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.
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.
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.”