Life

by Tim Flannery

  Green plants are far more efficient in their energy use than we humans with our fossil-fuelled power stations. Each year green plants manage to convert around one hundred billion tonnes of atmospheric carbon into living plant tissue, and in so doing they remove 8 per cent of all atmospheric CO2. This is a truly extraordinary figure. Just imagine if no CO2 found its way into the atmosphere. In just twelve years plants would then absorb and use almost all of the atmospheric CO2.

  Plants capture about 4 per cent of the sunlight that falls on Earth’s surface, which gives life a primary energy budget (excluding sulphur bacteria and other non-photosynthetic pathways) of approximately one hundred terawatt hours (one hundred trillion watt hours) annually. It’s the size of life’s primary energy budget and the resilience of its ecosystems (which is determined in part by biodiversity) that define a healthy planet. Scientists have only begun to think about Earth in these terms, so measurements of productivity and diversity remain approximate. Yet it’s clear from major extinction events in the fossil record that if Earth’s energy budget and ecosystem resilience fall below certain thresholds, a fully functioning Earth system cannot be maintained.

  Useful parallels can be drawn between the way energy flows in economies and Earth’s ecosystems. The size of economies is measured in dollars, while Earth’s energy budget is measured in terawatt hours. Dollars and terawatt hours clearly differ, but both represent potential resources that can be put to productive ends. Although an area of active study and dispute, it seems that the stability of both economies and ecosystems is related to their diversity, which itself is partly a function of size: the larger an economy or an ecosystem, the more diverse it can be. The presence of certain elements in economies and ecosystems can also help foster productivity. Banking is a good example. In economies well-run and well-regulated banks aid the flow of capital, thus stimulating productivity. In ecosystems certain species act rather like bankers by facilitating energy and nutrient flows. Earth’s ecological bankers include the big herbivores, those weighing a tonne or more. As we’ll soon see, in marginal ecosystems such as deserts or tundra, these ecological bankers speed the flow of resources through the ecosystem, allowing a substantial ‘biological economy’ to be built on a slender resource base. If humans destroy megafauna, they can induce the equivalent of a never-ending recession on such ecosystems, limiting their productivity and stability. And that impacts Earth function as a whole, just as a recession in the US can affect the global economy.

  So how does life spend its capacious energy budget? Basically, it is deployed to modify our planet so as to make it more habitable, and just how that is done is best understood by comparing Earth with the dead planets, such as Venus and Mars. Planets can have up to three principal ‘organs’, which correspond to the three phases of matter: a solid crust, a liquid (or frozen) ocean and a gaseous atmosphere. A living planet uses its energy budget to kick the chemistry of its organs out of balance with each other. No greater example of this exists than oxygen. Earth’s atmosphere is full of this highly reactive element, but if life was ever extinguished oxygen would quickly vanish by combining with elements in the rocks and oceans, forming molecules such as CO2. The chemical composition of the organs of dead planets, in contrast, exists in a state of equilibrium. As Lovelock realised in the 1970s, a planet whose atmosphere consists almost entirely of CO2 is a planet whose life force, if there ever was one, is long exhausted—a planet at eternal rest.

  Carbon is the indispensible building block of life. You and I are made up of 18 per cent carbon by dry weight, and plants have a much higher percentage. Almost all of that carbon was once floating in the atmosphere, joined in a ménage à trois with oxygen to form CO2. Billions of years ago, when life was a weak infant struggling to survive, there was more CO2 in the atmosphere than there is today, for living things had not yet discovered a means to use it. Back then, perhaps, life nestled as microscopic bacteria in the bosom of the deep sea, or hid in sediments around hot springs. Wherever it found a refuge, its energy budget must have been small, as most of Earth was still untouched by its power. Today, however, CO2 forms just four parts per ten thousand of the gaseous composition of Earth’s atmosphere, while a by-product of photosynthesis, oxygen, forms 21 per cent. This is the ultimate measure of life’s triumph.

