A cutaway view of the megavolcano in Siberia that led to the Permian mass extinction. (illustration credit ill.3)
(Click here to see a larger image.)
THE BERKELEY GEOCHRONOLOGY CENTER, a lab devoted to studying the ancient ages of Earth, is located on a pleasant tree-lined ridge overlooking UC Berkeley. Somewhat puzzlingly, it shares a building with the Church Divinity School of the Pacific. While seminary students strolled by in the hall outside, I met Paul Renne, the center’s head geologist, a big, jovial man in a T‑shirt. As he walked me through a warren of labs—full of the lasers and mass spectrometers that I’d come to expect in such places—I realized there was a peculiar kind of symmetry to the Geochronology Center’s tenancy arrangement with the Divinity School. After all, I had come to ask Renne about the closest thing to a total apocalypse the planet has witnessed.
Two hundred fifty million years ago, at the end of the Permian period, Earth spent thousands of years dying. At the end of those millennia of carnage, almost 95 percent of the species on the planet were dead. It was the worst mass extinction in our planet’s history, earning it the moniker “the Great Dying.”
The first phase of the mass extinction was caused by a disaster that has left an indelible and easily deciphered mark upon the Earth. If you visit the vast area known as the Siberian Traps today, you’ll find a beautiful, hilly terrain covered in short grasses. But 250 million years ago, the region was drowning under liquid rock spewing from the ragged mouth of a megavolcano. As its name implies, a megavolcano is far more powerful than your typical lava-filled mountain. Renne and other geologists estimate that as much as 2.7 million square kilometers of basaltic lava swept across the land in a fiery deluge. Almost a million square kilometers of the hardened basalt rock still remains here, smoothed by erosion into plateaus and valleys. It’s unclear whether this almost unimaginable ocean of lava was unleashed by one or two enormous eruptions, or a single, ongoing eruption that lasted for centuries.
But the Great Dying wasn’t caused by flaming tides of death. Volcanic eruptions on a large scale release a lot of gases, including greenhouse gases like carbon dioxide and methane. Jonathan Payne, a geologist at Stanford, estimates that the eruptions unleashed 13,000 to 43,000 gigatons (a gigaton is 1 billion tons) of carbon into the atmosphere. As if that wasn’t enough, they also released highly reflective sulfur particles that remained suspended in the atmosphere, scattering light away and cooling the climate very rapidly. The culprit responsible for the Great Dying was climate change.
Ironically, the roiling fires from this Siberian megavolcano may have caused a brief ice age. As glaciation locked coastal waters into ice sheets, the sea level dropped, and another source of greenhouse gas was unleashed. It’s possible that the water dipped low enough to expose methane clathrates, huge deposits of frozen methane that cling to the edges of continental shelves deep beneath the ocean. The clathrates melted and released ancient methane, a powerful greenhouse gas. As quickly as it began, the Permian ice age would have ended with a more intense greenhouse than before. These radical transformations in the atmosphere and climate made it impossible for most creatures to survive. Food sources dwindled. Species upon species died out.
It was an ugly ending for the Permian, which had been a time of rapid animal evolution on land. When the megavolcano began erupting, the earliest ancestors of today’s mammals were walking the Earth. Gingkos and conifers covered the coasts in forests, while seeded ferns evolved, uncurling their leafy fronds beneath tall pines. Mammal-reptile hybrids called synapsids roamed the land, some looking like giant lizards, and some like small rhinos. One of them, the enormous, dragon-like predator dimetrodon, had a tall sail attached to its back like a bony fin, and was such a badass hunter that paleontologists believe it may have fed upon sharks. These creatures all thudded around on the same continent because plate tectonics had finally pushed the planet’s landmasses together into one enormous continent called Pangaea, which stretched from pole to pole. A globe-wrapping ocean called Panthalassa teemed with sea creatures, from tiny single-celled organisms to corals and large fish.
A cutaway view of the megavolcano in Siberia that led to the Permian mass extinction. (illustration credit ill.3)
(Click here to see a larger image.)
