If this eruption could create one of Earth’s biggest oceans, just imagine the high volumes of carbon, methane, and sulfur it was pumping into the water and the atmosphere. A superhot greenhouse gripped the planet as the Triassic wound to a murky close. Whiteside ticked off the deaths that followed: As the temperatures climbed higher, the world-spanning tropical forests of the Triassic dried out and succumbed to enormous wildfires. The burned remains of forests slipped into the oceans along with carbon-rich soil. The oceans became acidic, which led to anoxia and die-offs there. Coral reefs were the first to go, and their deaths set off a cascading effect where anything that fed higher in the food chain died too. It was the perfect storm for destroying food webs, starting in the oceans and creeping up onto a warming land whose trees were being eaten by fire. Once again, climate change was killing the world.
The rise (and fall and rise) of the dinosaurs’ world
There are many well-preserved plant fossils from this era, so it’s possible to visualize how the extreme greenhouse conditions changed the environment between the Triassic and the Jurassic. Jennifer McElwain, a paleobotanist at University College Dublin, has spent years studying this transition in Greenland, excavating everything from leaves and flowers to microscopic bits of pollen, to reconstruct the world where dinosaurs ultimately triumphed. Today, coastal Greenland is hard tundra that’s too cold for trees, but in the late Triassic and early Jurassic, it was full of lush vegetation. McElwain called it “a cross between New Zealand conifers and the Florida Everglades.” It was a world of “broad, meandering rivers” and “big, wide floodplains” bordering forests full of towering trees and stubby, thick-trunked plants called cycadeoids with palmlike fronds bursting from their tops. And then came CAMP, with its carbon emissions and rising global temperatures.
Tens of thousands of years of greenhouse conditions led to fire after fire. Ultimately, McElwain believes, the environment of diverse trees, shady forests, and thick vegetation was reduced to swamps full of ferns. “There would have been ferns as far as the eye can see, with hardly any trees, and lots of fire,” McElwain said. There was no complex, multitiered canopy in the forests, so the landscape would have been much brighter. But within another 100,000 years, the region went back to being conifer-dominated. What emerges from this fast-motion vision of ancient forests rising, burning, and rising again is something approaching the truth of where the dinosaurs began. They were among the only survivors of radical environmental changes that drove their competitors to extinction. Most crurotarsans were extinguished in the burned threads of food webs, but those small, early dinosaurs were able to spread out and adapt to the new environments and continents.
When forests at last returned to the land, dinosaurs evolved to be much larger. They diversified into armored herbivores like triceratops and plate-backed stegosaurus, sneaky scavengers, and predators like the 40-foot-long T. rex that we’ve seen in movies from the 1933 version of King Kong to Jurassic Park. Dinosaurs were as diverse as mammals are today, and their behavior probably varied a great deal from species to species. Many of them walked on two legs, with body postures similar to birds—their heads would have been thrust far forward, their spines nearly horizontal, and their tails held out stiffly behind them rather than dragging on the ground. Indeed, most paleontologists today accept that birds evolved from therapods, a group of bipedal, feathered dinosaurs that included the infamous T. rex. If you ever want to imagine what it would be like to face down a dinosaur, imagine a hulking, 40-foot-long crow whose beak has become a toothy mouth.
Recent evidence suggests that many dinosaurs weren’t feathered in quite the way birds are today. Most had dark gray or reddish proto-feathers (often called dinofuzz) that looked something like spiny down. Indeed, dinosaurs may have had proto-feathers for millions of years before birds evolved the ability to fly. Also like their bird relatives, many dinosaurs made nests and laid eggs. Though it’s hard to piece together how these different Cretaceous animals might have behaved, some paleontologists theorize that they may have been social, like birds, forming flocks and possibly mating for life.
