The Earth has lost many creatures as a result of human activity: The dodo and the passenger pigeon are among the best known. But in more recent years, one of the most spectacularly tragic losses in nature was Thylacinus cynocephalus, commonly called the thylacine, or Tasmanian tiger.

The Tasmanian tiger was a truly unusual species. Shaped like a wolf or hyena, it had a ginger-colored coat streaked with gorgeous, Dr. Seussian black stripes. A predator, it could open its jaws astonishingly wide, so that its toothy gape resembled that of a crocodile. Most intriguing of all, this striped creature was a marsupial: It carried its young — as many as four at a time — in a pouch like a kangaroo.

For thousands of years, T. cynocephalus lived in Tasmania, an island ark 150 miles south of the Australian mainland, which is still home to some of Earth's most extraordinary animals. And it lived well into the 20th century.

So what happened to this glorious beast?

Tasmania was colonized by the British in 1803, and the settlers soon brought in a large-scale sheep-raising industry. Tasmanian tigers occasionally raided their livestock, and as a result, a government bounty was set up and money paid to anyone who brought in a dead tiger. Shooting and snaring these rare marsupials became a lucrative business.

By the mid-19th century, naturalists were sending up red flags about the creature's imminent extinction. In 1863 the famed British wildlife artist John Gould warned, "When the comparatively small island of Tasmania becomes more densely populated, and its primitive forests are intersected with roads from the eastern to the western coast, the numbers of this singular animal will speedily diminish; extermination will have its full sway." He noted that the demise of the Tasmanian tiger would be a "source of much regret."

By 1914, Gould's prediction was coming true. In that year, biologist T. Thomas Flynn (the father of Tasmanian-born movie star Errol Flynn) proposed the establishment of a wildlife sanctuary to save the tigers from annihilation. But his warnings, like those of other naturalists, were ignored. Tigers continued to be shot and trapped. By the 1930s, they were hardly ever seen, and in 1936, the last known Tasmanian tiger died at a Tasmanian zoo.

The extinction of the Tasmanian tiger is a source of much regret. And it's a source of mystification, too. Looking back, it's hard to comprehend how the Tasmanian settlers could have been so cavalier about the tiger's biological uniqueness, so heedless of the consequences. It would have been easy to save the animal, but the only law ever passed to protect the species was enacted the same year the last captive one died.

Today the tiger remains the island of Tasmania's symbol, pictured on coins, flags and license plates. But it has, sadly, fallen into the realm of myth and mystery. Search parties are still sent out to find any surviving wild tigers, but in 70 years, no physical evidence has been uncovered. This magnificent animal survives primarily in people's hopes and imaginations.

In 2006, many of us still have trouble foreseeing the consequences of our actions — and scientists continue to predict a disconcerting future. Last month, the World Conservation Union issued its 2006 "Red List," which identified more than 16,000 plants and animals — a greater number than ever before — as threatened with extinction, largely due to human activities. These include habitat destruction, the introduction of invasive species, overhunting and overfishing, pollution, and climate change.

On the Red List for the first time this year was Ursus maritimus, the polar bear — not surprising, given that the polar bear has become the emblematic species for global warming. The World Conservation Union's polar bear specialist group estimates that because of the melting of Arctic pack ice, polar bear populations will decline by "more than 30 percent within the next 35 to 50 years. The principal cause of this decline is climatic warming."

Their language is not as poetic as Gould's, but the message is the same. Will the polar bear cross from the realm of the living into the realm of myth, too? And what will be the fates of the hippopotamus (also appearing on the list for the first time) and thousands of other red-listed species? Will we regret our actions? History tells us that when it comes to preserving natural resources, human beings can be very foolish. But hindsight tells us we can change course and save these animals.

Margaret Mittelbach and Michael Crewdson are the authors of "Carnivorous Nights: On the Trail of the Tasmanian Tiger." They wrote this article for the Washington Post.