Preface to the First Edition

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This book was born during a lunchtime conversation at the University of Washington faculty club, and then it simply took off. It was stimulated by a host of discoveries suggesting to us that complex life is less pervasive in the Universe than is now commonly assumed. In our discussions, it became clear that both of us believed such life is not widespread, and we decided to write a book explaining why.

Of course, we cannot prove that the equivalent of our planet's animal life is rare elsewhere in the Universe. Proof is a rarity in science. Our arguments are post hoc in the sense that we have examined Earth history and then tried to arrive at generalizations from what we have seen here. We are clearly bound by what has been called the Weak Anthropic Principle—that we, as observers in the solar system, have a strong bias in identifying habitats or factors leading to our own existence. To put it another way, it is very difficult to do statistics with an N of 1. But in our defense, we have staked out a position rarely articulated but increasingly accepted by many astrobiologists. We have formulated a null hypothesis, as it were, to the clamorous contention of many scientists and media alike that life—barroom-brawling, moral-philosophizing, human-eating, lesson-giving, purple-blooded bug-eyed monsters of high and low intelligence—is out there, or that even simple worm-like animals are commonly out there. Perhaps in spite of all the unnumbered stars, we are the only animals, or at least we number among a select few. What has been called the Principle of Mediocrity—the idea that Earth is but one of a myriad of like worlds harboring advanced life—deserves a counterpoint. Hence our book.

Writing this book has been akin to running a marathon, and we want to acknowledge and thank all those who offered sustaining draughts of information as we followed our winding path. Our greatest debts of gratitude we owe to Jerry Lyons of Copernicus, who invested so much interest in the project, and to our editor, Jonathan Cobb, who fine-tuned the project on scales ranging from basic organization of the book to its numerous split infinitives.

Many scientific colleagues gave much of themselves. Joseph Kirschvink of Cal Tech read the entire manuscript and spent endless hours thrashing through various concepts with us; his knowledge and genius illuminated our murky ideas. Guillermo Gonzalez changed many of our views about planets and habitable zones. Thor Hansen of Western Washington University described to us the concept of "stopping plate tectonics." Colleagues in the Department of Geological Sciences, including Dave Montgomery, Steve Porter, Bruce Nelson, and Eric Cheney, discussed many subjects with us. Many thanks to Victor Kress of the University of Washington for reading and critiquing the plate tectonics chapter. Dr. Robert Paine of the Department of Zoology saved us from making egregious errors about diversity. Numerous astrobiologists took time to discuss aspects of the science with us, including Kevin Zahnlee of NASA Ames, who patiently explained his position—one contrary to almost everything we believed—and in so doing markedly expanded our understanding and horizons. We are grateful to Jim Kasting of Penn State University for long discussions about planets and their formations. Thanks as well to Gustav Ahrrenius from UC Scripps, Woody Sullivan (astronomy) of the University of Washington, and John Baross of the School of Oceanography at the University of Washington. Jack Sepkoski of the University of Chicago generously sent new extinction data, Andy Knoll of Harvard contributed critiques by E-mail; Sam Bowring spent an afternoon sharing his data and his thoughts on the timing of major events in Earth history; Dolf Seilacher talked with us about ediacarans and the first evolution of life; Doug Erwin lent insight into the Permo/Triassic extinction; Jim Valentine and Jere Lipps of Berkeley gave us their insights into the late Precam-brian and animal evolution; David Jablonski described his views on body plan evolution. We are enormously grateful to David Raup, for discussions and archival material about extinction, and to Steve Gould for listening to and critiquing our ideas over a long Italian dinner on a rainy night in Seattle. Thanks to Tom Quinn of UW astronomy for illuminating the rates of obliquity change and to Dave Evans of Cal Tech, with whom we discussed the Pre-cambrian glaciations. Conway Leovy talked to us about atmospheric matters. With Bob Berner of Yale, we discussed matters pertaining to the evolution of the atmosphere through time. Steve Stanley of Johns Hopkins gave us insight into the Permo/Triassic extinction. Walter Alvarez and Allesandro Montanari talked with us about the K/T extinction. Bob Pepin gave us insight into atmospheric effects.

Ross Taylor of the Australian University provided useful information to us, and Geoff Marcy and Chris McKay discussed elements of the text. Doug Lin of U.C. Santa Cruz discussed the fate of planetary systems with "Bad" Jupiters. We are grateful to Al Cameron for use of his lunar formation results.

