IF YOU THINK that humans are destroying the planet in a way that’s historically unprecedented, you’re suffering from a species-level delusion of grandeur. We’re not even the first creatures to pollute the Earth so much that other creatures go extinct. Weirdly, it turns out that’s a good thing. If it hadn’t been for a bunch of upstart microbes causing an environmental apocalypse over 2 billion years ago, human beings and our ancestors never would have evolved. Indeed, Earth’s history is full of apocalyptic scenarios where mass death leads to new kinds of life. To appreciate how these strange catastrophes work, we’ll have to travel back in time to our planet’s beginnings.
Earth is roughly 4.5 billion years old, and for most of its life the atmosphere would have been noxious for humans and all the creatures who live here now. Vast acidic oceans roiled in what today’s environmental scientists would call an extreme greenhouse climate: the air was superheated and filled with methane and carbon. Our planet’s surface, now covered in cool water and crusty soil, was bubbling with magma. The solar system had formed relatively recently, and chunks of rock hurtled between the young planets—often landing on them with fiery explosions. (One such impact on Earth was so enormous, and threw off so much debris, that it formed the Moon.) It was on this poisonous, inhospitable world that life began.
About 2.5 billion years ago, early in an eon that geologists call the Proterozoic, a few hardy microbes who could breathe in this environment drifted to the surface of the oceans. These microbes, called cyanobacteria (or blue-green algae), knit themselves into wrinkled mats of vegetation. They looked like black, frothy coats of slime on the water, trailing long, feathery tendrils beneath the waves. All that remains of this primordial ooze are enigmatic fossils that hide inside a distinctive type of ancient, spherical rock called a stromatolite. If you slice a stromatolite down the middle, you’ll see thin, dark lines curving across its inner surface like the whorls in a fingerprint—these are all that remain of those algal mats. Only a few people in the world would recognize them as the traces of impossibly old life that they are, and Roger Summons is one of them. He’s a geobiologist at the Massachusetts Institute of Technology who has spent decades studying the origins of life on Earth, as well as the extinction events that wipe it out.
An Australian with a dry sense of humor, Summons has an office you can only reach by walking through his lab, a big, airy room full of tanks of hydrogen and bulky mass spectrometers that look like old-school Xerox machines covered in tubes. When I visited him to talk about ancient Earth, he plucked some slices of stromatolite from the top of a filing cabinet to show me the traces of algae that spidered across their surfaces. “This one is eight hundred million years old, and this one is two-point-four billion,” he said, pointing at each ragged half sphere of rock. “Oh, and this one is probably three billion years old, but it’s a crap sample.”
Even with a “crap sample,” Summons can pin a date on the fossils of creatures who lived more than 2 billion years ago by examining the sediments that have preserved them. In his lab, researchers grind up ancient rocks, subjecting them to vacuum, freezing, lasers, and a strong magnetic field before running them through the mass spectrometers. At that point, often nothing remains of a stromatolite but ionized gas. And that’s exactly what mass spectrometers need to decode the atoms in each sample. Atoms in minerals decay at a fixed rate, and reading the state of a rock’s atoms can tell scientists how long it has been since it formed. Geologists don’t put fossils themselves beneath the laser. They use machines like the ones in Summons’s lab to figure out the ages of the rocks next to the fossils. Call it dating by association.
Knowing when the oldest stromatolites were created helps us date an event which changed Earth forever. The mats of algae that became stromatolites weren’t just methane-loving scum. They were also filling the atmosphere with a gas that was deadly to them: oxygen. This is how the first environmental disaster on Earth began.
Just like plants today, ancient blue-green algae nourished themselves using photosynthesis, a molecular process that converts light and water into chemical energy. Cyanobacteria were the first organisms to evolve photosynthesis, and they did it by absorbing photons from sunlight and water molecules from the ocean. Water molecules are made up of three atoms—two hydrogen atoms and one oxygen atom (hence the chemical formula H2O). To nourish themselves, the algae used photons to smash water molecules apart, taking the hydrogen to use as an energy source and releasing the oxygen molecules. This proved to be such a winning adaptation to Earth’s primordial environment that cyanobacteria spread across the face of the planet, eventually exhaling enough oxygen to set off a cascade of chemical processes that leached methane and other greenhouse gases from the atmosphere. The dominant form of life on Earth ultimately released so much oxygen that it changed the climate dramatically, soon extinguishing most of the life-forms that thrived in a carbon-rich atmosphere. Today we worry that cow farts are destroying the environment with methane; back in the Proterozoic, it’s certain that algae farts ruined it with oxygen.