What we do know is that when the catastrophes of the Cretaceous period hit, dinosaurs were in a position similar to their old rivals, the mega-gator crurotarsans. A lot of dinosaurs had evolved into specialists, and were therefore deeply connected to food webs that were all too easy to unravel with a few shifts in global temperature. This time, a group of mouse-like, furry animals called mammals—the descendants of the Permian survivor Lystrosaurus—were the survivors.
The Earth these mammals began to colonize with their strange paws and non-feathered faces had come into being through extremely complex events, whose true impact can only be measured in tens of millions of years. Environments had died and been reborn from the effects of liquid rock deep in the Earth and flaming balls from space; the mixture of gases in the atmosphere had been altered by microbes, mountains, and plants; temperatures had fluctuated between extreme icehouses and greenhouses; and the very shape of the planet’s landmasses and oceans had transformed quite radically dozens of times. If there had been a paleogeologist among the last of the dinosaurs, she could hardly have pinned the blame for her peers’ demise on any single factor. The entire ambiguous history of the planet would have had to stand trial for murdering brachiosaurus and letting a bunch of little monkeys take over.
The dinosaurs who survived
Adding to our paleogeologist dinosaur’s conundrum would be another issue, which is that the dinosaurs didn’t entirely die out. An evolutionary offshoot of therapods—birds—survived into the present to become one of the most successful animal classes on the planet. They are highly diverse, exhibit a wide variety of social behaviors, and undertake some of the most incredible migratory journeys of any creatures we know. Of course their dinosaur forebears are extinct, much the way humans’ forebears are. But the dinosaur evolutionary line appears to have continued unbroken.
Perhaps the single most common misconception about dinosaurs among humans is that these creatures and their world have disappeared. Like mammals, dinosaurs are survivors. But their children, who flash through the skies and leave us in awe, are so different from their ancestors that we find it hard to draw a connection between them. What we should ponder, as we move from geological history into the world where human evolution takes place, is whether our understanding of survival is as clouded as our understanding of dinosaurs.
The dinosaurs survived two mass extinctions, but the crows who like to hang out in the tree next to my house are nothing like those dog-sized dinosaurs who beat out the crurotarsans. In fact, it’s not entirely accurate to say the crurotarsans have been pushed off the environmental stage either. Are we not witnessing a strange tableau of survival whenever a bird alights on the head of a crocodile, bringing together the evolutionary offspring of Triassic and Jurassic? Instead of saying the dinosaurs died out, it might be more accurate to say that dinosaurs changed.
Can humans possibly expect to remain unchanged as we face the next mass extinction? History suggests that it’s unlikely. But if survival means that our species will evolve into creatures like ourselves, but with new abilities—like, say, flight—that’s not so bad. Some would even call it an improvement. Survival may be far weirder, and better, than we ever imagined.
5. IS A MASS EXTINCTION GOING ON RIGHT NOW?
IN THE OREGON high desert, a dark, fissured ridge bulges above the broad expanse of dusty ground and blue-green scrub. Observed from a hundred yards away, it looks like nothing more than a rocky outcropping. But viewed from the perspective of thousands of years in the past, it’s a landmark of incredible import. Hiding beneath the brows of this ridge are the wide, low entrances to the Paisley Caves, generous shelters that humans used as a rest stop for thousands of years. Over the past decade, the University of Oregon archaeologist Dennis Jenkins has led excavations in these caves that unearthed evidence of human habitation dating back over 14,000 years. That makes this one of the oldes
t known human campsites in the Americas.
The Paisley Caves mark a significant moment in human history. Many scientists identify the people who first came here as harbingers of a new mass extinction, authored by Homo sapiens, that’s started to accelerate during the past three centuries. The remains that litter the Paisley Caves include bones from some of the first animals that humans may have driven to extinction: mastodons and mammoths (often dubbed “megafauna” or “megamammals”), as well as American horses and camels. For millions of years, such creatures had dominated the continents’ vast plains and forests; soon, humans would claim these environments as their own. When humans were building cooking fires in the Paisley Caves, our species was on the verge of becoming populous enough to push other creatures out of their native habitats. Over the next few millennia, the population exploded. Humans invaded new habitats, pushing the bigger mammals out. We also killed these animals outright. Megafauna were a big part of the Neolithic diet. We find evidence of this in charred, gnawed bones that early settlers left behind, as well as in cave paintings that depict mammoth hunts.
