45,000 years ago our species was not alone on this planet. Alongside us, Homo sapiens, was a second member of our genus, Homo neanderthalensis, with its own tools, society and cultural practices.

At one time it is thought that there were around 70,000 Neanderthals living on Earth, mainly in what we now know as Europe and southwest and central Asia. How much our species interacted with this sapient cousin is not fully known, but there was certainly some interbreeding: while Neanderthals are long deceased, their DNA lives on in many Europeans and Asians.

But now, with the advances of genetic technologies, Neanderthals could return. Recent advances of gene editing tools such as CRISPR, as well as the sequencing of DNA taken from the bone of a female Neanderthal who is thought to have walked the Earth some 50,000-100,000 years ago, mean that what was once pure science fiction could soon become a reality.

Legendary geneticist George Church, the Robert Winthrop Professor of Genetics at Harvard Medical School who is currently spearheading the project to de-extinct the woolly mammoth, has said that he thinks the de-extinction of Neanderthals will occur in his lifetime.

“The reason I would consider it a possibility is that a bunch of technologies are developing faster than ever before,” he told Spiegel Online in 2013. “In particular, reading and writing DNA is now about a million times faster than seven or eight years ago. Another technology that the de-extinction of a Neanderthal would require is human cloning.

“We can clone all kinds of mammals, so it’s very likely that we could clone a human. Why shouldn’t we be able to do so?”

Bringing Neanderthals back from the dead

When we consider de-extincting Neanderthals, it is important to note that we would not be bringing back a precise, perfect copy of the Neanderthals that lived on Earth up until their extinction some 40,000 years ago.

As Douglas McCauley, assistant professor in the University of California Santa Barbara’s Department of Ecology, Evolution and Marine Biology, explains, the question of whether we can bring Neanderthals back from extinction “depends upon how much of a purist you are about the definition of Neanderthal”.

I expect we will be more interested in engineering bigger brains than bigger brow ridges

In the simplest terms, any scientists who set out to de-extinct Neanderthals will do so by cobbling together modern human and extinct Neanderthal DNA.

“The technique that many de-extinction scientists are now using to bring back extinct species is to sequence the genome of the dead species – line it up next to the genome of the nearest living relative – then use CRISPR gene editing techniques to modify elements of the genome of the living relative to approximate elements of the genome of the dead species,” explains McCauley.

This is the approach being taken by the Harvard team currently attempting to de-extinct the woolly mammoth.

“Here they are using the genome of the extinct woolly mammoth and the genome of a living Asian elephant. The goal, however, isn’t to bring back a perfect replica of the woolly mammoth. A success would be to genetically engineer a hairy, cold-tolerant Asian elephant.

“This would also remain the same strategy for any group attempting to bring back a Neanderthal. Again, this would be more like engineering increased Neanderthal-ness into the human genome – not like cranking out a carbon copy of a Neanderthal.”

This approach should be technically possible for Neanderthals in the near future. But, as McCauley explains, that doesn’t mean it will actually happen.

“Technically engineering more Neanderthal into the human genome will indeed be possible very soon,” he says. “Practically, I don’t really see this happening. People will most certainly use CRISPR and next-generation gene editing techniques to edit the human genome – but I think this is much more likely to be tuning humans up, rather than tuning down.

“I expect we will be more interested in engineering bigger brains than bigger brow ridges.”

Criteria for de-extinction

De-extinction is, in general, a topic that is set to be the subject of ever-greater discussion in the coming years, as hypothetical concepts become scientific reality.

“It is on the precipice of moving from a crazy idea we once mused about over coffee, to a real possibility we can actually make happen in the lab. From science fiction to real science,” summarises McCauley.

However, with such abilities come significant moral questions. De-extinction could be a vital tool for conservation, but it could also be used to produce creatures that are more reminiscent of science fiction horror stories than of scientific value.

As a result, efforts are already being made to build a moral framework within which de-extinction scientists can work. As part of this, McCauley authored a paper along with several colleagues that recommended using three specific criteria for the selection of candidates for the de-extinction process.

“I am a conservation biologist and an ecologist. The three criteria we issued were created from that vantage point: what species would we bring back if we genuinely wanted de-extinction to combat the ecological crisis being created by the ongoing human-driven mass extinction?” he explains.

“We suggested recovering species that: 1) performed ecological jobs that were highly unique and were not replicated by other surviving species; 2) recent extinctions for which the technological and ecological barriers for recovery and restoration were lower; and 3) species that we could meaningfully recover to historic levels of abundance.”

If following this approach, scientists would therefore favour species to de-extinct that could not only fulfil a role in the ecosystem that another species had not taken over, but were likely made extinct fairly recently and would survive and flourish in the current environment. And under these criteria, Neanderthals would be a poor choice.

“Neanderthals most importantly fail the first test,” explains McCauley. “Their ecology is very similar to another species that survived and thrived – our own.

“To put it bluntly, from a conversation biologists point of view: the last thing our planet needs right now is more hungry Hominids.”

Neanderthal revival: the moral issue

This is not to say, as some have suggested, that Neanderthals would pose any particular threat to modern humans.

“Quite the opposite,” argues McCauley. “The greatest challenge would be keeping de-extincted Neanderthals alive and safe from us, not worrying about them taking over.”

As these newly engineered Neanderthals would not be true replicas of their past equivalents, they would be likely to suffer from genetic issues, as well as being potentially highly ill-suited to the human-occupied modern world.

There are likely to be a host of developmental issues associated with looking after imperfectly genetically re-engineered Neanderthals

“There are likely to be a host of developmental issues associated with looking after imperfectly genetically re-engineered Neanderthals (e.g. birth defects), they are likely to be quite susceptible to modern disease, and it is unclear what habitats they would slot back into,” he adds. “Our species has taken over all of the once prime habitat of Neanderthals.”

Then there is the matter of Neanderthals’ original demise; something that could easily play out again if we were to bring back a group of the species. It’s hard to see the scientific value of de-extincting a species that would be at high risk of quickly becoming extinct again.

“It is important to remember that we likely played an important role in the original extinction of Neanderthals,” explains McCauley. “We competed heavily with them for food and homes and we may have given them lethal diseases. Reviving Neanderthals might simply be an act of recreating history.”

Value in de-extinction

For McCauley, there is currently no circumstance under which bringing back Neanderthals would be a good idea. But that does not mean that de-extinction as a wider practice does not have value – in fact, it could offer significant benefits, provided we select the right species to focus on.

“There is a very long list of other species that I think would be smarter to bring back before we started in on Neanderthals,” he says.

“As an ecologist that looks out at a world with species being driven extinct in all directions around us, I am all ears for smart new conservation tools.

“The challenge here will be carefully selecting targets that meaningfully help the planet, not using this new-found power to create oddities for zoos or bio-bazaar.”