There are physical limitations on what kind of species can be brought back, of course, based on how long ago they went extinct. Even if blood or tissue is preserved (say in amber, ice or tundra) the DNA in the sample will still degrade with a half-life of just 521 years.
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For dinosaurs, we can forget it. They died out 65 million years ago, and all their DNA would have completely broken down in less than 7 million years. For mammoths, they possibly went extinct as recently as 3,000 years ago, but even that's still not very good news. Even after that much time, only a tiny fraction of their DNA would be intact, so getting lucky enough to find just the right intact DNA isn't very likely. For the passenger pigeon, there's a bit of a better chance. It's only been around 100 years since the last of that species died off. So, we could probably do it with this species, and we could probably do so with species that are on the brink of extinction now.
So, we know our limitations, but the question then becomes 'should we'?
One of the big concerns is that these species died off for a reason, whether it was natural or caused by human activity. They weren't able to survive as the world around them changed, so we'd have to question whether or not they'd survive now, or if we'd just have them go extinct again.
The other main concern is with conservation efforts, and this really comes down to protecting us against ourselves and our tendency towards the 'quick fix'. If we had the ability to resurrect species at will, it might become far too tempting to destroy a species' habitat for its mineral wealth, or simply hunt the species to extinction, with the intention of cloning them later on. However, rebuilding an ecosystem to put those cloned animals back into would be a far more difficult (maybe even impossible) task.
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If it can be done responsibly, this kind of science does have some excellent potential. It could bring back species driven to extinction through our negligence. It could be used to restore endangered and 'on the brink' species to a population level where they could actually sustain themselves again. It could undo many of our mistakes. We would just have to be very careful that we didn't just end up making more of mistakes in the process.
(Photo courtesy: Wikipedia, via Royal BC Museum)
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- Living Nature
- Michael Crichton