Peter D. Ward, Donald Brownlee

Seattle, August 1999

Introduction: The Astrobiology Revolution and the Rare Earth Hypothesis

On any given night, a vast array of extraterrestrial organisms frequent the television sets and movie screens of the world. From Star Wars and "Star Trek" to The X-Files, the message is clear: The Universe is replete with alien life forms that vary widely in body plan, intelligence, and degree of benevolence. Our society is clearly enamored of the expectation not only that there is life on other planets, but that incidences of intelligent life, including other civilizations, occur in large numbers in the Universe.

This bias toward the existence elsewhere of intelligent life stems partly from wishing (or perhaps fearing) it to be so and partly from a now-famous publication by astronomers Frank Drake and Carl Sagan, who devised an estimate (called the Drake Equation) of the number of advanced civilizations that might be present in our galaxy. This formula was based on educated guesses about the number of planets in the galaxy, the percentage of those that might harbor life, and the percentage of planets on which life not only could exist but could have advanced to exhibit culture. Using the best available estimates at the time, Drake and Sagan arrived at a startling conclusion: Intelligent life should be common and widespread throughout the galaxy. In fact, Carl Sagan estimated in 1974 that a million civilizations may exist in our Milky Way galaxy alone. Given that our galaxy is but one of hundreds of billions of galaxies in the Universe, the number of intelligent alien species would then be enormous.

The idea of a million civilizations of intelligent creatures in our galaxy is a breathtaking concept. But is it credible? The solution to the Drake Equation includes hidden assumptions that need to be examined. Most important, it assumes that once life originates on a planet, it evolves toward ever higher complexity, culminating on many planets in the development of culture. That is certainly what happened on our Earth. Life originated here about 4 billion years ago and then evolved from single-celled organisms to multicel-lular creatures with tissues and organs, climaxing in animals and higher plants. Is this particular history of life—one of increasing complexity to an animal grade of evolution—an inevitable result of evolution, or even a common one? Might it, in fact, be a very rare result?

In this book we will argue that not only intelligent life, but even the simplest of animal life, is exceedingly rare in our galaxy and in the Universe. We are not saying that life is rare—only that animal life is. We believe that life in the form of microbes or their equivalents is very common in the universe, perhaps more common than even Drake and Sagan envisioned. However, complex life—animals and higher plants—is likely to be far more rare than is commonly assumed. We combine these two predictions of the commonness of simple life and the rarity of complex life into what we will call the Rare Earth Hypothesis. In the pages ahead we explain the reasoning behind this hypothesis, show how it may be tested, and suggest what, if it is accurate, it may mean to our culture.

The search in earnest for extraterrestrial life is only beginning, but we have already entered a remarkable period of discovery, a time of excitement and dawning knowledge perhaps not seen since Europeans reached the New World in their wooden sailing ships. We too are reaching new worlds and are acquiring data at an astonishing pace. Old ideas are crumbling. New views rise and fall with each new satellite image or deep-space result. Each novel biological or paleontological discovery supports or undermines some of the myriad hypotheses concerning life in the Universe. It is an extraordinary time, and a whole new science is emerging: astrobiology, whose central focus is the condition of life in the Universe. The practitioners of this new field are young and old, and they come from diverse scientific backgrounds. Feverish urgency is readily apparent on their faces at press conferences, such as those held after the Mars Pathfinder experiments, the discovery of a Martian meteorite on the icefields of Antarctica, and the collection of new images from Jupiter's moons. In usually decorous scientific meetings, emotions boil over, reputations are made or tarnished, and hopes ride a roller coaster, for scientific paradigms are being advanced and discarded with dizzying speed. We are witnesses to a scientific revolution, and as in any revolution there will be winners and losers—both among ideas and among partisans. It is very much like the early 1950s, when DNA was discovered, or the 1960s, when the concept of plate tectonics and continental drift was defined. Both of these events prompted revolutions in science, not only leading to the complete reorganization of their immediate fields and to adjustments in many related fields, but also spilling beyond the boundaries of science to make us look at ourselves and our world in new ways. That will come to pass as well in this newest scientific revolution, the Astrobiology Revolution of the 1990s and beyond. What makes this revolution so startling is that it is happening not within a given discipline of science, such as biology in the 1950s or geology in the 1960s, but as a convergence of widely different scientific disciplines: astronomy, biology, paleontology, oceanography, microbiology, geology, and genetics, among others.

In one sense, astrobiology is the field of biology ratcheted up to encompass not just life on Earth but also life beyond Earth. It forces us to reconsider the life of our planet as but a single example of how life might work, rather than as the only example. Astrobiology requires us to break the shackles of conventional biology; it insists that we consider entire planets as ecological systems. It requires an understanding of fossil history. It makes us think in terms of long sweeps of time rather than simply the here and now. Most fundamentally, it demands an expansion of our scientific vision—in time and space.

