Maps of the continents during different geological eras. (illustration credit ill.2)
(Click here to see a larger image.)
NEARLY TWO CENTURIES ago, scientists trying to learn about Earth’s history visited England’s famous “white cliffs of Dover,” where the wind and water had eaten away at the land’s edge, revealing rocks unseen for millennia. There, these early geologists discovered that the cliffs weren’t just big, crumbling slabs of chalk. They were actually formed from distinct layers of rock, each containing very different sets of fossils, providing a chronological record of how the land was built up from mineral deposits and ecosystems through the ages.
Each geological period is named for one of these rocky layers. They’re chunks of frozen time, identified by their unique combinations of life-forms and mineral deposits. Generally, fossils change dramatically from layer to layer because there has been an extinction event. Though only five of these demarcations qualify as mass extinctions, there have been dozens of smaller extinction events where, say, 20 or 30 percent of all species die out. You might say that geological time is measured in extinctions. But if you were to visit Dover, and allowed your eyes to wander up the cliff face, you’d also see layer after layer of evidence that life always emerges from mass death.
The more we learn about these layers, the more it seems that there are two basic causes that can set a mass extinction in motion. The first is an unexpected calamity from the inanimate physical world, often taking the form of fatal climate patterns, megavolcanoes, or even debris from explosions in space. As for the second, as we learned in the last chapter from the cyanobacteria that poisoned the planet, biological life can transform the physical world so much that extinctions are inevitable. Of course, many extinctions are a combination of the two causes—one often leads to the other.
To see examples of both, we’ll journey back hundreds of millions of years to the first two mass extinctions that gripped the Earth. The Ordovician (beginning roughly 490 million years ago) and Devonian (beginning roughly 415 million years ago) were both periods when life was exploding with unprecedented diversity. And both ended in holocausts. The Ordovician was scourged by natural disasters from Earth and space; the Devonian was choked to death by invasive species that turned the planet into an environmental monoculture.
Before the fecund Ordovician period, the seas had stopped looking like goo-covered murk and flowered into underwater forests full of aquatic plants, shellfish, coral, and lobsterlike arthropods called trilobites. New species were evolving at a rapid clip. It was a greenhouse world, with carbon dioxide levels in the air at fifteen times higher than they are today. But a warm, high-carbon climate was exactly what those Ordovician plants and animals needed.
Peter M. Sheehan, a geologist with the Milwaukee Public Museum, describes the Ordovician as having “the largest tropical shelf area in Earth’s history.” Put another way, it was a world of sultry beaches. Earth blossomed into this tropical paradise partly due to the climate, and partly due to continental drift, the process by which massive plates of the Earth’s crust slowly move around on top of the planet’s superheated molten layers. Lava gushing from underwater volcanoes applied so much pressure to the Earth’s crust that it pushed all the continents together, into the low latitudes of the warm southern hemisphere. Slowly drifting over the South Pole was a supercontinent called Gondwana, made up of land that became, among other places, Africa, South America, and Australia. Its balmy, world-wrapping coastline teemed with life.
Maps of the continents during different geological eras. (illustration credit ill.2)
(Click here to see a larger image.)
Ordovician life was confined almost entirely to the oceans, though a few plants spread to the land. Trilobites scuttled into many different territories, evolving into a range of species: some became swimmers, while others wandered the floors of the shallow seas, developing sharp, defensive spines or shovel-shaped heads for rooting food out of the sediment. Shelled creatures and sea stars attached themselves to enormous coral reefs, and strange colony animals called graptolites built complicated, beehive-like structures out of proteins secreted from their bodies. Their hives, which looked like thorny, interconnected tubes, floated beneath the ocean surface while the graptolites poked their feathery heads out and snarfed up plankton.
The ancestors of sharks prowled the waters and fed on everything that moved (and some things that didn’t). Joining the sharks were jawless fish called agnathans, whose soft mouth slits and heads were covered in bony plates that probably looked like turtle shells. These armored fish were the first vertebrates, or animals with internal skeletons like we have. Plus, there were thousands of new kinds of plankton evolving all the time, creating an abundant food source for all the multicelled newcomers looking for easy-to-reach food floating through the waters.
