The Sixth Extinction: An Unnatural History (2014)

A naturalist and teacher at the Natural Museum of History in Paris, Jean Leopold Nicolas Frederic Cuvier made a name for himself by studying ancient mastodon bones from America and arguing that this lost species, like many others, went extinct because of catastrophic events. By examining fossils, rock formations, and ancient texts, Cuvier further developed his cataclysmic hypothesis. Eventually, the catastrophe that Cuvier said had occurred was proven to have taken place, in the form of the spread of human beings around the world.

Charles Lyell, a geologist and friend of Cuvier’s, did not believe in Cuvier’s cataclysmic theory. Instead, he argued that an extinct species could reappear again in the future. Lyell wrote the Principles of Geology, which inspired a very young Charles Darwin. Lyell’s primary interest was in how new species developed rather than in how species died out. Although many of his theories were disproven, Lyell made a considerable impact on the early years of geological study.

Walter Alvarez was a geologist who found the remains of a giant asteroid that brought the Cretaceous period to a catastrophic end. Alvarez’s study of Foraminifera within the limestone and layers of clay in a gorge in Gubbio, Italy, prompted him to state that the limestone was a historical record of points in time when significant extinction events had happened.

His father, Luis, a Nobel Prize winner, helped him to use iridium to hypothesize the time frames indicated by layers of clay and limestone in the gorge. Both men eventually wrote a paper stating that the asteroid’s impact had caused temperatures to drop rapidly and extinction to occur.