Paleontologists have identified five points in Earth’s history when, for whatever reason (asteroid impact, volcanic eruptions and atmospheric changes are the main suspects), mass extinctions eliminated many or most species.
The concept of extinction took a while to sink in. Thomas Jefferson saw mastodon bones from Kentucky, for example, and concluded that the giant animals must still be living somewhere in the interior of the continent. He asked Lewis and Clark to keep an eye out for them.
Today, according to many biologists, we’re in the midst of a sixth great extinction. Mastodons may have been some of the earliest victims. As humans moved from continent to continent, large animals that had thrived for millions of years began to disappear—mastodons in North America, giant kangaroos in Australia, dwarf elephants in Europe. Whatever the cause of this early wave of extinctions, humans are driving modern extinctions by hunting, destroying habitat, introducing invasive species and inadvertently spreading diseases.