Five Billion Years of Solitude (21 page)

First appearing in rocks from the latter half of the Archean, cyanobacteria were sea-green prokaryotes that, like Devonian plants or Holocene people, would go on to profoundly alter the world. In this case, the cyanobacteria decisively shaped the subsequent evolution of all life on Earth, and defined the final eon of the Precambrian—the Proterozoic, the two-billion-year stretch of “early life” prior to the Phanerozoic. Unlike the photosynthetic life of the planet’s previous billion years, which used sunlight to gain chemical energy from hydrogen, sulfur, iron, and various organic molecules, cyanobacteria evolved a
metabolic pathway to use sunlight to split water, a substance that was far more abundant and offered more chemical energy. The pathway seems to be a fluke of evolution—as far as scientists can tell, it only emerged this single time throughout Earth’s long history. Its most obvious innovation was chlorophyll, a distinctively green class of light-absorbing molecules that more efficiently absorbed sunlight than the more ancient photosynthetic pigments, which were often pink or purple in color. After using chlorophyll to channel sunlight into water, cyanobacteria combined the harvested hydrogen with CO
₂
to synthesize sugars, and jettisoned the leftover oxygen. Cyanobacteria also possessed the rare ability to draw chemically inert nitrogen gas out of the air to incorporate into the biochemical building blocks of DNA and proteins. Armed with the capacity to produce their own fertilizer, cyanobacteria could uniquely thrive wherever water, CO
₂
, and sunlight were present, and were poised to quite literally conquer the world. Late Archean and early Proterozoic rocks show that’s exactly what they did, flourishing in vast open-ocean blooms and in concentrated communal mats blanketing the shallows and shorelines.

By 2.4 billion years ago, the Earth’s new masters produced oxygen so prodigiously that they began to irreversibly transform the planet, as iron that had been dissolved in ocean water oxidized, solidified, and precipitated to the seafloor. It settled in thick layers of ferric sludge that were destined to someday become engine blocks, skyscraper beams, and battleship hulls. Early on, most of that oxygen was mopped up through reactions with organic carbon, volatile gases from volcanoes, and the rusting oceans. The oxidized material sank to the bottom of the seas, creating stratified and stagnant global oceanic conditions similar to those that would, much later, form black shales like the Marcellus. Experts endlessly debate what decisively shifted the planet’s geochemistry away from sequestering all the surplus oxygen, but the result is indisputable: over the course of a few hundred million years, most of the ocean became saturated with the stuff, and ever after the gas flowed up into the atmosphere. In the upper atmosphere, the oxygen molecules
clumped together to form a layer of ozone that absorbed large portions of the Sun’s biologically harmful ultraviolet radiation, shielding life far below on the planet’s surface.

Oxygen’s rise was the world’s first great pollution crisis, long before the invention of internal combustion engines and chlorofluorocarbon-spewing refrigerators. Despite the benefit of the Earth’s new ozone layer, Earth’s oxygenation was an unmitigated ecological disaster for the anaerobic biosphere that had developed and flourished during the Archean. To those creatures, oxygen’s extreme chemical reactivity made it a terrible poison. Untold numbers of microbial species were annihilated as oxygen suffused the planet, and most of the surviving anaerobes retreated from the sunlight, initially into the anoxic muds found in the dark bottoms of deep seas and lakes, and much later into the low-oxygen digestive tracts of complex animals, including humans. They lurk in both kinds of shelters to this day. Oxygen also almost spelled the end for the cyanobacteria that produced it—the associated declines of anaerobic methanogens and heat-trapping atmospheric methane sent global temperatures plummeting and caused at least three Proterozoic ice ages, the first at 2.4 billion years ago, the second at 750 million years ago, and the third around 600 million years ago. Each was so prolonged and severe that glaciers reached the equator, repeatedly locking the oceans beneath a crust of ice that nearly eradicated all photosynthetic surface life. Joseph Kirschvink, a Caltech geologist who helped discover evidence of these extreme Proterozoic glaciations, called them “Snowball Earth” events after the likely appearance of our planet from space during each glacial episode. Near the equator or isolated volcanic hotspots, the ice might have been only a few meters thick, translucent enough to allow a twilight glow into the otherwise light-starved oceans where life hung on by the slimmest of margins. That the ice eventually thawed each time is clear, for otherwise we would not be here.

