct types of matter. A common set of principles would henceforth govern the chemistry of both the living and the nonliving world. We now know that atoms are cycled through the biosphere, in and out of living organisms, all the time. Every carbon atom in your body is identical to a carbon atom in the air or in a lump of chalk. There is no mysterious "zing" that renders your carbon atoms "alive" while those around you are dead; no lifelike quality that a carbon atom acquires when you eat it, and gives up when you exhale it.
In spite of the blurring of the distinction between organic and inorganic chemistry, vitalism lived on, popularized by some well-known philosophers such as Henri Bergson in France. In fact, it entered a more scientific phase with the work of a German embryologist, Hans Driesch, in the early 1900s. Driesch was impressed that embryos could be mutilated early in their growth yet still recover to produce a normal organism. These and other remarkable properties of organic development led him to propose that the emergence of the correct form of the organism, in all its intricate complexity, must be under the control of a guiding life force, which he termed entelechy. Driesch realized that the ordering properties of entelechy would place it in conflict with normal physical forces and the law of conservation of energy. He suggested that entelechy operates by affecting the timing of molecular interactions in a way that introduces a cooperative, holistic pattern.
Although embryo development remains incompletely understood, enough is known about it, and biological pattern formation in general, to convince biologists that entelechy, like any other version of the life-force concept, is an unnecessary complication. This hasn't prevented many nonscientists from clinging to vitalistic ideas today. Beliefs range from the quasi-scientific, such as Kirlian photography, where a photographic image showing a sort of corona glow around a person's hand is produced by placing it in a strong electric field, to the unashamedly mystical ideas of yin and yang energy flows, karmas, and auras that appear only to gifted psychics. Unfortunately for the mystics, no properly conducted scientific experiment has ever demonstrated a life force at work, nor do we need such a force to explain what goes on inside biological organisms.
A further reason to reject vitalistic explanations of life is their totally ad hoc character. If the life force manifests itself only in living things, it has little or no explanatory value. To make this point clear, let me use the analogy of a steam locomotive. Ask: what is a steam locomotive and how does it work? An engineer could give a very detailed reply to this question. He could tell you about pistons and governors and steam pressure and the thermodynamics of combustion. He could say which bits moved what to make the wheels turn. He might also wax lyrical and describe the gleaming brass and belching smoke.
Now, it might be objected that the engineer's account, however complete, would still leave out the essential traininess of the locomotive, the thing that endows a mere heap of connected metallic parts with the thrilling power, the majesty, the elegance of movement, the sense of presence that one associates with a steam locomotive. So are we to suppose that, in addition to being a collection of metal components, a locomotive must also be infused with "traininess" to make it the genuine item?
Of course, that is absurd. Where else are we to find traininess other than in a train? The steam locomotive simply is the bits and pieces of which it is composed, arranged together in the manner that they are. That is all. There is no extra ingredient, no traininess, that the manufacturer must add to "bring the machine alive" for its intended function. Likewise, in seeking to understand the origin of life, scientists look to normal molecular processes to explain what happened, and not to an external life force to enliven dead matter. What makes life so remarkable, what distinguishes the living from the nonliving, is not what organisms are made of but how they are put together and function as wholes.
Even though vitalism is discredited, a germ of the idea is correct. There is a nonmaterial "something" inside living organisms, something unique and, literally, vital to their operation. It is not an essence or a force or an atom with a zing. That extra something is a certain type of information or, to use the modern jargon, software.
The tale of the ancient molecule
Inside each and every one of us lies a message. It is inscribed in an ancient code, its beginnings lost in the mists of time. Decrypted, the message contains instructions on how to make a human being. Nobody wrote the message; nobody invented the code. They came into existence spontaneously. Their designer was Mother Nature herself, working only within the scope of her immutable laws and capitalizing on the vagaries of chance. The message isn't written in ink or type, but in atoms, strung together in an elaborately arranged sequence to form DNA, short for deoxyribonucleic acid. It is the most extraordinary molecule on Earth.
Human DNA contains many billions of atoms, linked in the distinctive form of two coils entwined in mutual embrace. This famous double helix is in turn bundled up in a very convoluted shape. Stretch out the DNA in just one cell of your body and it would make a thread two meters long. These are big molecules indeed.
