A study of the extinct Christmas Island rat suggests we cannot bring back the woolly mammoth and other lost species without important differences
It is impossible to bring extinct animals back to life exactly as they were, according to a study of the extinct Christmas Island rat. Even though researchers were able to recover a very high-quality genome from preserved specimens, it was impossible to recreate many key genes, meaning any resurrected animal would differ in some important ways.
“You may be missing what’s most important for the extinct form,” says Thomas Gilbert at the University of Copenhagen in Denmark. “If you think you are going to create a mammoth that’s exactly like the mammoth that went extinct, well, you are not really.”
Gilbert isn’t opposed to de-extinction. It is feasible to create animals that can perform the same role in ecosystems as extinct ones, he says. “If you’re happy with the end product, awesome.”
A few research groups are trying to resurrect extinct animals by sequencing the DNA in preserved samples, then genetically editing the genome of a close living relative to make it like that of the extinct species. They include Colossal, a company that wants to create a woolly mammoth, and the TIGRR lab at the University of Melbourne, Australia, which aims to bring back the thylacine.
The fundamental problem is that old DNA breaks up into lots of tiny pieces that are impossible to completely reassemble, says Gilbert.
In the case of the Christmas Island rat (Rattus macleari) – also known as Maclear’s rat – which went extinct in the early 20th century, the team was able to reassemble most of the pieces by using the genome of the related Norway brown rat (Rattus norvegicus) as a guide, but they couldn’t assemble all of them.
“Every bit of DNA that we could recover, we got,” he says. “There’s a 5 per cent fraction we can’t make sense of.”
Crucially, it is the parts of the extinct genome that differ most from the living relatives that are hardest to match and reassemble. This 5 per cent includes the genes that have been evolving the fastest, which are the ones that make closely related species different to each other.
In other words, the most important pieces of the puzzle are the parts that can’t be put back together, because those parts of the guide picture have been lost.
With the Christmas Island rat, the team was able to recreate near-complete versions of around half of its genes. This includes genes related to its hair and ears, suggesting that it would be possible to create an animal with the long black hair and round ears characteristic of this species.
However, many other genes, including those involved in the rat’s immune system and its sense of smell, could only be partially reconstructed. Smell plays a key role in behaviours such as finding food, avoiding predators and choosing mates, says Gilbert, so any recreated Christmas Island rat might behave very differently to the original species.
He has no plans to try to resurrect the Christmas Island rat. The team studied it only as a way of exploring what is possible.
“This paper nicely shows that the more evolutionary distance there is between the extinct species [and living relatives], the more of the genome won’t be correctly assembled,” says Beth Shapiro at the University of California, Santa Cruz.
“Does this mean that we will never, ever be able to reconstruct a genome using gene editing that is 100 per cent identical to a specific extinct organism? Yes,” she says. “But that is not surprising, and nor does it mean that Colossal will never be able to create an Arctic-adapted elephant that some might call a mammoth or that the TIGRR lab won’t be able to create a marsupial that has physical and behavioural traits that reflect the evolution of the Tasmanian tiger.”
“The goal of de-extinction has always been to create functional equivalents,” says Ben Novak at Revive & Restore, a US conservation non-profit whose initiatives include efforts to resurrect the passenger pigeon and the heath hen.
“Ultimately, the paper changes nothing about how de-extinction works in practice or how the world’s four projects are proceeding,” he says.
Journal reference: Current Biology, DOI: 10.1016/j.cub.2022.02.027
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