These new forms of life, the forerunners of so many animals and plants we take for granted today, almost didn’t make it. What was especially unusual about the Permian mass extinction was that it took out nearly every form of life. Unlike in other mass extinctions, which sometimes hit sea creatures but not land creatures, or animals but not plants, this extinction was absolute. As many species were lost at sea as on land. When the megavolcano pumped carbon into the atmosphere, a lot of that got dissolved into the oceans. The water grew warmer, which destroyed the habitats of shellfish, who are sensitive to temperature changes. It also grew more acidic. The shells of shellfish are made of calcium carbonate, which dissolves in acid. Many sea creatures didn’t survive simply because their offspring couldn’t form shells in a highly acidic ocean environment.
Meanwhile, on land, so many trees and plants died that the continent’s surface was “denuded,” as Payne put it. The result was shockingly rapid weathering. As acid-tinged rain poured from the sky, followed by hot winds, more soil poured into the oceans, further raising the levels of carbon and acid. Vast areas of the coastal seas became anoxic dead zones—regions completely purged of oxygen. With oxygen supplies low in the water, large fish could not survive, especially ones that lived close to the deeply damaged ocean’s surface.
Even insects, which generally survive everything, suffered extinctions. An estimated 9 out of 10 marine species and 7 out of 10 land species went extinct. Across the planet, carbon levels suddenly skyrocket in rocks from this period examined by Renne and his colleagues, which suggests that the dead bodies of plants and animals were quite literally piling up on land and at sea. As the plants rotted, they released even more carbon into the environment. The devastation was so complete that we see a “coal gap” in the layers of rock left behind from this era. Plant life, which decays into coal, was so sparse in the 10 million years following the end of the Permian that none of the fossil fuel could form.
The planet had already endured ice ages, greenhouses, cosmic rays, and speciation depression. But only in the Permian mass extinction were almost 95 percent of all species cut down. And it happened in just 100 thousand years—the blink of an eye in geological time.
Still, there were survivors. The Stanford geologist Payne showed me a rock that’s a slice of geological time from this period, where a layer of ocean-floor sediment filled with tiny shells is topped by a black layer of what looks like pure sludge. It’s easy to see that a diverse community of creatures was abruptly replaced by nothing but, well, slime. Payne and his colleagues have nicknamed this era Slime World, because the oceans were dominated by dark, oozing bacterial colonies, feasting on the dead bodies of their multicellular cousins.
On land, one of the great survivors was Lystrosaurus, an animal that managed to thrive. A heavy, clubfooted creature with a beaked snout and two tusk-like teeth, Lystrosaurus was a four-legged synapsid, or mammal-reptile hybrid. About the size of pigs, lystrosaurs were burrowing animals whose muscular hindquarters ended in short, wiggly tails. And they somehow managed to endure when even the precursors of the hardy cockroach were dying. They were herbivores, and their beaks probably allowed them to chomp on rough vegetation and dig for roots to eat.
For several million years after the end of the Permian, lystrosaurs were alone on a dead world. But they didn’t cower or retreat. Instead, they spread out as far as they could across the landmass that would one day fracture into the continents we know today. Their fossils have been found in Africa, Asia, and even Antarctica, which was a tropical region at the time. With no predators and no competition for their favorite foods, lystrosaurs could waddle anywhere they liked. They are, as far as we know, the only creatures ever to dominate our world so thoroughly: For millions of years, most four-legged land creatures were one type of lystrosaur or another.
Lystrosaurus was one of the few land animals to survive the Permian mass extinction, and its progeny spread across the Southern Hemisphere during the early Triassic. (illustration credit ill.4)
Why did these creatures—our distant ancestors—survive when so many of their fellow creatures didn’t? Theories abound. The Permian expert Mike Benton said it’s possible that they were “just lucky.” More likely, he added, they were well adapted for a world with depleted oxygen. They lived in underground tunnels, so they had a natural way to escape the heat and fire of the initial volcanic eruptions. Plus, the air they were used to breathing in their burrows was likely to be low in oxygen and full of dust—sort of like the air after carbon has been saturating it for a few centuries. Their barrel chests held lungs of a tremendous capacity, which meant more oxygen uptake. Lystrosaurus had the right respiratory system at the right time.
