The following is an excerpt from Becoming Earth: How Our Planet Came to Life by Ferris Jabr.

When you purchase products through the Bookshop.org link on this page, Science Friday earns a small commission which helps support our journalism.

Buy the Book

Becoming Earth: How Our Planet Came to Life

Buy

I knew the tower would be intimidatingly tall. I’d seen dizzying photos of its needlelike profile viewed from afar and sweeping footage from drones that traced its full height. Yet it was not until I arrived at the tower’s base, stared up at its spare metal skeleton, and confronted the imminent prospect of climbing to the top that I began to wonder if I had made a mistake.

Situated deep within a relatively pristine region of the Amazon rainforest in northern Brazil, the tower is part of a research facility aptly named the Amazon Tall Tower Observatory. Scientific instruments attached to the structure and its shorter siblings continuously collect airborne particles and gases at different altitudes. Researchers from around the world visit the station to study how the rainforest influences both local ecology and global climate.

The largest tower, the one I was standing beneath, is the tallest structure in South America, stretching 1,066 feet into the sky, about the same height as the Eiffel Tower. Its rectangular steel frame is painted in alternating blocks of orange and white, like a giant safety cone. As I imagined the journey to the top, however, safe was not the word that leapt to mind.

To ascend the tower, I would have to climb nearly 1,500 narrow steps with large gaps between them while harnessed to a railing by a short cord. The harness made it unlikely that anyone would plummet to their death, but, I was told, someone could break a limb if they mis-stepped and fell through one of the many openings in the tower’s latticelike framework. I’d heard that, on a few occasions, people climbing the tower for the first time had been unable to continue due to overwhelming fear, a situation that can entail convoluted procedures to return them safely to the ground. Several of the scientists I was accompanying repeatedly asked me whether I was afraid of heights. No, I told them. I’d explored cliffs, mountains, and the decks of skyscrapers without difficulty. Okay, they said. But are you sure?

In truth, I was not entirely sure. The tower seemed incomplete: not so much a finished structure ready for human use as the mere suggestion of a staircase in bare-bones scaffolding incapable of masking vertiginous views. I worried that my brain would not fully register the absurdity and danger of the situation until partway up, at which point I would panic and cling to the railing like a cat whose curiosity had outpaced his courage. But I had not traveled all that way to give up without trying. Once I made it to the top of the tower, I would be halfway between the trees and clouds. I had come to this place to learn about the relationship between them. I had come to see the Amazon make its own rain.

On an obligingly sunny morning in April toward the end of the Amazon’s rainy season, I joined a group of visiting scientists in a gear-filled container near the base of the tower. We pulled on climbing harnesses furnished with ropes and carabiners and strapped on mandarin orange hardhats. Each of us had a primary lifeline: a thick cord tethering our harness to a four-wheeled trolley designed to roll along the grooves of a curving rail that flanked the tower’s staircase. Sipko Bulthuis, a Manaus-based technician with a shaggy flop of blond hair, was the first to start climbing, followed shortly by Uwe Kuhn, an atmospheric chemist from the Max Planck Institute for Chemistry with foggy blue eyes and a tapered soul patch. I was next. I slid my trolley into the spiraling rail and took the first few steps, gripping the banister tightly with one hand and helping the trolley along with the other. Kuhn’s colleague Christopher Pöhlker—a tall, soft-spoken, moon- faced man—followed a little while later.

To my surprise, the ascent was immediately and continuously exhilarating. Thrill and wonder easily overpowered fear. Within ten minutes, we reached the rainforest canopy, about 80 to 115 feet above the ground. Here, the distinct features of individual trees—the yellow- flowered crowns of guayacans, the nearly horizontal branches of kapoks—were still discernible. The death metal screams of howler monkeys rumbled through the air, accompanied by the squawks and chitters of macaws. Above us, the sky was bright and blue, save for a few smudges of white in the distance.

By the time we’d climbed half the tower—533 feet—the view was quite different. We could no longer perceive the majesty of any particular tree. Instead, we saw a vast knurled mat of gray and green stretching to the horizon in every direction. From this height, I under- stood, more clearly than ever before, that each tree was part of an immense living network covering the planet’s surface. As we climbed, the trees pulled an invisible ocean from the soil through their roots into their trunks and tissues. The sun drew what the trees did not use through their leaves into the air, where the water vapor condensed on a floating potpourri of dust, microbes, and organic detritus derived from the forest, eventually forming visible clouds. Lakes of shadow now drifted across the canopy, echoing their woolly counterparts, as if to remind us that the trees were in the clouds and the clouds were in the trees—that forest and sky were twinned phrases in the same ancient chorus.

