Do you remember the loggerhead turtle and the bonobo? That’s a question our grandchildren may ask. Scientists expect to confirm a mass extinction — defined as a loss of 75% of species — over the next few centuries if current trends continue.
There have been five mass extinction events in Earth’s history. In the worst, which occurred 250 million years ago and was caused by rising carbon dioxide levels, nearly all marine species and almost three quarters of all land species died off. The last mass extinctions occurred 66 million years ago, when a six-mile-wide asteroid is believed to have collided with Earth, wiping out the dinosaurs.
Due to past extinction events, of all the species that have ever populated Earth at some time over the past 3.5 billion years, more than 95% have vanished. The Sixth Extinction, more formally designated the Holocene extinction, describes the extinction event that is occurring during the present Holocene epoch (since around 10,000 BCE). A special analysis carried out by the journal Nature indicates that 41% of all amphibians on the planet now face extinction while 26% of mammals and 13% of birds are similarly threatened.
But the culprit involved this time is no cataclysmic event like an asteroid. It is, plain and simply, human behavior.
Extinction, a historical perspective
These days, we hear regular warnings of this or another animal or plant or organism whose days are numbered. The recently-reported death of one of the last six white rhinos left in the world pierced our complacency, if only for a moment. We may be growing inured to such news, as if under “natural selection,” creatures necessarily come and go.
Extinction is a relatively new idea. Well into the 18th century, people found it impossible to accept that nature could simply allow an entire species to disappear. If a species was no longer to be found in one location, it must still exist
elsewhere. “Such is the economy of nature,” argued Thomas Jefferson, “that no instance can be produced of her having permitted any one race of her animals to become extinct; or her having formed any link in her great work so weak as to be broken.”
When fossils of strange creatures – such as the mastodon – were first dug up, they were assumed to belong to creatures that still lived in other lands. Jefferson himself backed numerous expeditions—including the famed Lewis and Clark expedition in what is now thewestern portion of the United States—in the hopes of finding mastodons still roaming somewhere in America.
Then the French anatomist Georges Cuvier showed that the elephant-like remains of the mastodon were actually those of a lost species. “On the basis of a few scattered bones, Cuvier conceived of a whole new way of looking at life,” notes Elizabeth Kolbert in her book, The Sixth Extinction: An Unnatural History. “Species died out. This was not
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