This is an extraordinary time to be alive. Look around you, take in the intricate complexities of life on Earth, and then consider this: complex life is a very recent invention. Our home planet spent most of its long history coated in nothing but simple, primordial slime. For billions of years, the only earthlings were made of goo.
Then, suddenly, everything changed. At one abrupt moment roughly 600 million years ago, something shook the Earth out of its complacency. From this came the beginnings of eyes, teeth, legs, wings, feathers, hair and brains. Every insect, every ape and antelope, every fish, bird and worm. Whatever triggered this new beginning was ultimately responsible for the existence of you and everyone you've ever known.
So what was it?
Paul Hoffman, part-time marathon runner, full-time geologist, and obsessive, intense seeker of glory, thinks he knows. He believes he has finally struck science gold. Now a full professor at Harvard University and a world-renowned scientist, he has uncovered evidence for the biggest climate catastrophe the Earth has ever endured. And from that disaster, according to Paul, came a remarkable new redemption.
SHARK BAY shows up from the air as a snag in the smooth coastline of Western Australia. Five hundred miles north of Perth, it lies just at the place where tropical and temperate zones rub shoulders. The area around the bay is a powerful reminder of how far we have come since primordial slime ruled the world. It is full of varied, vivid life.
This is one of the few places in the world where wild dolphins commune with humans, every day, regular as clockwork. At 7:00 A.M. each day a park ranger dressed in khaki uniform emerges from a wooden hut to focus a pair of binoculars on the horizon. Perhaps half an hour later, he'll spot the first dolphin fin. Somehow the word immediately spreads. Where there were only four or five people on the sandy beach, suddenly fifty or sixty appear.
Three harassed rangers do their best to marshal them into an orderly line. Everyone will get a chance to see the dolphins. No one will be permitted to touch them. No one must go more than knee-deep in the water. Another ranger deftly diverts the enormous wild white pelicans away from the beach by flicking on a water sprinkler. The birds flock around with gaping jaws—in this desert landscape, fresh water is irresistible.
The dolphins and their calves arrive. One of the rangers, a wireless headset amplifying her voice, wades up and down in front of the spectators, introducing the dolphins ("This is Nicky and Nomad, Surprise and her calf Sparky") and reciting useful dolphin facts. The crowd surges into the water, like acolytes seeking a Jordanian baptism, their expressions beatific.
The dolphins are the crowd pullers—more than six hundred of them live here. But Shark Bay is also famous for the rest of its wildlife. The bay contains more than 2,600 tiger sharks, not to mention hammerheads and the occasional great white. The tigers show up in the water as streamlined shadows up to twelve feet long; often they are skulking beside patches of sea grass in the hope that dinner will emerge in the form of a blunt-nosed, lumbering gray dugong. Dugongs, or sea cows, are supposedly the creatures behind the mermaid myths, though I can't see it myself. They are too prosaic, placidly chewing away at the end of a "food trail," a line of clear water that they have cut, caterpillar-like, through the fuzzy green sea grass. They're exceptionally shy and rare, but here, among the largest and richest sea-grass meadows in the world, are a staggering ten thousand of them—tiger sharks notwithstanding.
Then there are sea snakes, green turtles, and migrating hump-back whales. And just a little to the north, where the tropics begin in earnest, lies Coral Bay—one of the world's top ten dive sites. Come and dive the Navy pier! See more than 150 species offish! Also sea sponges and corals, brilliant purple flatworms, snails and lobsters and shrimp. And the vast, harmless whale sharks, the world's biggest fish. And on land there are wallabies and bettongs and bandicoots, emus and kangaroos and tiny, timid native mice.
There's everything in this region, from the wonderful to the plain weird. Evolution has been tweaking, adapting and inventing new forms of complex life for hundreds of millions of years, and here in Western Australia it surely shows.
But this is also a place where you can travel back in time, to see the other side of the evolutionary equation—the simplest, most primitive creatures of all. They come from the very first moments in the history of life, just after the dust from the Earth's creation had settled. And when these first fumblings of life appeared on Earth's surface, their form was exceedingly unprepossessing. Throughout oceans, ponds and pools, countless microscopic creatures huddled together in a primordial sludge. They coated the seafloor, and inched their way up shore with the tide; they clustered around steaming hot springs, and soaked up rays from the faint young sun. Dull green or brown, excreting a gloopy glue that bonded them together into mats, these creatures were little more than bags of soup. Each occupied a single cell. Each had barely mastered the rubrics of how to eat, grow and reproduce. They were like individual cottage industries in a world that had no interest in collaboration or specialization. They were as simple as life gets.
