Delving into Earth's history, Elizabeth Kolbert reveals the profound impact of mass extinctions on our planet. She identifies five major extinction events that have occurred over the past 450 million years, each resulting in the loss of at least 75% of species. These events, triggered by natural causes such as volcanic eruptions, asteroid impacts, and climate change, have shaped the course of evolution and redefined ecosystems.

Kolbert draws attention to the compelling evidence left behind by these mass extinctions. For instance, she highlights the end-Permian event 252 million years ago, which wiped out nearly 96% of marine species and 70% of terrestrial species due to massive volcanic activity. This catastrophic event led to the rise of the dinosaurs, which dominated the planet for millions of years until the end-Cretaceous event – another mass extinction caused by an asteroid impact – 66 million years ago.

Through engaging anecdotes, the author takes us on a journey to explore these extinction events and their lasting impressions on the fossil record. She shares the story of Luis Alvarez, the Nobel Prize-winning physicist who discovered iridium, an element abundant in meteorites, at the end-Cretaceous extinction boundary. This finding provided crucial evidence for the asteroid impact hypothesis.

Kolbert also discusses the significance of the Great Ordovician Biodiversification Event, which saw an explosion of new species and set the stage for future mass extinctions. By examining these past events, she emphasizes the importance of understanding the factors that lead to extinctions and the consequences they have on the natural world.

In summary, the first key idea in Elizabeth Kolbert's work focuses on mass extinctions throughout Earth's history, highlighting their causes, impacts, and the evidence left behind. This exploration of the past sets the stage for a deeper understanding of the current extinction crisis and its potential consequences.