Until recently, the arrow of natural selection seemed to go only one way. A species could form, then it could flourish, then it could go extinct. And once it was extinct, it could not come back.

Now, though, some scientists say they see a new path.

“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 that prowled the cliffs in the Pyrenees between Spain and France until the last one died in 1999. The method used was cloning — using frozen cells of the last of the animals to try to create a new one, much like Dolly the sheep was cloned from a frozen udder cell of a sheep that had died years before.

Last week at a conference in Washington, scientists from Australia reported on 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.