The Mysterious, Deep-Dwelling Microbes That Sculpt Our Planet
Earth’s crust teems with subterranean life that we are only now beginning to understand.
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Ferris Jabr is a contributing writer at the magazine and the author of “Becoming Earth: How Our Planet Came to Life,” from which this article is adapted.
In the middle of North America, there is a portal to the deep recesses of Earth’s rocky interior. The portal’s mouth — a furrowed pit about half a mile wide — spirals 1,250 feet into the ground, exposing a marbled mosaic of young and ancient rock: gray bands of basalt, milky veins of quartz and shimmering constellations of gold. Beneath the pit, some 370 miles of tunnels twist through solid rock, extending more than 1.5 miles below the surface. For 126 years, this site in Lead, S.D., housed the Homestake Mine, the deepest and most productive gold mine on the continent.
In 2006, the Barrick Gold Corporation donated the mine to the state of South Dakota, which converted it into the largest subterranean laboratory in the United States, the Sanford Underground Research Facility. Although the lowest tunnels flooded after mining ceased, it is still possible to descend nearly a mile beneath the planet’s surface. Most of the scientists who do so are physicists conducting highly sensitive experiments that must be shielded from interfering cosmic rays. But a few biologists also venture into the underground labyrinth, typically seeking its dankest and dirtiest corners — places where obscure creatures extrude metal and transfigure rock.
On a bitingly cold December morning, I followed three young scientists and a group of Sanford employees into “the cage” — the bare metal elevator that would take us 4,850 feet into Earth’s crust. We wore neon vests, steel-toed boots and hard hats. Strapped to our belts were personal respirators, which would protect us from carbon monoxide in the event of a fire or explosion. The cage descended swiftly and surprisingly smoothly. Our idle chatter and laughter were just audible over the din of unspooling cables and whooshing air. After a controlled plummet of about 10 minutes, we reached the bottom of the facility.
Our two guides, both former miners, directed us into a pair of small linked rail cars and drove us through a series of narrow tunnels. Within 20 minutes, we had traded the relatively cool and well-ventilated region near the cage for an increasingly hot and muggy corridor. Whereas the surface world was snowy and well below freezing, a mile down it was about 90 degrees with nearly 100 percent humidity. Heat seemed to pulse through the rock surrounding us, and the air was thick and cloying; the smell of brimstone seeped into our nostrils. It felt as though we had entered hell’s foyer.
The rail cars stopped. We stepped out and walked a short distance to a large plastic spigot protruding from the rock. A pearly stream of water trickled from the wall near the faucet’s base, forming rivulets and pools. Wafting from the water was hydrogen sulfide — the source of the chamber’s odor. Kneeling, I realized that the water was teeming with a stringy white material similar to the skin of a poached egg. Caitlin Casar, a geobiologist, explained that the white fibers were microbes in the genus Thiothrix, which join together in long filaments and store sulfur in their cells, giving them a ghostly hue. Here we were, deep within Earth’s crust — a place where, without human intervention, there would be no light and little oxygen — yet life was literally gushing from rock. This particular ecological hot spot had earned the nickname Thiothrix Falls.
On a different level of the mine, we sloshed through mud and shin-high water, stepping carefully to avoid tripping on submerged rails and stray stones. Here and there, delicate white crystals, most likely gypsum or calcite, ornamented the ground and walls, glimmering like stars. We eventually reached another large spigot mired in what looked like wet clay, which varied in color from pale salmon to brick red. This, too, Casar explained, was the work of microbes — in this case a genus known as Gallionella, which thrives in iron-rich waters and excretes twisted metal spires. At Casar’s request, I filled a jug with water, scooped microbe-rich mud into plastic tubes and stored them in coolers, where they would await analysis.
Casar and her colleagues have visited the former Homestake Mine at least twice a year for many years. Every time they return, they encounter enigmatic microbes that have never been successfully grown in a laboratory and species that have not yet been named. Their studies are part of a collaborative effort whose leaders include Magdalena Osburn, a professor at Northwestern University and a prominent member of the relatively new field known as geomicrobiology.
