The Canon (41 page)

Oh, yes, a breathable atmosphere. How easily we overlook what we need the most. You can live without water for three days, even a week to ten days if you're well hydrated at the start and you stay in a cool, shady spot. But if you stop breathing, you can die within minutes. Our obligingly inspirable air may seem simpler and less substantive than water, more insipid and inadvertent and with a more pronounced tendency to stare vacantly into space, yet appearances can be deceiving, especially the ones that you can't see. In fact, Earth's atmosphere is a richer and more complex resource than is our water, and it has taken comparatively longer to evolve into the specific mix that we inhale at a rate of 2 gallons every minute or so, for a total of about 3,000 gallons per day—enough to fill 100 bathtubs not currently en route to the sun. Because gas has mass, a pound for every 100 gallons, we pull in and blow off about 30 pounds of personal space daily.

The atmosphere is really an extension of Earth, a geopolitical player right up there with the core, mantle, and crust. Like so much else about our planet, the air was born on the inside and then brusquely turned inside out. From the moment Earth managed to cohere into a reputable sphere, it began outgassing the rudiments of an atmosphere, releasing plumes of hot vapors that had gotten trapped in its rocks during the natal melee. The first atmosphere was mostly hydrogen and helium, and it wasn't long for this world. Earth lacked sufficient gravitational mass to hold on to such lightweight gases, and its young core had yet to settle into the two-part inventor of a magnetic buffer against the solar wind. Before Earth had reached the half-billion-year mark, its primordial atmosphere had drifted or been abraded away. And while there are traces of helium in our skies today—a tiny fraction of 1 percent of the total atmosphere—there is virtually no free hydrogen gas to be found. Terrestrial hydrogen is in bondage with other elements—with oxygen in water, with carbon and nitrogen in the chains of our genes and proteins. If you want hydrogen straight up to put to work in some way, say as hydrogen fuel to power high-concept hydrogen cars, you must wrest it away from its molecular setting, and that takes energy, too.

Earth's second atmosphere would not be so easily spooked as the first. The crust cooled, and pertussive volcanoes loosened other volatiles from subterranean stone, outgassing water vapor, nitrogen, carbon dioxide, and ammonia at a furious pace, until the skies held one hundred times more gas than they do today. From this toxic, lofted broth, water vapor condensed as rain, and that was the start of the seas we see, and the earliest inklings of the air we need. Once poured, the oceans began absorbing some of the other gases in the atmosphere, dissolving with particular relish the carbon dioxide at its surface and transforming it into a seltzered froth. Ocean currents stirred carbon dioxide microbubbles wide and deep, until fully half the carbon dioxide from the atmosphere had been sucked into the sea. Bubbles: Everybody loves bubbles! Blowing them, drinking them, bathing in them, bursting someone else's. Bubbles are like puppies, always bouncy and happy and ready to play. What a shame if there were nobody around to grab the leash and take all those lively carbon-based bubbles out for a romp, running and running until somebody, somewhere, stops and takes a deep breath.

We don't know how life began on this planet. We don't know where it started—in surface waters littered with sunlight, or on the black ocean bottom by a piping hot vent, sheltered in a calm crescent of clay or slapped sentient by intertidal spray. We don't know when life started; estimates range from 3.2 billion to 3.8 billion years ago. We don't know what the very earliest life forms were like. But we do know that once life got started on this restless, bibulous Goldilocks planet, it did as Goldilocks would, upending everything in sight until the place looked and felt and smelled like home.

