A Brief History of Earth Read online

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  Dedication

  To Marsha.

  For everything.

  Contents

  Cover

  Title Page

  Dedication

  Prologue: An Invitation

  1: Chemical Earth

  2: Physical Earth

  3: Biological Earth

  4: Oxygen Earth

  5: Animal Earth

  6: Green Earth

  7: Catastrophic Earth

  8: Human Earth

  Acknowledgments

  Further Reading

  Index

  About the Author

  Also by Andrew H. Knoll

  Copyright

  About the Publisher

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  Prologue: An Invitation

  YOU LIVE YOUR LIFE tethered by gravity to the Earth. Every step brings you in contact with rock or soil, even if hidden by a veneer of macadam or floorboards. You may think you’ve escaped gravity’s clutches when you lift off in an airplane, but any exhilaration is fleeting; within a few hours, gravity will win, and you’ll settle back onto terra firma.

  Our attachment to the Earth extends well beyond gravity. The food you eat is made from carbon dioxide in the atmosphere or oceans, along with water and nutrients taken up from soil or sea. With every breath you bring oxygen-rich air into your lungs, enabling you to gain energy from your dinner. At the same time, carbon dioxide in the atmosphere keeps you from freezing. Moreover, the steel in your refrigerator door, the aluminum in your “tin” cans, the copper in your pennies, and the rare-earth metals in your smartphone all come from within the Earth. Given all this, it is remarkable how incurious most of us are about this great sphere that sustains us and occasionally, during earthquakes or hurricanes, places us in harm’s way.

  How can we understand Earth’s place in the universe? How did the rocks, air, and water that define our existence come to be? How do we explain our continents, mountains and valleys, earthquakes and volcanoes? What controls the composition of the atmosphere or of seawater? And how did the immense diversity of life all around us come to be? Perhaps most important, how are our own actions changing both Earth and life? In part these are questions of process, but they are also historical inquires, and that’s the framework of this book.

  This is a story about our home, the Earth, and the organisms that spread across its surface. Everything about the Earth is dynamic, ever changing despite common but false impressions of permanence. Boston, for example, has a temperate climate, with warm summers, cold winters, and moderate precipitation distributed more or less evenly throughout the year. The seasons are predictable and if, like me, you’ve been around for a few decades, you may get the feeling that you’ve seen it all before. Meteorologists, however, will tell you that the mean annual temperature in Boston has increased by more than a degree Fahrenheit (0.6 degree Celsius) during the lifetimes of its older citizens. We also know that the amount of carbon dioxide in the atmosphere—a major regulator of surface temperature—has increased by about a third since the 1950s. Similarly, measurements tell us that global sea level is rising and the amount of oxygen dissolved in the oceans has declined by about 3 percent since the Beatles catapulted to fame.

  Small changes add up through time. A plane flight from Boston to London lengthens by about one inch (2.5 centimeters) each year, as new seafloor slowly pushes North America and Europe apart. If we could run the tape backward, we’d see that 200 million years ago, New England and Old England were part of a single continent, with rift valleys like those seen today in eastern Africa just beginning to initiate an ocean basin. On the longest timescales, Earth’s transformations are truly profound. For instance, free to roam on the early Earth, you would have suffocated quickly in our planet’s oxygen-free air.

  The story of Earth and the organisms it sustains is far grander than any Hollywood blockbuster, filled with enough plot twists to rival a bestselling thriller. More than four billion years ago, a small planet accreted out of rocky debris circling a modest young star. In its early years, Earth lived on the edge of cataclysm, bombarded by comets and meteors, while roiling magma oceans covered the surface and toxic gases choked the atmosphere. With time, however, the planet began to cool. Continents formed, only to be ripped apart and later collide, throwing up spectacular mountain ranges, most of which have been lost to time. Volcanoes a million times larger than anything ever witnessed by humans. Cycles of global glaciation. Countless lost worlds we are only beginning to piece together. Somehow on this dynamic stage, life established a foothold and eventually transformed our planet’s surface, paving the way for trilobites, dinosaurs, and a species that can speak, reflect, fashion tools, and, in the end, change the world again.

  Understanding Earth’s history helps us appreciate how the mountains, oceans, trees, and animals around us came to be, not to mention gold, diamonds, coal, oil, and the very air we breathe. And in so doing, our planet’s story provides the context needed to grasp how human activities are transforming the world in the twenty-first century. For most of its history, our home was inhospitable to humans, and indeed, among the enduring lessons of geology is a recognition of how fleeting, fragile, and precious our present moment is.

  THESE DAYS, the headlines often seem to have been ripped from the book of Revelation: unprecedented wildfires in California and the Amazon aflame; record heat in Alaska and accelerating glacial melt in Greenland; giant hurricanes devastating the Caribbean and Gulf Coast, while “hundred year” floods inundate the American Midwest with increasing regularity; Chennai, India’s sixth-largest city, running out of water, with Cape Town and São Paulo coming close. The news from biology is hardly better: a 30 percent decline in North American bird populations since 1970; insect populations halved; massive coral mortality along the Great Barrier Reef; rapid declines of elephants and rhinos; commercial fisheries under threat around the world. Population decline is not extinction, but it is the road down which species travel on their way to biological endgame.

