Obtaining an exact replica is too ambitious even for this rat – which disappeared from the face of the earth less than 120 years ago.

De-extinction is hot† There has been wild speculation recently about the retrieval of iconic extinct species such as the huge woolly mammoth and mighty dinosaurs. However, in a new study, scientists are taking a different tack and exploring whether it’s possible to “extinct” a significantly smaller animal: Rattus macleari† A hefty rat that lived on Christmas Island, but died 119 years ago – probably due to the import of diseases.

Genetic Engineering

There are several ways scientists can try to bring back extinct animals. So cloning is an option. That is, if intact DNA is available (which is certainly a major hurdle for species that have been extinct for some time). Another possibility is genetic engineering. “First you describe the sequence of the extinct species and then you adapt the genome of living cells of a living related species, for example using CRISPR-Cas9 (so that it becomes identical to the genome of that extinct species, ed.),” this is how the researchers explain in the magazine Current Biology from. But that sounds simpler than it is, as they show in the same study on the basis of R. macleari at.

Easy peasy lemon squeezy…

At first glance, the de-extinction through genetic engineering of R. macleari quite feasible. First, the researchers succeeded in obtaining almost the entire genome of the extinct rat. What also helps is that the lineage of R. macleari relatively recently separated from that of a still living rat species: the brown rat. As a result, the genome of R. macleari corresponds to as much as 95 percent with that of the brown rat. “That’s perfect, because when you sequence the genome, you have to be able to compare it with a very good, modern reference genome,” explains researcher Tom Gilbert. In this way you can determine in which places the genome of the extinct species differs from that of the surviving relative. The next step is to modify the parts of the genome that are different in the living species than in the extinct species using genetic engineering so that they match those of the extinct species.

..or not?

As mentioned, the researchers succeeded in converting the genome of R. macleari largely sequenced. For the most part. Because a number of important genes could not be sequenced. These are genes that play an important role in the rats’ sense of smell. But what does that mean now if scientists made an attempt to R. macleari – by tinkering with the brown rat genome – to bring it back to life? Even if they managed to modify all the other brown rat genes to be identical to those of R. maclearithere wouldn’t be a real one yet R. macleari are being created. Because the resulting rat would not be able to perceive smells like real ones due to the lack of those few genes R. macleari did that. “With current technology, it is impossible to obtain the complete sequence of the extinct rat and thus it is also impossible to generate a perfect replica of it,” Gilbert said.


And that doesn’t just apply to the rat. Also in an attempt to bring other extinct animals back to life – when using genetic engineering – we run into this problem. “It is very, very clear that we will never be able to collect all the information necessary to obtain a perfect version of an extinct species. It will always be some kind of hybrid (with genes from the extinct species and genes from a living species that, due to a lack of knowledge, we cannot modify in such a way that they become identical to the genes of the extinct species, ed.).”


It might be a bit of a disappointment for people who hoped one day a real woolly mammoth or R. macleari to bump into. But it doesn’t have to be, Gilbert says. Because bringing back a perfect woolly mammoth or R. macleari may be troublesome; thanks to gene technology it is certainly possible to create something that at least resembles a woolly mammoth or R. macleari seems. The big question remains, however, whether you should want that. “It kind of depends on your end goal,” Gilbert says. “If you want to make a crazy woolly elephant for the zoo, it probably doesn’t matter that it’s missing some behavioral genes. But that raises a lot of ethical questions. And if you want to make something that takes on the same ecological role as the extinct variety, it might not be a big deal if it’s not perfectly identical. But if you want to bring back a species so that it can really completely replace the disappeared species, then that won’t work.”


However, that conclusion would not prevent Gilbert from unleashing gene technology on the brown rat himself. “I would like to try making some small (well-chosen) changes in the genome and then see what effect they have. I would like to determine how many genes we need to change to get something that people accept, at least in appearance as the missing species.” Concrete plans to R. macleari to bring back, however, there are currently none. And if Gilbert ever starts tinkering with the rat genome, the first thing he would like to do is work with living species and try to genetically engineer a black rat into a brown rat. “Because when we change a black rat by introducing mutations from the brown rat, at least we know what the original two species originally looked and behaved like.” And that obviously makes it a bit easier to map out the effect of the genetic adaptations.

Although such experiments somehow appeal to Gilbert, he realizes that some restraint is in order. “I think it’s a fascinating idea, but you always have to ask yourself whether it’s not better to use the money to maintain species that still exist here.”

Clearly, several trade-offs must be made when considering genetically engineering extinct species (or any version thereof). And that’s exactly what Gilbert and colleagues are doing in their study. They carefully examined the extent to which they would be able to produce a real through genetic engineering R. macleari to obtain. And their analysis shows that – despite the fact that the rat became extinct not so long ago and has fairly closely related living relatives – they will never succeed. “With modern genetic engineering, people will never regain the real, lost life form,” Gilbert said. “But they can do the analysis we’ve done to find out if they could be satisfied with what can be made of it and then decide whether they should really give it a try.”