Chapters 4–5 Summary

In the 1970s, in Gubbio, Italy, American geologist Walter Alvarez discovered an asteroid, which he believed to be the cause of the fifth mass extinction—the one that killed the dinosaurs. Between layers of limestone, Alvarez had found a layer of clay that contained material from meteorites. He sent samples to his father, American physicist Luis Alvarez, who was working at the University of California, Berkeley. The two scientists hypothesized that the six-mile-wide asteroid had hit Earth 65 million years ago. Scientists at the time believed extinction happened slowly, so most rejected this theory, despite a gap in the fossil record.

Ammonites (extinct cephalopods, or marine mollusks) populated oceans for over 300 million years. Though it isn’t clear why they became extinct, scientists theorize that ocean changes caused by the asteroid strike led to their demise.

The Alvarezes continued to find evidence to support their hypothesis. Oil drillers located a large crater. The Alvarezes determined this to be the asteroid’s impact point. With this evidence, the scientific community became more open to the idea of sudden mass extinction.

Everything that is alive today descended from an organism that survived the impact. Scientists describe the likelihood that a species will become extinct as its “preservation potential.” While Darwin’s model of extinction focused heavily on organisms that evolve to better function in their environment, the asteroid catastrophe demonstrates that preservation potential can be meaningless during mass extinction.

In 1962, American philosopher Thomas Kuhn coined the term “paradigm shift” to explain how disruptive scientific information eventually leads to new scientific concepts. Kolbert describes extinction’s modern definition as “long periods of boredom interrupted occasionally by panic.”

In 2000, Dutch chemist Paul Crutzen proposed that humanity is in a new epoch marked by human impact: the Anthropocene. People have affected every part of the planet. Carbon dioxide (CO₂) in the atmosphere is higher than it’s been in a million years and continues to rise, while oceans are more acidic. Crutzen argues that the Anthropocene will result in the sixth mass extinction.

Chapters 4–5 Analysis

The discovery of the asteroid that killed the dinosaurs demonstrates the scientific progress of the last century. Kolbert uses this anecdote to further the main idea that scientific thought is constantly changing and, much like the environment, is subject to long periods of stability interspersed with bursts of discovery. The asteroid theory, not even conceived until the 1970s, is well-established today.

Kolbert develops this idea by showing that until the Alvarezes’ discovery, scientists willingly accepted that there was a hole in the fossil record. Here, Kolbert provides a concrete example of paradigm shift before even explaining the concept. She compares uniformitarianism with catastrophism but ultimately paints them as two sides of the same coin, both important in people’s understanding of how the world has changed.

In Chapter 5, Kolbert continues the idea of the paradigm shift using Kuhn’s theories about how humans adapt to changing environments. This further supports the main idea that scientific discovery on mass extinction has occurred slowly over time, often with each new theory rejected at first and then assimilated into scientists’ general understanding.