Human activity and other factors have annihilated countless species over the last few hundred thousand years, but researchers now believe some of those creatures aren't necessarily gone forever, the New York Times finds. Cloning requires an intact cell, but advances in technology mean that it could be possible to bring back an extinct species by using genetic material to create hybrid cells to clone from a closely related species. Researchers using cloned cells have already tried to bring back the Pyrenean ibex, though the baby animal created survived just minutes. Efforts to bring back Australia's extinct Southern gastric brooding frog have so far yielded only embryos that failed to survive.
The efforts to reverse extinction raise many ethical questions, and it may not be desirable to bring back some species, including the huge flocks of passenger pigeons that used to carpet American cities in an inch-deep layer of guano, says the director of the Stanford Center for Law and Biosciences. But ethical questions aside, he is among many scientists who would love to see mammoths, saber-toothed tigers, and giant ground sloths roam the world once again. "We are not talking Jurassic Park," he says. "We are talking Pleistocene Park, 100,000 or 200,000 years ago," and "there are an awful lot of cool things that died within the past 200,000 years." (Read more cloning stories.)