Although these primitive slime creatures have now been out-competed in all but the most hostile environments, a few odd places still exist where you can experience the primeval Earth firsthand. The acidic hot springs of Yellowstone National Park, for instance, or Antarctica's frigid valleys. And here, in Western Australia, where countless microscopic, single-celled, supremely ancient creatures are making their meager living in one small corner of Shark Bay: a shallow lagoon called Hamelin Pool. The pool's water doesn't mix much with the rest of the bay, and it's twice as salty as normal. Since few modern marine animals will tolerate so much salt, this is one of the last refuges of ancient slime.
THE SIGN pointing to Hamelin Pool is easy to miss, even on the desolate road running south from Monkey Mia. On the second pass I finally spot it, turn left, and bump along a sand track with scrubby bush to either side. For this first visit I avoid the restored telegraph station with its tiny museum and tea shop, and head straight for the beach. I want to experience primordial Earth without a guide.
There's an empty parking lot of white sand, with wattles and low-slung saltbushes clinging to the surrounding dunes, and a path threading through the bushes toward the sea. Though I've come to find the world's simplest creatures, the complexities of life are everywhere. From one of the bushes a chiming wedgebill incessantly reiterates its five-note melody. From another, a gray-crested pigeon regards me unblinkingly. The shells of the beach crunch underfoot; they are tiny, bone-white, and perfectly formed, and the bivalves that grew them are eons of evolution ahead of the simple creatures that I'm seeking. I step onto the boardwalk, which stretches like a pier out into the water. Each weathered plank of wood contains row upon row of cells that once collaborated in a large, complex organism. Signs on all sides show pictures of the slime creatures with smily faces and cheery explanations of their origin. Flies buzz infuriatingly around my head, landing on my face to drink from the corners of my eyes. Black swifts swoop between the handrails, and butterflies the color of honey, with white and black tips to their wings. Time travel is harder than it looks. The modern world is right here even in Hamelin Pool, and it's stubbornly refusing to leave.
I retreat to the telegraph station to plead with the ranger for permission to leave the boardwalk and wade out into the pool. He hesitates and then relents. "Go along the beach to the left," he says. "Don't step on the mats. Be careful." The mats he's talking about are one of the signs of primeval Earth. They are slimy conglomerates of ancient cyanobacteria, and they grow painfully slowly. At the beginning of the last century, horse-drawn wagons were backed into the sea over the mats, to unload boat cargo. A hundred years later the tracks they left are still visible as bare patches in the thin black sludge. An injudicious footprint here will last a long time. I promise to watch my step.
I return to the beach and this time walk carefully toward the water's edge. More striking than the ubiquitous patches of sludgy, foul-smelling bacterial mats are the "living rocks" in between. These strange denizens of Slimeworld are everywhere, an army of misshapen black cabbage heads marching into the sea.
The ones highest up the shore are now nothing more than dead gray domes of rock, shaped like clubs, perhaps a foot tall. They once bore microbial mats on their surfaces, but these have long since shriveled, abandoned by the receding water. Closer to the Pool's edge the domes are coated with black stippling that will turn to dull olive green when the tide washes over them. Most of the stromatolites, though, lie in the water, stretching out as far as I can see. Between them the sand is draped with black-green mats of slime, and checkered with irregular patterns of sunlight as the waves ripple overhead. I wade up to my knees among these strange formations, basking in the sunshine. There is nobody else in sight.
