Forget dinosaurs – de-extinct the Christmas Island rat

Since becoming a popular concept in the 1990s, de-extinction efforts have focused on grand animals with mythical stature. Despite the dire warnings of Hollywood blockbusters like Jurassic Park, we persist in imagining our modern landscapes restocked with beasts many thousands – or even millions – of years absent.

But even though the process of de-extinction is moving rapidly from the realm of fiction into reality, a team of palaeogeneticists say we should be focusing our attentions on more achievable targets than dinosaurs. Documenting their genomic work in Current Biology, they say they may have found an ideal candidate – Rattus macleari, the Christmas Island rat.

Dinosaurs went extinct 65 million years ago. Mammoths have been missing for 4,000 years. But Christmas Island rats? We lost them only 119 years ago, when disease brought by European ships decimated their populations.

Having recently vanished, the Christmas Island rat still has many closely related species living today. Key among these is the Norway brown rat, with whom it shares about 95% of its genome – a factor potentially dictating the ability to bring the lost rat back from the dead.

De-extinction work is defined by what is unknown. When sequencing the genome of an extinct species, scientists face the challenge of working with degraded DNA, which doesn’t yield all the genetic information required to reconstruct a full genome of the extinct animal. But an animal such as the Christmas Island rat, alongside its genetic cousin the Norway brown rat, is something of a goldmine for evolutionary geneticists.

In much the same manner as the thylacine and the dunnart, the genome of the living rat provides a guidebook for how to assemble that of the extinct rat. Having obtained almost all of the Christmas Island rat’s genome from well-preserved skin samples, researchers can use the genome of the Norway brown rat to identify any gaps. Then, in theory, they can use CRISPR technology to gene edit the DNA of the living rat to match that of the extinct one.

“It was a quite a nice test model,” says Dr Tom Gilbert from the University of Copenhagen. “It’s the perfect case because when you sequence the genome, you have to compare it to a really good modern reference.”

Theoretically, the researchers say there is enough genetic information available to resurrect this recently deceased species – though it wouldn’t be a perfect copy.

“With current technology, it may be completely impossible to ever recover the full sequence, and therefore it is impossible to ever generate a perfect replica of the Christmas Island rat,” says Gilbert.

The replica rat would differ by a few key genes, most of which are related to olfaction – so while to our eyes the revived rat would be indistinguishable from its extinct companions, it probably wouldn’t process smells the same way it once would have, potentially leading to minor behavioural differences.

This is a problem that we’re unlikely to get past any time soon.

“It is very, very clear that we are never going to be able to get all the information to create a perfect recovered form of an extinct species,” Gilbert says. “There will always be some kind of hybrid.”

But in practical terms, is such a small degree of hybridisation meaningful? Perhaps not, says Gilbert. The key, he says, is not in how exactly the resurrected species approximates its lost counterparts, but how functionally different it is from living species of today.

Gilbert says that in order to make an ecologically functional mammoth, for example, it might be enough to edit elephant DNA to make the animal hairy and able to live in the cold.

“If you’re making a weird fuzzy elephant to live in a zoo, it probably doesn’t matter if it is missing some behavioural genes,” he says. “But that brings up a whole lot of ethical questions.”

For now, researchers are thinking of looking even closer to home still – working solely with species that are still living. Although it sounds oxymoronic to test de-extinction technologies on living species, Gilbert says that trialling CRISPR gene edits on a black rat genome to make it that of a Norway brown rat could be an important way to feel out the process before making the leap to resurrecting the Christmas Island rat.

Though Gilbert sees huge potential in the teams’ research, he says the process still gives him pause.

“I think it’s a fascinating idea in technology, but one has to wonder if that’s the best use of money as opposed to keeping the things alive that are still here.”