Scientists like Osburn have shown that, contrary to long-held assumptions, Earth’s interior is not barren. In fact, a majority of the planet’s microbes, perhaps more than 90 percent, may live deep underground. These intraterrestrial microbes tend to be quite different from their counterparts on the surface. They are ancient and slow, reproducing infrequently and possibly living for millions of years. They often acquire energy in unusual ways, breathing rock instead of oxygen. And they seem capable of weathering geological cataclysms that would annihilate most creatures. Like the many tiny organisms in the ocean and atmosphere, the unique microbes within Earth’s crust do not simply inhabit their surroundings; they transform them. Subsurface microbes carve vast caverns, concentrate minerals and precious metals and regulate the global cycling of carbon and nutrients. Microbes may even have helped construct the continents, literally laying the groundwork for all other terrestrial life.
Like so much about Earth’s earliest history, exactly where and when life first emerged is not definitively known. At some point not long after our planet’s genesis, in some warm, wet pocket with the right chemistry and an adequate flow of free energy — a hot spring, an impact crater, a hydrothermal vent on the ocean floor — bits of Earth rearranged themselves into the first self-replicating entities, which eventually evolved into cells. Evidence from the fossil record and chemical analysis of the oldest rocks ever discovered indicate that microbial life existed at least 3.5 billion years ago and possibly as far back as 4.2 billion years ago.
Among all living creatures, the peculiar microbes that dwell deep within the planet’s crust today may most closely resemble some of the earliest single-celled organisms that ever existed. Collectively, these subsurface microbes make up an estimated 10 to 20 percent of the biomass — that is, all the living matter — on Earth. Yet until the mid-20th century, most scientists did not think subterranean life of any kind was plausible below a few meters.
The oldest scientific reports of subsurface life date only to the 1600s. In 1684, while traveling through central Slovenia, the naturalist Janez Vajkard Valvasor investigated rumors of a dragon living beneath a spring near Ljubljana. Local residents believed the dragon forced water to the surface every time it shifted its body. After heavy rains, they sometimes found baby dragons washed up on rocks nearby: slender and sinuous with blunted snouts, frilled throats and nearly translucent pink skin. It was not for another century that naturalists formally identified the creatures as aquatic salamanders that lived exclusively underground in water flowing through limestone caves. They are now known as olms.
In the early 20th century, scientists started to get glimpses of the true abundance of life deep underground. Around 1910, while trying to determine the source of methane gas in mines, German microbiologists isolated bacteria from coal samples collected 3,600 feet below the surface. In 1911, the Russian scientist V. L. Omelianski discovered viable bacteria preserved in permafrost alongside an unearthed mammoth. Not long after that, Charles B. Lipman, a soil microbiologist at the University of California, Berkeley, reported that he had revived ancient bacterial spores trapped in chunks of coal obtained from a Pennsylvania mine.
Although these early studies were tantalizing, many scientists remained skeptical because of the possibility that surface microbes had contaminated the samples. In subsequent decades, however, researchers continued to find microbes in rock and water obtained from mines and drill sites all over the world. By the 1980s, attitudes had started to shift. Studies of aquifers clearly indicated that bacteria populated groundwater, even thousands of feet below the surface. And scientists developed more rigorous methods for preventing the accidental introduction of surface microbes, such as disinfecting drill bits and tracking the movement of fluids through the crust to make sure surface water was not mingling with their samples.
Ultimately, the results of this research confirmed that, if anything, early proponents of a subterranean biosphere had been too conservative. Wherever scientists looked — within the continental crust, beneath the seafloor, under Antarctic ice — they found unique communities of microbes collectively containing thousands of unidentified species. In certain pockets of the crust, there appeared to be as few as one microbe per cubic centimeter, equivalent to a country with only one person every 400 miles. The underworld was real, but its inhabitants were much smaller and stranger than anyone had imagined.
In the 1990s, Thomas Gold, an astrophysicist at Cornell University, published a series of provocative claims about Earth’s inner microbial wilderness. Gold proposed that micro-organisms permeated the entire subsurface, living in fluid-filled pores between the grains in rocks. Although scientists had not yet found microbes farther than 1.86 miles underground, Gold suspected that they lived as deep as six miles and that the biomass within the crust was at least equal to, if not greater than, that on the surface. He further suggested that at least some branches of life may have originated in the planet’s interior; that other planets and moons might also harbor subterranean ecosystems; and that deep-dwelling microbes were likely to be the most common form of life throughout the cosmos.