  Earth’s continental crust is far thicker than its oceanic crust and it’s made of lighter, silica-rich rock. The continents originated from erosion of the oceanic crust (which is made of basalt) and, remarkably, they may be a product of life. This might seem to be a large claim, but it’s worth keeping in mind that living things provide 75 per cent of the energy used to transform Earth’s rocks, while heat from within the Earth provides a mere 25 per cent.4 We tend to think about the transformation of rocks in Earth’s crust as the result of volcanoes, earthquakes and such like. It’s easy to overlook the silent work of lichens, bacteria and plants, which create grains of soil from intransigent basalt and other rocks by reaching deep into the strata, leaching and breaking down the rock with the acids they exude. Their work, while microscopic in scale, is ceaseless, and thrice greater in effect than that of all the world’s volcanoes combined.

  We have no evidence of life for the first half-billion years or so of Earth’s existence. Back then our planet was a water-covered sphere with little or no dry land. When life originated, those ancient living things, it has been suggested, produced acids that sped up the weathering process of the basaltic crust, separating the lighter elements in the basalt from the heavier ones. When these lighter elements are compressed and heated by movements in Earth’s crust they become granite, the foundation-stone of the continents and the essence of the earth beneath our feet. Perhaps, given enough time, energy from within the Earth could have affected the same transformation, but so vast was the amount of basalt weathered to create the first continents that recent research indicates it could have occurred only if life was capturing energy and using it to produce compounds that help break down rocks.5

  We can think of Earth’s rocky crust as a huge holdfast, like the lower shell of an oyster, which life has formed to anchor itself. And if we imagine the rocks as life’s holdfast, then we can think of the atmosphere as a silken cocoon, woven by life for its own protection and nourishment. Just consider what the atmosphere does for us. Its greenhouse gases keep the surface of the planet at an average of around 15° Celsius, rather than –18° Celsius. All of the principal greenhouse gases are produced by life (though some, such as CO2, can be produced in other ways as well), and without them Earth would be a frozen ball. Ozone, a form of oxygen composed of three atoms bonded together, is a product of life, for all free oxygen is derived from plants. While it makes up just ten parts per million of our atmosphere, it captures 97 to 99 per cent of all ultraviolet radiation heading our way. Without this protection, our DNA and other cellular structures would soon be torn apart and life at Earth’s surface would cease to exist. Then there is the more common form of oxygen (two atoms bonded together), which fuels our inner metabolic fires, providing the breath of life itself.

  As Wallace knew, our atmosphere is truly wondrous. We may think of it as big, but it is by far Earth’s smallest organ. To compare it with the oceans, we need to imagine compressing its gases around eight hundred times, until it becomes liquid. If we could do that, we’d see that the atmosphere is just one-five hundredth the size of the oceans. It’s a delicate, dynamic and indispensible wrapping to the planet, a cocoon that is constantly being repaired and made whole by life itself, a cocoon that intimately wraps around every living thing and connects chemically with a great rocky shell that life has forged as its support. And sandwiched between holdfast and cocoon is the liquid circulatory system of the beast: Earth’s oceans and other waters. Earth is truly the water planet, for water in its three states—vapour, liquid and solid—defines and sustains it. Liquid water covers 71 per cent of Earth’s surface while solid water, mostly in the form of glacial ice, covers a further 10.4 per cent. Water is
essential to life because the electrochemical processes that are life can occur only within it; fluids as salty as the ancient oceans flow through our veins. The ocean was almost certainly the cradle of life, and it remains life’s most expansive habitat. With a volume of 1.37 billion cubic kilometres it is eleven times greater in volume than all of the land above the sea. But unlike the land, which is populated by life only at its surface, the entire volume of the oceans is a potential habitat.6

  At the very beginning of our planet’s existence, Earth was lifeless and its three organs were in chemical equilibrium. No rocks survive from that distant time 3.9 to 4.65 billion years ago. That’s because our restless planet has been continuously recycling itself, so that almost all the physical evidence testifying to the nature of Earth’s original crust has been ground to dust, melted and formed anew. But by examining rocks that date to a slightly later time, when Earth’s life force was still weak, we can gain deep insights into what the enlivening of our planet meant.

