Life on a Young Planet (2 page)

I don’t mean to suggest a
plus ça change
view of the past decade; with each month, our understanding of the early Earth and the life it sustained continues to deepen and broaden. But the big picture—the major patterns of earth system development visible across eons of time—is pretty well limned out in terms of pattern, while underlying questions of causation remain active targets of research. One topic assayed in
Life on a Young Planet
, however, has changed fundamentally.

In my final chapter, titled “Paleontology ad astra,” I described then gathering interest in life beyond the Earth, catalyzed by an important, if now widely abandoned, claim of biological fingerprints in a meteorite from Mars. Since that time, planetary exploration has been transformed by rovers and orbiters within our solar system and by revolutionary discoveries of planets orbiting other stars. In 2003, as my book was going to press, NASA launched a pair of rovers toward Mars; Spirit and Opportunity have provided unprecedented insights into the environmental history of Mars, for the first time providing a sustained geologist’s-eye view of our planetary neighbor. Remarkably, ten years after landing, Opportunity continues to explore ancient martian sedimentary rocks, telemetering physical and chemical records of
another
young planet. More recently, a newer and better equipped rover, christened Curiosity, has joined in the exploration of Mars, providing geological evidence for the once unthinkable (or at least, unverifiable) notion that a lake
formed early in martian history was habitable. At the same time, a satellite called Kepler has fundamentally changed our conception of the universe, discovering thousands of candidate planets orbiting nearby stars. Most are relatively small, and at least a few look a lot like Earth, confirming that planets like ours—and perhaps life, as well—are widely distributed in our galaxy.

Books may look forward or backward, but inevitably they register the time and place where they were written.
Life on a Young Planet
records that moment in time when biologists and Earth scientists began working together to build an integrative picture of our planet’s history. We now know that we can only understand life’s evolutionary trajectory by embedding it in the chronicle of Earth’s dynamic environmental history. That is the lesson of deep Earth history, and it is also a lesson for today, when human technology has emerged as an environmental force of geological prominence. Whether we look outward, searching for a second example of life, or forward, hoping to navigate wisely through an era of mounting global change, the record of Earth and life through time provides a fundamentally important catalog of experience that can help guide our actions.

Prologue

I
N HIS BRIEF
poem “When I heard the learn’d astronomer,” Walt Whitman recounts an evening spent at a scientific lecture. Proofs and figures fill the hall, oppressively weighting the air,

Til rising and gliding out I wander’d off by myself,
In the mystical moist night-air, and from time to time,
Look’d up in perfect silence at the stars.

Although written more than a century ago, Whitman’s poem resonates with a surprisingly large contemporary audience. Earlier attempts to understand the universe and our place in it distilled nature’s mystery into powerful narrative. Science, Whitman implies, replaces awe with statistics.

But does ignorance really outstrip understanding as the preferred route to wonder? As a paleontologist, I don’t think so. To me, the scientific account of life’s long history abounds in both narrative verve and mystery. Lucy’s skull and diminutive bones, carefully displayed in a museum drawer, transport me to the warm African savanna where humanity took shape 3 million years ago. Dinosaurs take me back twenty to seventy times further, to Mesozoic forests patrolled by astonishing beasts—if I can’t share the awe that
Tyrannosaurus
inspires in my son, it is, quite simply, a failing of maturity. Older yet are the trilobites, those joint-legged monarchs of the Cambrian seas, skittering around a tropical reef some 500 million years ago.

The fossils of animals, claimed by popular culture as much as by science, provide a biological chronicle of remarkable proportions. And yet, they record only the most recent chapters in Earth’s immense evolutionary epic. The complete history of life ranges over four
billion
years, through alien worlds of sulfurous oceans beneath asphyxiating air, past iron-breathing bacteria and microscopic chimeras, to arrive at last at our
familiar world of oxygen and ozone, forested valleys, and animals that swim, walk, and fly. Scheherazade could hardly have invented a more engaging tale.