The impact of life on Earth was dramatic, a tectonic shift of its own,
and nothing illustrates that impact better than the things life did to the air. The atmosphere in which life arose, that special blend of next-generation outgassings from our generously ulcerous underground, was wondrous for its time, and likely offered the ideal setting for the chemistry of life to get its first hesitant footing, but it was not the sort of air the vast majority of modern organisms would describe as "fresh." Most notably, the atmosphere had no free oxygen in it. Yes, there were oxygen atoms bobbing about with their hydrogen earpieces, in the ambient water vapor, but of the paired, pure oxygen, the O
₂
gas we need to breathe, the air was almost entirely bare. Today, the atmosphere is about 20 percent O
₂
. Who put the pairs there? Our self-sacrificing ancestors, the cyanobacteria: large, floating mats of sun-eating microbes, ur-solar cells that made sweetness from light. Cyanobacteria, also called blue-green algae, were among the earliest known life forms, and a great success story. They were probably the first to master the art of photosynthesis, the stepwise transformation of solar energy, water, and carbon into sugar, the all-purpose cell food. Sunlight fell in abundance, and water—well, they
were
aquatic. And for the carbon source, they had the bubbles, the carbon dioxide that fizzled into the water from the air, and the matted flats of cyanobacteria greedily gulped it in. From the CO
₂
, they took the Cs they needed to bake their carbohydrates, their daily bread, and excreted the parts they couldn't use, the oxygen couplets, the mighty O
₂
s. Yet the air remained long unruffled, as all the oxygen waste spilling from Earth's booming archaeobacterial farms went instead, quite literally, to rust. The oceans were rich in iron—dissolved in the water or veining submerged rocks—and iron has a great affinity for oxygen. For the first billion-plus years of photosynthetic activity, oceanic iron handily sopped up the oxygen, and to this day most of the free O
₂
ever made in Earth's history remains locked up in ancient reservoirs of red, rusted rock.

Still, life kept up the pace, and the bacterial mats spread, until, beginning about 2 billion years ago, the sea's supply of exposed iron was oxidized, pig-sick of O
₂
, and the excess oxygen started filtering into the atmosphere. As it built up, some of it occasionally reacted with itself to form O3, an ozone layer, which in turn helped block out ultraviolet waves from the sun. Life below was growing steadily better for growing, in number, in kind, and in setting. The ozone shield would allow life to colonize the land without fear of frying, while the mounting count of oxygen duets in the air would spark the great aerobic revolution.

Cyanobacteria are still around today, numbering some 7,500 species, and many of those strains are, as their ancestors were, anaerobic, performing all their daily tasks with no need of oxygen. Indeed, exposure to oxygen will kill them, as it does other exclusively anaerobic microbes, including some of the symbiotic bacteria that live in our intestines, and other, less genial germs that cause tetanus and botulism. An anaerobically styled metabolism has its uses: it allows microbes to survive where nothing else can, and in our own bodies it gives muscle cells a chance to flex for short bursts of intense activity when our blood can't deliver the requisite oxygen in a timely fashion. Yet oxygen is an excellent fuel if you know how to burn it, and aerobically powered cells run far longer and more efficiently than their anaerobic counterparts. Aerobic strains of bacteria can divide thirty to fifty times more quickly than anaerobic ones. And while you can sprint for just a minute or two on the fruits of anaerobic metabolism alone, if you slow to a measured pace that gives your circulatory system a chance to supply oxygen as needed you can keep running for hours, the whole day if you're training for the Olympics, or owe a lot of money to an unofficial lending source in New Jersey.

Some time around 1.5 to 2 billion years ago, as oxygen concentrations climbed toward 1 percent of the atmosphere's gaseous mix, the first aerobic microbes arose, the first unicellular organisms that could exploit free-floating oxygen to power their internal operations. Dividing at a quickened pace, the oxygenic microbes began their sometimes rocky climb to dominance. They'd crowd out the anaerobes or subsume them beneath their more busily bulging blankets, only to begin exhausting the oxygen that their blue-green rivals supplied. The aerobics would crash, and the anaerobics revive, and oxygen levels rise again. The benefits of a dual survival plan, of burning oxygen when possible and switching to an alternative, oxygen-free strategy when necessary, must have occurred to one of these ancestral life forms, occurred in the sense of, yes, it happened, it occurred, and it was good. The first eukaryotic cells, the first cells to have their genetic material cloaked in a nucleus and to be otherwise well organized and compartmentalized compared to bacterial cells, are thought to be the result of an archaic merger between the distinct cell types. It may have been an accident, it may have been wolfish engulfment with a fairy-tale ending, we don't know, but the molecular and metabolic makeup of our cells, of all eukaryotic cells, suggests that early on some sort of large anaerobic cell—not a blue-green algae but an anaerobe that ate other cells rather than synthesizing its food from scratch—either fused with, swallowed, or was infected by a smaller aerobic cell. Rather than be digested down for spare parts, the smaller cell survived in the cytoplasmic sanctuary of the larger cell, and
therein arose one of the world's first great symbiotic partnerships. The larger cell protected the smaller cell and fed it anaerobically whenever oxygen proved scarce, while the smaller cell powered its patron through aerobic respiration whenever oxygen molecules diffused into the amalgamated microbe's gelatinous interior and aroused the aerobe's interest. These early switch-hitting cells were a bit clumsy, and they must have stumbled down a number of dead ends and blind alleys as they struggled to sort out the business of cell division complicated by the need to replicate and properly parse two cellular species rather than one. But their newfound metabolic plasticity and chemical deftness lent them sufficient advantage that they thrived despite taking longer to divide than purely aerobic microbes.