The living rocks of Slimeworld are called "stromatolites," a word that comes from the Greek meaning "bed of rock." Though the interiors of the stromatolites are plain, hard rock, their outer layers are spongy to the touch. Here on the surface is where the ancient microbes live. They're sun-worshipers: by day they draw themselves up to their full filamentous height—perhaps a thousandth of an inch—soak up the sun, and make their food; by night they lie back down again. The water that surrounds them is filled with fine sand and sediment stirred up by the waves. Gradually this sand rains down on the organisms, and each night's bed is a fresh layer of incipient rock. The stromatolites are inadvertent building sites; the sticky ooze that the organisms extrude acts as mortar and the sand acts as bricks. Every day, as the microbes worm their way outward, another thin layer of rock is laid down beneath them.1
It's a slow process. Stromatolites grow just a fraction of an inch each year. The ones in Hamelin Pool are hundreds of years old and would be astonishing feats of engineering, had they been created by design. For these microorganisms to erect a stromatolite three feet high is like humans building something that reaches hundreds of miles into the sky, and scrapes the edges of space. I wade a hundred yards, two hundred yards offshore, and the slope is still so gentle that the deepening is barely perceptible. Mercifully, the flies and butterflies have dropped back, and the birdsong is out of earshot. At last I begin to feel that I've traveled back to life's earliest days.
HAMELIN POOL'S mats and stromatolites look utterly alien, but they were once ubiquitous. Time was, this scene of stromatolites and stippled microbial mats would have greeted you everywhere you went. Forget dolphins and wallabies. This is how the Earth looked for nearly three and a half billion years. The imprints of the stromatolites and their mats show up still wherever sufficiently ancient rocks poke through to the Earth's surface. I've seen them in Namibia, in South Africa, in Australia and California. They are sometimes dome-shaped like these in Shark Bay, sometimes cones, sometimes branching like corals. There are places where you can walk among ancient petrified stromatolite reefs, rest your feet on their stone cabbage heads, and see where they have been sliced through to reveal rings of petrified growth. And you can run your fingers over fossilized mats, which give rock surfaces the unexpected texture of elephant skin. This slime used to be everywhere, and now it's almost nowhere.
How did we get from there to here? This is at once a simpler and more powerful question than it seems. Of course, life took many separate evolutionary steps on its way from stromatolites to wallabies. It had to invent eyes and legs and fur and feet, and everything else that distinguishes marsupials from slime. But there was one particular step that was more important than all the others, one that made all the difference.
The step was this: learning to make an organism not from just one cell, but from many. Though the first microbes on Earth were woefully unsophisticated, they did gradually learn new tricks to exploit the planet's many niches. But they all still had one thing in common. Each individual creature was packaged in its own tiny sac, a single microscopic cell. Then at some particular point in Earth's history, everything changed. One cell split into two, then four. From that time onward, organisms could be cooperative, and above all their cells could specialize. There could be eye cells and skin cells, cells to make up organs and tissues and limbs.
For life, this was the industrial revolution. Forget the old cottage industries. Now you could have factories with production lines. Parceling out tasks and specializing is always more efficient than trying to do everything yourself. And there are some things, wallabies for instance, that can only be made with a massive collaborative effort.
In just the same way, when organisms developed the ability to become multicellular, they gained a world of possibilities. Your body is made up of trillions of cells. Every hair is packed with them. You shed skin cells whenever you move. Your blood cells carry energy around your body, to feed the organs made up of still more cells. This multiple identity is the one criterion that's vital for any complex creation. Every dolphin and dugong, every shark, pelican and wombat depends for its existence on that crucial leap from one cell to many. This was the point when simple slime yielded its preeminence to the complex creations that heaved their way out of the sludge and started their march toward modernity.
But why did it take so long? The Slimeworld lasted for almost the whole of Earth history. Let's put in some numbers. Our planet had been around for 4 billion years before the first complex earthlings emerged from the ooze. That's nearly 90 percent of Earth's lifetime.
Four billion years is an insane amount of time, almost impossible to contemplate. There have been many attempts to capture this spread of time in ways that we can comprehend. If the history of life on Earth were crammed into a year, slime would have ruled through spring, summer and fall, continuing well past Halloween into the beginnings of winter. If it were squeezed into the six days of creation, slime ruled until six o'clock on Saturday morning. If it stretched over a marathon course, slime would have led the field past the twenty-three-mile mark.2
But my favorite image is this one, borrowed from John McPhee. 3 Stretch your arms out wide to encompass all the time on Earth. Let's say that time runs from left to right, so Earth was born at the tip of the middle finger on your left hand. Slime arose just before your left elbow and ruled for the remaining length of your left arm, across to the right, past your right shoulder, your right elbow, on down your forearm, and eventually ceded somewhere around your right wrist. For sheer Earth-gripping longevity, nothing else comes close. The dinosaurs reigned for barely a finger's length. And a judicious swipe of a nail file on the middle finger of your right hand would wipe out the whole of human history.