  Has the world run amok? In a word, yes. And we know why: the culprit is us. It is humans who pump greenhouse gases into the atmosphere, not only warming the Earth but increasing the magnitude and frequency of heat waves, drought, and storms. And it is humans who have driven species to the brink through changing land use, overexploitation, and, increasingly, climate change. With this in mind, possibly the most depressing news of all is the human response: widespread indifference, perhaps especially in my home country, the United States of America.

  Why do so many people care so little in the face of planetary changes that will reshape the lives of our grandchildren? In 1968, Baba Dioum, a Senegalese forest ranger, provided a memorable answer. “In the end,” he said, “we will conserve only what we love, we will love only what we understand, and we will understand only what we are taught.”

  This book, then, is an attempt at understanding. An invitation to appreciate the long history that has brought our planet to its present moment. An exhortation to recognize how profoundly human activities are altering a world four billion years in the making. And a challenge to do something about it.

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  1

  Chemical Earth

  MAKING A PLANET

  Todd Marshall

  IN THE BEGINNING WAS . . . well . . . a jot, a speck, a fleck at once incomprehensibly small but unimaginably dense. It wasn’t a localized concentration of stuff in the vast emptiness of the universe. It was the universe. How it got there, no one knows.

  What, if anything, came before is equally mysterious, but about 13.8 billion years ago, this primordial kernel of universe began to expand rapidly—a “Big Ba
ng” that unleashed an immense outward tide of energy and matter. Not the rocks and minerals of our daily existence; not even the atoms from which rocks, air, and water are built. At the dawn of the universe, matter consisted of quarks, leptons, and gluons, a curious cast of subatomic particles that would eventually coalesce into atoms.

  Our understanding of the universe and its history comes largely from the most ephemeral of sources: light. The luminous pinpricks that give shape to the night sky may seem unlikely history books, but two properties of light help us to understand how the universe has evolved. First, the intensity of different wavelengths in incoming radiation points to the composition of its source. Our eyes can detect only a narrow range of wavelengths, but stars and other heavenly bodies emit or absorb a broad spectrum of radiation, from radio- and microwaves to x-rays and gamma rays, each with a story to tell. And, importantly, light obeys a strict speed limit: 299,792,458 meters per second, or 186,276 miles per second, in space. Sunlight is emitted eight minutes and twenty seconds before we see it, and for stars and other bodies farther away, the light we record emanated still earlier—much earlier for the most distant objects. That’s what makes our starry sky a celestial history book.

  Microwaves distributed evenly across the sky speak of the Big Bang and its immediate aftermath, and radiation from the first generation of stars, formed a few hundred thousand years after time began, is just reaching us today. How did these early stars form? It all has to do with gravity, the architect of the universe. Gravity describes the attraction between different objects, with the strength of the attraction determined by the masses of the objects and the distance between them. As atoms formed within the early, expanding universe, gravity began to pull them together. Local aggregations grew, strengthening their gravitational pull, and eventually they collapsed into hot, dense balls, so hot and so dense that hydrogen nuclei fused to form helium, releasing light and heat. When that happens, a star is born. Large, hot, and short-lived, those primordial stars set the course of all that would come later, including us.

  The matter generated by the Big Bang consisted mostly of hydrogen atoms, the simplest of elements, along with some deuterium (hydrogen with an added neutron) and helium. A tiny bit of lithium formed as well, along with still smaller amounts of other light elements, but there wasn’t much else. Actually, there was something else, but we don’t quite know what it is. In the 1950s, astronomers began to use the motions of stars and galaxies (a collection of stars, gas, and dust held together by, once again, gravity) to calculate gravitational attraction in deep space, but when they summed up the mass of all known objects in the sky they found it insufficient to account for their observations. There had to be something else out there, something that interacts with normal matter through gravity but doesn’t interact with light; astronomers dubbed it dark matter. Astronomers have thoughts about what dark matter might be, but no one is certain. Even more mysterious is dark energy, also deemed necessary to explain the workings of the universe. Together, dark matter and dark energy are thought to make up some 95 percent of all that exists, enigmatic constituents that we can’t detect but which are thought to have played a major role in shaping the universe. We still have a lot to learn.

  Let’s get back to conventional matter. As the age of starlight began, the universe was a cold, diffuse cocktail of (mostly) hydrogen atoms. Early stars generated more helium, but there was nothing you could make into an Earth (see table). Where did the iron, silicon, and oxygen needed to build our planet come from? And what about the carbon, nitrogen, phosphorus, and other elements that make up your body? These and all other elements originated in succeeding generations of stars, foundries of the atoms that would one day form our planet. At the high temperatures and pressures within large stars, light elements fused to form carbon, oxygen, silicon, and calcium; iron, gold, uranium, and other heavy elements were forged in the giant stellar explosions called supernovae. The face you see in the mirror may be decades old, but it is made of elements formed billions of years ago in ancient stars.

