Much as we like to think our legacy will be the sum of our great works of art and science, humans might go down in geologic history as the force behind a tiny, extraordinary line of dirt. Even a casual observer of the fossil record looking back 100 million years from now could not miss the stark and sweeping decline in biodiversity that shows up alongside the advent of Homo sapiens. It will look like the fossil demarcation caused by the asteroid that killed the dinosaurs. Only this time the asteroid is us. We're a geologic force, careening through natural history.
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When all of human civilization thus far has been "compressed into a layer of sediment not much thicker than a cigarette paper," as Elizabeth Kolbert describes it in her excellent new book "The Sixth Extinction," what will a future paleontologist (human or otherwise) see? A mass die-off the likes of which has only occurred five other times in the last half billion years.
In that time, extinction has been relatively rare. Species disappear at a "background extinction rate" all the time, usually so low that you'd be unlikely to witness one in your lifetime. But so far scientists have discovered only five times when that rate exploded, wiping out life-forms tens of thousands of times faster than normal, over the course of just a few hundred thousand years — a geologic instant. We're living through just such a period of precipitous decline now. "The Sixth Extinction" shows us that it might be our fault.
Traversing four continents and animating scientists both living and long dead, Kolbert's narrative can be mesmerizing and awe-inspiring. It's also a bit terrifying. As evidence of our role in the current mass extinction event mounts, Kolbert illuminates this scientific mystery with a mix of history and field reporting. She weaves together the story of biological calamity, from the concept's first articulation in revolutionary France to the front lines of numerous extinctions today. Tellingly, these stories traverse land and sea, from remote Oceania to the author's own backyard.
The catastrophic events that did in today's fossils were caused by a variety of factors — climate change, ocean acidification, an extraterrestrial collision — but today's culprit appears to be, in the words of scientists David B. Wake and Vance T. Vredenburg, "one weedy species."
It's not exactly a whodunit, although the Homo sapiens we find behind the trigger of our current mass extinction event is unfamiliar. Sort of like the portrait you get of someone by only looking at their trash. It's not how we like to see ourselves, but maybe this mirror is more honest. By spewing greenhouse gases, altering great swaths of land, and shuffling species around the globe with heretofore unheard-of speed, we're effectively running geologic history in reverse.
Ours is a "mass invasion event," in the words of invasive species specialist Anthony Ricciardi. The McGill University professor describes our remixing of the world's flora and fauna as "without precedent" in the planet's history. During any given 24-hour period, it's estimated that 10,000 species ride around the world just in the ballast water that stabilizes large ships.
We're creating what some biologists call the New Pangea, referring to the ancient landmass home to all terrestrial species until plate tectonics broke that supercontinent apart. With the continents spread out as they are today, evolution ran its course in parallel — isolated islands like Australia and Madagascar developed entirely different creatures than Africa and Asia, which were different still from Europe and the Americas. What we're doing today is having disastrous results for biodiversity.
In her four years of research for "The Sixth Extinction," Kolbert found evidence of past and present extinction events everywhere. There's Panama's El Valle Amphibian Conservation Center, where a meticulously maintained refuge for imperiled frogs resembles "an ark mid-deluge." In Vermont, a limestone cave where bats have likely hibernated by the hundreds of thousands since the end of the last Ice Age is littered with tiny brown corpses.
Fungal invaders are to blame for the plight of the Central American frogs and the bats of New England — in the latter case, the aptly named Geomyces destructans causes "white nose syndrome" in bats, which has spread halfway across North America since its discovery in 2007. Like the chytrid fungus wreaking havoc on amphibians worldwide, Geomyces destructans has now spread to corners of the globe where native species have no natural defenses against it. Their best chance is more human intervention, as in the scientific fortress at El Valle, now home to the last remaining Panamanian golden frogs.
Kolbert observes with typical shrewdness that the frog is, or was, a lucky symbol in Panama: "its image is (or at least used to be) printed on lottery tickets." It disappeared from the vicinity of El Valle, an area that includes Thousand Frog Stream, faster than local gift shops could replace their golden frog tchotchkes.
Kolbert, a staff writer for The New Yorker, explored climate change in her last book, 2006's "Field Notes From a Catastrophe," which put her on the map as one of our best science writers. Climate change features prominently in "The Sixth Extinction." We're forcing species to move faster than many can muster and lowering the pH of the world's oceans, quite literally dissolving the underwater rainforests that are coral reefs.
But the fallout of global warming alone is not enough to rank the current extinction among the Big Five. In that sense, Kolbert makes a good case that our role as the ambivalent villains behind the biggest story on Earth is due to something more fundamentally human.
Our outsized influence on the planet's natural systems might seem like a modern affliction. But against the backdrop of the 3.5-billion-year history of life on Earth, ancient human history and the present day are practically indistinguishable. Every continent except Antarctica was once home to its own set of massive mammals — beavers the size of bears, wooly rhinos, mastodons — predecessors of the present-day victims of an ongoing "megafauna extinction." Our modern poisons pack more punch, but we killed off many of the biggest mammals the Earth has ever known simply by hunting them with Stone Age tools. Their evolutionary strategy was to grow "too big to quail," an advantage for which they endured long gestation periods between reproduction. Even primitive human predation was a huge pressure on slowly reproducing species.
"From an earth history perspective," Kolbert writes, "several hundred years or even several thousand is practically no time at all. From a human perspective though, it's an immensity. For the people involved in it, the decline of the megafauna would have been so slow as to be imperceptible."
Kolbert draws out the humor and the heroism of the great lengths some conservationists take to preserve endangered species; can we both praise and pity the scientists who administer ultrasounds elbow-deep in a Sumatran rhino's rectum, or manually coax DNA from captive Hawaiian crows? But she does not spend much time expounding on whether they represent a real way out of the existential crisis we've created for wildlife around the world. Jon Mooallem's wonderful "Wild Ones" (2013) devotes several hundred pages to this topic, teasing out our complicated response to the destruction of wilderness by our own hands.
Kolbert is reluctant to offer the promise of an escape. The real revelation of "The Sixth Extinction" is much darker. Its 13 chapters zero in on species past and present that are emblematic of some aspect of mass extinction events. Their subjects become more familiar as the chapters progress, ending in our evolutionary brethren — Neanderthals — and hinting at the fate of Homo sapiens. Maybe we have always been out of harmony with nature. In this context we start to see humanity as an aberration.
"As soon as humans started using signs and symbols to represent the natural world," Kolbert writes, "they pushed beyond the limits of that world."
In this geologic instant, we are the witnesses to and agents of something truly remarkable. Should we be humbled or horrified?
Chris Bentley is a freelance journalist based in Chicago.
"The Sixth Extinction"
By Elizabeth Kolbert, Henry Holt, 320 pages, $28