Should we bring back the passenger pigeon and the woolly mammoth?

• De-extinctioning is a movement to bring back species humans have killed off
• Scientists are working to revive passenger pigeons%2C woolly mammoths and other species
• Conservationists worry if extinction isn%27t forever%2C efforts to preserve habitat and species today will lose steam

STANFORD, Calif. – Just 99 years ago, the last passenger pigeon on the planet died at the Cincinnati Zoo. Today the Revive and Restore project is working to bring them back from the dead. They've also got their eyes on woolly mammoths. Other groups are pondering reviving everything from Tasmanian tigers to the dodo.

They're part of a growing movement among scientists, naturalists and visionaries working to bring back species that humans killed off over the past several thousands years. They call it de-extinctioning.

Ethicists, scientists, lawyers and biologists gathered at Stanford University Friday for a day-long conference to discuss a broader question: Even if we can, should we?

There are three ways scientists can bring back extinct species, said Beth Shapiro, who directs the paleogenomics lab at the University of California-Santa Cruz.

• Back breeding. An example is efforts to recreate the auroch, a European cattle ancestor that died off in Poland in 1627. By choosing cattle with characteristics close to what aurochs looked like, the TaurOs Project in Holland is aiming to breed auroch-like cattle.

• Cloning. DNA taken from the frozen cells of recently extinct animals is used to create embryos that are then brought to term in a surrogate mother. That was actually accomplished in 2009. Spanish scientists used preserved skin samples from the Pyrenean ibex, a wild mountain goat that went extinct in 2000, to create an embryo that was brought to term in a goat surrogate mother. Unfortunately, the newborn ibex kid only lived a few minutes before it died because of lung defects.

• Genomic editing. This uses preserved pieces of ancient DNA to make an educated guess about the genome of an extinct species. For example DNA from a frozen wooly mammoth found in the arctic could be sequenced. It wouldn't be complete because of degradation over time but the missing pieces could be filled in with elephant DNA. The basic technology is already commonly used in synthetic biology labs.

One animal species that won't be de-extinctioned is dinosaurs. "Jurassic Park will not happen," Shapiro said firmly. "Fossils are too old, there's no DNA left."

The topic has become hot over the past year and a half. There was a conference on the passenger pigeon at Harvard in February 2012. The National Geographic Society in Washington, D.C., held a small meeting of relevant scientists, mostly conservation and molecular biologists, in October 2012.

In March, National Geographic held a much larger TEDx conference on de-extinctioning "when it all went public" said Stewart Brand, who is spearheading the Revive and Restore project to bring back the passenger pigeon. The Stanford conference, focused on the ethical and legal issues, is the fourth, said Hank Greely, director of the Stanford Center for Law and the Biosciences and its organizer.

The de-extinction process isn't benign, said Matthew Liebman, an attorney with the Animal Legal Defense Fund of Cotati, Calif. Animals must be hormonally altered to produce extra eggs, the pregnancies are dangerous for the surrogate mothers and the clones themselves often die very quickly, he said. In the case of the Spanish ibex, the kid "lived for only 10 to 12 minutes before dying and the entirety of her existence outside the womb was in intense respiratory distress," he said.

Even so, there aren't laws that would stop it, Shapiro said. Although state animal cruelty laws prohibit unjustifiable animal suffering, most exempt biomedical research, "so I don't think it's likely we'll see criminal prosecution of de-extinction researchers."

The legal issues that will surround revived species are very unclear, said Andrew Torrance, a law professor at the University of Kansas and former biotech patent lawyer. But in general, he thinks "there are no solid legal barriers yet to de-extinction."

One question raised by several audience members at the forum was why extinct species should be brought back at all. Brand spoke up. "Because we don't want to be party to death that large. When something goes extinct, it hits us in the marrow."

Some say a better use for all this money, time and expertise would be to use the same techniques and technologies towards creating greater genetic diversity through cloning and other programs. Species that are down to just a few individuals and need help include the Chinese giant salamander, the Javan rhino or the kakapo owl parrot of New Zealand, said Kate Jones, chair of ecology and biodiversity at the University College London.

Biologists worry that de-extinctioning is dangerous because it could make people think extinction isn't forever, said Terry Root, a senior fellow at the Woods Institute for the Environment at Stanford University. "People are concerned that if the idea is out there that we can bring them back, we may not work so hard to preserve the species we have."

The prospect of reviving extinct species strikes fear in the heart of Jamie Rappaport Clark, president of the non-profit Defenders of Wildlife. After 20 years working for the U.S. Fish and Wildlife Service, she's seen how little political will there is to protect endangered species. De-extinctioning could "tip the balance to allowing habitat destruction while the economy is in trouble" and let politicians say they'll bring any species that might die out back later when times are better, she said.

Even so, she's convinced that it's too late to put this particular genie back in the bottle, she said. Already a group in Australia has revived the gastric-brooding frog, which died out in 1983. This year the Lazarus Project, a group working with Mike Archer at the University of New South Wales in Australia, announced that it had created embryos of the extinct frog using tissue collected in the 1970s.

Chuck Bonham, the director of the California Department of Fish and Wildlife in Sacramento, is convinced it's going to happen, so "we need to start figuring it out how to regulate it now." He spoke as an individual, not as a government employee, but asked the assembled scientists and visionaries to start talking to regulators now. "We need to be careful. We need to make sure we get this right because if we get it wrong it could be very bad."