Woolly rhinos went extinct at the end of the last ice age in Siberia about 14,000 years ago, and now ancient DNA is helping to shed light on what really happened to them and other large mammals.
Previously, it was believed that humans hunted these giant animals as they spread across the globe. But new research has suggested that climate change is the culprit, according to a new study based on sequencing ancient DNA from the well-preserved remains of 14 woolly rhinos.
“That we can read the DNA sequences, even the entire genomes, from these long-extinct animals is really amazing,” senior study author Love Dalén said in an email. Dalén is a professor of evolutionary genetics at the Centre for Palaeogenetics, a joint venture between Stockholm University and the Swedish Museum of Natural History.
“It’s a bit like having a time machine where we can travel back through time and study evolutionary change as it is happening in real-time.”
Given the climate where these animals lived and died, the cold conditions helped preserve their DNA.
“While obtaining high quality DNA is difficult, we are lucky to work on specimens that have been preserved in the permafrost for thousands of years,” said Nicolas Dussex, study coauthor and a postdoctoral researcher at the Centre for Palaeogenetics, in an email. “In a way, it’s like opening a freezer that was closed during the last Ice Age.”
The study published Thursday in the journal Current Biology.
The cold never bothered them anyway
Rather than disappearing due to overhunting by early humans, woolly rhino populations actually seemed to thrive and remain incredibly diverse before they went extinct. Recent research has also shifted back the timeline for humans living in Siberia. Originally, it was believed they arrived between 14,000 and 15,000 years ago.
New evidence has pushed human occupation back to sites that are at least 30,000 years old. So the arrival of humans no longer coincides with the demise of woolly rhinos, the researchers said.
Instead, the DNA they studied revealed more of a population boom for woolly rhinos during that time.
The DNA was retrieved from tissue, bone and hair samples from 14 woolly rhino specimens that lived across Siberia. The scientists were able to determine information about the population sizes and genetic diversity of these rhinos stretching back for tens of thousands of years before they went extinct.
The researchers were surprised to discover that woolly rhinos had a much higher genetic diversity than any living rhino, woolly mammoths or even modern humans.
The rhinos also appeared to go extinct suddenly, rather than gradually and did not experience much inbreeding. Inbreeding tends to increase as populations decline, and it occurred in the last woolly mammoths before they went extinct.
The researchers also found genetic mutations in the rhino DNA that helped them adapt to life in the bitterly cold weather of the last ice age, including a receptor in the skin that could sense temperature variations. Woolly mammoths also had this adaptation.
About 29,000 years ago, the woolly rhino population swelled as the ice age intensified and remained stable with little inbreeding. The data provided by the DNA followed the rhino population until about 18,500 years ago, which was about 4,500 years before they went extinct.
This tells the researchers that the cause for their extinction occurred during that 4,500-year gap.
A sudden but brief period of warming temperatures occurred toward the end of the last ice age. This event, called the Bølling-Allerød interstadial, happened between 12,890 and 14,690 years ago.
“The temperature change was fast,” said Dalén. “Some records from ice cores taken on Greenland suggest an increase in temperature by 10 degrees Celsius (18 degrees Fahrenheit), possibly within as little as a few decades.”
The large grasslands where the wooly rhinos roamed, called a steppe environment, would have been replaced by trees and shrubs in response to the warming as well.
Giant creatures from the last ice age
Like woolly mammoths, woolly rhinos were covered in thick fur and perfectly suited to their cold environment, grazing across the Siberian tundra. Both had adaptations that helped them thrive during the last ice age.
But mammoths were about three times bigger, had a more flexible diet and lived in matriarchal herds. The rhinos were likely more solitary, the researchers said.
And the mammoths didn’t experience an increase in population size as the rhinos did 29,000 years ago.
Now the researchers want to study DNA from woolly rhinos that lived during those last 4,500 years before they went extinct.
The scientists also want to investigate other large animals that had adapted to such cold conditions to see how they were affected by a warmer and less stable climate. This includes cave lions, wolves, mammoths, horses and steppe bison.
“We’re coming away from the idea of humans taking over everything as soon as they come into an environment,” said Edana Lord, study coauthor and postdoctoral student at the Centre for Palaeogenetics, in a statement. “Although we can’t rule out human involvement, we suggest that the woolly rhinoceros’ extinction was more likely related to climate.”