  In 2009 I visited the man who pioneered the still controversial idea that life might have helped create the continents. Minik Rosing is the director of the Geological Museum in Copenhagen and one of the foremost authorities on the origin of life. A ponytail-and jeans-wearing Inuit, he’s possessed of immense hospitality, and as we sat in his office drinking tea and watching the snow fall outside, he spoke of his love of old rocks. The most venerable surviving parts of Earth’s rocky crust are, he said, between 3.3 and 3.8 billion years old. They’re precious relics of the youngest Earth we can directly know, formed less than a billion years after the planet itself came into existence. And the very oldest can be found in Greenland.

  Minik rose from his seat as he spoke and handed me a rock from his desk. It was, he said, around 3.8 billion years old, and I was astonished to see that it was not folded, battered and scarred as you might expect, but undistorted, its layers as smooth as sheets on a hospital bed. In one layer was a slender black smear, which Minik said marked the start of Earth’s carbon cycle, a cycle that largely defines and maintains our planet. Instantly my mind was swallowed by the gulf of time that separates us from that moment when the living Earth-machine first ticked over. Today the carbon cycle runs at full roar, but back then, in a shallow ocean on a planet as fragile as an unshelled egg, it was as delicate and fluttering as a quickening.

  Geologists have learned a great deal about the infancy of our Earth through studying such rocks, and no lesson is more marvellous than the strong grip that life has exerted on our planet over its 3.5-billion-year existence. At the time those rocks were formed, and for long after, Earth’s atmosphere was toxic, incapable of supporting life as we know it. The oceans also were a toxic brew, with high concentrations of metals such as iron, chromium, copper, lead and zinc, as well as carbon and other elements. All of this changed when microscopic plants and bacteria began to break CO2 into oxygen and carbon, and to use the metals dissolved in the seawater to speed up the chemical reactions that were essential to their existence. As they died and sank to the ocean floor, they carried their minute cargoes of metals with them, and so, over aeons, the oceans were purged of their dissolved metals, becoming chemically similar to the oceans of today. The metals buried in the sediments had a different fate. Often, they were carried deep into the crust, where heating and compression further concentrated them, leading to the formation of ore deposits. Sometimes these ore bodies became incorporated into the continents and were thrust high into mountain ranges, forming the fabulous golden wealth of places like Telluride, Nevada, or the Incan mines of Peru. A similar process gave rise to Earth’s coal, oil and gas deposits, though these formed as a result of living things pulling CO2 from the atmosphere, rather than from them taking metals into their bodies.

  This distant Earth history has profound implications for our modern industrial society. It accounts not only for the state of our atmosphere and oceans, and the good fortune of some countries in possessing valuable mineral deposits, but for our bodies’ often-calamitous love of toxic metals as well. All of that matters because today we are digging up these elements at an unprecedented rate, and redistributing them through our air and waters, and that can have surprising consequences. As we will learn later, this is a tale of fundamental planetary disorder, which helps explain why some of us develop disorders such as intellectual disabilities and schizophrenia, and even perhaps why murder rates are high in some communities.

  It may seem a paradox that living things should take in toxic metals such as cadmium and lead as avidly as if they were the most precious nutrients on Earth. Assay any one of us and you’ll find a treasure-trove of toxic metals at concentrations many times greater than they occur in the natural world around us. The answer to the paradox lies in those oceans of long ago. Back then life consisted of little more than bags of chemical reactions floating in an ocean packed with metals. The laws of chemistry dictate that some of the reactions most crucial to life are enhanced by the presence of metals. In technical parlance, metals are catalysts and co-factors—substances that either permit or accelerate chemical reactions. Catalysts are perhaps most familiar to us from the catalytic converters in cars, which work by using a metal, often platinum, to hasten reactions that remove pollutants from the car’s exhaust. In our bodies catalysts hasten enzymic reactions and, in an ocean full of potential catalysts, early life became dependent upon them. So unchanging has life’s chemistry been over the past two billion years that the majority of the seven hundred-odd chemical reactions that run our bodies today are identical to those that occurred in those bags of chemical reactions that were early life.

