Undeniable (14 page)

You have probably heard terms like Mesozoic Era, Jurassic Period, and the Stone Age. These phrases come from geology. For seven eighths of Earth's history—basically, for most of it—living things were just revving up. Most life was single-celled organisms or relatively simple, soft-bodied animals. The last 500 million years are when almost all of the interesting stuff happened. Pretty much every creature you have ever heard of, from trilobites to dinosaurs to Neanderthals, appeared during that time. All of those years are currently considered to be part of just one single geologic eon, which goes by the wonderful name: the Phanerozoic Eon (Greek for the visible eon, the one we can see). The Phanerozoic is in turn divided into three geologic eras: Paleozoic, Mesozoic, and Cenozoic. That last one is the one we're living in. The names mean, roughly, old animals, middle animals, and new animals (which includes us). Finally, those eras are divided into epochs. Ours is the Holocene or “recent” Epoch, covering just the last 10,000 years—about 1/500,000th of the whole history of the planet. I drew a sketch.

All of those names and dates are essential to the paleontologists who are analyzing the history of evolution. They have painstakingly analyzed the available fossil evidence and assessed the number of living things on Earth, especially in oceans, at different times over the last 500 million years. In the process, they've discovered relatively short intervals of time when the amount of diversity has dropped abruptly. Those are the mass extinctions. I'll talk here about the five we know about. But I hope we'll all soon acknowledge that there really is a sixth mass extinction, one that is happening right now.

The oldest of the mass extinctions is called the Ordovician-Silurian, from the names of the geologic periods when it occurred (444 million years ago). Here we lost about 85 percent of ocean species. At the time, there was essentially no life on land. The ocean was where all the action was. It was the second greatest of the mass extinctions. Next we had the Late Devonian extinction, about 364 million years ago. This time we lost about half of Earth's animals and plants.

The most traumatic episode in life's Phanerozoic history was the Permian-Triassic extinction, which went down 251 million years ago. As a young engineer, I worked in the Permian Basin in Texas. I had no trouble observing the ancient seashells on the ground, right next to the oil-rig plumbing I was servicing. It's the same Permian you may have heard about if you enjoyed the television show
Friday Night Lights
, about the drama that is west Texas high school football. Near as we can tell, Earth lost about 75 percent of all species during the Permian-Triassic extinction. Think of it! At the end, only 25 percent of everything was still alive. You and I are descendants of that lucky minority, the 25 percent.

Where I went to college in central New York State, it's not so difficult to walk in the beautiful shale gorges cut by streams and find a trilobite fossil. In a week, you can find a dozen. It is sobering and remarkable to realize that trilobites lived here for more than 250 million years, from the Cambrian right up through the Permian. Yet they are all, every one of them, gone, extinct. I'd rather that didn't happen to us, at least not for a few more periods. On the other hand, the Permian-Triassic extinction cleared the way for the rise of the dinosaurs. It shows how robust living things are, and how strongly living things can rebound … given enough time, time, time.

Next we have the End-of-the-Triassic extinction, about 200 million years ago. During this period, we seem to have lost about half of the world's species. By this point in the story, it almost sounds routine.

Then at last, we arrive at the most well-known and significant mass extinction, the Cretaceous-Tertiary extinction, 66 million years ago. Geologists like to use single letters to designate the geologic periods. Who wouldn't? It saves a mouthful. As we move from the deep past into the more recent past, we already have two periods that begin with the letter C, the Carboniferous and Cambrian, which are written as C and
C-
. So the Cretaceous came to be abbreviated as K, and the die-off between the Cretaceous and Tertiary came to be called the K-T extinction. The K has a legitimate pedigree: It comes from
Kreide
, the German word for chalk, which is where Cretaceous got its name in the first place. More recently, geologists have given the early part of the Tertiary a more specific name—the Paleogene. So here in the early twenty-first century, we mark the Cretaceous-Paleogene boundary in the geologic record. The mass extinction that included the ancient dinosaurs is called, in abbreviated form, the K-Pg extinction.

By its old or new designation, this extinction is the dramatic one that marked the end of the age of dinosaurs (although I like to remind people that we are still surrounded by the modern descendants of the feathered dinosaurs—the birds). It's famous for most of us who are young at heart and fascinated with dinosaurs. It also, not incidentally, ushered in the age of mammals. It made space for us.

We cannot be absolutely certain what caused any of these mass extinctions, but we have a great many excellent clues. Just as important, we have a great many mathematical models of Earth's climate. We do our best to estimate what it would take to bring about traumas of these magnitudes. We look at the rocks, the fossils, and the chemistry. We run the numbers and come up with very thoughtful hypotheses about what happened. Studying the past is very helpful; it enables us to make better predictions about what could happen on Earth again, perhaps very soon.

We also have much more direct access to the here and now. Looking at Earth from space, as we can nowadays with our sophisticated satellites, we can observe climate as it is changing today. Not only that, but we can compare the climate of Earth with climates of our nearby planetary neighbors, Mars and Venus. We can infer just what it might take to change the climate of a whole planet so radically, and in such a short span of time, that half or even almost all of the living things there die and disappear.

We collect additional clues by studying Earth as a complex living system, a planet-wide ecosystem. If you consider any ecosystem that you've ever lived in—a forest, a city, a farm, or perhaps you've sailed on the ocean for a time—you can see these systems are complicated. Living things interact with their environments in countless ways. When the environment changes quickly, ecosystems change as well. When I worked for Boeing, I spent many wonderful hours hiking and climbing in the Seattle area. In the mountainous western part of North America, you can walk right up and over areas where rockslides have occurred in the last few hundred years. You can see the radical differences in plant life and wildlife between the top of the slide, the bottom or toe of the slide, and the unchanged boundaries, where the trees and fauna are living pretty much as they did before falling rocks crushed and scraped a great many living things down the mountain.

