The Sixth Extinction Background

The “Big Five”

Some general information about extinction is beneficial to readers of The Sixth Extinction. It is typical for plant and animal species to die out, or go extinct. Species go extinct continually, about one species every 700 years, in what is called background extinctions; however, on occasion, mass extinctions occur, in which vast numbers of species go extinct at the same time, within a concentrated time frame. Scientists have identified five major mass extinctions in Earth’s history. Known as the “Big Five,” these extinctions eliminated large portions of Earth’s species in cataclysmic events. They are:

• The Ordovician-Silurian Extinction: The first-known mass extinction began around 443.8 million years ago and had multiple phases, killing around 85% of all species, especially decimating marine species. It is believed that an ice age caused this extinction. It is also theorized that the ice age was caused by the formation of the North American Appalachian Mountains.
• The Devonian Extinctions: The Devonian extinctions included a series of events across a period of roughly 50 million years, some 400 million years ago. Not currently attributed to any one cause but likely the result of several environmental factors, this series of extinctions wiped out between 70% and 80% of animal species.
• The Permian Extinction: The Permian extinction was the most severe. It began around 265.1 million years ago and lasted about 13 million years, but some scientists argue that most extinctions occurred during the last 200,000 years of this period, with the majority in the last 20,000—very fast in geological time. More than 95% of marine species and 70% of land species were eliminated. The causes of this extinction are highly disputed. Theories range from asteroid impact to massive volcanic eruptions.
• The End-Triassic Extinction: The End-Triassic extinction occurred around 201.3 million years ago and eliminated about 76% of all species, perhaps within only a 10,000-year time frame. The causes of this event, too, are hotly contested, with some researchers even arguing it should not be considered a mass extinction at all. This event is generally considered to have allowed for the rise of the dinosaurs.
• The Cretaceous-Tertiary Extinction: The Cretaceous-Tertiary extinction is the most famous. It occurred around 66 million years ago and ended life for most dinosaurs, but other reptile groups survived and flourished, as did many amphibians and mammals. It is likely that the impact of a celestial body—and the dust cloud resulting from it—caused this extinction, though some paleontologists argue that other terrestrial factors, such as volcanic activity, were at least partially responsible.

Charles Darwin’s On the Origin of Species

Charles Darwin (1809–82) was a British naturalist who developed the theory of evolution by natural selection. Darwin spent several years circumnavigating the globe aboard the HMS Beagle, during which he examined fossils, noted geologic land changes, and collected specimens.

After this voyage, Darwin fleshed out his theory that living things—like land and the environment—are always changing. In On the Origin of Species (1859), he wrote that organisms that are better able to adapt produce more offspring and therefore have greater odds of surviving. This constant process of selecting and rejecting attributes to ensure survival propels the evolutionary process.

Author Elizabeth Kolbert uses Darwin’s thinking and influence as a central message in The Sixth Extinction, both to illustrate how his theories left an imprint on science’s understanding of extinction principles and to demonstrate the scientific process. Kolbert is not shy about pointing out holes in Darwin’s 19th-century views, such as his belief that extinction was not caused by catastrophic events but happened slowly.