Ten extinct beasts that could walk the Earth again
THE recipe for making any creature is written in its DNA. So last November,
when geneticists published the near-complete DNA sequence of the long-extinct
woolly mammoth, there was much speculation about whether we could bring this
behemoth back to life.
Creating a living, breathing creature from a genome sequence that exists only
in a computer's memory is not possible right now. But someone someday is sure
to try it, predicts Stephan Schuster, a molecular biologist at Pennsylvania
State University, University Park, and a driving force behind the mammoth
genome project.
So besides the mammoth, what other extinct beasts might we coax back to life?
Well, it is only going to be possible with creatures for which we can retrieve
a complete genome sequence. Without one, there is no chance. And usually when a
creature dies, the DNA in any flesh left untouched is soon destroyed as it is
attacked by sunshine and bacteria.
There are, however, some circumstances in which DNA can be preserved. If your
specimen froze to death in an icy wasteland such as Siberia, or snuffed it in a
dark cave or a really dry region, for instance, then the probability of finding
some intact stretches of DNA is much higher.
Even in ideal conditions, though, no genetic information is likely to survive
more than a million years - so dinosaurs are out - and only much younger
remains are likely to yield good-quality DNA. "It's really only worth studying
specimens that are less than 100,000 years old," says Schuster.
The genomes of several extinct species besides the mammoth are already being
sequenced, but turning these into living creatures will not be easy (see
"Revival recipe"). "It's hard to say that something will never ever be
possible," says Svante Pääbo of the Max Planck Institute for Evolutionary
Anthropology in Leipzig, Germany, "but it would require technologies so far
removed from what we currently have that I cannot imagine how it would be done."
But then 50 years ago, who would have believed we would now be able to read the
instructions for making humans, fix inherited diseases, clone mammals and be
close to creating artificial life? Assuming that we will develop the necessary
technology, we have selected 10 extinct creatures that might one day be
resurrected. Our choice is based not just on feasibility, but also on each
animal's "megafaunal charisma" - just how exciting the prospect of resurrecting
these animals is.
Of course, bringing extinct creatures back to life raises a whole host of
practical problems, such as where they will live, but let's not spoil the fun...
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Sabre-toothed tiger
(Smilodon fatalis)
Extinct: ~10,000 years ago
DNA preservation: 3/5
Suitable surrogate: 3/5
This fabled beast with its extraordinary canines would be a sight to behold.
There are some spectacularly preserved sabre-toothed specimens from the La Brea
tar pits in Los Angeles, but the tar makes extracting DNA tricky, so nobody has
been able to isolate decent sequences. However, there are also some
permafrost-preserved specimens that might be a better source of DNA. If we
could obtain a genome, a close living relative of the sabre-tooth, the African
lion, should be a good egg donor and surrogate mother. Californians, beware!
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Neanderthal
(Homo neanderthalensis)
Extinct: ~25,000 years ago
DNA preservation: 1/5
Suitable surrogate: 5/5
A draft sequence of the Neanderthal genome should be published sometime this
year. "To have a reasonable-quality genome, say comparable to the chimpanzee,
will then be another two years of work or so," says Svante Pääbo of the Max
Planck Institute for Evolutionary Anthropology in Leipzig, Germany. While he
and his colleagues hope the genome will offer unique insights into the
differences between us humans and our mysterious cousins, there is speculation
it could also be used to resurrect the Neanderthal. Because of our very close
shared ancestry, humans would make ideal egg donors and surrogate mothers.
However, while Soviet scientists might once have tried to create a human-ape
hybrid, today it is hard to imagine even the most crazed of mad scientists
entering such taboo territory. "I find the idea of resurrecting the Neanderthal
so ridiculous that any speculation on surrogate mothers is superfluous," says
Pääbo. At most, researchers might replace some human genes with the Neanderthal
versions in cells growing in a dish to see what the effect is, he says.