But at the time humans first spent the night in the Paisley Caves, megafauna roamed the Oregon mountains and the environment was much lusher and wetter than today. The Americas, and indeed the planet, had not yet been significantly transformed by human incursions. Jenkins described what he imagined was a typical view from the caves 14,000 years ago. The vast, dusty plains around the cave mouths today would have sparkled with water, where camels and mastodons came to drink. Even then, the caves wouldn’t have made an ideal village—they were too far from the water’s edge. “People would come to these caves periodically, but it wasn’t home,” Jenkins explained. It was a Neolithic rest area between two swampy regions that were packed with food and water. Enough people traveled between the two areas for thousands of years that their overnight stopovers left layers of detritus from campfires, tools, and waste. These caves are evidence that humans were trekking all over the place 14,000 years ago. While their brethren back in the lands that later became Syria and Turkey were erecting some of the first temples and proto-cities, these people were the first explorers in rich, uncharted land.
As humans spread across the American continents, starting from boats and coastal outposts along the Pacific Rim and working their way inland, they pushed the native wildlife out. By 10,000 years ago, most of the American megafauna were dead.
UC Berkeley biologist Anthony Barnosky has been at the forefront of research into megafauna extinctions and their relationship to a possible sixth mass extinction today. A careful scholar, he’s also an activist who is as at home talking to environmentalists on Twitter as he is in the pages of the prestigious journal Nature. He believes that the signs of mass extinction are all around us, and have been for millennia— which is really the only scale on which we can measure mass extinctions anyway. This sixth extinction began with the megafauna, which Barnosky believes weren’t simply victims of human hunting and expansion—there was also climate change from a minor ice age called the Older Dryas that would have decimated the beasts’ favored grazing grounds. If we are in a mass extinction, he concluded, it was kicked off by a “synergy of climate change and humans … the combination was evidently very bad.” Over the past few centuries, industrialization and human population explosions have changed the landscape further. There is ample evidence that we’ve driven dozens of species to extinction. But can we really call this a mass extinction, comparable to the end of the Cretaceous or Permian?
In a widely read article published in the March 3, 2011, issue of Nature, “Has the Earth’s Sixth Mass Extinction Already Arrived?” Barnosky and many of his colleagues (including the statistician and paleontologist Charles Marshall) argued that we can. In it, they explained that the extinction rates on Earth today are far above the background rate. If today’s endangered species all go extinct, our planet will be in the grip of a mass extinction within the next 200 years. Within 1,000 years, Earth might be a world as changed as it was after each of the previous mass extinctions we’ve discussed. The problem, Barnosky admits, is trying to pin down whether we’re in the middle of a mass extinction when such events are usually measured on an extremely long timescale.
“I think we’re on a leading edge,” Barnosky told me. “My take on it is that we’re actually not far into it. A true mass extinction is losing seventy-five percent of species that are recorded. We’ve lost maybe one or two percent of those we can count. So everything we want to save is still out there.” Still, he cautioned, the big problem is not our world right now but the world we’re heading toward over the next century. Looking at the data, he and his colleagues believe that extinction rates for mammals are far above the typical extinction numbers we’d expect for a background rate. “It’s happening too fast,” he sighed. “We’re somewhere between three and twelve times too high.” Given that humans are only likely to expand our territories further into those of endangered animals, he expects these numbers to grow. And when you add in all the carbon we’re pumping into the atmosphere, it’s possible that we’re re-creating the conditions that led to previous mass extinctions.