By the early 2000s, scientists started devising new ways to plunge even farther into Earth’s crust. Mines were particularly promising because they provided access to the remote subsurface without requiring much additional drilling or infrastructure. Tullis Onstott, a professor of geosciences at Princeton University, and his colleagues traveled to ultradeep gold mines in South Africa and retrieved samples of groundwater from nearly two miles underground. Within some of the deepest samples, they found a single species: a baguette-shaped bacterium with a whiplike tail that endured temperatures up to 140 degrees and acquired energy from the chemical byproducts of radioactively decaying uranium embedded in its sunless home.
Onstott and his colleagues decided to name the microbe Desulforudis audaxviator, after a passage in Jules Verne’s “Journey to the Center of the Earth,” which reads “descende, Audax viator, et terrestre centrum attinges” — “descend, bold traveler, and you will attain the center of the Earth.” The water in which D. audaxviator was discovered had not been disturbed for tens of millions of years at a minimum, suggesting that a population of these daring microbial terranauts may have sustained itself for at least as long. “We do not normally think of rock as harboring life,” Onstott writes in his book “Deep Life.” “Like most geologists, I too have viewed rocks as inanimate entities.” But now, he continues, as a geomicrobiologist, he sees all rocks as little worlds unto themselves, composed of micro-organisms, “some of which may have been living in the rock since its formation hundreds of millions of years ago.”
Some communities of subsurface microbes may be even older. The Kidd Creek Mine in Ontario, Canada, is one of the largest and deepest mines in the world. Extending about 1.86 miles below ground, it contains rich veins of copper, silver and zinc that formed nearly three billion years ago on the ocean floor. In 2013, the University of Toronto geologist Barbara Sherwood Lollar published a study demonstrating that some parcels of water in the Kidd Creek Mine have been isolated from the surface for more than a billion years, making it the oldest water ever discovered on Earth. Transparent when first collected, the iron-rich water turns a pale orange when exposed to oxygen; it has the consistency of maple sap, contains at least twice as much salt as modern seawater and, in Sherwood Lollar’s judgment, “tastes terrible.” In 2019, Sherwood Lollar, Magdalena Osburn and several colleagues confirmed that just like much younger fluids circulating through the pores and fissures in rock a few thousand feet below the surface, the billion-year-old water miles deep in Kidd Creek Mine is also populated by micro-organisms. Although some of these microbial communities are likely a few hundred million years old at most, it is possible that others have continuously inhabited the deep crust for an eon or more.
“This research really is a form of exploration,” Sherwood Lollar says. “Some of the findings are causing us to rewrite the textbooks about how this planet works. They are changing our understanding of Earth’s habitability. We don’t know where life originated. We don’t know if life arose on the surface and went down or whether life emerged below and went up. There’s a tendency to think about Darwin’s warm little pond, but, as my colleague T. C. Onstott likes to say, it could just as easily have been some warm little fracture.”
Many years ago, I learned an astonishing fact that began to change the way I think about life’s relationship to the giant, tempestuous, half-molten rock we call home. What I learned is this: Life on our planet does not simply experience the weather — it creates it. Consider the Amazon rainforest. Every year, the Amazon is drenched in about eight feet of rain. In some parts of the forest, the annual rainfall is closer to 14 feet, more than five times the average yearly precipitation across the contiguous United States. This deluge is partly a consequence of geographic serendipity: Intense equatorial sunlight speeds the evaporation of water from sea and land to sky, trade winds bring moisture from the ocean and bordering mountains force incoming air to rise, cool and condense. Rainforests happen where it happens to rain.
But that’s only half the story. Within the forest floor, vast symbiotic networks of plant roots and filamentous fungi pull water from the soil into trunks, stems and leaves. As the nearly 400 billion trees in the Amazon drink their fill, they release excess moisture, saturating the air with 20 billion tons of water vapor each day. At the same time, plants of all kinds secrete salts and emit bouquets of pungent gaseous compounds. Mushrooms, dainty as paper parasols or squat as doorknobs, exhale plumes of spores. The wind sweeps bacteria, pollen grains and bits of leaves and bark into the atmosphere. The wet breath of the forest — peppered with microscopic life and organic residues — creates conditions that are highly conducive to rain. With so much water vapor in the air and so many minute particles on which the water can condense, clouds quickly form. In a typical year, the Amazon generates around half of its own rainfall.