What happened after the rise of oxygen was an event shrouded in mystery until the late 1980s, when a Caltech geologist named Joe Kirschvink asked his student Dawn Sumner to research a rock whose existence seemed to be impossible—at least, given the prevailing theories about early Earth. Found near the equator, the rock’s surface was scored with marks that suggested it had once been scraped by the weight of a slow-moving glacier. In a short paper that eventually revolutionized geologists’ understanding of climate change, Kirschvink suggested that this rock offered a window on a late-Proterozoic phenomenon he called Snowball Earth.
Snowball Earth is what happens when our planet’s climate enters a very extreme “icehouse” state, the opposite of a greenhouse. A carbon-rich atmosphere can heat our climate up into a sweltering greenhouse, but an oxygen-rich atmosphere cools it down and causes what’s called an icehouse. Throughout its life, the planet has vacillated between greenhouses and icehouses as part of a geological process called the carbon cycle. Put in the simplest possible terms, a greenhouse happens when carbon is free in the air, and an icehouse occurs when carbon has been locked down or sequestered in the oceans and rocks. During an icehouse, ice collects at the poles, sometimes creeping down into lower latitudes during an ice age. But our recent ice ages were nothing compared with Snowball Earth.
Two billion years ago the sun was dimmer than it is today. As more and more cyanobacteria pumped out oxygen, the whole place began to cool down. Because the sun was a relatively weak heat source, this effect was magnified into a “runaway icehouse.” Ice from the poles began to spread outward, solidifying the top layer of the oceans and burying the land beneath vast, frozen sheets. The more ice that formed, the more it reflected sunlight—lowering the planet’s temperature further. Finally, ice stretched from the poles nearly all the way to the equator, pulverizing rocks beneath its weight. If you looked at Earth from space at that time, you’d have seen a slushy white ball, its circumference banded by a narrow equatorial ocean of algae-infested sludge. At that moment in geological history, our planet resembled Saturn’s icy moon Europa. It was an alien world called Snowball Earth.
I visited Kirschvink at the California Institute of Technology to find out what happened next. In the basement of the geology building, his generously sized desk was piled with fossils, family photographs, papers, and his prized possession, a cheap plastic vuvuzela from South Africa. “This is real!” he enthused, gesturing at the instrument whose droning sound annoyed and delighted audiences during the 2010 World Cup. Kirschvink lit up when he talked about the provenance of objects, whether pop culture ephemera or 3-billion-year-old fossils. Maybe it was his off-kilter imagination that allowed him to look for environmental patterns in Earth’s history that nobody had thought possible.
Kirschvink believes that there may have been as many as three snowball phases on Earth. “It was the longest, weirdest perturbation in the carbon cycle,” Kirschvink said. “And my explanation for it is simple. It’s the time between when the biosphere learned to make atmospheric oxygen and the time when everybody else learned to breathe it and use it.” Without any creatures around to breathe oxygen, the cyanobacteria likely created an atmosphere far more oxygenated than any we’ve ever known.
For 1.5 billion years after cyanobacteria evolved, Earth’s biosphere was in chaos. At least two more snowballs crept across the face of the planet, followed by intensely hot greenhouse conditions caused when volcanoes pumped carbon back into the air. Meanwhile, microbes were slowly learning to use oxygen to their advantage. A new kind of cell called a eukaryote began to populate the seas. Unlike cyanobacteria, which are basically just genetic material contained inside a membrane, a eukaryotic cell contains a nucleus packed with DNA as well as tiny organs called, appropriately enough, organelles. One of those organelles, called a mitochondrion, could turn free oxygen and other nutrients into energy. At last, Earth was inhabited by oxygen-breathers. The planet we know today was taking shape.
While the eukaryotes got busy swapping genetic material and sucking oxygen from the air, the old methane-breathers were dying out. A few migrated to the sea floor, finding niches near superheated volcanic vents where they could live in the remaining fragments of a once-global methane ecosystem. But the rest went extinct. It was the most extreme form of atmospheric pollution in Earth’s history, soon killing off almost every form of life that couldn’t breathe oxygen.
