18 Miles

by Christopher Dewdney

  Yet despite its critics, Oparin’s theory still stands, mainly because there are so many naturally occurring amino acid proteins. It would appear that given enough time, in this case hundreds of millions of years, proteins could indeed have combined and gradually become more complex. Recently, the Miller-Urey experiment has been successfully revisited. Other researchers have added volcanic gases to the Miller-Urey mixture and they too have brewed amino acids as a result. Not only that, it appears that we have been importing some of our complex proteins, including amino acids, from outer space. A large meteorite that fell in Murchison, Australia, in 1969 was found to contain 20 types of amino acid with no terrestrial source. So if you add the infall of amino acids from meteorites and comets to the stew of proteins already brewing in the early oceans, then you have quite a broth of life-builders in the primordial soup.

  But self-replicating proteins had to be reinvented many times over millions of years before one of them, again by chance, developed a membrane that gave it protection from the elements. It was the extraordinary good fortune of these proteins to emerge on a watery planet that orbited the sun at just the right distance — the Goldilocks Zone, as atmospheric scientists refer to it. Too close to the sun, like Venus, and water boils away; too far from the sun, like Mars, water freezes. And water, as it turns out, has a particular quality that jump-started intracellular transport and cell membranes.

  Water is bipolar, not in the manic depressive sense, but in the electrical sense. One side of a water molecule has a positive charge, the other a negative charge. Like little magnets, they attract each other with just enough strength to stay grouped together but not so much as to turn into a solid. This makes water an excellent transport medium for dissolved minerals and chemicals. It’s also why it has a meniscus, that layer of surface tension at the top of water you can see in an aquarium. Water molecules attract each other in all directions in deeper water, but at the surface they can only be attracted across the surface and downward, which aligns them into a temporary membrane. The first self-encapsulated proteins mimicked this property. Their membranes had water-loving molecules on the outside and water-repelling molecules on the inside. These joined in a circle to form a membrane that protected the delicate, nanomachinery of their interior.

  Self-encapsulated proteins flourished and grew more complex, eventually crossing the line by which we define life a little more than four billion years ago, less than 500 million years after the birth of the planet. These first simple, single-cell creatures were called prokaryotes and used sulfates as a source of energy. They were anaerobic, meaning they flourished in the absence of free oxygen. Prokaryotes dominated the oceans for hundreds of millions of years. During their reign, unicellular life established itself and achieved planetary distribution, though a time traveler standing on the shore of that ancient ocean would see no evidence of life. Only a microscope would reveal the ubiquity of unicellular organisms. Anyway, you wouldn’t have much time to collect samples because three billion years ago, when prokaryotic life had become firmly established, the environment was anything but temperate.

  A Typical Weather Report Three Billion Years Ago

  First of all, the days were shorter. The Earth was spinning three times faster than it is now. A full day-night cycle was eight hours long, with a little more than four hours of darkness and four hours of pale sunlight because, even though UV levels were high, the young sun was fainter than today. You’d definitely have needed an oxygen mask — the atmosphere was almost entirely composed of carbon dioxide. And when the moon rose, you’d have known it. It was much closer to Earth and would have appeared 12 times larger than it does now. Today, the moon looks to be the same size as a dime held at arm’s length. Three billion years ago, it would have looked the size of a cantaloupe. And you could hardly have called it moonrise. It would leap above the horizon and careen into the heavens, wheeling dizzily through the sky. Shadows cast by the moon, or the sun for that matter, would stretch and slide visibly as you watched them, like a time-lapse film.

  Certainly, moonrise over the primeval ocean would have been a wondrous sight, but you wouldn’t want to have been anywhere close to the water. In fact, the only safe vantage on the ocean would have been from the summit of a mountain somewhat inland. The tides were 1,000 feet high and arrived as quickly as a tsunami. Those prokaryotes living in the primeval oceans mustn’t have had much rest.

