An Ocean of Air (8 page)

The result, enlivened by some complex equations from Laplace, were exactly what Lavoisier had hoped for. The more work the guinea pigs did, the more oxy-gene they used, and the more heat they gave off. Lavoisier was convinced. "Respiration is a process of combustion," he wrote, "which,
though it takes place very slowly, is perfectly analogous to the combustion of coal." The same way that coal supplies the fuel for a fire, some derivative of food must also provide the raw material for the energy by which we all live. And as oxy-gene feeds the glowing flame, so, too, must it release the energy stored somewhere inside us.

Lavoisier had discovered something truly important. Flames do indeed use oxygen to generate energy from candles or wood, and he was right that when we breathe, we're using it to burn our food in much the same way. That's one reason we talk about "burning calories." If this sounds dangerous, it is. As Priestley had suspected, and Lavoisier had now begun to prove, breathing oxygen is what allows us to live such vivid, active lives. But we pay a heavy price, for oxygen is also the reason why we grow old and die.

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All living things need to breathe. That is, they have to generate energy when they need it from the food reserves stored in their bodies. In our case, we have reserves of sugar, protein, and fat, which sit around inside us like a pile of logs waiting to be burned. Every breath we take uses oxygen to convert some of those reserves into the energy we need to move, stay warm, and do everything else that makes us human.

But oxygen isn't the only chemical that living things can use for breathing. Indeed the bacteria that constituted the first life on Earth were forced to use something much less efficient for the simple reason that, when the world first formed more than 4.5 billion years ago, the atmosphere contained no oxygen at all. Oxygen didn't appear in the atmosphere for more than two billion years, and it finally showed up only because of a dramatic but inadvertent case of planetary pollution. Without that accidental airspill, there would be no life on Earth bigger than a pinhead.

When the planet was born, it came blanketed in an ocean of air. Like the sun and the other planets in our solar system, Earth was formed when a shapeless cloud of gases, dust, and fragments of rock began to collapse and coalesce. The rocks and dust trapped some of the gases between them like mortar between bricks, and much of the rest settled on the outside of the planets in a shroud held in place by the power of gravity.

This early ocean of air was just as dense as today's, and it would have looked very similar. But the lack of oxygen made a big difference to Earth's surface. The rocks, for instance, were a uniform dull gray color—without oxygen, the iron they contained couldn't rust to the lovely reds and ochres that we see today. Still, the young Earth wasn't without its beauties. The skies periodically shed a gentle yellow rain of elemental sulfur, and the earliest beaches sparkled with golden iron pyrite. Also known as "fool's gold," this exists today only deep underground, safely away from the oxidizing air, where its vibrant color can still confuse naïve miners on the hunt for nuggets of the real thing.

For animals like us, this early atmosphere would have been an impossibly suffocating place, but the world's first occupants had an alternative way of releasing their energy. Instead of oxygen, they "breathed" the gas that Priestley and his contemporaries called "inflammable air" and that we call hydrogen. In the process, they made methane—"natural gas." Since this wasn't nearly as efficient as using oxygen, the creatures breathing it couldn't grow large. Instead, they remained the way they had begun, as microscopic pinpricks in the fabric of life.

So it was, and so it would always have been if not for a new chemical reaction invented sometime between 2.5 and 3.5 billion years ago by microbes called cyanobacteria. These creatures are so tiny that a droplet of water can contain billions of them—as many as there are people on Earth. However, they are also ubiquitous. Today you can find them in drainpipes, puddles, or anywhere water is left to stand for a while and starts to go that distinctive green color that shows they are working their magic. For they are the microbes that learned how to use the sun's energy to split water and make food, in a process we now call photosynthesis. And in doing so, they give off delicate bubbles of a certain waste product: oxygen.

This is the reason that we can breathe today. Cyanobacteria and the green plants that later incorporated their invention are now part of a giant enterprise that acts as Earth's lungs. As fast as we animals use up oxygen by breathing, plants return it to the atmosphere. It's almost as if living plants are working to make the world habitable for us—as if the most important component of our atmosphere has been made by life, for life.

