19 March 2022 | New Scientist | 23IT IS impossible to bring extinct
animals back to life in their
original form, 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, they couldn’t
recreate many key genes, meaning
any resurrected animal would
differ in some crucial 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.”
A few research groups are trying
to resurrect extinct animals by
sequencing the DNA in preserved
samples, then editing the genome
of a close living relative to make
it like that of the extinct species.
These teams include Colossal,
a biosciences 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,
or Tasmanian tiger. Last week,
Colossal announced that it has
raised US$75 million in funding,
which it will spend on further
developing the technologies
needed for de-extinction.The fundamental problem
is that old DNA breaks up into
lots of tiny pieces that are
impossible to completely
reassemble, says Gilbert.
The Christmas Island rat
(Rattus macleari) – also known
as Maclear’s rat – was once
found on Christmas Island inthe Indian Ocean, before it went
extinct in the early 20th century.
The researchers were able to
reassemble most of the pieces
of its DNA 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,” says
Gilbert. “There’s a 5 per cent
fraction we can’t make sense of.”
Crucially, it is the parts of the
extinct animal’s genome that differ
most from the living relatives
that are the 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 jigsaw
puzzle are the parts that can’t
be put back together, because
those areas 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. These include
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 (Current
Biology, doi.org/hkpf).
The Christmas Island rat is
thought to have been wiped out
by a disease carried by the Norway
rat, so it might actually be an
advantage for a resurrected animal
to have Norway rat immune genes.
However, smell plays a key role in
behaviours such as finding food,
avoiding predators and choosing
mates, says Gilbert, so a recreated
Christmas Island rat might behave
differently to the original species.Gilbert has no plans to try to
resurrect the Christmas Island rat.
The team studied it only as a way
of exploring what is possible. But
he 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.”
“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
organisation whose initiatives
include efforts to resurrect
the passenger pigeon and the
heath hen. “De-extinction is
about restoring nature, not
individual species.”
If resurrected animals turn
out to have key traits missing, it
might still be possible to restore
those traits by taking genes from
other living or extinct relatives,
says Novak. “Ultimately, the paper
changes nothing about how
de-extinction works in practice
or how the world’s four projects
are proceeding.” ❚“The most important
pieces of the jigsaw
puzzle are the parts that
can’t be put back together”
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An effort to reconstruct the genome of the Christmas Island rat suggests we will
never be able to resurrect lost animals just as they were, finds Michael Le PageChristmas Island
and the extinct
Christmas Island ratNews