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Short-faced bear
(Arctodus simus)
Extinct: ~11,000 years ago
DNA preservation: 3/5
Suitable surrogate: 2/5
This towering beast would dwarf the world's largest living land carnivore, the
polar bear. The short-faced bear may have been a third taller than the polar
bear when standing upright, and it weighed up to a tonne. Recovering its DNA
should be possible as there are specimens encased in permafrost. The
short-faced's closest living relative is the spectacled bear of South America.
The two species parted evolutionary company only around 5 million years ago,
but unfortunately, at just a tenth the body mass of the short-faced bear, the
spectacled bear is unlikely to be a particularly good surrogate.
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Tasmanian tiger
(Thylacinus cynocephalus)
Extinct: 1936
DNA preservation: 4/5
Suitable surrogate: 1/5
The last Tasmanian tiger or thylacine - an individual that has become known as
Benjamin - died in Hobart Zoo in 1936. The existence of various preserved
tissues less than a century old means geneticists should be able to get
good-quality DNA and produce a complete sequence of the thylacine genome before
too long. When it comes to resurrection, marsupials like the thylacine might be
easier than most other mammals. Pregnancy in marsupials typically lasts just
weeks, and a simple placenta forms only briefly, meaning there might be less
risk of incompatibility between an embryo and a surrogate mother of another
species. For the thylacine, the surrogate would be the Tasmanian devil. After
birth, the fetus could be raised on milk in an artificial pouch.
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Glyptodon
(Doedicurus clavicaudatus)
Extinct: ~11,000 years ago
DNA preservation: 2/5
Suitable surrogate: 1/5
The Volkswagen Beetle-sized "colossal" armadillo, with its spiky, club-like
tail, once rumbled across the South American countryside, and some might fancy
seeing it do so again. Because there are no frozen glyptodons, obtaining usable
DNA will depend on finding well-preserved remains in a cool, dry cave. Beyond
that, there is an even bigger problem: the most suitable species to act as a
host for a developing glyptodon embryo would be the far smaller 30-kilogram
"giant" armadillo. The difference in size means it would struggle to carry its
extinct relative to term.
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Woolly rhinoceros
(Coelodonta antiquitatis)
Extinct: ~10,000 years ago
DNA preservation: 4/5
Suitable surrogate: 5/5
Resurrecting the woolly rhino has lots going for it. As with the mammoth, there
are plenty of specimens preserved in permafrost, and the availability of hair,
horns and hooves is a big plus. These tissues can be cleaned up with shampoo
and bleach to remove contaminant DNA from microbes and fungi before using
enzymes to release an abundance of near-pure rhino DNA. This makes it likely
that geneticists will publish the complete genome of this hirsute beast before
long. However, although the woolly rhino has close living relatives that might
make suitable surrogates, all contemporary rhino species are themselves on the
brink of extinction. As long as this remains the case, resurrecting a woolly
rhino is unlikely to be a top priority.
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Dodo
(Raphus cucullatus)
Extinct: ~AD 1690
DNA preservation: 1/5
Suitable surrogate: 3/5
In 2002, geneticists at the University of Oxford got permission to cut into the
world's best-preserved dodo specimen, a foot bone - complete with skin and
feathers - held under lock and key at the university's Museum of Natural
History. "It was one of the scariest things I've had to do," recalls Beth
Shapiro, an ancient DNA specialist now at Pennsylvania State University. This
yielded minute fragments of dodo mitochondrial DNA but nothing more. Since
then, no other specimen has yielded even a whiff of dodo DNA, but there is
still hope that some will one day be found. "We're still looking," says
Shapiro. If one turns up and a genome sequence could be produced from it, it
would then be down to pigeons to help bring their famous cousin back from the
dead.
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Giant ground sloth
(Megatherium americanum)
Extinct: ~8000 years ago
DNA preservation: 2/5
Suitable surrogate: 1/5
This giant stood around 6 metres tall and is estimated to have weighed a
whopping 4 tonnes. The sloth's relatively recent extinction means that several
specimens have been found with hair, an excellent source of DNA. So are we
likely to see the giant sloth genome published? "Absolutely," says Hendrik
Poinar of MacMaster University in Canada, who has extracted giant ground sloth
DNA from fossilised dung deposited some 30,000 years ago. The difficulty for
anyone intent on resurrection would be the lack of a suitable surrogate. Its
closest living relative - the three-toed tree sloth - is tiny by comparison. It
might be able to provide eggs with which to create a giant ground sloth embryo,
but the fetus would quickly outgrow its surrogate mother.