  Evolution was a slow-acting force at this time, but after many hundreds of millions of years, a momentous change finally did occur, a chance mutation that led to a new single-celled life-form, one that had a terrific edge over its anaerobic predecessors. Cyanobacteria. This newcomer took advantage of the relative abundance of carbon dioxide in the atmosphere as well as the sunshine. Cyanobacteria combined carbon dioxide and sunlight with water to produce carbohydrates for food. In essence, they survived exactly as plants do today. They were green, and, like plants, their unique metabolic process had a simple waste product — oxygen. Free oxygen, that formerly minor player in Earth’s oceans and atmosphere, became a major one somewhere between 2.8 and 2.5 billion years ago.

  Tiny Terraformers

  If humans eventually colonize other planets, they will rely on giant factories to process alien atmospheres into something breathable in a process called terraforming. Plans are already being drawn for the eventual terraforming of Mars. These mega-engineering projects will easily surpass any earthly achievements — the Pyramids, the Panama Canal, the Great Wall of China — but we have yet to build them.

  Fortunately for us, Earth has already been terraformed. But huge machines didn’t process our atmosphere; cyanobacteria did. Slightly less than three billion years ago, the dominant type of cyanobacteria lived in coral-like colonies called stromatolites. They formed knobby reefs in the oceans where they quietly bubbled away, releasing oxygen into the water. If you could stroll along the beach of a primeval ocean, you’d likely come upon wide, submerged ledges of these low reefs sitting just offshore, stretching alongside the ancient seaside as far as the eye could see. The air would be warm, but you’d still need an oxygen mask. The stromatolites and their allies had to pump out oxygen for hundreds of millions of years before the overflow leaked into the atmosphere.

  Surprisingly, stromatolites have survived. They are the kings of living fossils. Nothing — not the tuatara lizard of New Zealand, unchanged for 100 million years, or even the coelacanth, the missing-link fish from Madagascar that looks today as it did 350 million years ago, or the sponges, with their billion-year heritage — holds a candle to the stromatolites, unchanged for 2.8 billion years. There is a thriving colony of them in Shark Bay on the west coast of Australia and another at Exuma Cays in the Bahamas. These unprepossessing, gray, blob-like rocks smeared with a thin layer of cyanobacterial cells — so small that one billion are contained in a square foot — were the dominant life-form on our planet for almost two billion years. It was almost as if life had stopped evolving.

  The Oxygen Catastrophe

  But while cyanobacteria were transforming the oceans, they were also killing off their predecessors. Oxygen was lethal for the pioneers of life on Earth — the prokaryotes and extremophiles that had flourished for a billion years. In a mere 300 million years, cyanobacteria had saturated the oceans with so much oxygen that 99 percent of the prokaryotic organisms died out in one of the greatest extinction events the Earth has known. Only a few survived at the bottom of oceans near thermal vents or buried under mud where oxygen could not find them. As a result, the Great Oxygenation Event is also called the Great Oxygenation Extinction Event or, more briefly, the Oxygen Catastrophe. Even so, the prokaryotes had a good run, dominating the planet for almost 400 million years. Their descendants still subsist today, buried deeply in rock or mud.

  But those little bubbles of oxygen that cyanobacteria released not only exterminated most of the prokaryotes, they also initiated a vast, irreversible geochemical reaction. Underwater iron deposits
began to rust for the first time in Earth’s history. The oceans must have been tinged orange for millions of years during this great undersea rust bloom. The oxidizing iron left telltale bands in sedimentary rock that was laid down at the bottom of these newly oxygenated oceans. Today, geologists commonly find three-billion-year-old rocks with red band formations that contain layers of rust-dyed sediment, evidence of the first free oxygen on our planet.

  Mars on Earth

  For 300 million years, the cyanobacteria’s steady output of oxygen was absorbed by iron and buried in ocean sediments. When all the available iron had bonded with oxygen, around 2.5 billion years ago, there was nowhere for the excess oxygen to go, so it bubbled out of the water and into the atmosphere. Oxygen levels in the atmosphere began to rise precipitously, triggering another oxidation event — all the exposed iron on dry land started to rust. Just like the banded marine sediments in ancient seafloor strata, these land-based layers can be plainly seen in rocks dating from this period.

