The Sixth Extinction
Il Sole 24 (Italy), January 13, 2008
Last fall, the organizers of the Rome Science Festival asked me to come give a lecture about mass extinctions--about how our planet has experienced huge die-offs in the past, and whether it may be on the verge of another die-off today. Only after I began to prepare my talk did I realize how fitting it was to speak of such matters in Italy. For it is in Italy that some of the most important advances in understanding extinctions have taken place.
The first advance was the most simple and yet the most profound: the realization that rocks store a record of life’s history. In 1666 two fisherman caught a giant shark near Livorno. It was so extraordinary that it was delivered to Florence to be dissected by the great anatomist Nicholas Steno. As Steno cut apart the enormous fish, he was struck by how much its teeth resembled mysterious objects known as tongue stones. Tongue stones were triangular pieces of rock that had been known for centuries. They were generally thought to be “sports of nature.” Geometrical forms somehow impressed themselves on rocks, producing both the repeating structures of mineral crystals as well coils, stripes, or--in the case of tongue stones--triangles.
But Steno had a different idea. He wondered if the tongue stones were petrified teeth of long-dead sharks. In the twenty-first century, this seems like an inevitably obvious conclusion. But in the seventeenth century it was nothing of the sort. The journey from a shark’s mouth in the ocean to a rock on dry land was hard for most of Steno’s contemporaries to imagine. Fortunately, Steno lived during the scientific revolution, when some alchemists and natural philosophers were beginning to argue that matter was made up of invisible corpuscles--what we would call molecules and atoms. Steno argued that after animals died their teeth, bones, and shells could be transformed, corpuscle after corpuscle, into stone. As animal remains slowly fossilized, they were incorporated into rocks.
Steno traveled through the Italian countryside to ponder how this might happen. Gazing at cliffs and hillsides, he concluded that all rocks and minerals were originally fluid. Floating on the surface of the planet long ago, they gradually settled out of the ocean and formed horizontal layers, with new layers forming on top of older ones. Molten rock sometimes intruded into the rocks, spreading out on the surface of the Earth to form new layers of their own. As these rocks formed, they buried animal remains, allowing them to be transformed to fossils. Thus Steno could explain how, for example, a sea shell could end up on top of a mountain.
Later generations learned how to read those rock layers, to discover their relative ages, and to use the fossils they contained to create a history of life. And they were astonished to discover that some fossils belonged to species that no longer existed. In Italy, for example, fossils of elephant-like creatures were discovered in the eighteenth century. Georges Cuvier, the founder of modern paleontology, pointed out that elephant fossils were found in many places where no elephant lives today. Cuvier was also famous for being able to distinguish species based on subtle features. He demonstrated that the teeth of fossil elephants found in places like Italy were different from the teeth of elephants today. In fact, they were different enough to belong to a different species--a species, he claimed, that no longer exists. It had, Cuvier declared, become extinct.
Extinction, nineteenth-century paleontologists realized, was a dominant feature of the history of life. The fossil record is made up of mostly species that emerge and then disappear. Many of those species were nothing like any creature alive today. Scientists could divide the history of life into eras based on how emergence and disappearance of species. The Mesozoic Era, for example, stretched from 250 million years ago to 65 million years ago. The end of the era was marked by the extinction of giant dinosaurs, marine reptiles, and other strange creatures we don’t see around us today. Their place was taken by mammals, including our own primate ancestors.
Up until 30 years ago, most scientists believed these transformations were unimaginably slow. Extinction rates rose over the course of many millions of years and then subsided again. But the rocks of Italy once again revealed an important clue to the nature of mass extinctions: they can happen very quickly.
Walter Alvarez, a geologist at the University of California at Berkeley, found that a gorge near the town of Gubbio contained rocks that had formed precisely at the end of the Mesozoic Era. When he and his colleagues analyzed the chemistry of the rocks, they were surprised to find it loaded with a rare element called iridium. Alvarez proposed that an iridium-rich comet or asteroid had crashed into Earth 65 million years ago, and the impact showered iridium-laced dust around the planet.