Because it involves such disparate scientific fields, the Astrobiology Revolution is dissolving many boundaries between disciplines of science. A paleontologist's discovery of a new life form from billion-year-old rocks in Africa is of major consequence to a planetary geologist studying Mars. A submarine probing the bottom of the sea finds chemicals that affect the calculations of a planetary astronomer. A microbiologist sequencing a string of genes influences the work of an oceanographer studying the frozen oceans of Europa (one of Jupiter's moons) in the lab of a planetary geologist. The most unlikely alliances are forming, breaking down the once-formidable academic barriers that have locked science into rigid domains. New findings from diverse fields are being brought to bear on the central questions of astrobiol-ogy: How common is life in the universe? Where can it survive? Will it leave a fossil record? How complex is it? There are bouts of optimism and pessimism,- E-mails fly,- conferences are hastily assembled,- research programs are rapidly redirected as discoveries mount. The excitement is visceral, powerful, dizzying, relentless. The practitioners are captivated by a growing belief: Life is present beyond Earth.

The great surprise of the Astrobiology Revolution is that it has arisen in part from the ashes of disappointment and scientific despair. As far back as the 1950s, with the classic Miller-Urey experiments showing that organic matter could be readily synthesized in a test tube (thus mimicking early Earth environments), scientists thought they were on the verge of discovering how life originated. Soon thereafter, amino acids were discovered in a newly fallen meteorite, showing that the ingredients of life occurred in space. Radiotelescope observations soon confirmed this, revealing the presence of organic material in interstellar clouds. It seemed that the building blocks of life permeated the cosmos. Surely life beyond Earth was a real possibility.

When the Viking I spacecraft approached Mars in 1976, there was great hope that the first extraterrestrial life—or at least signs of it—would be found (see Figure I.1). But Viking did not find life. In fact, it found conditions hostile

Figure I.1 Percival Lowell's 1908 globe of Mars. Some thought that the linear features were irrigation canals built by Martians.

to organic matter: extreme cold, toxic soil and lack of water. In many people's minds, these findings dashed all hopes that extraterrestrial life would ever be found in the solar system. This was a crushing blow to the nascent field of as-trobiology.

At about this time there was another major disappointment: The first serious searches for "extrasolar" planets all yielded negative results. Although many astronomers believed that planets were probably common around other stars, this remained only abstract speculation, for searches using Earth-based telescopes gave no indication that any other planets existed outside our own solar system. By the early 1980s, little hope remained that real progress in this field would occur, for there seemed no way that we could ever detect worlds orbiting other stars.

Yet it was also at this time that a new discovery paved the way for the interdisciplinary methods now commonly used by astrobiologists. The 1980 announcement that the dinosaurs were not wiped out by gradual climate change (as was so long thought) but rather succumbed to the catastrophic effects of the collision of a large comet with Earth 65 million years ago, was a watershed event in science. For the first time, astronomers, geologists, and biologists had reason to talk seriously with one another about a scientific problem common to all. Investigators from these heretofore separate fields found themselves at the same table with scientific strangers—all drawn there by the same question: Could asteroids and comets cause mass extinction? Now, 20 years later, some of these same participants are engaged in a larger quest: to discover how common life is on planets beyond Earth.

The indication that there was no life on Mars and the failure to find extrasolar planets had damped the spirits of those who had begun to think of themselves as astrobiologists. But the field involves the study of life on Earth as well as in space, and it was from looking inward—examining this planet— that the sparks of hope were rekindled. Much of the revitalization of astrobi-ology came not from astronomical investigation but from the discovery, in the early 1980s, that life on Earth occurs in much more hostile environments than was previously thought. The discovery that some microbes live in searing temperatures and crushing pressures both deep in the sea and deep beneath the surface of our planet was an epiphany: If life survives under such conditions here, why not on—or in—other planets, other bodies of our solar system, or other plants and moons of far-distant stars?

Just knowing that life can stand extreme environmental conditions, however, is not enough to convince us that life is actually there. Not only must life be able to live in the harshness of a Mars, Venus, Europa, or Titan,- it must also have been able either to originate there or to travel there. Unless it can be shown that life can form, as well as live, in extreme environments, there is little hope that even simple life is widespread in the Universe. Yet here, too, revolutionary new findings lead to optimism. Recent discoveries by geneticists have shown that the most primitive forms of life on Earth—those that we might expect to be close to the first life to have formed on our planet— are exactly those tolerant life forms that are found in extreme environments. This suggests to some biologists that life on Earth originated under conditions of great heat, pressure, and lack of oxygen—just the sorts of conditions found elsewhere in space. These findings give us hope that life may indeed be widely distributed, even in the harshness of other planetary systems.