  As early life mined the ocean’s dissolved metals the waters became leached of catalysts and living creatures became desperately hungry for them. Even today, it is metals that limit life’s spread in the oceans. In the frigid Southern Ocean, for example, a lack of iron is the key factor limiting plankton growth. Add iron and life flourishes. After two billion years of coping in a world where metals are not easy to be had, life has become extremely adept at keeping hold of whatever metals come its way. And in a world where human activity is releasing metals into the air and seas in ever greater abundance, that can be dangerous, for, like many good things, too much metallic catalyst can be very dangerous. So it is that, despite the damage mercury does to us, our bodies absorb the mercury in the fish we eat even more avidly than the flesh of the fish itself. We store up the metal in our livers, skins and brains, even after we are mortally poisoned by it.

  The links between Earth’s oceans, crust and atmosphere are nowhere more elegantly exhibited than in the theory of continental drift. Every three hundred million years or so the continents coalesce, creating a single large continent surrounded by oceanic crust. Then the landmass breaks apart again, eventually to come together in another cycle. You can think of the continents acting like dollops of scum floating on a pot of boiling water. The dollops move around, joining together and breaking apart, driven by the convection in the boiling water. While no one understands precisely what drives the movements of the continents, convection within Earth’s molten mantle, Earth’s gravity, and the pull of the Moon all appear to be factors.

  There are two kinds of plates: continental and oceanic. When two continental plates are moving apart, new oceanic crust forms between them. When a continental and oceanic plate collide, however, the oceanic plate is thrust under the continent, and is melted. As a result, mountain ranges, volcanoes and mineral-rich rocks are formed. A good example of this is the Andes. When two continental plates collide, it’s far harder for one to slip under the other. Instead the plates buckle, and truly gigantic mountains, such as the Himalayas, are formed. Rivers erode the mountains, creating fresh new soil, and it’s this renewal, along with the slow grinding of glaciers, that fertilises life on Earth with the minerals that are essential to plant and animal growth. It’s no accident that some of our greatest civilisations sprang up on the plains laid down along rivers flowing from high mountains. If the continents w
ere spawned by life, then we must see this fantastic movement of Earth’s plates as at least partly a consequence of life itself.

  The most important thing about the movement of the continents in relation to life in the oceans is the effect it has on the recycling of salts. The waters of the ocean are recycled, by evaporation and precipitation and thence through Earth’s rivers, every thirty to forty thousand years, and with each recycling rivers leach salt from the continental rocks and carry it into the sea. You might deduce from this that the oceans are growing saltier, and in the nineteenth century this is exactly what scientists thought. They assumed that the oceans contained fresh water upon their formation, and, knowing the rate at which salt is carried into the oceans by rivers, they estimated Earth to be just a few tens of millions of years old. They then coupled this faulty finding with a prediction that a sort of salty doomsday awaited us a few million years hence, when the oceans would become as salty as the Dead Sea.

  The truth is far more remarkable. The saltiness of the oceans has remained relatively constant for billions of years, and the drift of the continents plays a vital role in this regulation. As the continents move apart, the ocean’s basalt crust is stretched ever more thinly, until it finally ruptures. These rupture lines are known as mid-ocean ridges. They are often located near the centre of ocean basins, and they allow the basins to grow wider. These remote, submarine mountain ranges are rich in life but are among the least known places on Earth. Greg Rouse, a friend of mine, explores them in a submersible, and he’s one of the few humans ever to have seen them at firsthand.

  In 2005 Rouse explored one of the last unknown submarine mountain ranges, deep in the South Pacific Ocean. He showed me video footage taken on the trip of a fantastical white octopus that he had captured using a robotic arm and put in a container on the outside of the submersible. He was wildly excited at the thought of naming and describing the amazing creature, but during the three-hour ascent the ghostly octopus managed to open the lid of its container and escape back to the depths. He also told me something completely surprising. On the evening before one dive, a filleted fish had been cast overboard by a crew member of the support vessel, and when Rouse arrived at the crest of the range four kilometres below, he discovered the filleted fish lying on the bottom, just where the submersible landed. For this to occur, the column of water below the vessel must have been completely serene. To us inhabitants of the turbulent atmosphere such things are utterly astonishing, and they underline how little we understand our planet and its workings.