From these calamities sprang new opportunities: each Proterozoic ice age placed immense evolutionary pressures on the biosphere while
also ratcheting up the level of energy-stoking atmospheric oxygen. A lucky few anaerobes, by dint of mutation and natural selection, adapted to tolerate the newly oxygenated atmosphere and ocean. Some of these new breeds of aerobic prokaryotes, in fact, took revenge on their conquerors by engulfing the cyanobacteria into their bodies as cellular slaves, making oxygenic photosynthesis their own. This process, called endosymbiosis, was what gave rise to the first eukaryotes, cells with centralized nuclei and specialized cellular structures. Modern plants are green because their cells contain chlorophyll-filled “chloroplasts”—structures that are scarcely distinguishable from cyanobacteria. The cells of modern plants and animals alike also contain enclosed structures called mitochondria, which are the cellular components that allow all eukaryotes to draw metabolic energy from oxygen—that is, to breathe. Chloroplasts and mitochondria each carry DNA independent of their host organism, confirming that both are captive descendants of prokaryotes incorporated into eukaryotic cells sometime in the latter half of the Proterozoic.

By the conclusion of the last great Proterozoic ice age some 600 million years ago, atmospheric oxygen was approaching the levels of today, and a new breed of fledgling eukaryotes waited in the wings to exploit its immense power to release chemical energy. For the first time, multicellular creatures could draw enough power from the air itself to support large, active bodies. The stage was set for the explosive diversification and growth of life, the rise of complex plants and animals, the colonization of land, and the eventual emergence of humans. We have now arrived where this pocket history began, at the oldest rocks of Pennsylvania, the root of the Cambrian, the great transition between Earth’s three eons of simple life and its subsequent half billion years of burgeoning biological complexity.

Even armed with this cause-and-effect chronology, it can be difficult to understand why the Earth wasn’t always the place we see before us, and just how its transition from an alien, hostile planet took place. Deep planetary time, in all its vastness, is the most fundamental thing
to recognize. A thousand years of shifting climate can produce a forest where a desert used to be. A million years of tectonic activity can thrust up a mountain from wide-open prairie plain. A hundred million years of evolutionary trial and error can transform a prokaryote into a eukaryote, or a mouse into a man. A billion years is time enough to entirely restructure the ways of the world. To most people, the Phanerozoic and the Proterozoic certainly
sound
the same, and the difference between a million and a billion years is less a quantity of time and more a matter of three extra zeroes at the end of a numeric string. But simple thought experiments reveal the truth.

Consider that the entire 542-million-year span of the Phanerozoic is only about an eighth of our planet’s total history. Imagine, as the writer Bill Bryson has, stepping into a time machine to venture back to the Phanerozoic dawn at a rate of 1 year per second. After ninety minutes, you would find yourself in the Bronze Age, around the time of the construction of Stonehenge, the domestication of the horse, and the founding of Abrahamic religions. A day later, you would be in the middle of the Stone Age, just as small bands of foraging humans began to migrate out of Africa. To reach the beginning of the Cambrian, the base of the Phanerozoic, would take you about 17 years. Now remember that almost a decade of earlier Precambrian time underlies each and every passing Phanerozoic year—departing from the far-distant Cambrian, your year-per-second time machine would take another 125 years to transport you to our planet’s first moments.

Or try mapping the Earth’s 4.5 billion years onto a calendar year. The Precambrian commences with the New Year’s Day coalescence of Earth from the primordial nebula and persists until the Cambrian explosion in mid-November. Life gets going sometime in late February, but cyanobacteria only begin pumping oxygen into Earth’s atmosphere in mid-June. The Marcellus shale forms a few days after Thanksgiving, and Pennsylvania’s coal measures are all laid down in the first week of December. The following week, dinosaurs appear, but they succumb to extinction by Christmas Day. Anatomically modern humans show
up late to the party, just after a quarter to midnight on New Year’s Eve. One minute before midnight, the last glacial pulse ebbs back to the poles and the Holocene interglacial begins. Approximately one second before midnight, Earth enters the Anthropocene.

