A hundred miles north of Rome, there’s a small town called Gubbio. Gubbio is notable for its beautiful limestone; indeed, there is a massive limestone gorge with steep, smooth walls. It was here, in the 1970s, that a geologist named Walter Alvarez discovered the traces of a huge asteroid—the asteroid, which, scientists later decided, hit the Earth during the Cretaceous period, causing the mass-extinction of the dinosaurs.
The most famous extinction in history is the extinction of the dinosaurs millions of years ago. While many schoolchildren grow up hearing that an asteroid killed the dinosaurs, such a theory was controversial only forty years, and unheard of half a century ago.
Walter Alvarez had come to Italy to study plate tectonics. Beneath the surface of the earth, he found layers of marine fossils. Yet, curiously, he also noticed that there were thick layers of clay, containing no fossils, imbedded in the limestone of Gubbio. Back in California, Walter Alvarez’s father, Luis Alvarez (a Nobel Prize-winning scientist himself), suggested that Walter test the layers of clay for iridium, a radioactive element. Walter found that clay contained huge amounts of iridium, suggesting that the clay may have originated in an asteroid. Further tests showed that there were thick layers of iridium dating back to the end of the Cretaceous period. In 1980, Luis and Walter Alvarez co-wrote an influential paper arguing that an iridium-rich asteroid struck the Earth at the end of the Cretaceous era, killing the dinosaurs.
Walter and Luis Alvarez originated the now-popular theory that the dinosaurs were wiped out by an enormous asteroid that struck the Earth in the Cretaceous period. The asteroid is a classic example of the kinds of global catastrophe that are central to the catastrophist interpretation of extinction. It’s also an example of the process of paradigm shift—new evidence emerged (iridium in clay) that suggested a completely different version of events from what was previously believed, and the Alvarezes created a theory that explained the new evidence.
From the beginning, biologists and paleontologists rejected the Alvarez theory of mass-extinction. They pointed out that extinction is a slow, gradual process, not the result of a sudden catastrophe like an asteroid collision. For more than a hundred years, scientists had known about sudden discontinuities in the fossil record, particularly the marked decrease in the number of fossils in soil layers corresponding to the end of the Cretaceous period. However, most scientists explained this decrease by positing that paleontologists would eventually discover more fossils from the period. Few scientists believed that there had been a sudden, global change at the end of the Cretaceous era—on the contrary, they took a uniformitarian view of the distant past.
Like many ideas that would ultimately prove to be powerful explanations of natural history, the Alvarezes’ theory of the dinosaurs’ extinction was initially met with disdain and indifference. However, the asteroid theory was strong because it offered an elegant explanation for a longstanding problem—the “fossil gap” that coincided with the extinction of the dinosaurs. The asteroid theory killed two birds with one stone—it explained why certain layers of the Earth were rich in iridium, and it explained what happened to the dinosaurs at the end of the Cretaceous period.
In spite of the unpopularity of the Alvarez theory, evidence continued to build that an asteroid had hit the Earth long ago. The first important piece of supporting evidence was the discovery of “shocked quartz” — quartz that has been exposed to sudden changes in pressure — that dated back to the end of the Cretaceous era. The next clue was the discovery of a layer of sandstone, seemingly caused by a huge tsunami wave that dated back to the same period. Finally, scientists uncovered an enormous crater in present-day Mexico, in which there were layers of melted rock from the end of the Cretaceous era.
Like all good scientific theories, the Alvarez theory of dinosaur extinction was “falsifiable,” meaning that further evidence could be offered to support or disprove the theory. In the decades following the appearance of the theory, new evidence surfaced that seemed to support the Alvarezes. As a result, the scientific community came to accept the asteroid theory.
Kolbert meets with Neil Landman, a paleontologist who specializes in ammonites, or Discoscaphites iris (prehistoric, nautilus-like creatures). Landman shows Kolbert a fossil site near Princeton, New Jersey. There is a thick, iridium-rich layer of rock and soil at the site, along with hundreds of ammonite fossils.