But over a few hundred thousand years, over 80 percent of the species in the Ordovician coastal waters would go extinct.
We can place part of the blame for the slaughter on the Appalachian Mountains, a gently curving spine of peaks that stretched from Canada’s Newfoundland down to Alabama in the southern United States. These mountains were formed during the Ordovician when a smashup between two continental plates pushed ancient volcanic rock into jagged peaks above the continent. Almost immediately, rain and wind began eroding the soft, dark rock. The newly formed mountains ran with thick slurries of water and mud, which turned into rivers that picked up even more soil on their way to the seas. This natural process, called weathering, is actually one of the most powerful ways to change our planet’s atmosphere. As exposed earth crumbles beneath the weather’s onslaught, tiny rocks pull carbon dioxide from the air and take it with them into sediments deep beneath the sea. Sliding into the sea along with all that carbon was the Ordovician’s warm, life-nourishing climate.
Seth Young, a geology research associate at Indiana University, observed, “We are seeing a mechanism that changed a greenhouse state to an icehouse state, and it’s linked to the weathering of these unique volcanic rocks.” The Ordovician Appalachians weathered so rapidly, in fact, that they were worn down to a flat plain within a few hundred million years. The Appalachians we know today are the result of a second tectonic-plate smashup, which raised a new set of mountains about 65 million years ago. Washing carbon out of the atmosphere sounds like a good dream in our fossil-fueled times, but it was the worst thing that could happen in the Ordovician. Without greenhouse gases to keep the planet warm, disaster struck in the form of the fastest glaciation in the planet’s history. About 450 million years ago, ice caps began to spread outward from the poles. Gondwana and its hot, humid shorelines were at ground zero of the ice apocalypse.
As the glaciers grew, they locked up liquid water and lowered sea levels dramatically, drying out the lush coastal areas beloved by corals, graptolites, and shelled creatures. Most affected were stationary animals like the shellfish in a coral reef, which remain anchored in place for most of their lives. Because they couldn’t move, they died with their habitats. In all, Peter Sheehan estimates that about 85 percent of marine species died over a million years as massive ice sheets sucked the liquid out of their environments. Not all the Ordovician species died at once. There were two “extinction pulses,” as geologists put it. The first came when ice abruptly destroyed sea life. The second came when the ice melted just as suddenly as it had come, causing sea currents to slow and stagnate. Fewer currents meant that less oxygen was churned into the water and vast “dead zones” of anoxic (low-oxygen) water suffocated life throughout the oceans. First came ice, then came stagnation. Together, they created a mass extinction.
Despite what we know about the Appalachian Mountains, the wholesale slaughter at the end of the Ordovician remains a mystery. We understand why rapid freezing and thawing would kill so many life-forms. But typical ice ages are millions of years in the making, and this one lasted for less than a million, making it ridiculously rapid in geological time. Could weathering alone have caused the rapid glaciation in the first place? Probably not. It’s possible that the ice formation was hastened by invisible rays from space.
Adrian Melott, a professor of physics and astronomy at the University of Kansas, has long been fascinated by a weird fact about mass extinctions. They seem to fall roughly every 63 million years. Trying to explain why this might be, he stumbled upon one possible explanation for the swiftness of the Ordovician ice age. It has to do with the motion of our star through the swirling galactic disk of the Milky Way.
Every star in the galaxy has an orbit around the edges of the galactic disk. As our sun makes its vast circuit around the Milky Way, it bobs up and down, floating above or below the galaxy’s flat plane about every 60 million years. When it does this, our solar system brushes the edge of the protective magnetic field that envelops the galaxy, deflecting dangerous cosmic rays zooming through deep space (on a smaller scale, the Earth’s magnetic field protects us from these same particles). Cosmic radiation could help explain why extinction events are more likely to happen every 63 million years or so.