The fossil record of life on our own planet is also a major source of relevant information. One of the most telling insights we have gleaned from the fossil record is that life formed on Earth about as soon as environmental conditions allowed its survival. Chemical traces in the most ancient rocks on Earth's surface give strong evidence that life was present nearly 4 billion years ago. Life thus arose here almost as soon as it theoretically could. Unless this occurred utterly by chance, the implication is that nascent life itself forms— is synthesized from nonliving matter—rather easily. Perhaps life may originate on any planet as soon as temperatures cool to the point where amino acids and proteins can form and adhere to one another through stable chemical bonds. Life at this level may not be rare at all.

The skies too have yielded astounding new clues to the origin and distribution of life in the Universe. In 1995 astronomers discovered the first extrasolar planets orbiting stars far from our own. Since then, a host of new planets have been discovered, and more come to light each year.

For a while, some even thought we had found the first record of extraterrestrial life. A small meteorite discovered in the frozen icefields of Antarctica appears to be one of many that originated on Mars, and at least one of these may be carrying the fossilized remains of bacteria-like organisms of extraterrestrial origin. The 1996 discovery was a bombshell. The President of the United States announced the story in the White House, and the event triggered an avalanche of new effort and resolve to find life beyond Earth. But evidence—at least from this particular meteorite—is highly controversial.

All of these discoveries suggest a similar conclusion: Earth may not be the only place in this galaxy—or even in this solar system—with life. Yet if other life is indeed present on planets or moons of our solar system, or on far-distant planets circling other stars in the Universe, what kind of life is it? What, for example, will be the frequency of complex metazoans, organisms with multiple cells and integrated organ systems, creatures that have some sort of behavior—organisms that we call animals? Here too a host of recent discoveries have given us a new view. Perhaps the most salient insights come, again, from Earth's fossil record.

New ways of more accurately dating evolutionary advances recognized in the Earth's fossil record, coupled with new discoveries of previously unknown fossil types, have demonstrated that the emergence of animal life on this planet took place later in time, and more suddenly, than we had suspected. These discoveries show that life, at least as seen on Earth, does not progress toward complexity in a linear fashion but does so in jumps, or as a series of thresholds. Bacteria did not give rise to animals in a steady progression. Instead, there were many fits and starts, experiments and failures. Although life may have formed nearly as soon as it could have, the formation of animal life was much more recent and protracted. These findings suggest that complex life is far more difficult to arrive at than evolving life itself and that it takes a much longer time period to achieve.

It has always been assumed that attaining the evolutionary grade we call animals would be the final and decisive step: that once this level of evolution was achieved, a long and continuous progression toward intelligence should occur. However, another insight of the Astrobiological Revolution has been that attaining the stage of animal life is one thing, but maintaining that level is quite something else. New evidence from the geological record has shown that once it has evolved, complex life is subject to an unending succession of planetary disasters that create what are known as mass extinction events. These rare but devastating events can reset the evolutionary timetable and destroy complex life, while sparing simpler life forms. Such discoveries again suggest that the conditions hospitable to the evolution and existence of complex life are far more specific than those that allow life's formation. On some planets, then, life might arise and animals eventually evolve—only to be quickly destroyed by a global catastrophe.

To test the Rare Earth Hypothesis—the paradox that life may be nearly everywhere but complex life almost nowhere—may ultimately require travel to the distant stars. We cannot yet journey much beyond our own planet, and the vast distances that separate us from even the nearest stars may prohibit us from ever exploring planetary systems beyond our own. Perhaps this view is pessimistic, and we will ultimately find a way to travel much faster (and thus farther), through worm holes or other unforeseen methods of interstellar travel, enabling us to explore the Milky Way and perhaps other galaxies as well.

Let's assume that we do master interstellar travel of some sort and begin the search for life on other worlds. What types of worlds will harbor not just life, but complex life equivalent to the animals of Earth? What sorts of planets or moons should we look for? Perhaps the best way to search is simply to look for planets that resemble Earth, which is so rich with life. Do we have to duplicate this planet exactly to find animal life, though? What is it about our solar system and planet that has allowed the rise of complex life and nourished it so well? Addressing this issue in the pages ahead should help us answer the other questions we have posed.

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