The author John McPhee has devised a more visceral visualization: Throw your arms out wide to represent the span of all Earthly time. Our planet forms at the tip of your left hand’s longest finger, and the Cambrian begins at the wrist of your right arm. The rise of complex life lies in the palm of your right hand, and, if you choose, you can wipe out all of human history “in a single stroke with a medium-grained nail file.”

Deep time is something that even geologists and their generalist peers, the earth and planetary scientists, can never fully grow accustomed to. The sight of a fossilized form, perhaps the outline of a trilobite, a leaf, or a saurian footfall can still send a shiver through their bones, or excavate a trembling hollow in the chest that breath cannot fill. They can measure celestial motions and list Earth’s lithic annals, and they can map that arcane knowledge onto familiar scales, but the humblest do not pretend that minds summoned from and returned to dust in a century’s span can truly comprehend the solemn eons in their passage. Instead, they must in a way learn to stand outside of time, to become momentarily eternal. Their world acquires dual, overlapping dimensions—one ephemeral and obvious, the other enduring and hidden in plain view. A planet becomes a vast machine, or an organism, pursuing some impenetrable purpose through its continental collisions and volcanic outpourings. A man becomes a protein-sheathed splash of ocean raised from rock to breathe the sky, an eater of sun whose atoms were forged on an anvil of stars. Beholding the long evolutionary succession of Earthly empires that have come and gone, capped by a sliver of human existence that seems so easily shaved away, they perceive the breathtaking speed with which our species has stormed the world. Humanity’s ascent is a sudden explosion, kindled in some sapient spark of self-reflection, bursting forth from savannah and cave to blaze through the biosphere and scatter technological shrapnel
across the planet, then the solar system, bound for parts unknown. From the giant leap of consciousness alongside some melting glacier, it proved only a small step to human footprints on the Moon. The modern era, luminous and fleeting, flashes like lightning above the dark, abyssal eons of the abiding Earth. Immersed in a culture unaware of its own transience, students of geologic time see all this and wonder whether the human race will somehow abide, too.

Immiscible emotions emerge from contemplating the dual realities of modern life and deep time—strange amalgams of apathy and anxiety that resist easy dismissal. Against the rich pageant of a planet and its past, the brief activity of a human life shrinks toward futility, even as human habits and behaviors, human choices in aggregate, so forcefully send much of Earth’s complex biosphere sliding into oblivion. Yet as wrenching as the Anthropocene’s changes may be, their permanence is as questionable as man’s dominion: to be sure, once extinct, a species cannot be readily resurrected, but there are few reasons to believe that with time the planet’s biodiversity could not recover, just as it has in the past. It can be seen as a blessing that modern civilization, in all its power, must struggle to even slightly perturb the robust microbial world that forms the basal roots of the Tree of Life. If it were otherwise, we could far more definitively disrupt the biosphere. Most of the more abiotic changes—alterations in geochemistry, atmospheric and oceanic circulation patterns, and so on—will eventually be reversed and erased by growing continental cratons, subducting oceanic crust, and erupting volcanoes. The fact that Earth’s renewal will only unfold over millions of years may be no consolation to people today and tomorrow, no help to the countless species trampled beneath our civilization’s tread, but that makes a recovery no less plausible. The grass will grow, the Sun will shine, and life on Earth will go on, with or without a wily band of tool-using primates. At least, that is, until the Sun, ever brightening through the eons as it fuses itself to death, brings an ultimate end to all things Earthly.

Whether all this makes the proximate probability of civilization’s fall
and the present destruction of the planet’s biological wealth something to become upset about depends, therefore, on your sense of scale, and on where you think humans reside in what might charitably be called the “Big Picture,” the assessment of things starting with the biosphere and extending out into the Milky Way. It is, in truth, only a minuscule fraction of nature’s much larger tableau. Further up the cosmic scale, where even an entire galaxy is but one nebular mote out of hundreds of billions, or further down, to the quantum world of fundamental particles, the significance of Earth’s spark of life, sentience, and technology grows indiscernible. But in all the uncertain sun-filled space between, it becomes just possible to see the promise of greater things, to envision our spark surpassing its billions of years of solitude upon a single planet circling a single star, flaring in ascension beyond planetary and stellar time to shine at the base of the endless, enduring galactic range.