The Amazon’s rain ritual challenges the way we typically think about life on Earth. Conventional wisdom holds that life is subject to its environment. If Earth did not orbit a star of the right size and age, if it were too close or too far from that star, if it did not have a stable atmosphere, liquid water and a magnetic field that deflects harmful cosmic rays, it would be lifeless. Life evolved on Earth because Earth is suitable for life. Since Darwin, prevailing scientific paradigms have likewise emphasized that the ever-shifting demands of the environment largely dictate how life evolves: Species best able to cope with changes to their particular habitats leave behind the most descendants, whereas those that fail to adapt die out.
Yet this truth has an underappreciated twin: Life changes its environment, too. In the mid-20th century, when ecology established itself as a formal discipline, this fact began to gain wider recognition in Western science. Even so, the focus was on relatively small and local changes: a beaver constructing a dam, for instance, or earthworms churning a patch of soil. The notion that living creatures of all kinds might modify their environments in much more significant ways — that microbes, fungi, plants and animals can change the topography and climate of a continent or even the entire planet — was rarely given serious consideration.
In recent decades, however, the scientific understanding of life’s relationship to the planet has been undergoing a major reformation. Contrary to longstanding maxims, life has been a formidable geological force throughout Earth’s history, often matching or surpassing the power of glaciers, earthquakes and volcanoes. Over the past several billion years, all manner of life forms, from microbes to mammoths, have transformed the continents, ocean and atmosphere, turning a lump of orbiting rock into the world as we’ve known it. Living creatures are not simply products of inexorable evolutionary processes in their particular habitats; they are orchestrators of their environments and participants in their own evolution. We and other living creatures are more than inhabitants of Earth. We are Earth: an outgrowth of its physical structure and an engine of its global cycles. The evidence for this new paradigm is all around us, although much of it has been discovered only recently and has yet to permeate public consciousness to the same degree as, say, selfish genes or the microbiome.
The history of life on Earth is the history of life’s remaking Earth. Nearly two and a half billion years ago, photosynthetic ocean microbes called cyanobacteria permanently altered the planet, suffusing the atmosphere with oxygen, imbuing the sky with its familiar blue hue and initiating the formation of the ozone layer, which protected new waves of life from harmful exposure to ultraviolet radiation. Today plants and other photosynthetic organisms appear to help maintain a level of atmospheric oxygen high enough to support complex life but not so high that Earth would erupt in flames at the slightest spark. Marine plankton drive chemical cycles on which all other life depends and emit gases that increase cloud cover, modifying global climate. Kelp forests, coral reefs and shellfish store huge amounts of carbon, buffer ocean acidity, improve water quality and defend shorelines from severe weather. Animals as diverse as elephants, prairie dogs and termites continually reconstruct the planet’s crust, facilitating the flow of water, air and nutrients and improving the prospects of millions of species. And micro-organisms, like those I observed deep within Earth’s crust, are now thought to be important players in many geological processes.
As I studied the interdependence of Earth and life, I continually returned to an ancient and controversial idea: that Earth itself is alive. It was not until the late 20th century that the idea of a living planet found one of its most popular and enduring expressions in Western science, the Gaia hypothesis. Conceived by the British scientist and inventor James Lovelock in the 1960s and later developed with the American biologist Lynn Margulis, the Gaia hypothesis proposes that all the animate and inanimate elements of Earth are “parts and partners of a vast being who in her entirety has the power to maintain our planet as a fit and comfortable habitat for life.”
Lovelock published his first book on Gaia in 1979 amid a growing environmental movement. Although his ideas found an enthusiastic audience among the public, many scientists criticized and ridiculed them. Those who bristled at the notion typically made the same protestations: Earth cannot be alive because it does not eat, grow, reproduce or evolve through natural selection like “real” living things.
Yet there has never been an objective measure or a universally accepted definition of life. There are numerous examples of things we consider inanimate that have traits of the living and vice versa. Life is more spectral than categorical, more verb than noun. Life is not a distinct class of matter but rather a process — a performance. Life is something matter does.
Although science has not yet arrived at a fundamental explanation of the phenomenon we call life, many experts in the past century have favored a variation of the following: Life is a system that sustains itself. This defining capacity for active self-preservation and self-regulation emerges at many different scales: at the scale of the cell, the organism, the ecosystem and, I would argue, the planet.
Gaia still retains something of a stigma in mainstream science, but in recent decades opposition has waned significantly. Although the claim that Earth itself is a living entity remains controversial, some scientists embrace it, and others are increasingly open to it. The idea that life transforms the planet and is intertwined with its self-regulatory processes has become a central tenet of mainstream Earth-system science, a relatively young field that explicitly studies the living and nonliving components of the planet as an integrated whole. As the Earth-system scientist Tim Lenton has written, he and his colleagues “now think in terms of the coupled evolution of life and the planet, recognizing that the evolution of life has shaped the planet, changes in the planetary environment have shaped life, and together they can be viewed as one process.”