  From the perspective of an adjacent planet like Mars, the Earth would have undergone a Technicolor transformation. Within centuries of the first atmospheric oxygen, the continents would have changed color from brown and gray to bright terra cotta. By then, the oceans would have regained their original color, and Earth would have been a blue-and-orange planet twinkling in the Martian sky. This change of planetary color was testament to life’s power. The primeval atmosphere had been transformed by life, and Earth’s fate was now unique. It had diverged from a standard, geological planetary process and was now setting out on its own individual path. Life had begun to shape the face of the planet.

  If our imaginary time traveler stood on the rust-red shore of that ancient ocean 2.5 billion years ago, she would no longer have needed an oxygen mask. The air would have been as deeply and fragrantly breathable as any seaside air today. Oxygen was in great supply, but there was not yet any multicellular life around to take advantage of its abundance.

  There was a wrinkle though — that first whiff of oxygen was a cool one. Just as the continents began to turn rust red, ice caps appeared at both poles. Within a few thousand years, these polar ice caps accumulated into continental ice sheets that drove both south and north until only a very narrow band of ice-free ocean and land girdled the Earth at the equator. The first planetary deep freeze, the Huronian glaciation, was taking center stage. It lasted 300 million years and ended during a period of intense volcanic activity. Critically for us, the stromatolites and other cyanobacteria had survived in their equatorial refugia. But the Huronian glaciation, as it turns out, was just a warning shot over the bow of life’s fragile boat: worse glaciations were to come.

  As our imaginary time traveler standing on that shore 2.5 billion years ago could tell you, the atmosphere was similar to the atmosphere we have now, although carbon dioxide levels were much higher. Today the atmosphere is composed of 13 gases, of which two dominate — oxygen at 21 percent and inert nitrogen at 78 percent. Those ratios are important. Take oxygen, for instance. Every single percentage point over 21 percent increases the likelihood of forest fires by 70 percent. If oxygen ever reached 25 percent, all land vegetation — from the high Arctic to the equatorial rainforests — would eventually burst into flame in a raging, planetary wildfire. Nitrogen also sits at a sweet spot. If nitrogen levels fell to 75 percent, the climate would spiral into a deep freeze from which the Earth would never recover.

  The other important gases are trace gases like argon at 0.9 percent and carbon dioxide at just 0.04 percent, neon at 0.001818 percent, hydrogen at 0.000055 percent, methane close to 0.00018 percent and helium at only 0.000524 percent. The other gases are very minor players, except for ozone, which, like carbon dioxide, has a disproportionate influence on Earth’s habitability. Ozone forms a diaphanous, ethereal umbrella over the Earth, shielding the planet from damaging ultraviolet radiation. When scientific research proved that chlorofluorocarbon from spray cans and refrigerators was destroying the ozone layer, international legislation was enacted in just over a decade. It was an open-and-shut case. Without ozone, all earthly vegetation and most creatures would burn and mutate in the intense ultraviolet radiation. Despite that, ozone, along with radon, krypton, xenon and nitrous oxide, adds up to merely 0.000004 percent of the atmosphere.

  Nitrogen is a majority shareholder in our atmosphere and yet, other than maintaining the Earth’s moderate temperature, it’s a silent partner. Certainly wine aficionados use pressurized containers of nitrogen to cap their opened bottles (apparently it works better than a vacuum seal), and filling car tires with nitrogen instead of air is a recent automotive trend, but compared with oxygen, nitrogen seems almost menial, commonplace. But don’t be fooled; nitrogen has a cosmic pedigree.

  If other planets harbor life, their atmospheres likely contain a significant percentage of nitrogen. It is the seventh most plentiful element in our universe and has been incorporated into every living thing on Earth. Its absence would doom us. Nitrogen makes up about 3 to 4 percent of the dry weight of all life and provides an essential element for cellular construction and amino acids. It is present as nitrates in animal waste and urine. Potash is essentially nitrogen, and we mine it because much of our food comes from plants that require nitrogen fertilizer.