Although DNA is a material structure, it is pregnant with meaning. The arrangement of the atoms along the helical strands of your DNA determines how you look and even, to a certain extent, how you feel and behave. DNA is nothing less than a blueprint -- or, more accurately, an algorithm or instruction manual -- for building a living, breathing, thinking human being.
We share this magic molecule with almost all other life forms on Earth. From fungi to flies, from bacteria to bears, organisms are sculpted according to their respective DNA instructions. Each individual's DNA differs from others in the same species (with the exception of identical twins), and differs even more from that of other species. But the essential structure -- the chemical makeup, the double-helix architecture -- is universal.
DNA is incredibly, unimaginably ancient. It almost certainly existed three and a half billion years ago. It makes nonsense of the phrase "as old as the hills": DNA was here long before any surviving hills on Earth. Nobody knows how or where the first DNA molecule formed. Some scientists even speculate that it is an alien invader, a molecule from Mars perhaps, or from a wandering comet. But however the first strand of DNA came to exist, our own DNA is very probably a direct descendant of it. For the crucial quality of DNA, the property that sets it apart from other big organic molecules, is its ability to replicate itself. Put simply, DNA is in the business of making more DNA, generation after generation, instruction manual after instruction manual, cascading down through the ages from microbes to man in an unbroken chain of copying.
Of course, copying as such produces only more of the same. Perfect replication of DNA would lead to a planet knee-deep in identical single-celled organisms. However, no copying process is totally reliable. A photocopier may create stray spots, a noisy telephone line will garble a fax transmission, and a computer glitch can spoil data transferred from hard disk to a floppy. When errors occur in DNA replication, they can manifest themselves as mutations in the organisms that inherit them. Mostly a mutation is damaging, just as a random word change in a Shakespeare sonnet would likely mar its beauty. But occasionally, quite by chance, an error might produce a positive benefit, conferring an advantage on the mutant. If the advantage is life-preserving, enabling the organism to reproduce itself more efficiently, then the miscopied DNA will out-replicate its competitors and come to predominate. Conversely, if the copying error results in a less well-adapted organism, the mutant strain will probably die out after a few generations, eliminating this particular DNA variant.
This simple process of replication, variation, and elimination is the basis of Darwinian evolution. Natural selection -- the continual sifting of mutants according to their fitness -- acts like a ratchet, locking in the advantageous errors and discarding the bad. Starting with the DNA of some primitive ancestor microbe, bit by bit, error by error, the increasingly lengthy instructions for building more complex organisms came to be constructed.
Some people find the idea of an instruction manual that writes itself simply by accumulating chance errors hard to swallow, so let me go over the argument once more, using a slightly different metaphor. Think of the information in human DNA as the score for a symphony. This is a grand symphony indeed, a mighty orchestral piece with hundreds of musicians playing thousands of notes. By comparison, the DNA of the ancient ancestor microbe is but a simple melody. How does a melody turn into a symphony?
Suppose a scribe is asked to copy the original tune as a musical score. Normally the copying process is faithful, but once in a while a quaver becomes a crotchet, a C becomes a D. A slip of the pen introduces a slight change of tempo or pitch. Occasionally a more serious error leads to a major flaw in the piece, an entire bar omitted or repeated perhaps. Mostly these mistakes will spoil the balance or harmony, so that the score is of no further use: nobody would wish to listen to its musical rendition. But very occasionally the scribe's slip of the pen will add an imaginative new sound, a pleasing feature, a successful addition or alteration, quite by chance. The tune will actually improve, and be approved for the future. Now imagine this process of improvement and elaboration continuing through trillions of copying procedures. Slowly but surely, the tune will acquire new features, develop a richer structure, evolve into a sonata, even a symphony.
The crucial point about this metaphor, and it cannot be stressed too strongly, is that the symphony comes into being without the scribe's ever having the slightest knowledge of, or interest in, music. The scribe might have been deaf from birth and know nothing whatever of melodies. It doesn't matter, because the scribe's job is not to compose the music but to copy it. Where the metaphor fails is in the selection process. There is no cosmic musician scrutinizing the score of life and exercising quality control. There is only nature, red in tooth and claw, applying a simple and brutal rule: if it works, keep it; if it doesn't, kill it. And "works" here is defined by one criterion and one criterion only, which is replication efficiency. If the mistake results in more copies made, then, by definition, without any further considerations, it works. If A out-replicates B, even by the slightest margin, then, generations on, there will be many more A's than B's. If A and B have to compete for space or resources, it's a fair bet that A will soon eliminate B entirely. A survives, B dies.