Like many living things, Earth absorbs, stores and transforms energy. Earth has a body with organized structures, membranes and daily rhythms. From the raw elements of our planet have emerged zillions of biological entities that ceaselessly devour, transfigure and replenish its rock, water and air. Such organisms do not simply reside on Earth; they are literal extensions of Earth. Moreover, organisms and their environments are inextricably bonded in reciprocal evolution, often converging upon self-stabilizing processes that favor mutual persistence. Collectively, these processes endow Earth with a kind of planetary physiology: with breath, metabolism, a regulated temperature and a balanced chemistry. Earth is not a single organism or a product of standard Darwinian evolution, but it is nonetheless a genuine living entity, a vast interconnected living system.
One early metaphor Lovelock deployed to explain Gaia was a redwood tree. Only a few parts of a tree contain living cells, namely the leaves and thin layers of tissue within the trunk, branches and roots. The rest is dead wood. Similarly, the bulk of our planet is inanimate rock, wrapped in a flowering skin of life. Just as strips of living tissue are essential to keep a whole tree alive, Earth’s living skin helps sustain a kind of global being. What Lovelock did not realize at the time, however, was that even Earth’s seemingly inert skeleton of rock was far more porous and alive than most people believed.
For hundreds of years, Lechuguilla Cave appeared to be little more than a long hole leading to dead-end passages in the Guadalupe Mountains of New Mexico. Prospectors visited to collect bat guano, which was prized as a fertilizer. Otherwise, no one paid it much attention. One day in the 1950s, however, cavers noticed wind streaming through rubble at the cave’s bottom, suggesting a hidden section. A series of excavations beginning in the 1980s uncovered several long passageways. Subsequent explorations eventually revealed about 150 miles of underground terrain. The tunnels and chambers were decorated with strange and beautiful formations: massive chandeliers of frostlike gypsum, lemon-yellow sulfur pods, pearly balloons of hydromagnesite, transparent selenite spears and calcite lily pads hovering over turquoise pools.
In the early 1990s, the astrobiologist and geomicrobiologist Penelope Boston watched a National Geographic TV special about Lechuguilla. She was fascinated by the prospect of a pristine subterranean wonderland. One of the researchers featured on the show, Kim Cunningham, had found some preliminary evidence of microbial life in the cave. Boston, who was particularly interested in the possibility of life beyond Earth, saw Lechuguilla as an analog for potential subsurface habitats on other planets. She called Cunningham and arranged to visit the cave with a team of scientists and cavers.
Boston and the other scientists, who did not have much caving experience, practiced for a few hours on cliffs in Boulder, Colo. The brief training was nowhere near adequate. Lechuguilla is not a simple series of interconnected rooms that a person can walk through; it’s a tangle of crystal labyrinths embedded in a tortuous maze of rock. To navigate it, Boston and her colleagues had to rappel down steep cliffs, climb over slippery towers of gypsum, traverse narrow ledges and squirm through stone honeycombs, all while towing their cumbersome gear. “We were in such an alien environment that we were just basically coping,” Boston recalls. “I kept thinking to myself, I just have to live long enough to get out of here.”
They did survive, but not without injuries. At one point, Boston sprained her ankle and gouged her shin. Her most revealing malady emerged not long before leaving the cave. As she prepared to scrape some curious rust-colored fluff from a portion of the ceiling into a bag, a smidgen fell into her eye, which soon swelled shut as though it were infected. Perhaps, she thought, the brown fuzz was made by microbes; perhaps it was microbes. Laboratory studies eventually confirmed her hunch: the cave was covered in micro-organisms that chewed through rock, chemically transforming iron and manganese for energy and leaving behind a soft mineral residue. Microbes were turning rock into soil more than one thousand feet underground.
Eventually, through many years of research, Boston and other scientists revealed that the microbes in Lechuguilla do much more than spit out a little dirt. Lechuguilla is ensconced in thick layers of limestone, the petrified remains of a 250-million-year-old reef. The manifold chambers in such caves are usually formed by rainwater that seeps into the ground and gradually dissolves the limestone. In Lechuguilla, however, microbes are also the sculptors: Bacteria eating buried reserves of oil release hydrogen-sulfide gas, which reacts with oxygen in groundwater, producing sulfuric acid that carves away limestone. In parallel, different microbes consume hydrogen sulfide and generate sulfuric acid as a byproduct. Similar processes happen in 5 to 10 percent of limestone caverns globally.
