De-extinction could soon become reality – and the International Union for the Conservation of Nature is already making plans to encourage proper use of the technology

Imagine taking a cruise to Mauritius to be greeted by waddling dodos. Maybe you would prefer a Siberian safari and a chance to catch a glimpse of a woolly mammoth, or a voyage to either Australia – for the Tasmanian tiger tour – or New Zealand, to look for Giant Moas.

There is just one small problem with these eco-tourism options: all of these animals are extinct. But imagine if they were not.

Perhaps one day this possibility will be realised. Scientists are developing technologies to bring extinct species back to life – a process termed "de-extinction".

The mention of de-extinction naturally conjures images of the most charismatic of extinct species. But what about the Saint Helena olive tree? Or Rabbs' fringe-limbed tree frog? The last member of this amphibious species, named Toughie, died in September 2016 in the Atlanta Botanical Garden in Georgia, US. How do you choose which species to bring back?

We could ask the experts – except that there none, yet. Technology has not advanced far enough to make the resurrection of extinct species a genuine possibility. For the moment, conversations around de-extinction largely involve crystal-ball forecasting. Still, the theoretical possibility is being taken seriously.

The International Union for the Conservation of Nature is responsible for assigning the conservation status of each species. The IUCN is confident that technology will inevitably progress to the point that de-extinction becomes a viable option. Confident enough, in fact, that they gathered a group of conservation experts to draft an unusual document – published in May 2016 – with guidelines for managing species that are currently extinct.

Axel Moehrenschlager, director of conservation and science at the Calgary Zoo in Canada, and Phil Seddon from the University of Otago in New Zealand helped draft these new IUCN guidelines. They are both experts in re-introduction – bringing (living) species back to landscapes in which they once lived. After all, de-extinction and re-introduction are conceptually similar.

Under what conditions should it be done and what would it mean for conservation?

The goal, they stress, should not be to resurrect species simply so that a single flashy individual can be put on show in a zoo.

Instead, de-extinction should be viewed in the same way as existing re-introduction programmes: the aim would be to generate a genetically-diverse, viable population, living in a robust habitat.

A 2013 TEDx conference on de-extinction stimulated people's imagination and much debate. Until that point, research had primarily tried to figure out whether it could be done.

"But we (the IUCN) asked under what conditions should it be done and what would it mean for conservation?" says Moehrenschlager.

"We've got a window of opportunity now to get people thinking about the complexities of what would be involved," says Seddon.

The IUCN does not endorse de-extinction. They drafted their 2016 guidelines on the subject pro-actively so that if the process eventually becomes a tool for conservationists, there are protocols in place. Since re-introductions involve uncertainty even with familiar species, a little forethought would not be amiss when introducing a species that maybe has not existed for tens of thousands of years.

We've got a window of opportunity now to get people thinking

"We know from re-introducing existing species in areas where they've been missing for a while that you can have all sorts of unintended consequences," Seddon says.

Each component species in an ecosystem has a role. For example, grazing animals keep vegetation in check, while top predators keep prey population numbers manageable.

"We're still grappling with the idea of functionality in ecosystems but we do understand that some species are less redundant than others," says Seddon. "De-extinction could be the art of trying to bring something back that would fill in the gap or serve the same purpose in that ecosystem."

The idea is similar to ecological replacement programmes. Moehrenschlager describes re-populating islands in the Seychelles and the Caribbean with tortoises. The replacement tortoises perform crucial browsing functions that extinct tortoise species used to do.

Replacement and re-introduction programmes around the world generally follow IUCN guidelines to decide which species to work with and where they should go. Moehrenschlager and Seddon wondered if they could use a similar template to theoretically pick a viable candidate species to resurrect.

In 2014 they created a ten-question screening test for potential de-extinction candidates. The questions range from the causes of extinction to habitat needs and the impact and potential risks of re-introduction.

The IUCN declared the Yangtze River dolphin functionally extinct in 2006

For instance, do we know why the species went extinct and can we address current or future triggers of extinction? If we do not know why it died out in the first place, then protecting it from extinction after it has been resurrected would be almost impossible.

Is there appropriate habitat for the species and will it still be available in the future? To answer this, conservationists need to understand the climate, physical space, and food requirements of the candidate species.

Finally, can the impacts and potential risks of resurrection be predicted, mitigated, and controlled? Re-introduced species could outcompete the current inhabitants of the ecosystem or spread diseases that infect livestock or people. They could interfere with agriculture or people's livelihoods. If any of these scenarios come to fruition, how easy would it be to fix the situation?

Before a species is chosen for de-extinction, it has to pass this lengthy test.

"If you fail the test, you're out," says Moehrenschlager, "but if you pass, you're only good enough to go to the next stage of assessment."

They put three candidate species to the test – the thylacine or Tasmanian tiger, the Yangtze River dolphin, and the Xerces blue butterfly. So how did the candidates fare?

The IUCN declared the Yangtze River dolphin functionally extinct in 2006 (although there was an unconfirmed sighting of one in October 2016). It lived in one of the most densely-populated areas on the planet and suffered from a combination of pollution, hunting and habitat loss. Some individuals died after becoming tangled in fishing nets.

Would the Xerces butterfly capture the public's attention if it became the first species chosen for de-extinction?

All the original threats to its survival remain. Industrial waste continues to be pumped into the river, which will only cause further degradation of the habitat. As with any species, efforts to bring the dolphin back are irrelevant if there is nowhere to put them.

