Did the Earth once undergo a super ice age, one that froze the entire planet from the poles to the equator? In Snowball Earth, gifted writer Gabrielle Walker has crafted an intriguing global adventure story, following maverick scientist Paul Hoffman’s quest to prove a theory so audacious and profound that it is shaking the world of earth sciences to its core.

In lyrical prose that brings each remote and alluring locale vividly to life, Walker takes us on a thrilling natural history expedition to witness firsthand the supporting evidence Hoffman has pieced together. That evidence, he argues, shows that 700 million years ago the Earth did indeed freeze over completely, becoming a giant “snowball,” in the worst climatic catastrophe in history. Even more startling is his assertion that, instead of ending life on Earth, this global deep freeze was the trigger for the Cambrian Explosion, the hitherto unexplained moment in geological time when a glorious profusion of complex life forms first emerged from the primordial ooze.

In a story full of intellectual intrigue, we follow the irascible but brilliant Hoffman and a supporting cast of intrepid geologists as they scour the planet, uncovering clue after surprising clue. We travel to a primeval lagoon at Shark Bay in western Australia, where dolphins cavort with swimmers every morning at seven and “living rocks” sprout out of the water like broccoli heads; to the desolate and forbidding ice fields of a tiny Arctic archipelago seven hundred miles north of Norway; to the surprising fossil beds that decorate Newfoundland’s foggy and windswept coastline; and on to the superheated salt pans of California’s Death Valley.

Through the contours of these rich and varied landscapes Walker teaches us to read the traces of geological time with expert eyes, and we marvel at the stunning feats of resilience and renewal our remarkable planet is capable of. Snowball Earth is science writing at its most gripping and enlightening.

GABRIELLE WALKER earned a Ph.D. in natural sciences from Cambridge University. She served as the features editor at New Scientist magazine for seven years and is currently a contributing editor there. She has also taught in the science writing program at Princeton University. Her travels in search of stories have taken her to all seven continents—including a stint at the South Pole.

ONE

First Fumblings

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 humpback 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 of fish! Also sea sponges and corals, brilliant purple Xatworms, 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 seaXoor, 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 outcompeted in all but the most hostile environments, a few odd places still exist where you can experience the primeval Earth firistband. 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 butterXies 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 interior of the stromatolites is 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 Wlamentous height--perhaps a thousandth of an inch--soak up the sun, and make their food; by night they lie bac...