Darwinism is the central principle around which our understanding of biology is constructed. It offers an economical explanation of how a relatively simple genetic message elaborates itself over the eons to create molecules of DNA complex enough to produce a human being. Once the basic manual, the precursor DNA, existed in the first place, random errors and selection might gradually be able to evolve it. Good genes are kept, bad genes are discarded. Later I shall discuss the adequacy of this austere explanation, but for now I am more concerned with the starting point. Obviously Darwinian evolution can operate only if life of some sort already exists (strictly, it requires not life in its full glory, only replication, variation, and selection). Darwinism can offer absolutely no help in explaining that all-important first step: the origin of life. But if the central principle of life fails to explain the origin of life, we are left with a problem. What other principle or principles might explain how it all began?
To solve this problem, we must seek clues. Where can we look for clues about the origin of life? A good place to begin is to ask where life itself began. If we discover the place where life started, we may be able to guess the physical conditions that accompanied its genesis. Then we can set about studying the chemical processes that occur in such conditions, and build up an understanding of the prebiotic phase bit by bit.
Microbes and the search for Eden
When I was a youngster I was occasionally coerced into attending Sunday school, an ordeal which I hated. The only positive memory I have is of browsing through a picture book describing the Garden of Eden. The image it conjured up was of a well-ordered parkland in which the sun always shone and exotic animals roamed without fear, presumably being entirely vegetarian. It was a nice contrast to life in a dreary London suburb. Unfortunately, the biblical Garden of Eden turned out to be a myth. Still, there must have been a place where Earth's earliest creatures lived, a sort of scientific Eden. Where was it located?
I am writing this section of the book on a showery spring day in the Adelaide hills. The winter rain has turned the countryside green, and everywhere I look a luxuriant canopy of trees towers over a profusion of smaller bushes, shrubs, and grasses. Birds swoop in the sky and flash colorfully between the branches. Hidden among the foliage are snakes, lizards, spiders, and insects. There will also be rabbits, possums, mice, echidnas, and the occasional koala or kangaroo. Even in this arid country, life is conspicuous and exuberant.
The sheer variety of living things has delighted people for thousands of years. But it is only comparatively recently, with the invention of the microscope, that the true diversity of life on Earth has been revealed. For, even as naturalists marveled at the biological richness of a rain forest or a coral reef, a still greater cornucopia lay unseen all around them. This invisible biosphere is the realm of the micro-organisms, single-celled specks of life that inhabit almost every available nook and cranny the planet can provide. Long dismissed as "mere germs," microbes are now known to dominate the tree of life. "You could go out into your back yard," says John Holt of Michigan State University, "and if you really put your mind to it, you could find a thousand new species in not much time." Holt's comment seems exaggerated until you realize that a spoonful of good-quality soil may contain ten trillion bacteria representing ten thousand different species! In total, the mass of micro-organisms on Earth could be as great as a hundred trillion tons -- more than all the visible life put together.
To be sure, the physical effects caused by micro-organisms are often very visible: through infectious diseases, the fermentation of alcohol, and the degeneration of food, for example. Even so, microbes have been persistently underrated by humans, perhaps because they are so much smaller than we. Stephen Jay Gould believes we should correct this chauvinism by referring to the present era as the Age of Bacteria, so thoroughly do these tiny creatures overwhelm all others in population numbers and variety. By contrast, so-called higher organisms like humans, dogs, and primroses occupy just a few of the peripheral branches of the tree of life.
Size is not the only reason why microbes tend to get overlooked. They aren't easy to culture in the laboratory, and in the wild a lot of them are inert. Also, many different species of bacteria appear superficially identical, and until recently microbiologists tended to lump them together in classification schemes. Now, with the powerful techniques of molecular sequencing, the real genetic differences are revealed. Bacteria that look the same under the microscope may turn out to share fewer genes with each other than they do with humans.
