Part 5, Chapter 22 Summary and Analysis

Extinction seems to be a natural part of the life cycle. During its history, there have been five major extinction episodes on Earth: the Ordovician, Devonian, Permian, Triassic and Cretaceous. In addition, there have been many other smaller ones. During the Permian period, at least 95 percent of all the animals known from the fossil record died out. On average, a species exists for about four million years, which is approximately how long humans have been around.

For most cases, scientists have little idea what killed off species. Prime contributors include changes in global climate, changing sea levels, oxygen depletion of the seas, meteor and comet impacts, huge hurricanes, volcanoes and catastrophic solar flares. There is just too little evidence to determine the exact causes for mass extinctions.

The impact of the KT meteor, which killed the dinosaurs, wiped out...