Over time, Lystrosaurus’s progeny repopulated the southern part of Pangaea, diverging into many subspecies. Their favored half of the supercontinent eventually broke off from the northern half and became its own continent, Gondwana (named after the southern Ordovician continent), packed with dinosaurs and proto-mammals. It took 30 million years for our planet to grow a robust ecosystem again, packed with predators and herbivores and a wide range of flora and fauna.
Those 30 million years of ecosystem struggles are their own story. Though every mass extinction unfolds differently, they all end when a new community of creatures has established itself—generally, a community that statistician Charles Marshall described as “completely different life-forms.” After the Permian, during the early millennia of the Triassic period, new communities of completely different life-forms rose and fell with alarming regularity. A new ecosystem would come together only to collapse in a few million years. Then another ecosystem would arise. This mass extinction just wouldn’t end.
Why did it take the planet so long to recover from the Great Dying? For answers, I visited Peter Roopnarine, a zoologist at the California Academy of Sciences who has a rather singular occupation among scientists. He’s developed a computer program that simulates food webs, the complex interplay between predators and prey within an ecosystem. Using this program, Roopnarine studies why the worst part of mass extinctions isn’t necessarily the fire, or the eruptions. It’s what comes afterwards, in the centuries of what scientists call “indirect extinctions” caused by food webs that are too unstable to support life.
In this food web illustration created by Peter Roopnarine, the arrows between life-forms indicate who eats whom. This is a Cretaceous-era food web. (illustration credit ill.5)
The old computer game Wator offers a perfect example of a simple food web simulation. In it, red pixels stand in for sharks (predators) and green pixels for fish (prey) as they battle it out for supremacy of the sea. You can set a few simple parameters, such as how many sharks and fish there are to start, how often they breed, and how long it takes before they starve. Then you press “start” and watch generations unfold in seconds. When there are too many sharks, or the fish breed too slowly, the population of sharks eventually dwindles to zero and the waters of “Wator” become a sheet of uniform green. And that means you fail. What Wator reveals is that predators are as much at the mercy of prey as the reverse. Food webs can be knocked out of balance by life-forms at any point in the food chain.
Roopnarine’s simulations are infinitely more complex than Wator, incorporating the smallest planktons to the largest predators, and everything in between. In them, he describes relationships between predators and prey that lived millions of years ago. And from these models, he’s generated a theory about why the Triassic burned through so many food webs.
He began by coming up with a way to generate a realistic food web for species that no longer exist. He included every known form of life from the fossil record, and then he extrapolated predator–prey relationships based on what we know about how animals behave today. “You can’t ever know exactly what a fossil animal was eating—you can’t even know that with animals today,” Roopnarine explained. “But we can use the body size, tooth shapes, and other things to decide who their prey might have been.” Predators’ body sizes are helpful because, obviously, a small predator will prefer small prey, while a larger predator might be a generalist who can eat creatures of many sizes.
There are always complications, Roopnarine admitted. Many species share the same potential prey or predators, and it’s hard to know which species might have been generalists with many food sources, or specialists with just a few. But the beauty of using computers to simulate food webs is that you can go through as many iterations as you like, creating different worlds each time paleontologists discover more about a fossil predator’s range or appetites. Plus, there are a few rules of thumb, including the fact that there are usually far more specialists than generalists. Once the ancient food web has been set up in his program, Roopnarine said, he can simulate food-web disturbances like the one in the early Triassic.
Based on what he’s figured out so far, Roopnarine’s theory is that a basic imbalance in early Triassic food webs led to millions of years of maimed ecosystems rising and collapsing in rapid succession. Initially, the problem was that so few creatures had survived the Permian mass extinction. Of the survivors, he said, “you have small carnivores and some seriously big, bad amphibians who are the precursors of crocodiles.” Among herbivores, he said, lystrosaurs were the only game in town. The problem was that nobody around seemed to be eating Lystrosaurus, perhaps because they were the wrong size or in the wrong environments for most predators. In fact, the food web began to unravel because there were so many carnivores and very few prey.