Gould points out that it has always been the Age of Bacteria. Indeed, for most of the duration of life on Earth, it has consisted of nothing but microbes. This sobering fact offers an opportunity, though. Because life began with microbes, we can expect to find important clues about the origin of life by studying living examples. The hope is that some of them will contain relics of their distant past in the form of unusual structures. Vestiges of ancient biochemistry may have been retained as redundant features -- the microbial equivalent of the human appendix. It is even possible that living microbes are carrying around within them molecular remnants of a prebiotic world.
By piecing together fragments of information from living microbes, we may be able to work out what the ancestral organism might have been like, and to guess where and how it lived. Unfortunately, you can't tell just by looking what the evolutionary history of micro-organisms might be. They have few anatomical features by which to classify them. No arms or legs, gills or lungs, eyes or ears present themselves for comparison. As I shall explain later, the evidence linking microbes to their ancient ancestors lies largely in their biochemistry -- in their genetic makeup and the metabolic pathways they employ. The techniques of modern molecular biology permit this evidence to be teased out. Like scraps of an ancient scroll covered in a half-forgotten text, this trail of molecular evidence, partly obliterated by the ravages of time, offers a seductive glimpse of an evolutionary past stretching back nearly four billion years.
Given that there are so many species of microbes, where should the search for molecular clues be concentrated? Today it is the aerobic and photosynthesizing bacteria that we most notice, but for over two billion years there was little or no free oxygen available on Earth. Yet microbes flourished in a variety of habitats, fermenting alcohol, producing methane, reducing sulfate. Some microbes maintain their ancient lifestyles today, and these are the ones most likely to offer clues to the earliest forms of life. Which suggests an intriguing idea: suppose there survives today an obscure niche, an exotic place, where conditions resemble the asteroid-battered, gas-shrouded, boiling inferno that was the primeval Planet Earth? If we look carefully, we might find relic organisms still living there, microbes that have changed little since the dawn of life.
Is this possible? Could there be such a place?
The answer is yes. And its location is as surprising as it is obscure. Deep beneath the sea, on the dark ocean floor, there are regions where the Earth's crust stretches and tears. Driven by powerful thermal forces deep inside the planet, the rocky strata of the seabed are continually shifting and straining. Here and there, along mid-ocean ridges, the crust is rent to expose molten rock to the icy ocean above. The oozing lava shrinks and cracks as it cools, creating a matrix of fissures and tunnels through which water circulates by convection, dissolving minerals as it goes. At the vents, the Earth spews forth a stream of searing fluid, liberally spiced with chemicals. The brutal encounter of superheated liquid with cold seawater creates chemical and thermal pandemonium.
It seems impossible to imagine that any form of life could inhabit such a harsh environment, more reminiscent of Hades than the Garden of Eden. Yet it does. Astonishingly, these volcanic ocean vents are home to a rich variety of microbes, some of them apparently relics of an ancient biology. Here in the black volcanic depths dwell the closest organisms we know to the first living creatures on Earth. In the coming chapters I shall describe how startling discoveries of submarine and subterranean superbugs are transforming our thinking about the origin of life and the possibility of life on Mars and elsewhere.
But first I need to explain some of the basic principles of biochemistry. Foremost among these are the laws of thermodynamics.
Copyright © 1999 by Orion Productions
The Search for the Origin and Meaning of Life
The Fifth Miracle
The Search for the Origin and Meaning of Life
In this provocative and far-reaching book, internationally acclaimed physicist and writer Paul Davies confronts one of science's great outstanding mysteries -- the origin of life.
Three and a half billion years ago, Mars resembled earth. It was warm and wet and could have supported primitive organisms. If life once existed on Mars, might it have originated there and traveled to earth inside meteorites blasted into space by cosmic impacts?
Davies builds on recent scientific discoveries and theories to address larger questions of existence: What, exactly, is life? Is it the inevitable by-product of physical laws, as many scientists maintain, or an almost miraculous accident? Are we alone in the universe, or will life emerge on all earthlike planets? And if there is life elsewhere in the universe, is it preordained to evolve toward greater complexity and intelligence?
Through his search for answers to these questions, Davies explores the ultimate mystery of mankind's existence -- who we are and what our place might be in the unfolding drama of the cosmos.
- Simon & Schuster |
- 304 pages |
- ISBN 9780684863092 |
- March 2000