  Through the immensity of time, stars formed and died, each cycle adding to the inventory of the elements concentrated today in Earth and life. Galaxies merged and black holes (regions so dense that no light can escape) emerged, slowly shaping the universe we observe today.

  We pick up the story about 4.6 billion years ago, focusing on an unassuming cloud of hydrogen atoms, along with small amounts of gas, ice, and mineral grains within the spiraling arm of a nondescript galaxy called the Milky Way. At first, the cloud was large, diffuse, and cold (really cold, with temperatures of 10–20 degrees Kelvin, or –460 to –420 degrees Fahrenheit). Probably nudged by a nearby supernova, this cloud began to collapse into a much smaller, denser, and hotter nebula. As had occurred billions of times elsewhere in the universe, gravity eventually drew most of the cloud into a hot, dense, central mass—our Sun. Most of the nebula’s hydrogen went into the Sun, but ice and mineral grains were partitioned into a disk that rotated around our fledgling star, broadly reminiscent of the rings of tiny particles that encircle Saturn today (Figure 1). At first, this disk was hot enough to vaporize the minerals and ices from which it formed. Over a few million years, however, it began to cool, faster in its outer reaches and slower close to the Sun’s heat.

  * * *

  ELEMENTAL COMPOSITION OF THE EARTH AND LIFE

  (percent, by weight)

  Earth

  Iron

  33

  Oxygen

  31

  Silicon

  19

  Magnesium

  13

  Nickel

  1.9

  Calcium

  0.9

  Aluminum

  0.9

  Everything else

  0.3

  Cells in the human body:

  Oxygen

  65

  Carbon

  18

  Hydrogen

  10

  Nitrogen

  3

  Calcium

  1.5

  Phosphorous

  1

  Everything else

  1.5

  * * *

  FIGURE 1. This remarkable image, taken by the Atacama Large Millimeter Array, shows HL Tauri, a young Sun-like star, and its protoplanetary disk. The rings and gaps evident in the image record emerging planets as they sweep their orbits clear of dust and gas. Our own solar system may have looked much like this 4.54 billion years ago. ALMA (ESO/NAOJ/NRAO)/NASA/ESA

  We know from our everyday experience that different substances melt or crystallize at distinct temperatures. At the Earth’s surface, for example, water will turn to ice at 0 °C (32 °F), but dry ice freezes from carbon dioxide at much lower temperatures (–78.5 °C). In much the same way, the minerals found in rocks crystallize from molten precursors at temperatures that range from hundreds to more than 1,000 °C. For this reason, as the planetary disk cooled, different materials crystallized into solids at different times and distinct places, all in relation to their respective distances from the Sun’s heat. Oxides of calcium, aluminum, and titanium formed first; then metallic iron, nickel and cobalt, and only later, beyond a distance from the Sun christened the frost line, ices of water, carbon dioxide, carbon monoxide, methane, and ammonia—the materials of oceans, air, and life. Bits of minerals and ice collided to form larger particles, and these coalesced into still bigger bodies. Within a few million years, only a handful of large spherical structures remained where the disk once rotated. The “third rock from the sun” was the Earth, a stony mass orbiting the Sun from a distance of about 93 million miles (150 million kilometers).

  HOW, SPECIFICALLY, DID the Earth take shape, and what can we know about its infancy? If light chronicles the history of the universe, rocks tell our planet’s story. When you gaze into the Grand Canyon or marvel at the peaks framing Lake Louise, you’re viewing nature’s library, with volumes of Earth history on display, inscribed in stone. Sediments—cobbles, sands, or muds form
ed by erosion of earlier rocks, or limestones precipitated from water bodies—spread across floodplains and the seafloor, recording, layer upon layer, the physical, chemical, and biological features of our planet’s surface at the time and place they formed. Igneous rocks—formed from molten materials deep inside the Earth—tell us more about our planet’s dynamic interior, as do metamorphic rocks forged from sedimentary or igneous precursors at elevated temperature and pressure deep within the Earth. Collectively, these rocks offer a grand narrative of Earth’s development from youth to maturity, of life’s evolution from bacteria to you, and—perhaps the grandest narrative of all—of the ways that the physical and biological Earth have influenced each other through time. After forty years as a geologist, I’m still amazed that cliffs along the Dorset coast of southern England allow me to conjure up a picture of the Earth as it existed 180 million years ago. Still more remarkable, as we’ll see, are those rocks that tell of Earth and life billions of years ago.

  If you look closely at imposing peaks in the Rocky Mountains or the Alps, another aspect of Earth history may snap into focus. Their tooth-like shapes don’t reflect deposition. On the contrary, they are being sculpted by erosion, physical and chemical processes that wear away rocks, eradicating their stories. Earth writes its history with one hand and erases it with the other, and as we go further back in time, erasure gains the upper hand. Our planet coalesced some 4.54 billion years ago, but Earth’s oldest known rocks date back only to about 4 billion years. Older rocks must have existed, but they’ve been eroded away or were buried and transformed through metamorphism into unrecognizable form. A few may still lie in some remote Canadian or Siberian hillside, waiting to be recognized, but largely, the first 600 million years of Earth history constitutes our planet’s Dark Age.