How to Resurrect Lost Species

Will we ever see a woolly mammoth again? What about the striped Tasmanian tiger, once-prolific passenger pigeon, or the imposing wild cattle called aurochs?

Our species has played a role in the extinction of these and many other species. But now some scientists are proposing a radical turn of the tables: Bringing lost species back from the dead.

Three main methods for "de-extinction" have been proposed. Cloning gets the most attention, thanks in part to the science fiction of Jurassic Park. We probably won't ever see a Tyrannosaurus—despite the discovery of degraded soft-tissue remnants in fossilized dinosaur bone, no one has ever found non-avian dinosaur DNA—but cloning is plausible for less ancient creatures whose genomes can be reconstructed.

And while mammoths tend to hog the spotlight when such proposals are discussed, researchers are also considering resurrecting other species that might not be as famous, but are equally charismatic. (See "Pictures: Extinct Species That Could Be Brought Back.")

The thylacine, also known as the Tasmanian tiger, is one such lost species. These striped carnivores appear superficially doglike but are actually marsupials that once roamed Australia, Tasmania, and New Guinea. A combination of causes—most prominently hunting and trapping by humans—totally wiped them out by 1936.

But as paleontologist Michael Archer has realized, the relatively recent extinction of thylacines means that museum specimens can still yield viable genetic material. When he was the director of the Australian Museum in the early 2000s, he and his colleagues extracted DNA samples from the teeth, skins, and dried tissues of thylacines in the museum's collection.

Obtaining those genetic sequences was just the first step, though. The DNA is in fragments, and the next major task will be assembling all those snippets into the whole genome of the thylacine.

Although Archer discontinued the project when he left the museum for a position at the University of New South Wales, he's optimistic that researchers can someday pick up where he and his team left off: "As technology improves, and sequencers get better and better, it would be a brave person to declare that the whole nuclear genome won't ever be able to be reconstituted."

If the thylacine genome is completed, the next hurdle would be using it to create a viable pup. Archer suggests two possible routes. One way would be to combine the nucleus of a lab-created thylacine cell with an egg of a Tasmanian devil, another carnivorous marsupial, that had its nucleus removed. The resulting embryo could be implanted into the uterus of a Tasmanian devil, potentially resulting in the birth of a thylacine.

Alternatively, Archer said, scientists could try a less direct approach. "A cell line of living Tasmanian devil cells could be used to progressively splice in thylacine sequences to replace the corresponding regions of the devil chromosomes." Repeated over and over again, such a method could eventually produce a thylacine. A 2008 study in which thylacine DNA was successfully spliced into a mouse hints that such a method might be successful.

"I wouldn't want to make light of the challenges involved in any of this," Archer said. Yet he believes such a project may become feasible in the future. "It can't be done now—but that was true 20 years ago for many things that can be done today." Indeed, he said the biggest hurdle for cloning "could be the widespread conviction that it's a waste of time to try."

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