Stephen Jay Gould set the discovery of these vast stretches of Earth time in a long line of findings that put humans firmly in our place.4 Galileo, said Gould, taught us that the Earth isn't the center of the universe. Darwin, that we're just another kind of animal. Freud, that we're not even aware of most of the things going on in our own heads. And geologists have now discovered that the Earth reached late middle age before we were so much as a glimmer in its eye.
Though we humans are certainly complex, also clever, perhaps even the highest form of life that Earth has so far produced, we're nothing like the most natural earthlings. Measured in units of staying power, Earth's first, most primitive experiment in life was also its best. With simple individual cells, nothing complex, nothing flashy, each creature out for itself, life had found a supremely winning formula. Why should it ever change?
That's the question that has plagued complex, clever, thinking, adaptable humans since they first uncovered this bizarre history of life. Earth looked set to stay locked in slime forever. Why did complex life appear at all, and why did it wait to emerge until that one point in time, just a few hundred million years ago, nearly at the end of the marathon, somewhere near your right wrist, late in the Earth's middle age?
To answer this, Paul Hoffman has seized on an idea that was first proposed sixty years ago, and was then dropped, halfheartedly resurrected, and dropped again several times over the intervening years. There's nothing halfhearted, however, about the resurrection Paul has now effected. He's marshaled new evidence, restored and amalgamated old ideas, and employed fierce argument to persuade the people around him. According to Paul, life's richness, diversity and sheer overwhelming complexity arose from a mighty catastrophe. It's called the "Snowball Earth."
FIRST CAME the ice. It crept from its strongholds at the north and south poles, freezing the surface of the ocean, spreading gradually over the Earth's surface. A blue planet inexorably made white.
Individual crystals of ice first appeared in the sea like tiny floating snowflakes. They were smashed together by wind and waves, their fragile arms broken and their debris turning the seawater into a greasy slick. The surface thickened and froze into a thin transparent layer. As this layer thickened, it grew gray and then opaque from salt and air bubbles that filled its inner voids. In some places the greasy ice congealed into large round pancakes, with raised edges like giant lily pads, where they bumped and smashed against each other. And, a nice touch this, the fresh young sea ice grew a coating of frost flowers, each one the size and shape of edelweiss.
Sea ice bends. Unlike freshwater ice, which can shatter like glass, the ice that forms first on the surface of the sea is elastic. When you try to walk on it, your legs unexpectedly buckle. But as it thickens it becomes reassuringly firm, like solid rock.
Though sea ice is gray when it first forms, it whitens year by year as its brine drains back into the sea. Even gray young ice is often dusted with white snow. But a frozen ocean is far from monochrome. Gashes of open seawater, created as the pack ice is ripped apart by wind and weather, expose the deep turquoise roots of the floating sea ice. And the dark ocean reflects in the clouds, streaking them the color of a bruise. "Water sky," this is called, and polar sailors have long used it as a clue for where to point their ship next as they navigate perilously through the pack.
Where waterways have frozen over, the ice is smooth and level. Where the edges of an old water wound have been cauterized together again, untidy piles of ice blocks are an astonishing bright blue. Ice cracks suddenly like a whip. Sometimes pack ice groans and creaks as the wind crams floes together or prepares to break them open. But for the most part, the frozen polar oceans are shrouded in silence—eerie and absolute. There is no scene more alien on Earth.5
For perhaps a few thousand years, the white menace stole unheeded toward the equator. Earth's primitive life-forms had neither the eyes to see the encroaching ice nor the wit to fear it. Most of them lived their dull lives in a band around the Earth's waist, and as the ice advanced steadily from the far north and south, they bathed unconcerned in the warmth of their shallow, equatorial seas.
An occasional storm might have whipped up waves near the shore. Perhaps the surf tore at the rubbery microbial mats that coated the seafloor and sprayed nearby rocks with scraps like soggy chicken skin. Stromatolites built up their stone reefs, layer by microscopic layer. Geysers blew. Rain fell. The sun shone again. There was no hint of the devastation to come.
But when the ice reached the tropics, its slow creep became a sprint. In a matter of decades, it engulfed the tropical oceans and headed for the equator.