(Oxygen didn't actually appear in the atmosphere until several hundred million years after it was created in this way as a by-product of photosynthesis. At first, it reacted with Earth's rocks and oceans as quickly as it was made. In the case of the oceans, the dissolved iron they contained turned to rust and fell to the seafloor, making vast mountains of debris that have turned into the world's biggest iron mines. Whenever you eat with a stainless steel fork, or drive a car, you're probably benefiting from this early rain of rust.)

Oxygen is fantastically reactive. When it engages in chemistry, it can release large amounts of energy, which can in turn be used to fuel the activity of living things. So, oxygen's arrival in the air had a dramatic effect on the course of evolution. As long as there was too little oxygen in the atmosphere to be useful to the creatures below, they were forced to remain both sluggish and microscopic. For billions of years, the planet was coated with nothing more than primordial slime.

But gradually, inexorably, more and more oxygen trickled into the sky until one point, nearly 600 million years ago, when the atmosphere tripped over its oxygen threshold. The result was the most dramatic burst of evolutionary change in Earth's history. Huge new creatures suddenly appeared, some of them more than a meter long. They weren't only big; they were inventive, and almost unbelievably varied after the dull slime that came before. These new creatures had shapes. They had eyes and teeth and legs and shells. They had learned to make their bodies out of not just one cell, but many. They were the world's first animals.

It's hard to overemphasize the importance of this evolutionary step. Think of the transition between cottage industries and the industrial revolution. Before this point, a single cell had to do everything that life needs—eat, excrete, breathe, reproduce, all in one tiny sac. Afterward, cells could specialize and share the load. Some became arms, some hair, brains, or bones. Creatures were no longer restricted to the size of pinheads. What's more, they had muscles to drive their new bodies, and that meant that at last they could move. Imagine a life without moving, and the difference it makes when suddenly you can. The new earthlings could seek out new sources of food, including other creatures. Some could chase and
others could flee. They developed armor to protect themselves and weapons to attack. They learned new skills, took on new shapes and colors, and ultimately became the vivid variable life forms we see around us on Earth today, including humans.

Nobody knows exactly the mechanism by which that final rise in oxygen triggered the appearance of animals, but what's certain is that there could be no complex life without it. To be big and multicelled requires huge amounts of energy, and it takes oxygen to generate that sort of power. Every other way of breathing is simply too feeble. We need oxygen because we need its spectacular reactivity. Without it, humans could never have existed.

This reactivity comes with its own dangers. As Priestley suspected when he saw how brightly candles flared up in his new gas, breathing oxygen is like playing with fire. And we are gradually, all of us, getting burned.

That's because whenever oxygen gets involved in chemical reactions, it releases tiny negatively charged particles called electrons. All atoms and molecules contain these particles and, like people, they are most stable when they are in pairs. A chemical entity that contains one of these single, footloose electrons is called a free radical, and it is one of the most reactive, and destructive, forces on the planet. Free radicals rip through everything in their path, splitting apart stable pairs and creating yet more free radicals, which head off on destructive paths of their own. That's what happens, for instance, if you are exposed to radioactivity. The damage comes not from radiation itself, but from the free radicals it generates.

And the trouble is that when we use oxygen to breathe, there are always some electrons that break free. Even if you're doing nothing but breathing, about 2 percent of the oxygen you consume still escapes as free radicals. If you're exercising vigorously, it's more like 10 percent. According to one calculation, the potential damage from simply breathing for one year is equivalent to the radiation from ten thousand chest x-rays.