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Moa
(Dinornis robustus)
DNA preservation: 3/5
Suitable surrogate: 2/5
There is plenty of moa DNA to be found in well-preserved bones and even eggs in
caves across New Zealand, so obtaining a moa genome should be doable. But which
one? It would be tempting to go for the massive Dinornis robustus, which stood
more than 3 metres tall, but starting with the more modestly sized Megalapteryx
didinus might make more sense. Although only distantly related to ostriches, it
might be possible to boot up the moa genome in an ostrich egg. As no bird has
yet been cloned, however, perhaps the most feasible approach would be to
engineer an ostrich embryo to be moa-like.
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Irish elk
(Megaloceros giganteus)
Extinct: ~7700 years ago
DNA preservation: 3/5
Suitable surrogate: 2/5
Deer-hunting enthusiasts would give almost anything for a chance to stalk this
Pleistocene giant, once found across Europe. A typical male Megaloceros stood
more than 2 metres tall at the shoulder and sported antlers 4 metres wide. It
is actually a deer rather than an elk, and its closest living relative is the
much smaller fallow deer, the two species having parted evolutionary company
around 10 million years ago. The gulf between the two species means it is hard
to see how a complete genome could be converted into a living, breathing animal.
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Giant beaver
(Castoroides ohioensis)
Extinct: ~10,000 years ago
DNA preservation: 2/5
Suitable surrogate: 1/5
There is fierce controversy over the reintroduction of normal beavers in some
countries, so imagine how much fuss there would be over the reintroduction of
the 2.5-metre-long giant beaver to North America. It's not too much to hope for
a genome sequence of this massive rodent, says Hendrik Poinar, a geneticist at
McMaster University in Hamilton, Canada. The capybara, which is about half the
mass, would probably be the most suitable surrogate, though it might still be
too distant a relative.
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Gorilla
(Gorilla gorilla)
Extinct: Almost
DNA preservation: 5/5
Suitable surrogate: 5/5
The first species to be brought back from extinction will most likely be one
that is alive today. Conservationists are freezing tissue samples from some
threatened species, so clones could be created with the help of a closely
related surrogate species if a suitable habitat becomes available. For
gorillas, the surrogate would be the chimpanzee.
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Revival recipe
YOU WILL NEED:
Well-preserved DNA
Several billion DNA building blocks
A suitable surrogate species
Some seriously advanced technology
HERE'S WHAT TO DO...
1. Extract the DNA from your extinct species, sequence the fragments and
assemble to obtain a complete genome.
REALITY CHECK: genome sequences from extinct animals are likely to be riddled
with lethal errors.
2. Now take your DNA building blocks and recreate the DNA of your extinct
beast, in the correct number of chromosomes.
REALITY CHECK: it is not yet possible to make such long DNA molecules from
scratch, but we should be able to one day.
3. Package the chromosomes up into an artificial nucleus and pop it in an egg
collected from your suitable surrogate species. This should then develop into
an embryo, which will be a clone of a long-dead animal.
REALITY CHECK: finding compatible species, let alone extracting eggs from them,
could be a huge problem. Plus, no one has yet managed to clone birds or
reptiles.
4. Grow a baby animal from the embryo. For mammals, implant the embryo in the
womb of a compatible surrogate mother. For a reptile or bird, incubate embryo
using yet-to-be-developed techniques. For an amphibian or fish where
fertilisation takes place outside the body, just sit back and watch.
REALITY CHECK: compatible surrogate mothers may not exist for many extinct
mammals.
HOW TO CHEAT:
Rather than synthesising the entire genome from scratch, you could take the DNA
of a closely related living species and modify it to be more like that of the
extinct species you are aiming for.
REALITY CHECK: some living species have already been made superficially more
like extinct ones, but with today's knowledge and technology they remain far
from the real thing.
Source: New Scientist.
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