Alvarez inspired a generation of geologists and paleontologists to search for evidence of the impact. We know now exactly where the impact took place--along the coast of the Yucatan. We now know that the impact triggered tidal waves, global forest fires, and environmental havoc. And most experts generally now agree that the impact was at least partly responsible--or perhaps entirely so--for the mass extinctions at the end of the Mesozoic Era. In a geological instant, half of all species on Earth became extinct.
The discovery of a dinosaur-killing impact prompted scientists to take a fresh look at how rapid environmental changes can cause biodiversity to collapse. Today, the most intriguing of these mass extinctions is the one that struck 250 million years ago, ending the previous era, known as the Mesozoic. It’s sometimes called the Great Die-Off, because it claimed over 90 percent of all marine animals and over 70 percent of species on land.
While many sites around the world preserve rocks from this age, some of the best-studied are, once more, in Italy--in particulary, in Butterloch Canyon in the Dolomites. These rocks record an epic of unparalleled ecological collapse. On land, for example, huge forests disappeared, replaced by vast expanses of weedy plants called club mosses that dominated ecosystems for millions of years.
In 2003, a team of scientists claimed to have found evidence that another impact coinciding with the Great Die-Off. But since then, other researchers have not been able to confirm the finding, and in general they remain skeptical. They don’t know exactly what caused the mass extinctions, but the strongest candidate is a colossal flow of lava that covered much of Siberia. It released heat-trapping gases, such as carbon dioxide and methane that may have created a stifling climate. The warmth then caused the oceans to become starved of oxygen.
Some researchers argue that these conditions triggered an explosion of a certain type of bacteria, known as sulfate-reducers. These microbes live today in swamps and ocean muck; they produce hydrogen sulfide, which creates the noxious odor of rotten eggs. Hydrogen sulfide also highly toxic. It’s possible that this gas killed or at least seriously harmed marine organisms. The gas that escaped the ocean may have harmed life on land directly. Or it may have wreaked havoc by helping to destroy the ozone layer. In the Dolomites, as well as in other places in Europe, Asia, Africa, and North America, paleontologists have found a huge number of deformed fern spores coinciding with the Great Die-Off. It’s possible that they mutated because the ozone layer had disappeared, allowing cosmic rays to reach the surface of the Earth.
This research has deep implications for the future of life on Earth. For the first time in the history of life, a single species is driving many others extinct. Humans are hunting species to oblivion, decimating them with pollution, and destroying their habitats. Scientists estimate that the current rate of extinctions is a thousand times higher than the ordinary rate of extinctions recorded by fossils. Some projects foresee the rate reaching ten thousand times higher.
Missing from many of these projections is the effect of global warming. We are now playing the part of volcanoes, putting heat-trapping gases into the atmosphere at a rapid rate. As temperatures rise, biologists are finding that the ranges of species are shifting. In some cases, the ranges may shrink, leaving species more vulnerable to extinctions. Estimating how many extinctions will occur thanks to climate-driven range shifts is difficult, as scientists are hobbled by many uncertainties. But the Intergovernmental Panel on Climate Change estimates that twenty to thirty percent will likely be at high risk of extinction if the global mean temperature rises 2 to 3 degrees C. That toll would come on top of the one we’re already inflicting by other means.
But the fossil record shows that to understand the full impact of what we’re doing to life on Earth, we must think on a bigger scale. Changing the Earth’s climate isn’t a simple matter of adjusting a thermostat. Many unexpected things can come from warming the planet. In ten thousand years, will our descendants have to defend themselves from a planetary miasma of sewer gas? Will we trigger another Great Die-Off? We can’t say, but we also can’t eliminate the possibility. It’s easy to ignore these possible long-term effects of what we do today. Politicians are not thrown out of office for what will happen in a thousand years. Indeed, the problem is neurological: our brains are not well-adapted to think more than a few years into the future.
But if Steno could look at the hills of Italy and begin to understand that how shark teeth could be turned to stone in the past, then we can at least try to understand that our impacts on life will linger long after we are dead.
Copyright 2008 Carl Zimmer