Human hunters were not responsible for killing off the woolly rhino, according to a new study of the extinct creature’s genome that found climate change is to blame.
Experts from Stockholm University sequenced ancient DNA taken from 14 woolly rhinoceros fossils to find out the state of their population near the end of their existence.
The extinction of prehistoric big beasts such as woolly mammoths, rhinos and cave lions at the end of the last ice age has previously been attributed to the spread of early humans around the globe and their hunting patterns.
While overhunting led to the demise of some species, research suggests that the extinction of the woolly rhino may have actually been down to a warming climate.
Study authors found that the woolly rhino population remained ‘stable and diverse’ until only a few thousand years before it disappeared from Siberia.
They said this was when temperatures likely rose too high for the cold-adapted species to cope with the warming environment and led to them dying off.
To learn about the size and stability of their population in Siberia, the researchers studied the DNA extracted from tissue, bone, and hair samples of fossilised remains.
Study senior author Professor Love Dalén said it was initially thought humans appeared in northeastern Siberia 14,000 or 15,000 years ago.
This was around the time we know that the woolly rhinoceros went extinct.
‘But recently, there have been several discoveries of much older human occupation sites, the most famous of which is around 30,000 years old,’ said Dalén.
‘So, the decline towards extinction of the woolly rhinoceros doesn’t coincide so much with the first appearance of humans in the region. If anything, we actually see something looking a bit like an increase in population size during this period.’
Study co-first author Edana Lord said they were able to sequence the complete nuclear genome of woolly mammoths to ‘look back in time’.
‘We also sequenced 14 mitochondrial genomes to estimate the female effective population sizes,’ according to Lord.
By looking at the genetic diversity – or ‘heterozygosity’ – of the genomes, the researchers were able to estimate the woolly rhino populations for tens of thousands of years before their extinction.
Co-first author Dr Nicolas Dussex said they also examined changes in population size and estimated levels of inbreeding within the active populations.
‘We found that after an increase in population size at the start of a cold period some 29,000 years ago, the woolly rhino population size remained constant and that at this time, inbreeding was low,’ said Dussex.
The researchers explained that the stability lasted until well after humans began living in Siberia, contrasting the declines that would be expected if the woolly rhinos went extinct due to hunting.
Doctoral student Ms Lord said: ‘That’s the interesting thing: we actually don’t see a decrease in population size after 29,000 years ago.
‘The data we looked at only goes up to 18,500 years ago, which is 4,500 years before their extinction, so it implies that they declined sometime in that gap.’
The DNA data also revealed genetic mutations that helped the woolly rhinoceros adapt to colder weather – which may have contributed to their decline.
One of these mutations, a type of receptor in the skin for sensing warm and cold temperatures, has also been found in woolly mammoths.
Adaptations like this suggest the woolly rhinoceros, which was particularly suited to the frigid northeast Siberian climate, may have declined due to the heat of a brief warming period, known as the Bølling-Allerød interstadial.
This period that coincided with their extinction towards the end of the last ice age.
‘We’re coming away from the idea of humans taking over everything as soon as they come into an environment, and instead elucidating the role of climate,’ said Lord.
‘Although we can’t rule out human involvement, we suggest that the woolly rhinoceros’ extinction was more likely related to climate.’
The team hope to study the DNA of additional woolly rhinos that lived in that crucial 4,500-year gap between the last genome they sequenced and their extinction.
‘What we want to do now is to try to get more genome sequences from rhinos that are between 18,000 and 14,000 years old,’ said Dalén, as at some point their population ‘surely must decline’.
The researchers are also looking at other cold-adapted megafauna to see what further effects the warming, unstable climate had.
‘We know the climate changed a lot, but the question is: how much were different animals affected, and what do they have in common?’
The findings have been published in the journal Current Biology.