Kolbert’s travels to Princeton are important because she sees, first-hand, the consequences of a sudden global catastrophe. The ammonites used to be a successful, flourishing species; after the asteroid hit, however, their useful evolutionary qualities proved to be liabilities.
Ammonites were spiral-shaped mollusks, although there is some debate about what, exactly, they looked like (some scientists argue that ammonites had long tentacles, but Landman maintains that they did not). Ammonites probably evolved their distinctive spiral shells because the shells were capable of withstanding intense water pressure. The ammonite species may have existed for many millions of years, and ammonite fossils can be found in many different parts of the world.
Ammonites had many evolutionarily useful traits. They could swim in the ocean, and they had thick, durable shells that allowed them to survive in many different temperatures and pressures. For many millions of years, ammonites survived; however, the sudden global catastrophe was too dramatic and rapid for the ammonites to prevail.
In their paper on the extinction of the dinosaurs, the Alvarezes argued that the true killer wasn’t the asteroid itself; it was the dust that the asteroid threw into the Earth’s atmosphere. The dust—or, to use the scientific term, “bolide”—was extremely hot, and it moved around the Earth at a phenomenal speed, blotting out the sun and either scorching or smothering most of the planet’s population. Large creatures like the dinosaurs were the first to go, followed by sea creatures and mammals. The mass-extinction of the dinosaurs brings up the concept of “preservation potential”—a way of measuring the likelihood that a species will go extinct. Scientists can approximate a species’ preservation potential by measuring its population, the number of different places where the species lives, and the overall composition of the species (e.g., thick-shelled mollusks have a higher preservation potential than hollow-boned birds).
One reason that it’s hard to measure the preservation potential of a species is that it’s almost impossible to predict the likelihood of different catastrophic events. Thus, an ammonite might seem to have a very high preservation potential, because its shell is thick and strong. However, the arrival of a sudden catastrophe like an asteroid would immediately lower the ammonite’s preservation potential (and, indeed, ammonites went extinct after the asteroid struck). This points to the tremendous uncertainty and vulnerability surrounding mass extinction.
Back in New Jersey, Kolbert watches Landman and his peers investigate the ammonite fossil site. Landman explains to Kolbert that, during the dinosaurs’ extinction, the ammonites died out very quickly. But, oddly, nautiluses—probably the animals that most resemble ammonites—survived and still exist today. Why the difference? Landsman speculates that ammonites produced small eggs that grew into tiny, weak infant ammonites. Nautiluses, by contrast, lay large, robust eggs that can survive sudden changes in temperature or water pressure.
This passage shows the vagaries of natural selection. While some people have interpreted the theory as suggesting linear progress in which nature consistently selects the “best” traits, the story of the ammonites and nautiluses shows that natural selection is more random than that. The mass-extinction at the end of the Cretaceous period turned the small, fragile ammonite eggs—once evolutionarily advantageous for their mobility—into a liability that made the species extinct.
The survival of the nautiluses brings up an important point: every single life form on the planet today is descended from a species that survived the dinosaurs’ extinction. However, the fact that today’s species are descended from species that survived a past extinction does not mean that modern animals are “extinction-proof.” On the contrary, the rules of survival are constantly changing—a creature that survives one mass-extinction might not be able to survive another. The ammonites, for example, had a high preservation potential leading up to the dinosaurs’ extinction (because they laid small eggs that drifted throughout the ocean, increasing the number of places where ammonites lived), However, due to sudden changes in their habitat, ammonites’ evolutionary advantages became huge disadvantages.
Kolbert’s analysis of the Alvarezes’ theory of the dinosaurs’ extinction exemplifies the differences between catastrophism and uniformitarianism. Crucially, it also demonstrates that the two theories can coexist. For many millions of years, ammonites, dinosaurs, and other animals had competed in the slow, gradual process of natural selection. But the arrival of an asteroid altered the natural selection process, making it almost impossible to survive on the Earth. This shows a melding of two paradigms that seemed, at one time, to be at odds—another way that scientific thought can evolve.