Cosmic rays are highly energetic subatomic particles that have been bouncing around in deep space since the early days of the universe. They can shoot right through a living creature’s body, damaging its DNA and causing cancer. And these particles aren’t much kinder to the molecules that make up Earth’s atmosphere. Cosmic rays can damage the ozone layer, which leaves the planet more vulnerable to deadly radiation. Melott hypothesizes that cosmic-ray bombardment could also whip up a thick cloud layer in the atmosphere, lowering temperatures and helping the ice caps to form more quickly.
As the planet cooled, extinctions would have been worsened by radioactivity hitting the planet’s surface. “At this point we’re thinking that … the radiation dose for organisms on the surface of the earth, or in the top kilometer of ocean water, could be very large. This causes cancer and mutations.” Melott paused, as if imagining a planet with gray skies racked by cancers and catastrophic erosion. Then he chuckled. “Or, you know, it could lead to giant ants that rampage across the Earth.”
His joke about the 1950s atomic monster movie Them!, featuring giant ants that take up residence in the sewers of Los Angeles, underscores the degree to which he thinks of his work as speculative. Cosmic rays, he conceded, were only one part of the problem that animals faced at the end of the Ordovician. “The analogy I like to give is that it’s like you have the flu and then you get shot. Cosmic-ray stress is like the flu.” But other factors—the bullet in Melott’s analogy—need to be in play. And these would likely be the volcanic activity that led to the uplift of the Appalachians, the weathering that flattened them, and the resurgence of volcanoes that shut down the Ordovician ice age as quickly as it began.
The Ordovician ended with an extremely rapid version of what happened during the snowball phases of Earth’s history. A swiftly changing climate, vacillating between icehouse and greenhouse, made it impossible for most species to survive. Because those deadly climate shifts happened so fast, geologists have dubbed this horrific period the Ordovician mass extinction, marking the first time our planet witnessed the deaths of so many species at once.
By the time the planet’s temperatures had stabilized, the Ordovician biosphere was gone forever. A few survivors remained, like the hardy trilobites. But for the most part, new animals and plants evolved to rule the seas, and a few creatures even crept up on land. Life diversified and flourished for 100 million years, a fairly long time even for a geologist. Consider that modern humans evolved only about 200,000 years ago, and you have an idea how many species evolved and died out during the 100 million years before the planet suffered its next mass extinction. Our next rendezvous with mega death came during the Devonian period. This time there were no dramatic claws of ice, cosmic rays, or greenhouse extremes—but that’s because this was the first mass extinction caused by life itself. By the end of the Devonian, 50 percent of marine genera (groups of species) and an estimated 75 percent of species were dead. Oddly, these species died out at an ordinary rate, probably no higher than the typical background extinction levels. So why is this even considered a mass extinction at all? Because almost no new species evolved to take the extinct ones’ places for as many as 25 million years. It was mass extinction by attrition.
Scientists call this phenomenon a “depression in speciation,” meaning a low point in the evolution of new species. If you had been floating around for thousands of years in one of the Earth’s oceans during the late Devonian, about 374 million years ago, you wouldn’t see corpses piling up. Nor would you see vast stretches of lifeless water as you would have during the late Ordovician. Instead, you’d see the same species slowly spreading everywhere, darting around in enormous coral reefs that were ten times more expansive than the ones we have today. There weren’t fewer life-forms during this mass extinction. There were just fewer kinds of them.
How did invasive species destroy the planet? It all had to do with the period’s peculiar ocean ecosystems. The massive sea creatures of the Devonian earned the period its nickname, the Age of Fishes. The eminent geologist Donald Canfield conducted a study of the atmosphere during this period, after which he and his colleagues concluded that the Devonian oceans contained a high amount of oxygen, which allowed the period’s enormous animals to evolve. A group of hardy armored fish called placoderms vied with sharks to become the ocean’s most forbidding predators. Placoderms grew up to 36 feet long and had faces entirely covered in armor; they were also among the first creatures to develop jaws. (Sharks won the scary toothed predator contest in the end, though—placoderms went extinct.) Reefs made from algae and early sponges—all species lost to this extinction—were dramatically unlike the coral-dominated reefs we know today. The ocean floors crawled with ammonites, which looked something like octopuses with spiraled shells.