  But there really shouldn’t be as much nitrogen as there is. Normally, nitrogen and oxygen react with each other, and, over the eons, they should have combined. Most nitrogen should be sequestered in the oceans as stable nitrate ions. Yet that hasn’t happened. It’s part of the counterintuitive miracle of our atmosphere. Something, and it’s most likely life, is keeping the mixture from reacting.

  Nevertheless, in terms of climate control, nitrogen, for all its ubiquity and abundance, is almost completely overshadowed by carbon dioxide in terms of punching above its weight. Carbon dioxide represents only 0.04 percent of our atmosphere, or approximately 400 parts per million (ppm). If you added the same proportion of the poison strychnine to water, you could drink gallons without the slightest effect. These concentrations have remained fairly stable over the past 400,000 years, ranging from 180 ppm during the height of glacial ages to 290 ppm during interglacial periods. Yet despite that scarcity, it is critical for regulating the overall surface temperature of the planet and is therefore completely essential for life on Earth.

  Carbon dioxide is crucial for plant survival and therefore for all life-forms right up the food chain. After the first plants learned how to extract carbon dioxide from the atmosphere with their exquisitely complex nanomachinery, they enlisted photosynthesis to convert carbon dioxide into energy. Then they bootstrapped us out of the cradle of the ocean. Plants are our heroes. Today the global sum total of energy captured by photosynthesis is about 130 terawatts, six times more than the total amount of energy used by all human civilizations currently in existence. The carbon that’s not used for energy builds the branches, roots, leaves, flowers and stems. This fixes the carbon, and when the plant dies, the carbon is sequestered in the soil, eventually compressed into rock, which is then, over millions of years, subducted into the molten interior of our planet to be later released by volcanoes. A cycle of air, life, rock and fire.

  The world’s volcanoes, on average, release about 130 to 230 megatonnes of carbon dioxide yearly. It sounds like a lot, but it really isn’t next to the staggering amount that photosynthetic organisms contribute. Decaying vegetation, both underwater and on land, creates carbon dioxide. Seaweed and plankton in the oceans produce about 332 gigatonnes of carbon dioxide yearly, yet even that pales in comparison to land vegetation, which produces a staggering 439 gigatonnes of carbon dioxide every year. By contrast, humans only pump 29 gigatonnes into the atmosphere yearly. Problem is, that small percentage we’re adding doesn’t have a natural carbon sink to neutralize it. The 771 gigatonnes from the oceans and land, as well as the 180 megatonnes (on average) from volcanoes, are all accounted for within our planetary carbon cycle. So our surplus is accumulat
ing. We’re playing with fire, literally, by burning sequestered carbon and adding it to the atmosphere. In the mid-twentieth century, carbon dioxide levels were at 320 parts per million (ppm); now they are climbing over 400. We’ve been doing this for quite some time.

  And for all that, in geological terms, our dubious effort to add more carbon dioxide into the atmosphere is doomed. Carbon dioxide concentrations have been steadily decreasing since their highest levels in the primordial atmosphere, when it was the dominant gas. Gas newbies, oxygen and nitrogen, pushed out carbon dioxide to the extent that by the Cambrian period, 500 million years ago, carbon dioxide was already a trace gas, with concentrations at about 7,000 ppm. Concentrations of carbon dioxide decreased to 3,000 ppm during the Jurassic and Cretaceous periods, more than 60 million years ago. Then they fell lower, 34 million years ago, to 760 ppm. You can see where this is going. Today carbon dioxide concentrations stand at approximately 400 ppm. Over the long term, in a hundred million years or so, one of the most essential gases for the continued existence of life is going to run out. But that doesn’t let us off the hook, not by a long shot. Here and now, carbon dioxide management is a global dilemma.

  There’s one more atmospheric player I haven’t mentioned yet, mainly because it isn’t a gas. Water. It occupies about 2 percent of the atmosphere in the form of water vapor and clouds, though that 2 percent is distributed very unevenly. Warm air holds more moisture than cold air, so there’s more atmospheric water aloft over the tropics than above the polar regions. In its global entirety, on average, the atmosphere holds about 375,000 trillion gallons of water. There is an ocean above our heads.