By “soon,” I mean within a billion years, or possibly 2 billion—a period of time that’s almost impossible to wrap our minds around. Still, that is the timescale required to understand Earth’s environmental transformations. Many of the catastrophic changes we’ll discuss over the next few chapters took millions of years to unfold. To geologists, we are all living in fast motion, our lives so short that it’s usually impossible for us to personally experience environmental change. Often, these scientists will contrast “human-scale” time with what they clearly view as real time, or time that unfolds on a planetary scale.
One of our most incredible accomplishments as a species, however, is an ability to think beyond our own life spans. We may not live in geologic time, but we can know it. And the more we learn about our planet’s past, the more it seems that Earth has been many different planets with dramatically different climates and ecosystems. This idea offers a much broader perspective than what you find in the work of environmentalists like Bill McKibben, who argues in his book Eaarth that humans have burned so much fossil fuel that we’re turning our planet into something fundamentally different (requiring the new name Eaarth). In that book and elsewhere, he laments the loss of “nature,” by which he means the ecosystems that existed on Earth before human meddling. But before humans took center stage on Earth, there were many permutations of nature. Climate disasters were the norm. Indeed, the only way Earth could ever transform enough to merit a new name like Eaarth would be if the planet’s environment suddenly stopped changing.
Undeniably, our planet is undergoing potentially deadly environmental changes today. But it’s incorrect to say that this is the first or even the worst time it’s happened. For the creatures who perished during the Proterozoic, and other periods we’ll learn about in the coming chapters, McKibben’s ideal of nature would be deadly. Over the course of its history, Earth has always vacillated between a carbon-rich greenhouse and its opposite, the oxygen-rich icehouse where humanity is more comfortable. We’re simply the first species on Earth to figure out how this climate cycle works, and to realize that our survival depends on preventing the next environmental shift.
As bad as the oxygen apocalypse was, neither Kirschvink nor the geobiologist Roger Summons would call it a mass extinction. So how can an entire world full of life go extinct without it being a mass extinction? This brings us to the question of what mass extinction really is. In a remarkable paper published in Nature in the spring of 2011, a group of biologists from across North and South America exhaustively summed up all the data available from the fossil record and present-day extinctions and came up with a clear definition. They agreed that mass extinctions on Earth can be defined as events in which 75 percent or more species go extinct in less than 2 million years. The oxygen apocalypse didn’t happen fast enough to qualify.
The statistician and paleontologist Charles Marshall, a coauthor on that Nature paper, warns that the definition of “mass extinction” is highly contextual and slippery. Sitting with his back to an enormous window overlooking the UC Berkeley campus, Marshall told me that the key to understanding mass extinction always begins with a calculation of what researchers call the “background extinction rate.” Species naturally pass into extinction all the time, at a rate of about 1.8 extinctions per million species every year. On top of that, there are also natural cycles of elevated extinction rates that fall roughly every 62 million years in the fossil record. So just because a bunch of creatures are going extinct, even in numbers above the background extinction rate, doesn’t mean you’re looking at a mass extinction. The only time you’re really seeing a mass extinction, Marshall said, is when “you see a big spike sticking out of the background distribution.” While on Earth those big spikes tend to be times when 75 percent or more species go extinct, it’s all relative. “You could imagine a planet where the biggest spikes sat at thirty percent,” Marshall speculated. “On that planet, thirty percent of species dying out would constitute a mass extinction.”
There are some ways that the fossil record can trick us into seeing a mass extinction where there isn’t one. Take, for example, the bombs at Hiroshima and Nagasaki. The rates of death were high, but they were low in terms of the world’s population. If we looked at these atomic bomb strikes in the fossil record, it might appear that there had been a mass extinction, but that’s because we’d be mistaking the rates in one local area for a global phenomenon. When geologists study mass extinction in the fossil record, they constantly have to ask themselves whether the extinctions they’re seeing are a statistical anomaly like Hiroshima, or something more widespread. Mass extinction is not an absolute idea, and to measure it we have to prove that the extinctions aren’t just localized. Plus, we have to compare the rate of death to the normal background extinction rate.
Still, the oxygen apocalypse does resemble a mass extinction in one way. It ushered in a completely different world, populated by an entirely new set of life-forms. It gave rise to the atmosphere that allowed life as we know it to develop. The change was so dramatic, said Marshall, that “you’re measuring less by magnitude and more by the idea of a world changed forever.” In every mass extinction, the world is changed forever—but over a short, terrifying two million years, rather than a slow billion. In the next few chapters, we’re going to see exactly what that looks like.