Five times in Earth’s relatively recent history—meaning the period when multicellular animals occupied the oceans and/or the surface—at least half of all life-forms were suddenly obliterated. The most recent and famous of these annihilating events was caused by our planet’s collision with a giant comet or asteroid, as we’ll see in chapter 19.1
Not that every cataclysm is a bad thing. Perhaps none are, in the fullness of time. When violent upheavals erase the global stage, like dust in a car wash, something else follows in the niches that appear. To date, those new conditions, populated by new forms of life, have been more than merely interesting; they’ve brought us to our present situation. It is for this reason that major disasters that have befallen a largely uninhabited or single-cell-populated Earth will similarly be ignored in this book—especially since the record of such very early violence is now buried so deep it is hard to determine what happened with any precision.
Instead we begin with the earliest known cataclysm after the moon’s creation, one that very much affected the future of the planet and that was most central for the appearance of us multicelled creatures. We’re talking about the GOE.
It’s the great oxygenation event. It’s also commonly called the oxygen holocaust, the oxygen crisis, the oxygen revolution, and the oxygen catastrophe.
These may seem like unduly negative labels since the starring element isn’t one of the notorious bad guys like arsenic. You’ve likely not been surveyed about your favorite element by one of those robocall polls conducted just as you’re sitting down to dinner, and it’s hard to say whether most folks would choose oxygen over, say, gold or platinum. But it’s the only one of the ninety-two natural elements you can purchase at a coin-operated or card-swipe booth in places like Tokyo.
Our planet, like all others in the known universe, had little or no oxygen to begin with, even though it’s the third-most-common element in the cosmos. But free oxygen—the gaseous element sitting alone or else combining with itself as O2—is rare on planets for a good reason. Its electron configuration makes it unusually fond of sharing electrons. At the slightest provocation, it will link with another element in a process called oxidation. Its favorite target is the universe’s most abundant element, hydrogen. When it connects up to form H2O, it creates the most common compound in the cosmos (a compound, you’ll recall from high school, is a substance made of two or more elements).
So when life began on Earth, it had to make do with no free oxygen. In that original matrix 3.9 billion years ago, the creatures that arose—such as cyanobacteria that lived abundantly in the oceans and probably on land too—broke water into its constituent hydrogens and oxygen and thus released free oxygen.
At first only prokaryotic organisms created oxygen as a waste product. Later, eukaryotic creatures did too.2 But the rare, precious liberated oxygen wasn’t given much of a chance to be a free-floating part of the atmosphere. Instead it was quickly snatched up by such greedily reducing elements as iron. Millions and millions of years passed while Earth’s silicon, iron, hydrogen, and carbon slowly turned into silicon dioxide (quartz and sand), iron oxide (rust), hydrogen hydroxide (water), and carbon dioxide, which grew more abundant in the atmosphere.
Earth’s atmosphere was sent spiraling off balance, even though these were the first steps toward our planet becoming what it is today. You’d think the extra CO2 would have warmed the planet, since today it’s the primary villain in climate change. But the opposite happened. The newly minted oxygen attacked the methane that was initially abundant. Methane is CH4, and oxygen smashed this larger molecule into fragments as it latched onto its C and its H to form carbon dioxide and water vapor. Since the new CO2 and H2O were both less effective greenhouse gases than the CH4, the methane it replaced, Earth got cooler.
Thus, one of the early consequences of the oxygen holocaust was that ice ages began. But even before these took a toll on Earth’s life-forms, the oxygen was deadly to the prevailing anaerobic organisms. The cyanobacteria, by unleashing a very un-Darwinian mass homicide/suicide, created the first global cataclysm since the moon’s birth, wiping out most of the life on our planet.
Needless to say, this oxygen catastrophe had a plus side; it led to an event that was probably the most dramatic and important in the history of our world. Yet, in an indictment of our education system, if people are asked to name the single most influential occurrence in the planet’s history, few are knowledgeable enough to cite the Cambrian explosion. Most people probably don’t even know what it is.
The Cambrian explosion might sound like some sort of huge detonation, one more item in our recitation of cataclysms, but it’s almost the exact opposite—it’s the odd term for the sudden remarkable appearance and proliferation of multicellular animals and plants. Prior to this time, which was about five hundred and fifty million years ago, life was invisible. It was all microscopic. Now a strange, wild profusion of large creatures materialized quickly and globally.
And although the first creatures, like trilobites, looked alien, and the plants were largely different from today’s flora (there was no such thing as grass, for example), the planet was now a visibly alive biosphere. The oxygen holocaust had effectively erased a global situation that had endured for hundreds of millions of centuries.
We oxygen-adorers, or at least those of us with a them-versus-us mind-set, might hesitate to label that wipeout of non-oxygen-loving creatures a cataclysm. But you’d feel differently, of course, if you were the final anaerobic organism watching your extended family die off, knowing that this was really the end of an era that had endured for not millions but billions of years.
Enough violin music. Now that the scene has shifted to a planet that is more familiar, we can focus on the cataclysms that were the greatest of all in terms of affecting our kith and kin, the beloved multicellulars.