Whenever humans settle an area, local species tend to disappear either because of our encroachment of their habitat, overhunting, disease, or some other form of human-induced impact. This has been going for some time, which is why humans are often the prime suspects for the extinction of many megafauna species such as woolly mammoths and cave lions. But although we have a proven track record of driving species to extinction that doesn’t mean they’re all on us.
According to a fascinating study that sequenced the ancient DNA from 14 individual woolly rhinoceros, these giant beasts were still doing very well for themselves even though human presence had been relatively stable for thousands of years in their Siberian habitat. However, it was only after an abrupt shift in climate that warmed their habitat that the cold-adapted beasts began to enter a downward slope.
“It was initially thought that humans appeared in northeastern Siberia fourteen or fifteen thousand years ago, around when 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 thirty thousand years old,” says senior author Love Dalén , a professor of evolutionary genetics at the Centre for Palaeogenetics, a joint venture between Stockholm University and the Swedish Museum of Natural History.
“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.”
The woolly rhino (Coelodonta antiquitatis) first appeared some 350,000 years ago and may have survived until as recently as 14,000-10,000 years ago. The herbivore fed on grass, shrubby sprouts, lichens, and mosses — and it was very good at it too as its range extended all the way from South Korea to Scotland and Spain. Cave paintings in France depict them there as early as 30,000 years ago.
Its thick fur made it extremely well adapted to the cold weather of Siberia, as well as most other frigid areas that were parts of its habitat, particularly during the last Ice Age.
Dalén and colleagues wanted to investigate whether humans or a shifting climate ultimately wiped out these ancient rhinos. But how could one tell? Believe it or not, telltale clues can be gleaned from a creature’s DNA, which records molecular events that scientists can use to infer things such as population size or instances of inbreeding.
The researchers sequenced the DNA from the tissue, bone, and hair samples retrieved from 14 woolly rhinos. The nuclear genome, which contains genes from both parents, enabled the researchers to estimate population sizes over time, while the mitochondrial genomes, which are only passed from the mother’s side, revealed instances of inbreeding. Whenever frequent inbreeding occurs, it means that there are that many genetically diverse members in a population, pointing to the fact that their numbers have dwindled significantly.
“We examined changes in population size and estimated inbreeding,” says co-first author Nicolas Dussex, a postdoctoral researcher at the Centre for Palaeogenetics. “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.”
Although humans are believed to have become widespread across Siberia about 14,000 years ago, archaeological evidence suggests that the first people arrived there at least 30,000 years ago.
If humans had a major role in driving the woolly rhinos extinct, there should have been a marked decline in their population starting from when the two species began to have contact. However, the new study seems to indicate that the rhino population was stable for thousands of years living alongside humans.
The DNA sequencing also revealed specific genetic mutations that may have helped the woolly rhinoceros adapt to and thrive in very cold weather. One such mutation expresses a type of receptor in the skin that senses warm and cold temperatures.
Mutations such as these suggest that the rhinos may have found it challenging to adapt to a brief warming period, such as the Bølling-Allerød interstadial, which 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 in megafaunal extinctions,” says co-first author Edana Lord , a PhD student at the Centre for Palaeogenetics.. “Although we can’t rule out human involvement, we suggest that the woolly rhinoceros’ extinction was more likely related to climate.”
This story isn’t a wrap. The data from the genomes offer a timeline that only goes up to 18,500 years ago, meaning there’s a crucial 4,500-year-old gap between the last genome the researchers sequenced and the extinction of the woolly rhinoceros.
“What we want to do now is to try to get more genome sequences from rhinos that are between eighteen and fourteen thousand years old, because at some point, surely they must decline,” says Dalén. 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 were reported today in the journal Current Biology.
