The asteroid strike was bad. The timing was worse

Evolution

The Church on Spilt Blood in St. Petersburg, Russia, is like something out of a fairy tale. Perched on the edge of a frigid canal, it has a forest of onion domes that stretches toward the sky and pastel-colored mosaics that cover every square inch of the interior. This is not the type of place where paleontologists typically hang out, but I was in town to study a new dinosaur, and I insisted on taking a detour. The visit was personal. The church was built on the spot where Czar Alexander II was assassinated by revolutionaries in 1881, setting in motion a sequence of events that led, eventually, to me. The czar's death ushered in a frenzy of anti-Jewish pogroms. Jews on the edge of the Russian empire grew frightened, and a family in Lithuania panicked and sent their young son to safety in America. That man was my great-grandfather. If not for that chain of dominoes that began more than 100 years ago in St. Petersburg, I would not be here today.

All families have stories like this one—weird twists of fate in the distant past, without which the present would be very different. Evolution works this way, too. The history of life is one big contingent tale, liable to be rerouted at any time. Indeed, that is precisely what happened 66 million years ago, at the end of the Cretaceous period. For the preceding 150 million years dinosaurs had dominated the planet, growing to colossal sizes and thriving in nearly every conceivable environment on land. But then something changed, and Tyrannosaurus, Triceratops and their kin vanished.

The extinction of the dinosaurs is one of the greatest mysteries in all of science, and it hooked me on science as a teenager. Over the past decade, as I have collected dinosaur fossils around the world, it has gnawed at the back of my mind: How could such successful creatures just disappear? A popular theory, advanced in the 1980s, holds that an asteroid did them in. But skeptics have wondered whether other forces might have contributed to their demise. As researchers discover new dinosaurs and learn more about this group's evolution, we are getting closer to a conclusive answer.

I recently organized a large international gathering of paleontologists who met to hash out exactly what we know about why the dinosaurs went extinct. We used the most up-to-date inventory of dinosaur diversity to examine evolutionary trends over time, reviewed the latest information on the timing of the extinction and took a long look at the many environmental changes occurring around the time the dinosaurs disappeared. To our surprise, our team of nearly a dozen dinosaur experts—often an argumentative bunch—came to a clear consensus: as popular wisdom has it, the extinction was abrupt, and an asteroid was primarily to blame. But that story is incomplete: the asteroid happened to hit during what was already a horrible time for dinosaurs, when their ecosystems were vulnerable because of previous environmental change. It is a new and unexpected twist on an old tale and one that has surprising relevance to the modern world and our own evolutionary story.

An enduring mystery

Like most teenagers, I did some rash things in high school. Perhaps nothing was more brazen than picking up the phone one day in the spring of 1999 and cold-calling Walter Alvarez, a geologist at the University of California, Berkeley. I was a 15-year-old kid obsessed with dinosaurs; he was the eminent National Academy of Sciences member who nearly 20 years earlier had proposed the idea that a massive asteroid impact killed off the dinosaurs. His hypothesis began with a curious observation. The geologic record preserves a thin band of clay that marks the boundary between the dinosaur-dominated sediments of the Cretaceous period, which spans the time between 145 million and 66 million years ago, and the dinosaur-barren sediments of the Paleogene period, between 66 million and 23 million years ago. Alvarez found that the clay band was saturated with iridium, an element that is rare on Earth but common in extraterrestrial bodies such as comets and asteroids. He first noticed this anomaly in a rocky gorge near the medieval commune of Gubbio in Italy's Umbria region. As chance had it, my family was gearing up for a trip to Italy to celebrate my parents' 20th wedding anniversary. I nagged my parents to take a break from the basilicas and art museums and visit Gubbio for a day to see the geologic feature that spawned Alvarez's famous killer-asteroid scenario. But I needed directions, so I decided to go straight to the source.

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That Alvarez not only answered my call but also gave me detailed directions to the very spot in the gorge where he detected the iridium spike still floors me. I did not expect such a scientific giant to be so kind, so generous with his time. His asteroid theory, published in Science in 1980 with his Nobel Prize–winning physicist father, Luis, and two Berkeley colleagues, touched off a decade of frenzied debate. Dinosaurs and mass extinctions were constantly in the news; the impact idea appeared in countless books and television documentaries; and hundreds of scientific papers argued back and forth as to what really killed the dinosaurs, with paleontologists, geologists, chemists, ecologists and astronomers all weighing in on the hottest scientific issue of the day.

By the end of the 1980s it was undeniable that an asteroid or comet crashed into the planet 66 million years ago. The same iridium layer had been found around the world. And other geologic oddities known to stem from extraterrestrial impacts, including blobs of glass called tektites and deformed grains of quartz known as shocked quartz, turned up alongside the iridium. Furthermore, geologists even located a crater dated to the exact moment of the dinosaur extinction—the 180-kilometer-wide Chicxulub Crater in Mexico. Something unexpected and huge, about 10 kilometers across, had arrived from space and triggered a cataclysm of volcanic eruptions, wildfires, tsunamis, acid rain and sunlight-blocking dust, dooming the dinosaurs.

Still, scientists had precious little information on how dinosaurs were changing during the run-up to the impact and exactly how they and their ecosystems responded to this extraordinary environmental disaster. Debate thus continued to rage over whether that asteroid knocked out the dinosaurs suddenly, while they were still in their prime, or whether it delivered a final blow to a moribund group that was gradually wasting away and would have gone extinct anyway. After all, the asteroid did not strike a static planet but one that was experiencing dramatic sea-level fluctuations, temperature shifts and extreme volcanism. Maybe some of these things had factored into the extinction?

Fresh findings

I never made it to Gubbio during that family trip to Italy. Floods closed the main rail line from Rome, and I was devastated. Fate can be cruel (just ask the dinosaurs), but it goes the other way, too. So imagine my surprise when, five years later, I was back in Italy for a college geology field course. We were staying in a small observatory in the Apennine Mountains run by Alessandro Montanari, one of many scientists who made names for themselves in the 1980s studying the end-Cretaceous extinction. On our first-day tour we passed through the library, where a solitary figure was scrutinizing a geologic map under a flickering light. “I want you all to meet my friend and mentor, Walter Alvarez,” Montanari said in his singsong Italian accent. “Some of you may have heard of him.”

A few days later we were in the gorge in Gubbio, the Mediterranean sun beaming down and fast cars whizzing by. Alvarez stood in front of a class of college students, pointing to the exact place where the asteroid theory was conceived. My classmates were ragging on me because after I introduced myself to Alvarez and he remembered our discussion five years earlier, I could not stop smiling. That day is seared into my memory as one of the most important moments of my early career. I knew then that the riddle of the dinosaur extinction had a hold on me.

Somewhat paradoxically, as a graduate student my research focused mostly on the rise of the dinosaurs to dominance and the origin and early evolution of birds (which stemmed from dinosaurs and are thus the only dinosaur group that did not go extinct). But I finally had the chance to contribute to the dinosaur extinction debate in 2012, when I was finishing up graduate school. My colleague Richard Butler of the University of Birmingham in England, who uses statistics to study evolutionary trends, came up with a nifty idea: How about we pool our expertise on different dinosaur groups and different analytical techniques to take a fresh look at how dinosaurs were changing during the 10 million to 15 million years before their extinction?