About an hour after we began climbing, we reached the last flight of stairs. Roosting swallows had covered this part of the tower with droppings, which were now as dry and flaky as ash. Before we could take the final few steps, we had to remove our trolleys from the stair- case railing and slide them into a separate rail on the observation deck while remaining attached to the tower via a secondary carabiner. “Always be connected,” Bulthuis said as he explained the procedure.

“Sempre tem que estar conectado.”

I made the necessary adjustments and stepped onto the tower’s top-most platform, which was surrounded by nothing more than a series of thin, X-shaped panels with gaps wide enough to accommodate the average adult torso. I had anticipated that this would be the scariest part of the climb, yet even here I felt surprisingly secure. The heat of the sun was more intense, and the wind fierce at times, but the tower remained steadfast.

Gazing at the unbroken expanse of rainforest beneath us was a decidedly different experience than exploring it by foot. On the ground, I had been overwhelmed by the beauty and lushness of life all around me and the opportunity to examine it in detail: by the ferns and bromeliads frothing on every branch and the intricate tapestries of moss and lichen; by the tantalizing shimmer of a blue Morpho butterfly and the delicacy of a ghost plant’s milky flower trembling on a wiry stalk. At more than a thousand feet in the air, the concept of individual organisms began to blur and dissolve. From this vantage point, the forest seemed less like a place or ecosystem in and of itself than the skin, the fleece, of a much larger entity—one whose true scale I was only beginning to glimpse. It felt as though I’d been trapped in a drop of pond water on a microscope slide, confusing a strand of algae for a jungle, and was only now switching places with the eye behind the lens.

We are so used to thinking of the environment governing life’s evolution and sculpting its endless forms. Conventional wisdom maintains that rainforests and other highly biodiverse regions of the planet are the result of fortuitous circumstances. Yet just about everything I could see from the top of the tower, I was coming to understand, was to some extent created by life. Most of the tens of thousands of documented animal species in the Amazon, and all those as yet undiscovered, would not exist if plants and fungi had not populated and transformed the planet’s land surfaces half a billion years ago. Complex life may never have evolved, let alone emerged from the sea, if single- celled microbes had not started reforming the ocean and atmosphere several billion years earlier. The soil from which the trees below me grew, the rain-heavy clouds preparing to burst, the color of the sky, the air itself—we owe it all to life.

The development of a stable atmosphere was one of the most important events in Earth’s infancy. Without sufficient atmospheric pressure, any liquid water on a planet’s surface will eventually boil off into space. If young Earth had not retained liquid water on its surface, life as we know it would not exist. Yet it is also true that without life, Earth’s water would not be as, well, fluid. A defining feature of our planet today is not simply the presence of water but the simultaneous existence of water in all its possible states—vapor, liquid, ice—and its continuous movement between air, sea, and land. Over time, life became intertwined with the physics that makes this flux possible.

The hidden threads linking life and the atmosphere have fascinated Russ Schnell since his youth. Growing up in rural Alberta, Canada, he witnessed lightning, hail, and torrential downpours every summer. He liked to watch storm clouds form: these great swirling masses of vapor, like whirlpools in the sky, sucking up air, dust, and whatever was too light to escape their pull. As the clouds inhaled, they gradually grew larger and darker, fuming at their continually redefined borders. As a student at the University of Alberta, Schnell—a short, slender man of twenty with thick blond hair and a disciplined mind—spent his summers assisting a group of atmospheric scientists. One of the project leaders tasked him with investigating the formation of hail- stones. How exactly did clouds produce such large chunks of ice?

Water that evaporates into the atmosphere will not automatically freeze at 32°F. Pure water can remain liquid to about −40°F. To freeze at higher temperatures, water needs a seed, or what’s technically termed an ice nucleus: a tiny particle that acts as a geometric template, aligning water molecules into a highly organized solid crystal. At the time, in 1968, most scientists thought that airborne water vapor condensed on floating particles of dust and soot and that these kerneled beads of water could in turn freeze if the air were cool enough. But no one knew what kind of particles made the best ice nuclei or how embryonic ice crystals grew into hailstones the size of baseballs and grapefruits. Schnell’s task was to dissect the very heart of hail and find the mysterious mote that turned cloud water into ice.

Schnell thought back to the hailstorms he had observed as a kid and all the ones he’d witnessed in the intervening years. They always seemed to form over forests and other densely vegetated areas. What if, Schnell wondered, ice nuclei weren’t just inert bits of dirt? What if some were spewed out by trees, or vacuumed up from plants by churning storm clouds? When he told the senior scientists about his idea, they smiled as though amused by a schoolchild’s naïveté. Trees helped return water to the atmosphere, of course, but apart from that, what could they possibly have to do with clouds, much less hail? Still, if that was the hypothesis he wanted to pursue, he was free to do so.