In the case of mass extinctions, it must be like a rockslide on a global scale. What could bring about something like that? For me, there are two things I can imagine right away. Geologists have, too.

The first potential extinction triggers are volcanoes. If you ever get a chance, I strongly encourage you to visit Mount St. Helens National Volcanic Monument in Washington State in the U.S., where entire ecosystems vanished when the mountain blew its top on May 18, 1980. Countless birds, fish, insects, and hundreds of large animals like deer, voles, and raccoons were killed in an instant. All evidence of them was either buried under thousands of tons of massive icy, rocky, mud, or it was incinerated, burned to a crisp. If you can, go to Hawaii to see the state's volcanoes oozing red-hot molten rock. There's no stopping a lava flow; it incinerates everything in its path. Imagine dozens or hundreds of volcanoes spewing ash and fire over enormous areas of the planet's surface. The result could be such an abrupt change in Earth's atmosphere that no living thing, especially no complex system of living things, would be able to stay alive.

We know that mass volcanic eruptions occurred because we can still see the giant lava flows that resulted. These bursts of volcanism are comprised of the distinctive rock called basalt. When it cools, it often forms enormous blocks with right angles like gigantic grains of table salt. The lava flows are so expansive that geologists call them flood basalts. Some of these enormous flows could have erupted even under the sea surface—such as the giant Kerguelen Plateau in the south Indian Ocean—radically changing the oceanic and atmospheric chemistry. An enormous outpouring of lava in what is now Siberia is currently considered the best explanation for the devastating Permian-Triassic extinction. Researchers are blaming volcanoes for the end-Triassic extinction, too. But keep in mind: We're investigating an old, old crime scene.

In the Deccan region of what is now India, there is an enormous zone of volcanic rocks. The zone is bounded by India's east and west coasts and the Vindhya Mountains. Outcroppings of these layers of rock resemble stair steps, and the Scandinavian word for stairs is
trapp
, which when shortened is trap, so geologists embraced it. At the Deccan Traps we find several layers of rock that cover an area of 500,000 square meters (200,000 square miles). They comprise 1.2 cubic kilometers of lava. It was one hell of an eruption or series of eruptions.

In between the layers of frozen lava in the Deccan Traps are layers of sediment laid down by ancient seas. Geological dating of those layers shows that they formed between 60 and 68 million years ago and that the eruption(s) reached a peak around 65 million years ago—about the same time the ancient dinosaurs met their fate.

Could these flood basalts have something to do with the demise of the ancient dinosaurs? Some geologists, like Princeton University's Gerta Keller, have made a strong case. After some fieldwork in the area, she said, “It's the first time we can directly link the main phase of the Deccan Traps to the mass extinction.” She was talking about the ancient dinosaurs, et al.

When Keller studied the fossils in ancient sediments near the Deccan Traps, she saw that the biodiversity of foraminifera (a broad class of aquatic microorganisms) took a tumble right around the time of the eruptions. Apparently, there were at least significant local extinctions. The powerful volcanoes that built the Deccan Traps must have spewed toxic gasses and created enormous layers of atmospheric dust; those emissions reflected sunlight into space cooling Earth somewhat. During other episodes, volcanoes shot out tons of greenhouse gasses, heating the world up in a hurry. It was climate change with whiplash.

Even a less violent form of geologic change could have been devastating to life. For instance, the shifting of the continents and shorelines could have sent the global climate into an inhospitable new state. That seems to have happened in the Ordovician-Silurian, when most of the land on Earth was part of a single supercontinent that migrated to the South Pole. During this period Earth cooled, enormous glaciers formed, sea levels plummeted, and a lot of ocean life was left high and dry. But some extinctions seem to have happened quickly, not just in geologic terms but in human terms as well.

Which brings me to the second big extinction-driver: asteroids. If Earth is struck by an asteroid, everything can change in a flash. Such an event may have brought on the K-Pg extinction, when things seem to have changed far too quickly to explain with a volcanic eruption—at least, not with a volcanic eruption alone. The scientific consensus today is that the main blow to the ancient dinosaurs was a 10-kilometer-wide rock that struck our planet off the coast of what is now Mexico. The result is still faintly visible as a 180-kilometer-wide crater named Chicxulub (say it Mayan style: “CHIK-suh-loob”). It means “devil's flea.” I guess they can be ornery. Some earlier extinctions may have also been the result of an asteroid or group of asteroids hitting the ocean, leaving little evidence for us to find.

One afternoon, I met Walter Alvarez, the scientist who developed and championed the theory that an asteroid triggered the K-Pg extinction. We had a delightful lunch. He is a thoughtful, enthusiastic guy who loves to teach. He also has a rare ability to look at the world and see things that other people don't. He's a bit like Darwin in that way. Walter and his father, geologist Luis Alvarez, proposed the asteroid impact idea in 1980, at a time when most of his colleagues thought that impacts were unimportant in the history of geology. The theory was considered highly controversial. Since then it has been carefully evaluated. It is now considered, by almost all accounts, very reasonable and likely.

When Earth was forming and was comprised of molten minerals and metals, the heavier materials sank to the center. Geologists would not have expected to find much iridium—a hefty element, atomic number 77—in the rocks at the surface. And generally they do not, but it does show up in one distinctive layer: in rocks that are 66 million years old, formed just at the time of the extinction. Alvarez reasoned that the iridium came from an asteroid, because asteroids are rich in iridium compared to Earth's crust. Apparently the asteroid hit near Chicxulub and disintegrated, spreading its debris all around the world.