Is dead dead? Soon,not forever

Maybe we can no longer delay death,but we can reverse it, said George Church,a Harvard Medical School geneticist.

For now,only one extinct subspecies has been brought back,and the baby animal that was born lived just minutes in 2003. It was a Pyrenean ibex,a large goatlike creature from the Pyrenees between Spain and France last seen in 1999. The method used was cloningusing frozen cells of the last of the animals to try to create a new one.

Recently,at a conference in Washington,scientists from Australia reported their attempt to bring back a weird frog,the Southern gastric brooding frog,that went extinct about a quarter century ago. So far they have only made early embryos,which have died.

The science of bringing back extinct species is complex,and the task can seem a bit daunting. Actual cloning requires an intact cell from an extinct species,something that might not exist.

If cloning works,it results in an embryo that must be implanted in a closely related species to serve as a surrogate mother. But new DNA technologies have suggested another way to bring back extinct species,and all that is needed is some genetic material. The idea is to compare the DNA of the extinct species to that of a closely related existing species and then start substituting chunks of the extinct species DNA into the DNA in cells of the existing species. Then those hybrid cells could be used to clone. After a while,the resulting bird or animal would have enough of the extinct species DNA to closely resemble it.

Another method is to backbreed. That might work,for example,with the auroch,an ancient breed of wild cattle. It is thought that most of its distinctive genes still exist,scattered among existing cattle strains. Scientists could breed those existing strains,selecting an offspring with more and more and more of the auroch DNA until they get cattle close to aurochs.

In theorya wild theorybackbreeding humans might even enable scientists to bring back Neanderthals,says Hank Greely,the director of the Stanford Center for Law and the Biosciences at Stanford University.

Of course,there are many arguments against fooling with nature in this way.

One is the consequences for the Endangered Species Act. Its premise,Greely points out,is that extinction is forever. If you take away that argument,what happens? he asks.

In the end,a sense of wonder draws Greely and many others to the idea of bringing back species. For me,it would just be so cool to see a woolly mammoth or a saber tooth tiger or a ground sloth, Greely says. We are not talking Jurassic Park. We are talking Pleistocene Park,100,000 or 200,000 years ago.