For weeks, Schnell roamed nearby forests and fields, grabbing handfuls of grass and plucking leaves from poplars, aspen, and conifers. In the lab, he cut off a small piece of vegetation and sloshed it around in a vial of water to capture whatever invisible particles were on its surface. Using a syringe, he pulled some water from the vial and care- fully placed dozens of drops on a temperature-controlled copper plate. He then covered the plate with a glass dome and gradually lowered its temperature. If the drops froze before the plate reached 5°F, then he’d know they contained some kind of ice nucleus that was facilitating the formation of ice crystals. They never did.

One summer evening in 1970, in a rush to get to a party, Schnell left a plastic bag containing grass and water on a shelf in the lab and forgot about it. Ten days later, he discovered that the bag was full of a milky white emulsion. The grass had begun to decay. Rather than throwing it out, Schnell decided to test the rotten grass water on the copper plate. To his astonishment, the water froze at 29.66°F—a much higher temperature than anyone had ever reported in similar conditions. Something in that putrid brew—something biological—was turning water into ice.

Schnell moved to the University of Wyoming for graduate school, where he continued his studies on plant-derived ice nuclei. Suspecting that a plant-loving fungus was involved, Schnell asked a colleague in the botany department, Richard Fresh, to take a look at his leaf samples. Fresh discovered that the ice-forming molecules were in fact proteins clinging to the shell of a rod-shaped bacterium called Pseudomonas syringae that tended to live in soil and on plants. The proteins mimicked the shape of ice crystals, providing a perfect template to organize free-floating water molecules into a cohesive solid.

On the ground, the bacteria gave plants frostbite, rupturing their tissues in order to access their nutrients. When storm clouds sucked up air and dust from the ground below, they would inevitably pull in various microorganisms as well. Once in the clouds, P. syringae and its proteins could seed ice crystals and hailstones. No scientist had ever seriously proposed that a microbe could freeze water, let alone change the weather, yet here was the proof: a bacterial protein that acted like Kurt Vonnegut’s fictional ice-nine.

Enthralled by these discoveries, Schnell embarked on a globe- spanning research expedition. First, he traveled west from Canada across the central United States. Then he flew to England and traveled east across Europe and through Russia on the Trans-Siberian Railway. Before returning home, he toured Japan, Thailand, India, Nepal, Iran, and parts of Africa. Skinny and disheveled, Schnell lived frugally, eating and lodging for as little as $100 a month. Whenever he had the opportunity—on the side of a road, in a field, in a grove of trees—he would stop, collect some leaf litter, and store it in a plastic sandwich bag. Back in Wyoming, he tested dozens of samples from all manner of ecosystems and climates. In every single one, he found active ice nuclei produced by P. syringae and other microbes.

Ice-making microorganisms, Schnell realized, were not just important for hail formation. Once they got into the atmosphere, they would also increase the chance of rain. Only a small percentage of all clouds grow heavy enough to rain. The vast majority just evanesce. But the presence of ice nuclei can dramatically change the odds. An ice nucleus can start a chain reaction that quickly freezes lots of cloud water, drawing in even more water and swelling a cloud until it bursts. The proteins that P. syringae makes are the most effective ice nuclei ever discovered. Schnell thinks they are a crucial component of the water cycle in ecosystems across the planet. “Almost all rain that falls on land, even over the Sahara and along the tropics, is first an ice crystal,” he says.

The private sector quickly recognized the potential of Schnell’s discovery. By the 1980s, a company called Snomax had patented the process of creating artificial snow using sterilized proteins isolated from massive vats of P. syringae. Ever since, ski resorts around the world have relied on microbial proteins to blanket their slopes. In contrast, the scientific community largely ignored weather-changing microbes for several decades, regarding them as an intriguing but trivial aspect of meteorology not worthy of serious research. In recent years, however— as climate change has pushed scientists to reexamine the complexities of the atmosphere, and astonishing new discoveries have come to light—attitudes have started to change.

It’s now clear that P. syringae is far from the only organism that can turn water into ice. Numerous bacteria, algae, lichen, and plankton— on land and out at sea—produce ice-seeding proteins. Strong winds, updrafts, thunderstorms, and dust storms routinely whisk these tiny creatures into the atmosphere, where they form celestial colonies for weeks at a time before returning to the planet’s surface in the very precipitation they stimulate. In doing so, these accidental aeronauts may influence the planet in profound ways that until now have been largely neglected. “The whole concept has definitely gained a lot of traction,” says David Sands, a professor of plant pathology at Montana State University. “We need to recognize these microbes as a part—maybe even a major part—of meteorological processes.”

