Smithsonian - Alex Fox
Populations of the Ice Age icon were healthy right up until their extinction, suggesting they crashed precipitously as the planet warmed
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A lifelike restoration using the remains of a baby woolly rhinoceros recovered from the Siberian permafrost. The specimen was nicknamed Sasha after the hunter who discovered it. (Albert Protopopov)
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Some 30,000 years ago, the hulking woolly rhinoceros and its
curved, three-foot-long horn inspired ancient humans to streak the
limestone walls of
Chauvet Cave
in southern France with vivid charcoal portraits of the Ice Age beast.
But were humans’ artistic renderings of the woolly rhino accompanied by
hunting that drove the creatures extinct roughly 14,000 years ago?
The explanation for the demise of Ice Age megafauna, including the woolly mammoth, giant sloth and saber-toothed cat, has been
debated for
decades. Many researchers have proposed that the mammals were hunted into extinction by
Homo sapiens in what’s been termed the “
overkill hypothesis.” Now, in
research published today in
Current Biology,
scientists who analyzed the animal’s DNA suggest that it may have been
the planet’s swiftly changing climate that undid the species.
“Humans
are well known to alter their environment and so the assumption is that
if it was a large animal it would have been useful to people as food
and that must have caused its demise,” says Edana Lord, a graduate
student at the Centre for Palaeogentics and co-first author of the
paper. “But our findings highlight the role of rapid climate change in
the woolly rhino’s extinction.”
The woolly rhino was a fur-matted fortress of a creature that
grazed across the dry, frigid steppe ecosystem of northern Eurasia,
including modern-day France but particularly in Siberia, for hundreds of
thousands of years. They were roughly the same size as the white rhinos
of today, which can reach up to five tons. Scientists speculate that
woolly rhinos used their horns, which are thinner and more blade-like
than those of living rhinos, to sweep away snow and nibble at
frost-crusted tufts of greenery.
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A woolly rhinoceros skeleton. (Fedor Shidlovskiy)
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But suddenly, around 14,000 years ago the woolly rhino died out.
Humans are thought to have first made their way to the rhino’s Siberian
stronghold around 30,000 years ago, meaning they overlapped with woolly
rhinos for some 16,000 years. The animal’s last centuries of existence
coincided with a sudden and severe warming event called the
Bølling–Allerød interstadial,
which began around 14,700 years ago. During this roughly 2,000-year
period of warmth, the meltwater gushing from the planet’s immense ice
sheets raised sea level by approximately
50 feet.
For the paper’s senior author Love Dalen, a paleogeneticist at
Stockholm University and the Swedish Museum of Natural History, the goal
was to establish when the woolly rhino’s population started to decline
and what that lined up with. “Did the decline start 30,000 years ago
when humans appeared,” says Dalen. “Or 14,000 years ago when we know the
planet went through a period of rapid warming?”
To
find out when that decline likely began, Dalen and his colleagues
needed good quality samples of woolly rhino DNA. The ancient animal’s
DNA would reveal how inbred or genetically diverse the woolly rhino was,
which would allow researchers to infer when the population started
shrinking before going extinct. Finding samples meant scientists spent
years tramping around the
increasingly sodden Siberian permafrost
to track down pieces of bone, tissue and hair from specimens. They then
screened those samples to find the ones with the most pristine genetic
material.
The team selected and sequenced the mitochondrial genomes of 14 samples that included a
baby woolly rhino found mummified in the permafrost with much of its hide intact and from a scrap of rhino meat recovered from the stomach of an Ice Age
puppy found with its internal organs intact.
Animal cells contain both nuclear and mitochondrial DNA. While most
of the cell’s DNA is contained in the nucleus, the mitochondria also
has its own DNA.
These short, maternally inherited lengths of
genetic code are easier to sequence but provide less information about
ancestry than the much larger nuclear genome. Still, the multiple
samples of mitochondrial DNA would allow the researchers to expand and
corroborate results gleaned from a nuclear genome, which is considered
more reliable because it contains fewer mutations.
To sequence the woolly rhino’s nuclear genome, Dalen needed a
sample that carried relatively intact and unfragmented strings of
genetic code and low levels of contamination from the bacterial DNA that
often dominates ancient cells. The researchers finally settled on an
18,500-year-old humerus found in the Arctic Chukotka region of Russia.
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Study co-lead author Edana Lord, a paleogeneticist at the Centre for Palaeogenetics, extracting a DNA sample from woolly rhino tissue in the lab. (Marianne Dehasque)
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But to know how to piece together the disordered fragments of the woolly rhino’s ancient nuclear genome the researchers needed what’s called a reference genome, a genome from a modern relative of the extinct species being sequenced. The scientists sequenced the woolly rhino’s closest living relative, the endangered
Sumatran rhino. Dalen estimates the two species share nearly 99 percent of the same DNA, and, because they’re so genetically similar, the team was able to use the properly arranged Sumatran rhino genome to figure out how to arrange the recovered fragments of woolly rhino DNA.
The 18,500-year-old bone showed strong genetic diversity, indicating that the species’ population was relatively stable and was missing the duplicate sections of genetic code that result from inbreeding. “This was a healthy individual in terms of genetic diversity, which leads us to believe that the decline in population leading up to the woolly rhino’s extinction must have happened rapidly sometime between 18,500 years ago and 14,000 years ago,” says Lord.
“This paper shows that woolly rhino coexisted with people for millennia without any significant impact on their population,” says
Grant Zazula, a paleontologist for Canada’s Yukon territory and Simon Fraser University who was not involved in the research. “Then all of a sudden the climate changed and they went extinct.”
This doesn’t prove that human hunting didn’t help extinguish the
woolly rhino, cautions Dalen. It’s possible that either human
populations or human hunting capabilities reached some crucial tipping
point in the 4,500 years his team’s data doesn’t cover.
But
the more likely explanation, says Dalen, is that the massive change in
climate during the Bølling–Allerød interstadial drove the rhino into
oblivion. “I personally don’t think that the increase in temperature in
itself was a big problem for the rhinos, but what that warming does is
increase precipitation,” says Dalen.
An increase in precipitation would have resulted in more snow. If
the snow was many feet deep, it may have put the grasses out of reach of
the rhinos, placing them in danger of starvation. Eventually, the
warmer, wetter climate would have transformed the steppe ecosystem the
woolly rhino depended on from grasslands to shrubs to more forests.
Kay Behrensmeyer, the curator of vertebrate paleontology at the
Smithsonian National Museum of Natural History, isn’t totally convinced
that climate change alone slayed the woolly rhino. Extinction doesn't
usually have a single cause, she says, but instead occurs “when a range
of factors critical to a species' existence reach a tipping point.” She
says that even with a scant 4,500-year window, human hunting and climate
change may have each played a role in the rhino’s doom.
Lord and Dalen say they hope to find more recent samples with high
quality DNA so they can probe the millennia just prior to the loss of
the woolly rhino. Zazula says even if the final word on the woolly
rhinoceros adds human hunting to the mix, the massive changes wrought by
Earth’s fickle climate not so long ago should be a lesson for our
species to tread more carefully in the present. “It only takes a short
period of warming to drive cold adapted species extinct,” he says. “That
rapid warming at the end of the Ice Age is similar to what we’re seeing
now and it could have similarly devastating consequences for Arctic
species today.”
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