Ice spread out from shallow bays and grew first a skin, then a carapace over the oceans. It clung to the beaches and scraped the mats on the seafloor. In some places this shell was still thin enough to crack and seal again. In others it was thousands of feet deep.
For a few hundred, perhaps even a few thousand years after the oceans were capped with ice, the land remained bare. But ice began to accumulate, gradually, in the thin air of mountain ranges, creating great frozen rivers that flowed down to fill the surrounding valleys. In the end, the whiteout was complete. Earth's surface looked like the frigid wasteland of Mars, or one of Jupiter's ice-covered moons. Sunlight bounced off the bright surface and was dazzled back into space. The mercury hit a staggering minus 40 degrees C. (Or it would have, except that at those temperatures mercury itself would have frozen.) There was little wind or weather of any kind. Clouds by and large disappeared, save perhaps for tiny ice crystals high in the atmosphere, which scattered sunsets into strange, lurid colors, blue and green, rimmed with vibrant pink. No rain fell and little snow. Every day brought silent, unremitting cold.
The Snowball wasn't just another humdrum old "ice age" like those from more recent eras. The events we call ice ages were merely brief cold blips in an otherwise fairly comfortable world. There was ice in New York then, but none in Mexico. If you were in northern Europe during one of those ice ages, you shivered. But if you were in the tropics, you scarcely noticed.
Instead, Paul's Snowball was the coldest, most dramatic, most severe shock the Earth has ever experienced. It was the worst catastrophe in history. For perhaps a hundred thousand centuries, Earth was a frozen white ball, desolate and all but lifeless.
To the microbes, the Snowball must have seemed like the end of the world. Some survived, of course—they must have, or we wouldn't still see them today. Perhaps they huddled for warmth around undersea volcanoes. They might have survived near hot springs, or found fissures and cracks in the sea ice where the sun's rays could slip through. But for many, perhaps most, the Snowball was disastrous.
Eventually the Snowball empire began to founder. Volcanic gases gradually built up in the atmosphere, trapping the sun's heat and turning the air into a furnace. After millions of years of stasis, the ice finally succumbed, melting in a rapid burst of perhaps just a few centuries. Temperatures now soared to 40 degrees C. Intense hurricanes flooded the surface with acid rain. Oceans frothed and bubbled, and rocks dissolved like baking powder. Earth had leapt out of the freezer and into the fire.
There was at least one more of these Snowball-inferno lurches, and there may have been as many as four. But at the end of them all, after the last of the Snowballs and its attendant hothouse finally faded, some 600 million years ago, came the most important moment in the history of evolution. The rocks that appeared immediately afterward bear fossils showing the first stirrings of complex life. Out of the ice and the fire that followed had come the complexity that we see around us today.
THIS IS Paul Hoffman's vision, and he is enchanted by it. Most other geologists are horrified. Accept his story, they say, and you have to reconsider everything you thought you knew about the workings of the world. Geologists are taught from an early age that the Earth is a slow and steady place. The past looked pretty much like the present. Change happens only very slowly, nothing is terribly extreme. True, there have been a few occasions where they have been forced to admit, somewhat grudgingly, that this picture falls short. The idea that an asteroid came from space to wipe out the dinosaurs was once derided, but is now widely accepted. Okay, the argument goes, so the occasional extraterrestrial calamity can rock the Earthly boat. But broadly speaking, the geological picture of Earth's history is a settled, safe, comfortable one.
Compare that to Paul's picture of the Snowball. A global freeze. A planet that looked more like Mars than home. Ice everywhere. And then a sudden lurch from the coldest to the hottest that the Earth has ever been. Every way you look at it, his Snowball stretches the bounds of decency. It's as extreme and catastrophic as they come.
Small wonder, then, that the Snowball has become the most hotly contested theory in earth science today. Paul Hoffman, though, is resolute. He is the chief champion of the theory. By argument, evidence, and brute force of personality, he is determined to win over the unbelievers.
Paul is an obsessive man espousing an extreme theory. If he is proved right, we'll have learned something important about where we all ultimately come from. But there's a darker side to Paul's theory. He has uncovered behavior in our planet that's unsettling in the extreme. If his vision is true, Earth can experience sudden lurches in climate that are more violent, and deadly, than anyone had ever imagined, and such catastrophes may well happen again.
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