Figure P.1.
The geologic timescale, showing the time relationships of major events in Phanerozoic evolution and the Precambrian rock units discussed in this book. (Ma = million years before present)

Nor is the story complete in its current telling. It can’t be, because each hard-won fact raises a new question. John Archibald Wheeler, one of the twentieth century’s preeminent physicists, once remarked that we live on an island in a sea of ignorance. This metaphor comes with an insightful corollary: as the island grows, built piece after piece by the accumulation of knowledge, its shoreline—the interface between knowledge and uncertainty—expands proportionately. There is much we do not understand about the history of life, and the same will be true of our grandchildren. But, then, if we knew all there was to know, scientific interest would cease. Textbooks may portray science as a codification of facts, but it is really a disciplined way of asking about the unknown.

This, then, is a book about history—the history of life before the dinosaurs, before the trilobites, before animals of any kind. My story begins with the initial diversification of animals in Cambrian seas. From there, the scene shifts to older rocks formed in earlier oceans. Having established how we can study life’s deeper history, we explore the fragmentary record of Earth’s earliest organisms and ruminate on the origins of life, before ascending again through geological time, following a trail of fossils and molecules that leads back to the Cambrian “Explosion” of animal life, now seen as both the culmination of life’s long Precambrian history and a radical departure from it.

I have three goals in writing this book. First is the obvious one. “Narrative history,” wrote C. Vann Woodward, is “the end product of what historians do. The narrative is where they put it together and make sense for the reader.” To me, science’s creation story is a deeply engrossing narrative that, told correctly, helps us to understand not only our biological past, but the Earth and life that surround us today. Contemporary biological diversity is the product of nearly 4 billion years of evolution. We are a part of this legacy. Thus, by coming to grips with life’s long evolutionary history, we begin to understand something of our own place in the world, including our responsibility as planetary stewards.

My second goal is to tell the story of early evolution in a particular
way. The history of life is commonly recounted as a naturalist’s Generations of Abraham: bacteria begat protozoans, protozoans begat invertebrates, invertebrates begat fishes, and the like. Such catalogs of received wisdom can be memorized, but there isn’t a lot to think about. For this reason, I have chosen to relate my story as an enterprise—one in which rocks and fossils are encountered in remote corners of the globe, analyzed in the laboratory, and interpreted in light of processes (but not necessarily conditions) observable today. Discoveries in paleontology, the most traditional of scientific pursuits, weave together with emerging insights from molecular biology and geochemistry.

In some ways, conventional paleontology and the research described here seem poles apart, their practitioners squinting to view the past through the opposite ends of a telescope. Dinosaur bones are big and spectacular—they keep you
awake
at night. But, apart from the size of its inhabitants, the
world
of the dinosaurs was much like our own. In contrast, Earth’s deep history is recounted by microscopic fossils and subtle chemical signals. And yet, the story they tell is dramatic, a succession of vanished worlds that leads through atmospheric transformation and biological revolution to the Earth we know today.

If we want to understand events that took place a billion or more years ago, how do we go about doing it? It’s one thing to learn that photosynthetic bacteria lived on tidal flats 1.5 billion years ago, and quite another to understand how we recognize microscopic fossils as photosynthetic, how we determine that the rocks enclosing them formed on an ancient tidal flat, and how we estimate their age as 1.5 billion years. The epistemological leitmotif of how we know what we think we know recurs throughout this book. As human enterprise, this is also a story of exploration that extends from the inner space of molecules to the literal outer space of Mars and beyond. Cold nights in Siberia are part of the tale, as are warm friendships in China.

Finally, having excavated and evaluated the preserved shards of our biological past, I want to step back and ask whether we can identify any general principles that shine through the maze of historical particulars. What are the grand themes of life’s early history? The astrobiologist in me, eager for a glimpse of samples collected on Mars, asks what aspects of our terrestrial biology might be found wherever life exists and which
features are likely to prove the specific products of our particular planetary history? We don’t yet know the answer, but how we search for life elsewhere in the universe depends to a large extent on how we think about this question.

