Some decades ago, the idea of making extinct animals walk on Earth again seems like a mere fantasy. Then came Jurassic Park and those type of scientific efforts have since focused on the bigger, grander animals.
Well, what if we’re actually one step (out of, let’s say, a million) forward in terms of reviving extinct animals?
It seems a bit far-fetched when we’re talking about the great dinosaurs which went extinct some millions of years ago, or mammoths that ceased to walk the Earth about 4000 years ago. But what about Christmas Island Rat that was declared extinct almost 120 years ago?
If you’re unfamiliar with this species, Christmas Island Rat is believed to have gone extinct due to diseases brought over on European shops.
Now, last March, in the journal Current Biology, a team of paleogeneticists took interest in Rattus macleari. What they’ve discovered could provide insights into the limitations of de-extinction across all species.
When scientists sequence the genome of an extinct species, it becomes a rather difficult task because they have to work with degraded DNA. That is, it doesn’t yield whatever genetic information that the scientists need to reconstruct a full genome of the extinct animal.
But along with his colleagues, evolutionary geneticist at University of Copenhagen Tom Gilbert found something remarkable about the Christmas Island Rat.
The team was able to obtain almost all of the rodent’s genome. Moreover, since this species diverged from other Rattus species relatively recently, it shares about 95% of its genome with a living rat, the Norway brown rat.
“It was a quite a nice test model. It’s the perfect case because when you sequence the genome, you have to compare it to a really good modern reference,” said Gilbert.
What might be the limitations?
After the scientists sequenced the DNA and matched up the genome against the reference genome of the living species, they identified the parts of the genomes that don’t match up.
In theory, they would need to use CRISPR technology to gene edit the DNA of the living species to match that of the extinct one. Matching up between the brown rat to Rattus macleari is a good test case because the evolutionary divergence is similar to that of the elephant and the mammoth.
However, even though the sequencing process was mostly successful, a few key genes were missing. The genes were related to olfaction, and it means that a resurrected Christmas Island Rat would likely be unable to process smells in the way as it would have originally.
Gilber said, “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.
“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. There will always be some kind of hybrid.”
De-extinction with a caveat
So basically, the scientists say that even though we may able to revive extinct animals someday, there is a possibility and risk of not living the way they did back then.
Now, the insights mentioned above is that although an exact replica won’t be perfect, the DNA is editable. Scientists in the future could potentially make the animals de-extinct, with different functions from the current, living one.
According to Gilbert, it may be sufficient to edit elephant DNA to make it hairy, able to live in the cold, and ecologically functional mammoth. “If you’re making a weird fuzzy elephant to live in a zoo, it probably doesn’t matter if it is missing some behavioral genes. But that brings up a whole lot of ethical questions.”
What are the next plans then? Well for now, Gilbert wants to try doing the actual gene editing on rats. But he plans to start with species that are still living. He intends to begin by doing CRISPR edits on a black rat genome to change it to a Norway brown rat before attempting to resurrect the Christmas Island rat.
As excited as he is about his future research, his discovery became something that he reflects on. “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,” Gilbert said.
Could we bring back Tasmanian tiger to live?
With Gilbert’s finding in mind, raises the question if we could also reverse the extinction of Tasmanian tiger, which was erased from our planet in the 1930s. Again, this is another case that ‘it’s gonna take a long while, but we’re getting there.’
Researchers at DNA Zoo Australia who have mapped the genome of the endangered numbat have also announced that the species’ DNA could be used as a blueprint to bring its extinct cousin, Tasmanian tiger, also known as thylacine, back from the dead.
How the numbat DNA helps
Fundamentally, the process is similar to Gilbert’s: taking the genome of a living species and edit its DNA to more closely resemble that of an extinct relative.
So, the scientists this time needed intact genetic blueprint of the extinct animal. Back in 2017, Professor Andrew Pask at the University of Melbourne sequenced the genome from a juvenile specimen held in Museums Victoria’s collection.
Pask said, “It’s one of the best genome builds we have for an extinct animal. It’s in lots of little tiny pieces when we get it from an extinct specimen, so the DNA is broken up.”
Unfortunately, any mammal genome presents their challenges: repeat sequences of A, C, G and T nucleotide bases.
“Those bits are really hard to put back together when you’re building your puzzle, particularly because we don’t have the picture on the box of the puzzle – we don’t know what [the thylacine genome] is meant to look like,” said the professor.
This is where the numbat genome could provide a solution. Simply put, numbats and Tasmanian tiger are cousins. They shared a common ancestor that lived about 35m to 41m years ago.
The current technology to edit genes isn’t advanced enough to be able to change all of the differing sequences to thylacine DNA in a timely manner. According to Pask, it’d take hundreds of years to go through and do it with our current technology.
So, any de-extinction via this process would prioritize which DNA sequences to target, yielding a gene-edited animal genome that isn’t exactly the same as the extinct one.
“We then need to be able to turn a marsupial DNA-containing cell into a living marsupial, and we don’t have that technology developed yet either,” said Pask.
No possibility of cloning
Perhaps some of you have been asking, “What about cloning? Can’t we do that?”
Well, it is actually possible to revive the extinct animals using that method, called somatic cell nuclear transfer.
The process involves taking a nucleus (the structure containing the genome) from the cell of an extinct animal. Then, it’s inserted into an egg of a living relative species, which has had its own nucleus removed.
But there’s a limitation to that: it relies on intact cells from the extinct species. Therefore, this method is only applicable when the last living animal has recently died. In terms of reviving thylacines, it’s out of the question.
Now, scientists did use this method, and successfully. It’s just that the success was brief.
In 2003, researchers cloned a Pyrenean ibex, a subspecies of the Iberian wild goat. It went extinct when Celia, the last living individual, was killed by a falling tree.
The team cloned Celia with a fruitful result, and they were able to revive the subspecies from extinction. That happened only for several minutes though, because the newborn animal died of a lung defect.
Similarly, a project led by a paleontologist at the University of New South Wales aimed to bring back the southern gastric-brooding frog. The last known specimen of the frog died in 1983, but the team have some well-preserved tissues.
And even though the project has successfully produced early-stage embryos, no clones so far have survived to tadpoles or adult frogs.
Still a long way to de-extinction
Although there are some new discoveries and advancement towards bringing back extinct animals to live, the limitations are just greater—especially with our current technology.
I think what we can learn from this is that we should do our best to conserve and preserve the animals we’ve got right now. Because as we can see, de-extinction efforts are so hard to do, not to mention risky.