The possibility that the atmosphere teems with unseen life has intrigued scientists since the advent of microbiology in the seventeenth century. Antonie van Leeuwenhoek, one of the first people to observe microbes through a microscope, surmised the existence of “living creatures in the air, which are so small as to escape our sight.” In the 1800s, while aboard the HMS Beagle, Charles Darwin collected wind- swept dust over the Atlantic that was later revealed to be full of microbes. And in the early 1900s, Fred C. Meier, a plant pathologist with the U.S. Department of Agriculture, convinced Charles Lindbergh and Amelia Earhart to furnish their aircraft with metal cylinders de- signed to capture microorganisms.

Only in the late twentieth century, however, did researchers begin to regard airborne microbes as more than passive travelers. In 1978, searching for the origins of a P. syringae outbreak in Montana wheat fields, Sands flew a Cessna through the clouds above the crops, sticking petri dishes through a porthole. P. syringae soon grew in those dishes. In the 1980s, building on earlier research by Schnell, Sands formally proposed the theory of bioprecipitation: the idea that some bacteria disperse themselves through an elaborate rain dance. “At the time, a lot of people thought it was crazy,” says Cindy Morris, who works for France’s National Research Institute for Agriculture, Food and Environment, and has long collaborated with Sands. But “no one is telling us we are crazy anymore.”

In the mid-2000s, Sands, Morris, and their colleagues collected fresh snow on three continents. Nearly every sample contained ice- nucleating microbes, including plant-dwelling bacteria that had made it all the way to Antarctica. A few years later, scientists in Europe and the United States discovered a variety of microbes in the centers of hailstones. Other researchers analyzed cloud water and measured, on average, tens of thousands of bacteria in each milliliter. Using a century of weather data, Morris and a couple of colleagues have also found statistical support for a bioprecipitation feedback loop: the more in- tense a rainstorm, the more frequent and intense the storms in the days and weeks ahead, presumably because heavy downpours kick microscopic life up into the air.

Scientists once thought that ice-nucleating proteins evolved primarily as a way for P. syringae and its ilk to feed on plants and only secondarily as an opportunistic means of air travel. But P. syringae is not always harmful to plants and does not live on them exclusively; it’s also found in rivers and lakes. Evolutionary relationships among different ice-making bacteria indicate that ice-nucleating proteins evolved at least 1.75 billion years ago, long before the aquatic ancestors of modern plants began to explore the land. Back then, Morris and Sands propose, these proteins probably helped microbes survive freezing water and major glaciations, perhaps by sequestering damaging ice crystals outside their cells.

Over the eons, ocean waves and powerful winds would have carried microbes into the atmosphere, where they would have encountered DNA-warping ultraviolet light, a dearth of food, and the threat of desiccation. Bacteria with ice-nucleating proteins would have enjoyed a huge advantage over those without: a return ticket to the surface. And microbes that could survive long enough to travel great distances would have expanded their range and possibly found more favorable habitats, as the biologist W. D. Hamilton theorized. The types of bacteria that scientists are finding in precipitation today possess a number of skills that may be adaptations to an ancient familiarity with the high life: pigments that act like sunscreen, for example, and the ability to feed solely on molecules commonly found in cloud water. One study even concluded that certain bacteria can reproduce within clouds.

For most of its history—for somewhere between 2 and 3.5 billion years—Earth was an exclusively microbial planet. For much of that inconceivably long era, there were few if any organisms composed of more than one cell. When more elaborate multicellular organisms emerged and populated the sea and land, they did so within a living matrix of far smaller and more ancient creatures. The rise of plants, fungi, and animals profoundly increased the complexity of Earth’s ecosystems not simply by introducing larger and more sophisticated organisms but also by spawning countless new relationships between those organisms and their microbial predecessors.

As living creatures linking land and sky—effectively functioning as sponges and pumps—plants developed a particularly close relationship with the water cycle. At the same time, they became canvasses and conduits for their microbial partners. Wherever the confluence of geology and climate offered an abundance of light, heat, and moisture, plants had an opportunity to thrive. Wherever plants thrived, they did so only by associating with fungi and microbes, including those that seeded clouds and induced rain. The warm, wet regions of the continents grew soft and green with leaf and bud. As plants became stronger and taller, they lofted invisible societies of essentially weightless beings, amplifying their presence in the atmosphere. Together, they pulled water from the soil, pushed it into the air, and called it back again.

Schnell’s instincts were right: trees had everything to do with clouds.

From Becoming Earth: How Our Planet Came to Life by Ferris Jabr. Copyright © 2024. Published on June 25, 2024 by Penguin Random House. Excerpted by permission.