Those “big, bad amphibians,” known as crurotarsans, were in fierce competition with each other. With their huge, toothy mouths and muscular tails, they would have been deadly predators—and Roopnarine says that the competition between carnivores during the early Triassic was more intense than in any other food web he’s looked at. The carnivores competed with each other so intensely for the tiny amount of available prey that they wound up driving each other to extinction over and over. New creatures would evolve, then get crushed out of existence. Only the herbivore Lystrosaurus, and eventually other herbivores, really recouped their losses. It took tens of millions of years before there were a small enough number of carnivores for food webs to stabilize.
This raises the question of what makes for a stable food web over the long term. And there’s an easy answer. “Diversity,” Roopnarine said firmly. A food web needs to be “robust,” full of many kinds of carnivores, herbivores, and plants, in order to withstand an environment that can often hammer creatures with everything from volcanoes to drought and sea-level shifts. As long as there are many nodes in a food web, a healthy balance of predator and prey, you have a community of life-forms that can remain steady even when the environment wobbles.
“So does that suggest some communities are better than others when it comes to survival?” I asked.
Roopnarine offered a conspiratorial nod. “This can be controversial, but yes, you could say this is natural selection at the community level.” Food webs don’t compete the same way two species might because they don’t exist next to each other, trying to eat the same things and live in the same caves. Instead, they compete with each other temporally, replacing each other in the same geographical places over time. To “win” the natural-selection game, a food web must outlast other food webs, remaining stable for as long as possible in the same place. Looked at from this perspective, you might consider all of Earth’s geologic history a competition between food webs struggling to last through as many environmental disasters as possible, simply by retaining their robustness in the face of calamity.
Survival is never just a matter of one species being exceptionally adept. We only survive in the context of our food webs. And when a food web starts to unravel, the extinction of one creature will mean the “secondary extinctions” of others.
Roopnarine and his colleagues have run enough simulations of food-web collapse that they’ve discovered a pattern. You can take away up to 40 percent of the life-forms in a system, and the number of secondary extinctions doesn’t increase significantly. “But there’s a critical interval after that where things happen rapidly—a threshold effect,” Roopnarine said. “The secondary extinction numbers rise dramatically.”
Imagine a world like the one we live in today, with a variety of creatures in many different environments. Let’s say we begin to chip away at one of those environments, like the American prairies. People clear grasslands, kill both predator and prey animals, and destroy insect pests. Still, the food web seems stable. Creatures and plants go extinct in the region, but there seems to be no ripple effect. And then, after centuries, we hit a tipping point. Forty percent of the nodes in the prairie food web have been knocked out. Suddenly, there are predators with very few prey. Catastrophic deaths among predators result: They are competing for scarce or no resources. And then a drought hits, killing the few remaining prairie grasses. Now our tiny herbivore population goes mostly extinct. We are left with few predators and virtually no prey. The already unstable food web falls apart, one death leading to another—and making the community more vulnerable to climate fluctuations.
“Don’t expect the unraveling to be linear,” Roopnarine warned. The deaths will be exponential. Once we hit the threshold, our food web has lost in the war of community selection. A new food web will rise up to take its place, turning the American prairie into a whole new world full of strange predators and grasses unlike any we’ve ever seen.
So the Permian extinction event yields a double lesson in survival. First, it offers compelling evidence that climate change caused by greenhouse gases can kill nearly every creature on the planet. Regardless of how that greenhouse scenario starts—whether it’s a massive volcano or an industrial revolution—climate change can kill more effectively than a meteorite impact. Of course, atmospheric changes were only the first phase in a problem that lasted 30 million years. One could argue that food-web collapse is really what makes the Permian mass extinction a “Great Dying.” The scourge started by Permian megavolcanoes echoed for millions of years, rending food web after food web until at last equilibrium was achieved in the Triassic period.
Still, there were survivors. Humans and many other mammals on Earth owe our existence to a bunch of piglike creatures with beaks who loved the sunny southern climate. That Lystrosaurus survived for millions of years (much longer than Homo sapiens has been around) proves that complex life can make it through even the most terrible disasters. These lumbering proto-mammals also left behind a few tips for what to do when we hit that wall of toxic air. By following in the lystrosaurs’ footsteps, mammals dodged the next major mass extinction—even though many dinosaurs didn’t.