When oxygen first appeared on Earth some 2.2 billion years ago, it was certainly a deadly poison for many of the earliest microbes. The methane-producers simply couldn't cope with the free radicals that were suddenly ripping through their bodies, tearing apart their vital chemicals. To survive, these organisms had to find refuge. They persist today in places that are comfortably moist and yet are hidden from the probing fingers of the atmosphere. That's why paddy fields give off methane, why swamps yield marsh gas that sometimes ignites into the ghostly dancing flames of legend, and why animals, including humans, generate natural gas in their guts. We fart because our intestines are now airless sanctuaries for those poisoned earthlings.

Other organisms, the ones from which we're descended, developed various complex strategies to deal with the worst depredations of oxygen. In particular, our own bodies are permanently ready to deploy an army of chemicals called antioxidants. A full-scale war is taking place every second, in every cell of our bodies, to stop the free radicals from forming, mop up the ones that do, or commit cell suicide if the invading forces become overwhelming. But the powerhouses inside our cells spend their lives playing with fire, and the long, slow leakage gradually wears us down. All of the diseases of old age—dementia, cancer, heart disease—come from the accumulation of damage caused by escaped free radicals. That's one reason that eating fruits and vegetables helps protect us from these diseases—both are packed with antioxidants that help to mop up free radicals.

It's also why smoking brings these diseases on earlier in life than we would otherwise expect. Nicotine isn't the problem, except insofar as it's addictive and so encourages you to smoke more. The real damage comes from the smoke itself, which is crammed with chemicals that react with oxygen to generate bucket-loads of free radicals—something like a million billion in every puff.

So can we somehow stave off old age by consuming more antioxidants? It seems not. In spite of the evident benefits from fruits and vegetables, there's no clear sign that eating "antioxidant supplements" bought from the health-food section of your local grocery has the same beneficial effect. In fact too many packaged antioxidants might hurt, rather than help. It takes us so long to grow old because our bodies have evolved such careful strategies to protect us from the worst effects of free radicals. Eating extra antioxidants might interfere with these natural mechanisms, like unruly mercenaries disrupting the operation of a highly trained army.

The damage from oxygen is also one reason humans are made up of two sexes. Every cell in our bodies possesses tiny powerhouses called mitochondria, which are the locations where all the oxygen burning takes place. These mitochondria are at the forefront of all the free-radical damage, and it's crucial to make sure that the ones handed down to the next generation are free from the damage that comes with aging. A woman's eggs are born with her, and spend their lives using essentially no energy at all. Their mitochondria are kept in pristine cold storage, ready for the children to use, which is why eggs sit and wait to be fertilized rather than going off in search of a sperm.

Meanwhile, every time a man's sperm are regenerated, the mitochondria the new ones contain are a little older. They also use plenty of energy to swim along and find the stationary egg. But after that—and here's the clever part—the mitochondria from the sperm get jettisoned, like spent rocket stages. So every child inherits pristine mitochondria from its mother, and the aging clock doesn't start ticking until the fetus starts to form. If we had only one gender to play with, this couldn't happen. Hence the troubles, and glories, of romantic relationships between men and women are born in the chemistry of oxygen.

The lesson of oxygen shows that many things that are exhilarating have their own attendant dangers: making discoveries, making enemies, challenging the authorities, falling in love. Indeed, everything about our vigorous, dynamic lifestyles comes with its own terrible cost. For our agile minds, our strong bodies, our different sexes, for the power of movement itself, we have to accept the inevitability of old age and death. The oxygen in each breath you take brings you everything that's worth living for, but it will ultimately make you pay with your life. Within its chemistry lies the very heart of the human condition.

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Lavoisier didn't know the extraordinary ways that oxygen has shaped our world and our lives, but he did know that he had proved that the most essential and vibrant ingredient for life comes from the air. He had also discovered that we breathe to burn our body's fuel, something that came as
a great surprise to scientists of the eighteenth century. Until that point, eating and breathing were considered wholly unrelated activities. And this led the fair-minded Lavoisier to an uncomfortable conclusion. "As long as we considered respiration simply as a matter of consumption of air," he wrote, "the position of the rich and the poor seemed the same; air is available to all and costs nothing."