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But researchers are on the verge of piecing together complete genomes of long-dead species such as Neandertals and mammoths. (See a brief overview of human genetics.)

So now the big question is, Will we soon be able to bring such extinct species back to life?

Researchers are divided over how they might try to do this and whether it's even feasible. (Related: "Woolly Mammoth Resurrection, 'Jurassic Park' Planned [April 8, 2005].)

At the core of this issue is DNA, which encodes the thousands of genes that tell cells how to build themselves and keep running.

Researchers already have deciphered the complete gene sequences—or genomes—for many living species, including humans, dogs, and mice. (Related: "Dog Genome Mapped, Shows Similarities to Humans" [December 7, 2005].)

The DNA of long-extinct species can also be preserved—in bones or bodies found in dry caves or inside ice, for example.

"Retrieval of DNA from ancient specimens is relatively easy now," said Alan Cooper, of the University of Adelaide in Australia.

Even though such DNA has degraded into thousands of small pieces, researchers can still read these fragments and piece together much of the original genetic instructions.

Dead to Return?

So many researchers think that assembling the genome of Neandertals (often spelled "Neanderthals") or mammoths is just around the corner.

A team led by Stephan Schuster and Webb Miller at Pennsylvania State University and Tom Gilbert at the University of Copenhagen is working on the genome of woolly mammoths preserved in the Siberian permafrost.

"I think it's definitely feasible" to assemble these genomes, said Eske Willerslev of the University of Copenhagen in Denmark. But "it's going to be extremely hard work."

Svante Paabo, at the Max Planck Institute in Leipzig, Germany, and his colleagues are aiming to assemble a Neandertal genome from bones preserved in arid caves. (Related: "Neandertal DNA Partially Mapped, Studies Show" [November 15, 2006].)

In a paper appearing this week in Proceedings of the National Academy of Sciences, Paabo says that only certain types of errors appear in such ancient DNA, paving the way for scientists to more easily anticipate and correct gaps in their knowledge.

But ideas of resurrecting these animals "is for the most part science fiction," Paabo argued.

Cooper, of the University of Adelaide, agrees. "As far as I can see, it is not going to be practical," he said.

That's because researchers are reading little fragments of preserved DNA and guessing at what the original genetic instructions were, Cooper said.

"You're not actually physically putting the DNA together, and I can't see any way of doing that feasibly," Cooper said.

In large part, the problem is that living animals package their DNA with proteins that help it wind up into chromosomes. This packaging is crucial to making the DNA work properly, Cooper argues.

Willerslev, of the University of Copenhagen, said the only way he could see of bringing back an extinct species like a mammoth would be to find an extremely well-preserved cell.

That's extremely unlikely to happen, he added, because all parts of a cell break down over time, even in mammoths that have been encased in ice since they died.

But, he said, researchers working on cloning have contacted him, wanting to get a hold of mammoth tissue so they could try to clone a mammoth.

"I was surprised," Willerslev said. "I thought it was completely ridiculous."

These cloning researchers are "pros," he added. "But I don't think they will find anything they can use" in the frozen tissue.

Japanese researchers, meanwhile, have been searching for years for a preserved mammoth with intact sperm, which they say could be used to create a new mammoth.

But researchers who work on ancient DNA think this is also unlikely.

"This is not the way to do it," said Hendrick Poinar, of McMaster University in Canada.

Recipe for Resurrection

Miller, of Pennsylvania State University, however argues that we should never say never.

"Do they also say that synthesizing a virus will never be possible?" he asked.

This was accomplished for the first time in 2005, when researchers reassembled the deadly 1918 flu from preserved tissue samples.

"What about a bacterium? A yeast? A fruit fly?" Miller added. "I'm curious where the line can be drawn."

McMaster University's Poinar has his own ideas of how researchers might revive mammoths and other species—and he thinks it's only a matter of time before it's possible.

"It's theoretically possible, and I think it's going to be done at some point," Poinar said.

He says that once you have the genome of a mammoth, you could compare it with the genome of its closest relative, the Asian elephant. (Related: "Woolly Mammoth DNA Reveals Elephant Family Tree" [December 20, 2005].)

Then you could genetically engineer the elephant DNA, point by point, so that it matches the mammoth DNA.

Then, by inserting this modified DNA into an elephant's egg cell, and implanting it in an elephant's womb, you could create a modified elephant that's nearly identical to the original mammoth, Poinar says.

Or it could become possible to make entire chromosomes from scratch.

"I wouldn't be surprised if, in ten years, you'd be able to synthesize chromosome-length DNA," Poinar said.

"Five years ago everybody was saying you'd never be able to sequence the genomes of extinct animals ... but here we are. We're not that far away now."

But Poinar isn't sure we should bring these extinct animals back.

"The more poignant question is whether this should be done," he said. "This needs to be discussed way in advance. And the time is now, because it's going move very, very quickly."

It's not clear where we'd put a herd of mammoths, for example, and the natural predators that once hunted them—other than people—are also extinct, he added.

"I can't think of a good reason to do it, other than the 'wow' value."