Today we see the purest expression of this ancient alliance in yeast cells, considered the most "primitive" of eukaryotic cells but no less worth toasting for that. Yeast cells have their distinctive aerobic and anaerobic phases: the first phase begins to bubble your beer, the second ferments it. But all eukaryotic cells carry living proof of the primal alliance. Look at any one of your body cells under a powerful microscope, and find the mitochondria, the striped, sausage-shaped bodies where oxygen is burned and food molecules are transformed into energy packets, to be stored or spent as needed. Those mitochondria are the descendants of formerly free-swimming cells; and though they have long since forsaken the means to survive on their own, mitochondria keep pieces of past freedom in their small stash of genes. Mitochondrial DNA is distinct from the much larger cell genome stored in the nucleus, and its limited number of genes encode proteins devoted mostly to aerobic affairs and energy production. No other component of our large, crowded cells has even this modest measure of genomic autonomy. The mitochondrial exception was written into the original eukaryotic compromise, and through more than a billion years of evolution it has never been broken.

There were other early instances of a cellular pooling of talents. Today's plant cells are thought to be the result of an ancient encounter between a cyanobacterial cell, with its priceless sun-eating chemistry set, and an aerobic cell that could make good on the oxygen wealth in the air. True to the terms of that paleocoupling, modern plants live a Jekyll-and-Hyde sort of life. During the day, when solar energy galvanizes their photosynthetic machinery, plants breathe in carbon dioxide, make their sugars, and exhale oxygen gas, in a manner reminiscent of cyanobacteria. But at night, plants take small amounts of that oxygen
back, reabsorbing the gas through diffusion and using it to help transport their homemade food plantwide.

For all the seesaw cycling between aerobic and anaerobic life, the level of atmospheric oxygen gradually mounted until about 400 million years ago, when it reached a concentration much like what we see today, a fifth of the total ether—though there have been fluctuations up and down ever since. Scientists have cited the surging supply of oxygen as a likely stimulant for a number of evolutionary seismic shakes. One was the advent of multicellular life around 700 million years ago, when heretofore separate eukaryotic cells began banding together into interdependent clans and taking up specialties—I'll be the mouth-parts if you'll serve as the gut tube. Another was the so-called Cambrian explosion of 530 million years ago, the dramatic diversification of multicellular life into a bona fide bestiary, the fete of faunal body plans that included the ancestors of all the major animal groups alive today. Some researchers also attribute the formidably proportioned arthropods of the Carboniferous period, roughly 300 million years ago, when dragonflies had wings like falcons and scorpions were the size of skunks, to a sharp spike in atmospheric oxygen, the result of an exponential growth of vascular plants. Even today, regions of comparatively high oxygen concentrations are often home to unusually large invertebrate species. The biggest jellyfish and marine worms are found in the coldest, most oxygen-rich waters of the ocean. The correlation between giantism and oxygen is not absolute, however; and, as far as I can tell, urban insects that inhabit poorly ventilated spaces like cupboards and basements seem perfectly capable of turning Goliath on the spice of spite alone.

The ceaseless give and take between bio and geo doesn't stop at oxygen. Carbon is cycled in great, intersecting loops through water, air, mud, body plans living and dead, now drifting into the atmosphere as carbon dioxide gas, now sinking into sediment as rotting gymnosperm forests. Calcium snakes through rocks, water, seashells, our cells. Iron and other trace metals play pivotal roles in both the private biochemistry of the body and the public geochemistry of the oceans, and the amount monopolized by one party at a given moment affects the rhythms and possibilities of the other.