"If there's no suitable habitat and if the threats aren't addressed, then it's not worth even entertaining doing these things," says Moehrenschlager. "It begs questions of ethics, morality, logistics, and financial investment to tinker with species which might not have any hope of ever existing in the wild."

The situation looks more favourable for the Xerces butterfly and the thylacine.

The last thylacine died in 1936. Hunting, habitat loss, and lack of prey led to its extinction. It inhabited mixed forest, wetlands, and coastal heath – and parts of that habitat remain intact. In fact, they are even protected to ensure territory for the Tasmanian devil, whose range overlaps with that which the thylacine once inhabited. There was never any suggestion that the thylacine carried any unusual diseases, and resurrecting the species might meet few protests, other than from farmers worried about losing the occasional sheep.

The Xerces butterfly lived in the San Francisco area. Its habitat shrunk as the city expanded and it was declared extinct by 1941. Yet some suitable habitat remains in Golden Gate Park, where the trees it prefers still grow. And in the unlikely event that the butterflies trigger any harmful impacts, the adults tended to fly in a single group, which would make a resurrected population easier to collect if needed.

We should be using this technology for species that are on the cusp of extinction or maybe just tipped over

But would the Xerces butterfly capture the public's attention if it became the first species chosen for de-extinction? Iconic species like mammoths or sabre-toothed cats might generate more excitement.

Mammoths might not be a bad choice for the procedure, given the important role they played in their ecosystem as grazers on the arctic steppe. Sabre-toothed cats, on the other hand, were apex hunters. The largest collection of their fossilised remains was found in the La Brea Tar Pits – which lies smack in the middle of Los Angeles city limits. Neither the animals nor the city's residents would enjoy that re-introduction.

Guilt often drives our desire to bring back certain animals. Humans hunted New Zealand's moas and the passenger pigeon to extinction. The dodo likely died out thanks to ship rats brought in by sailors to Mauritius. Re-introducing these animals might make us feel better, but that is hardly the point.

"We should be using this technology for species that are on the cusp of extinction or maybe just tipped over," Seddon says. "When we know a lot about their habitats, we've got the right kind of genetic material, and know how to raise them in captivity."

The more we know about the species, the greater the likelihood of producing a viable, genetically-diverse population that can exist in the wild.

The end result is a population that looks similar physically to the extinct species

So how far away is the technology from resurrecting extinct species? Much closer than most people might think. In fact, species resurrection has already been attempted.

In 2000 a Pyrenean ibex (or "bucardo") was cloned from sample cells taken from the last surviving individual. But this early attempt at delaying the extinction of the species was unsuccessful: the clone succumbed to a lung defect just seven minutes after birth.

It is also worth bearing in mind one key point that is often neglected: true de-extinction is impossible. None of the methods researchers are currently pursuing will lead to a carbon copy of the extinct species. The best the technologies can give us is a proxy.

One method, "selective back-breeding", involves working with a species that is related to the extinct species. Scientists choose individuals with traits that resemble those seen in the extinct species and selectively breed them. Then they choose the offspring that have slightly more developed versions of those "extinct" traits and selectively breed them, and so on. The end result is a population that looks similar physically to the extinct species – even though it is not actually the same at the genetic level.

Other methods involve piecing together genetic material extracted from the remains of extinct species, injecting it into an egg of a related existing species, and then bringing the egg to term in a suitable surrogate mother.

None of the methods researchers are currently pursuing will lead to a carbon copy of the extinct species

Then there is "somatic cell nuclear transfer", or cloning. This only works for recently-extinct species with enough preserved tissue samples. Of course, it can also be used in healthy populations – think Dolly the Sheep.

Genome engineering is required if scientists are to bring back anything that went extinct before adequate samples could be preserved. DNA breaks apart with age and the older a biological sample is, the more pieces its DNA will be in. It is like a giant puzzle with many thousands of pieces – and to make things more complicated, some of those pieces will be missing. The gaps need to be filled by bits of genome from a related living species.

Both cloning and genetic engineering result in an embryo that needs a surrogate mother – one from a species as closely related as possible to the extinct species. Again, this introduces another way through which the species to be resurrected will be modified. The surrogate mother's uterine hormones and growth factors will changes the way the embryo develops, switching genes on or off in a pattern inconsistent with what would have been typical for the embryo's own species. So, for instance, a baby mammoth brought to term by an elephant will not be identical to its Ice Age forerunners.

Then, when the "resurrected" mammoth is born, it is the only one of its kind. How will a mammoth learn to be a mammoth if it is raised by humans or elephants?

The proxy may be close genetically and even behaviourally, but it can never be identical to the original. In other words, de-extinction is not a way to reverse the damaging ecological consequences of human activities.

Some scientists fear that this important point is overlooked. They worry that the enticing allure of "de-extinction" could even do real harm to conservation efforts because they might lead to the impression that extinction is not forever. Despite these concerns, Moehrenschlager thinks that if the tools exist, we should use them.

The planet could lose a thousand species in the same time that we could maybe de-extinct one

"We should never blindly stick with what we are most familiar and comfortable with," he says. "The fact is, the planet's on fire and we need to do it all."

Besides, de-extinction cannot replace traditional conservation practices.

"The scale of biodiversity loss is huge," says Moehrenschlager. "The trajectory of extinction will so incredibly outpace even the theoretical possibility around de-extinction that the two numbers will never mesh." The planet could lose a thousand species in the same time that we could maybe de-extinct one.

"For now," agrees Seddon, "we really need to keep what we've got. The first rule of intelligent tinkering is to keep all the parts."

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