The Sixth Extinction: Chapter 4 Summary & Analysis

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.

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.

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.

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.

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.

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.

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).

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.

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.