Peter Ward, a geologist at the University of Washington, who has written about mass extinctions in several books, including his influential work The Medea Hypothesis, believes carbon emissions mean that environmental change is almost inevitable. “We’re going back to the Miocene,” he said, then laughed darkly. The Miocene, a geological age that ended roughly 5.3 million years ago, was the last time that the planet had no Arctic ice cap. It was a period of intense heat when greenhouse conditions reigned and our hominid ancestors had not yet evolved. Though many animals might thrive in the Miocene climate, humans wouldn’t. We are the products of a cold Earth, just like many of our mammal brethren. “We need to keep those ice caps,” Ward said.
The question for scientists like Barnosky and Ward is whether somebody living millions of years from now could look back on our own geological period, the Quaternary, and say that it ended with the sixth mass extinction event on our planet. If so, that would put humans in a class with cyanobacteria as the only life-forms that ever single-handedly brought on an environment-changing event with widespread deadly effects. However, as we’ve seen from looking at previous mass extinctions, it’s impossible to pin the blame for such an enormous event on just a single catastrophe—or a single species’ meddling.
If we are in the early days of a mass extinction, the main thing that sets it apart from the five previous ones is the presence of a species that has the ability to stop it. We are tenacious survivors, incredible inventors, and we’ve demonstrated an ability to plan for the future collectively—even, sometimes, for the good of all rather than the good of the few. One of our most powerful skills in making those plans is our knowledge of history. Not only have we kept records of human history for thousands of years, but we’ve also developed scientific methods of discovering what happened to the planet before we evolved. The geological history we’ve just shot through at top speed is full of information about the kinds of life-eradicating dangers that Earth has confronted over and over. By remembering this history, we can make informed decisions about what to do next in order to ensure our survival as a species.
In the next section of the book, we’ll explore how humans have already made it through tens of thousands of years of environmental catastrophe, disease, and famine. With each blow to our species, we’ve crafted better and better methods of surviving.
PART II WE ALMOST DIDN’T MAKE IT
6. THE AFRICAN BOTTLENECK
MOST OF US are familiar with the basic outlines of the human evolutionary story. Our distant ancestors were a group of apelike creatures who started walking upright millions of years ago in Africa, eventually developing bigger brains and scattering throughout the world to become the humans of today. But there’s another story that has received less attention. Advances in genetics have given us a sharper understanding of what happened between the “wal
king upright” and the “buying the latest tablet computer” chapters of the tale.
Written into our genomes is the signature left behind by an event when the early human population dwindled to such a small size that our ancient ancestors living in Africa may have come close to extinction. Population geneticists call events like these bottlenecks. They’re periods when the diversity of a species becomes so constrained that evidence of genetic culling is obvious even thousands of generations later. Sometimes the shrinking of a population is the result of mass deaths, and indeed, there is evidence that humans may have been fleeing a natural disaster when we walked out of Africa roughly 70 thousand years ago. But our species probably experienced multiple genetic bottlenecks beginning as far back as 2 million years. And those earlier bottlenecks were caused by a force far more powerful than mass death: the process of evolution itself.
In fact, the African bottlenecks are an example of the paradoxical nature of human survival. They provide evidence that humans nearly died out many times, but also tell a story about how we evolved to survive in places very far away from our evolutionary home in Africa.
The Fundamental Mystery of Human Evolution
Given our enormous, globe-spanning population size, humans have remarkably low genetic diversity—much lower than other mammal species. All 6 billion of us are descended from a group of people who numbered in the mere tens of thousands. When population geneticists describe this peculiar situation, they talk about the difference between humanity’s actual population size and our “effective population size.” An effective population size is a subgroup of the actual population that reasonably represents the genetic diversity of the whole. Put another way, humanity is like a giant dance party full of billions of diverse people. But population geneticists, elite party animals that they are, have managed to find the one ideal VIP area that contains a small group of people who very roughly capture the diversity of the party as a whole. In theory, that room contains the party’s effective population size. If they all started randomly having sex with each other, their children might loosely reproduce the diversity and genetic drift of our actual, billions-strong population.
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