Since Boston’s initial descent into Lechuguilla, scientists around the world have discovered that micro-organisms transform the planet’s crust wherever they inhabit it. Alexis Templeton, a geomicrobiologist at the University of Colorado, Boulder, regularly visits a barren mountain valley in Oman where tectonic activity has pushed sections of the earth’s mantle — the layer that sits below the crust — much closer to the surface. She and her colleagues drill boreholes up to a quarter of a mile into the uplifted mantle and extract long cylinders of 80-million-year-old rock, some of which are beautifully marbled in striking shades of maroon and green. In laboratory studies, Templeton has demonstrated that these samples are full of bacteria, some of which change the composition of Earth’s crust: They eat hydrogen and breathe sulfates in the rock, exhale hydrogen sulfide and create new deposits of sulfide minerals similar to pyrite, also known as fool’s gold.
Through related processes, microbes have helped form some of Earth’s stores of gold, silver, iron, copper, lead and zinc, among other metals. As subsurface microbes break down rock, they often free the metals stuck within it. Some of the chemicals microbes release, such as hydrogen sulfide, combine with free-floating metals, forming new solid compounds. Other molecules produced by microbes grab soluble metals and bind them together. Some microbes stockpile metal inside their cells or grow a crust of microscopic metal flakes that continuously attract even more metal, potentially forming a substantial deposit over long periods of time.
Life, in particular microbial life, has forged a large quantity of Earth’s minerals, which are naturally occurring inorganic solid compounds with highly organized atomic structures, or, to put it more plainly, very elegant rocks. Today Earth has more than 6,000 distinct mineral species, most of which are crystals such as diamond, quartz and graphite. In its infancy, however, Earth did not have much mineral diversity. Over time, the continuous crumbling, melting and resolidifying of the planet’s early crust shifted and concentrated uncommon elements. Life began to break apart rock and recycle elements, generating entirely new chemical processes of mineralization. More than half of all minerals on the planet can occur only in a high-oxygen environment, which did not exist before microbes, algae and plants oxygenated the ocean and atmosphere.
Through the combination of tectonic activity and the ceaseless bustle of life, Earth developed a mineral repertoire unmatched by any other known planetary body. Comparatively, the moon, Mercury and Mars are minerally impoverished, with perhaps a few hundred mineral species among them at most. The variety of minerals on Earth depends not merely on the existence of life but also on its idiosyncrasies. Robert Hazen, a mineralogist and astrobiologist at Carnegie Science, and the statistician Grethe Hystad have calculated that the chance of two planets having an identical set of mineral species is one in 10³²². Given that there are only an estimated 10²⁵ Earthlike planets in the cosmos, there is almost certainly no other planet with Earth’s exact complement of minerals. “The realization that Earth’s mineral evolution depends so directly on biological evolution is somewhat shocking,” Hazen writes in his book “Symphony in C.” “It represents a fundamental shift from the viewpoint of a few decades ago, when my mineralogy Ph.D. adviser told me: ‘Don’t take a biology course. You’ll never use it!’ ”
The continents themselves may also be partial constructs of microbial terraforming. No one knows precisely how the continents were born, but a widely supported theory proposes that continental crust is a distillation of oceanic crust. The continents are made of granite, which, as far as we know, is abundant only on Earth; it has rarely been found in substantial quantities anywhere else in the known universe. In contrast, oceanic crust is composed of basalt, a cosmically common rock. Basalt is dark, dense and rich in magnesium and iron, a particularly heavy metal. More than three billion years ago, as Earth’s earliest ocean crust aged and cooled, it eventually became heavier than the mantle on which it floated and started to sink, a process called subduction. During its descent into the mantle, ocean crust and its overlying sedimentary layer released the water trapped within them, which lowered the melting point of the surrounding mantle. Certain components of the mantle began to melt into buoyant magma, which eventually erupted from volcanoes and cooled into new rock.
