Endangered & Extinct
Extinction is always happening. It’s a natural part of the history of life. But every once in a long while, great numbers of species disappear rapidly—over a few thousand or a few million years. Scientists call this a mass extinction.
Mass extinctions have happened at least five times during the past 500 million years. One of the most famous occurred about 65 million years ago, when all non-flying dinosaurs disappeared. Starting about 100,000 years ago, large mammals and other species began to disappear more swiftly than normal, as humans spread from Africa to other continents.
During the past 500 years, at least 90 mammal species are known to have died out, with many more barely hanging on. Today, about 25 percent of the living mammal species are threatened with extinction. Many scientists think these extinctions may be part of another mass extinction event—called the Sixth Extinction.
One remarkably successful mammals species—our own—is contributing to increasing extinction rates today. By altering habitats, polluting, and overhunting, humans put other mammals at risk. Natural forms of climate change—as well as human-induced global warming—are also factors.
Mammal Mass Extinctions
Humans first arrived in North America about 15,000 to 20,000 years ago. Not too long after, numerous large mammal species went extinct. This pattern occurred on many other continents and islands, too: as humans arrived, other mammals disappeared.
But did humans cause these extinctions? Scientists have proposed several competing hypotheses, such as overhunting, climate change, killer plagues, and comets, but many think a combination of factors—not only humans—must be responsible.