One clear theme of evolutionary history is the cumulative nature of biological diversity. Individual species (of nucleated organisms at least) may come and go in geological succession, their extinctions emphasizing the fragility of populations in a world of competition and environmental change. But the history of guilds—of fundamentally distinct morphological and physiological ways of making a biological living—is one of accrual. The long view of evolution is unmistakably one of accumulation through time, governed by rules of ecosystem function. The replacement series implied by the Generations of Abraham approach fails to capture this basic attribute of biological history.

Another great theme is the coevolution of Earth and life. Both organisms and environments have changed dramatically through time, and more often than not they have changed in concert. Shifts in climate, in geography, and even in the composition of the atmosphere and oceans have influenced the course of evolution, and biological innovations have, in turn, affected environmental history. Indeed, the overall picture that emerges from our planet’s long history is one of
interaction
between organisms and environments. The evolutionary epic recorded by fossils reflects, as much as anything else, the continuing interplay between genetic possibility and ecological opportunity.

This long view of biological history provides what may be the grandest theme of all. Life was born of physical processes at play on the young Earth. These same processes—tectonic, oceanographic, and atmospheric—sustained life through time as they shaped and reshaped our planet’s surface. And, eventually, life expanded and diversified to become a planetary force in its own right, joining tectonics and physical chemistry in the transformation of air and oceans. To me, the emergence of life as a defining—perhaps
the
defining—feature of our planet is extraordinary. How often has this happened in the vastness of the universe? That’s what
I
think about when I look up “in perfect silence at the stars.”

Awe and humility attended the telling of earlier creation stories. They are appropriate companions to science’s version, as well.

Fossils found along the Kotuikan River in northern Siberia document the Cambrian “Explosion,” the remarkable flowering of animal life that began some 543 million years ago. As Charles Darwin recognized more than a century ago, Cambrian fossils raise fundamental questions about life’s earlier evolution. What kind of organisms preceded these already complex animals? Can we find older rocks, and if we can, will they preserve a record of Earth’s earliest biological history?

Sometimes the past was shot with a hand-held camera; sometimes it reared monumentally inside a proscenium arch with moulded plaster swags and floppy curtains; sometimes it eased along, a love story from the silent era, pleasing, out of focus and wholly implausible. And sometimes there was only a succession of stills to be borrowed from the memory.

—Julian Barnes
Staring at the Sun

T
HE CLIFFS ALONG
the Kotuikan River glow fawn and pink in the late afternoon sun (
figure 1.1
). Elsewhere, in North America or in Europe, a vista like this would be celebrated as a national park, its approaches flanked by campgrounds and souvenir shops. But here, in the forested wilderness of northern Siberia, its pastel beauty is both unremarkable and largely unseen. From a sheltered niche halfway up the cliff, I look up at my friend Misha Semikhatov perched high above the river, his large frame barely supported by a narrow ledge. The drop beneath his feet is precipitous, but Misha’s attention is elsewhere, fixed on a layer of sedimentary rocks just above his head. To his experienced eye, the bed of crinkly laminated limestones tells of an ancient tidal flat that bordered a vanished ocean, a broad expanse of shoreline exposed at low tide, covered by thickly matted bacteria, and occasionally crossed by small animals. As I rest against the rock face, observing marginally
older beds, jotting in my notebook, and swatting mosquitoes (not necessarily in that order), I reflect on what has brought Misha and me to this remote spot high above the Arctic Circle (
figure 1.2
). The literal answer is a giant Soviet-era military helicopter that deposited us, a small group of colleagues, and a ton of gear some seventy miles upstream. From there, small rubber rafts floated us like Huckleberry Finn slowly down the river, through canyons of limestone, beneath circling falcons, past wolves that howl at the midnight sun, to this wild and beautiful place.