This process continues today. In Earth’s earliest chapters, however, the mantle was significantly hotter than it is now; in addition to squeezing water out of sinking ocean crust, the mantle melted the crust itself. When this hybrid magma rose to the surface, it cooled into a new kind of rock, granitoid rock, which was largely depleted of magnesium and iron and thus was much less dense than basalt. Over time, granitoid rock was subducted and recycled into true granite. Because granite was less dense than basalt, it accumulated on top of the ocean crust, forming thick patches of early continental crust that gradually breached the water’s surface. Later, with the emergence of plate tectonics, protocontinents coalesced into microcontinents and eventually formed immense tracts of land high above sea level. By about 2.5 billion years ago, nearly a third of the planet’s surface was land.
Several Earth scientists, including Robert Hazen and his colleagues, have investigated the possibility that life helped create the continents by promoting the subduction of oceanic crust and sediments and their transformation into granite. The more water the crust and sediments contain, the more easily this process occurs. When Earth was young, microbes inhabiting the ocean crust were likely dissolving the basalt with acids and enzymes in order to obtain energy and nutrients, producing wet clay minerals. By lubricating the crust with those wet byproducts, the microbes may have accelerated the dissolution of both mantle and crust and their eventual transfiguration into new land.
The geophysicists Dennis Höning and Tilman Spohn have published similar ideas. They point out that water trapped in subducting sediments escapes first, whereas water in the crust is typically expelled at greater depths. The thicker the sedimentary layer covering the crust, the more water makes it into the deep mantle, which ultimately enhances the production of granite. In Earth’s earliest eons, micro-organisms and, later, fungi and plants dissolved and degraded rock at a rate much greater than what geological processes could accomplish on their own. In doing so, they would have increased the amount of sediment deposited in deep ocean trenches, thereby cloaking subducting plates of ocean crust in thicker protective layers, flushing more water into the mantle and ultimately contributing to the creation of new land. Some computer models suggest that had life never evolved, the expansion of the continents would have been severely stunted, and the planet might have remained a water world flecked with islands: an Earth without much earth.
To recognize that deep subsurface life not only exists but also is engaged in a continuous alchemy of earth — that it may have helped create the very land on which all terrestrial life depends — is to redefine the modern understanding of life’s influence on the planet. Yet even today, some scientists, especially in geology and related fields, continue to describe life as a relatively inconsequential layer of goo coating a vastly greater mass of inanimate rock.
Such characterizations belie life’s true power. Life significantly expands the surface area of the planet capable of absorbing energy, exchanging gases and performing complex chemical reactions. The Earth-system scientist Tyler Volk has calculated that all the plant roots on Earth, finely furred with tiny absorptive hairs, make up a surface area 35 times greater than the entire surface of our planet. Microbes are collectively equivalent to 200 Earth areas. And if there were a layer of fertile soil three feet thick spread across the continents, all the tiny particles within it would have a combined surface area more than 100,000 times that of the bare planet.
There’s simply no comparison between an Earth without life and Earth as we know it. Life’s ubiquity endows our planet with an anatomy and physiology. Together, Earth and life form a single, self-regulating system, one that has endured and evolved for more than four billion years. We have as much reason to regard our planet as a living entity as we do ourselves: a truth no longer substantiated by intuition alone, or by one man’s vision, but by a preponderance of scientific evidence.
When we learn to see our species as part of a much larger life form — as members of a planetary ensemble — our responsibility to Earth becomes clearer than ever. Fossil fuels, industrial agriculture and widespread pollution have not simply raised global temperature or “harmed the environment”; they have severely imbalanced the largest known living entity, hurling it into crisis. The speed and magnitude of this crisis are so great that, without the necessary interventions, Earth will require anywhere from thousands to millions of years to fully recover on its own. In the process, it will become a world unlike any we have experienced, one potentially incapable of supporting modern human civilization and the ecosystems on which we currently depend.
For more than two centuries, Western science has regarded the origin of life as something that happened on or in Earth, as if the planet were simply the setting for a singular phenomenon, the manger that housed a miracle. But the two cannot be separated in this way. Life does not merely reside on the planet; it is an extension of the planet. Life emerged from, is made of and returns to Earth. Earth is not simply a terrestrial planet with a bit of life on its surface; it’s a planet that came to life. Earth is a rock that broiled, gushed and bloomed: the flowering callus of a half-sealed Vesuvius suspended in a bubble of breath. Earth is a stone that eats starlight and radiates song, whirling through the inscrutable emptiness of space — pulsing, breathing, evolving — and just as vulnerable to death as we are.
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