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 oldest 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.