Watery habitats were everywhere—even on the continents. Enormous tropical inland seas dominated the landmass that later became North America. Most of the Midwest and the central United States were fully submerged, which is why paleontologists today find some of the best fossilized fish in the middle of the Midwest’s rolling prairies, which are about as far from the coast as you can get. Yet by the end of the Devonian, almost none of the gigantic armored fish and swarms of tentacled ammonites were left. What happened?
One paleontologist, Ohio University’s Alycia Stigall, has a theory that could explain why life during this period went from diverse to homogenous. She believes that invasive species took over the world’s oceans and inland seas, the same way cockroaches, kudzu, rats, and humans have spread across the globe today.
Stigall lives in Ohio, at the bottom of what was once a shallow Devonian sea. In fact, the vista from her windows is the former seafloor of an inland ocean hit particularly hard by the mass extinction. “We don’t have a good modern analogue for these types of oceans,” she said. They are a completely vanished ecosystem, though she imagines they might have been something like Hudson Bay. At the end of the Devonian, it’s likely that sea levels were high, pooling several inland oceans together. Earthquakes thrust new mountains from the land, which also brought previously separated ecosystems into contact with each other.
Many highly adaptable or generalist species began invading new watery territories. That meant they were competing with the local specialist species, like trilobites, for food. A specialist species requires very specific temperatures or food sources. They couldn’t cruise all over the Devonian oceans eating anything that came into view the way sharks could. So when invasive species came into their territories and stole their food, the specialists had nowhere to go. Their populations dwindled and they went extinct. By the end of the Devonian mass extinction, the planet was covered with giant, homogenous inland oceans where you’d see the same generalist species no matter where you looked. And out of those circumstances, Stigall believes, you had the makings of a mass extinction.
Though Stigall’s account is only one of many theories about mass extinction in the Devonian, her claims are backed up by evidence that many of the creatures who survived the period were generalists like sharks—creatures who could live anywhere and eat almost anything. Another survivor was the humble crinoid, a starfish-like creature with several feeding arms surrounding its mouth that make it look a little like the “face-hugger” stage of the creature in Alien. The crinoids went through a floating larval stage that allowed them to drift into many new environments before attaching to the ground and feeding on the abundant plankton in the water.
Still, homogenous ecosystems like the ones in the Devonian would have left all life-forms primed for disaster. Generalist species may be hardy, but they also share the same vulnerabilities. Say, for example, a drought hits an ecosystem in the Midwest. If there is only one type of wheat species, and it can’t deal with higher temperatures or less moisture, a short-term climatic change could kill off every blade of wheat in a given region. Without a diverse range of grain species, which might have different moisture tolerances, drought slays all the wheat. This in turn kills the animals who feed on that wheat, whose deaths leave predators hungry, too. Soon, you have multiple extinctions because the whole food chain has been ravaged. “The more we cull diversity, the more we are vulnerable to extinction,” Stigall concluded. It’s very possible that this is how the Devonian came to a close, with only a few invasive species scraping by until speciation, or new species evolution, made the ecosystem diverse again.
For Stigall, there’s a lesson in the fossils that surround her home, remnants of an inland sea unlike anything that exists now. “We’ve got the same problem with invasive species today,” she said. “It’s not caused by sea levels, but by humans, because we like to move things around.” She described how invasive species like pigeons and rats, as well as some trees and grasses, have gone from a few local regions to expand across the Earth. If this trend continues, she predicted that “long term, what you expect is a huge decimation of total biodiversity.” We could be on our way back to the late Devonian, in the early stages of a mass extinction that begins with a depression in speciation and ends with deadly homogenization.
Sometimes, the urge to live by expanding into as many territories as possible can backfire. As the invasive species of the Devonian reveal, life does not always beget more life. Some ways of living can actually kill just as handily as climate change and radiation.
As devastating as the Ordovician and Devonian mass extinctions were, they were nothing compared to what ravaged the planet about 75 million years later. The “Great Dying,” as it is known among geologists, was the worst period of mass death the Earth has ever known. It’s very likely that this event had no single cause; it was set off by a combination of disasters from the physical world and the biological.