The Fifth Miracle: The Search for the Origin and Meaning of Life - Couverture rigide

CHAPTER 1: The Meaning of Life

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.

FRANCIS CRICK

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 ...

Are We Alone in the Universe?

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.