Read the text and choose the best group of words for each gap in the text.

Bringing back the woolly mammoth and other extinct creatures may be impossible

An extinct rat that once lived on an island in the Indian Ocean
may have put an end to
once aroused
has clearly strengthened (3 p.)
scientists’ dreams of resurrecting more famous extinct animals like the woolly mammoth. The Christmas Island rat disappeared just over 100 years ago, but researchers now say even its detailed genome isn’t complete enough to bring it back to life.

No species has yet been revived, but de-extinction appeals to many geneticists and futurists. “I know a lot of biologists who think,
"Why should we even try?"
"Can I do this?"
"This is impossible!" (3 p.)
says Karen Wendling, an ethicist at the University of Guelph. Part of the fascination is simply the promise of seeing a vanished species come to life. But putting a key animal back into its original habitat could
have unforeseen consequences.
also help restore ecosystems.
slow down climate change. (3 p.)
The mammoth once kept arctic shrubs and trees under control and fertilized grasses with their manure.

To decide which animal to revive,
To bring back an extinct species,
To discover which species are related, (3 p.)
scientists would first need to sequence its genome, then edit the DNA of a close living relative to match it. Next comes the challenge of making embryos with the revised genome and bringing them to term in a living surrogate mother. So far, scientists have sequenced the genomes of about 20 extinct species, including a cave bear, passenger pigeon, and several types of mammoths and moas. But no one has yet reported re-creating the extinct genome in a living relative.

In the new study, Tom Gilbert, an evolutionary geneticist at the University of Copenhagen, and his colleagues, thought it best to start small. “If we want to try something so crazy,
let's take a very unusual animal,"
why not start with a simple model,"
it's best to use an extinct species," (3 p.)
Gilbert reasoned. So, the researchers focused on the Christmas Island rat (Rattus macleari), which disappeared by 1908, because it is closely related to the Norway rat, a well-studied lab animal with a complete genome sequence that scientists already know how to modify.

The researchers extracted DNA from the skins of two preserved Christmas Island rats and sequenced it many times over to get as much of the genome as possible and used the genome of the Norway rat as a reference. Comparing the two genomes revealed that
almost 5% of the Christmas Island rat's genome was still missing.
relying on the Norwegian rat as the only point of reference was problematic.
the Norwegian and Christmas Island rats had almost identical genomes. (3 p.)
Many of those genes will be the ones that make each species unique, says Victoria Herridge from the Natural History Museum in London. She notes that the more time that has passed since an extinct species and its living relative have diverged, the more genes are likely to remain unknown. The work “really highlights the difficulties, maybe even the ridiculousness, of de-extinction efforts,” she says.

But Andrew Pask, a developmental biologist at the University of Melbourne who is spearheading an effort to bring back an extinct marsupial called the thylacine, or Tasmanian tiger, is unfazed by the new report and remains optimistic. “The thylacine is
perhaps the best candidate for de-extinction,"
even a more challenging species to study,"
interesting as it, too, lived on an island," (3 p.)
he says. His team sequenced this predator’s genome as well as the genomes of several potential surrogate species, including the dunnart or marsupial mouse. They find that although 5% of the thylacine’s genome cannot be reconstructed, that 5% is primarily repetitive regions that
may make the thylacine look somewhat similar to the dunnart.
likely won't affect how the transformed animal looks or behaves.
could result in some repetitive behaviors in the animal. (3 p.)

Conversely,
Moreover,
Instead, (3 p.)
sequencing is “steadily improving” for both modern and extinct DNA, says George Church, the Harvard University geneticist who helped found a company called Colossal Laboratories and Biosciences. “Many 100% animal genomes will be arriving faster and faster.”

Gilbert now thinks creating an exact replica of a mammoth or a passenger pigeon will be “impossible.” But such efforts might lead to ‘proxies’, animals close enough to carry out the same function in the extinct species’ old ecosystem.

Herridge cautions that in most cases scientists won’t know in advance how the edited genes will affect the animal’s behavior and ability to survive. Some researchers also think de-extinction efforts
can become increasingly more difficult technically.
may take needed resources away from conservation.
might result in species whose behavior is unpredictable. (3 p.)
“You can save eight extant species for the cost of one that you make de-extinct,” Wendling points out.

But even those who are not sure that de-extinction could or should happen still fantasize about the possibility. Wendling wants to know what a dodo bird is like. And, Herridge says, “Personally, I would love to see a giant ground sloth or a saber-toothed cat!”