At the end of the last ice age, the spread of early humans into new regions placed too much strain on some animal species due to overhunting. While it has been assumed that this is why the woolly rhinoceros vanished, a new study published by Cell Press suggests that global warming was responsible.
The analysis of ancient DNA revealed that the woolly rhinoceros population in Siberia remained stable until a few thousand years before it was wiped out. The experts speculate that temperatures simply became too warm for the cold-adapted species.
Study senior author Love 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 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,” said Professor 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.”
The study was focused on the DNA testing of tissue, bone, and hair samples collected from the remains of 14 individuals.
“We sequenced a complete nuclear genome to look back in time and estimate population sizes, and we also sequenced fourteen mitochondrial genomes to estimate the female effective population sizes,” said study co-first author Edana Lord.
The genetic diversity that was revealed through genomic testing made it possible for the team to estimate the woolly rhino populations for tens of thousands of years before they disappeared.
“We examined changes in population size and estimated inbreeding,” said co-first author Nicolas Dussex. “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.”
The stability of the rhino numbers persisted until well after humans appeared in Siberia, which means that hunting was not responsible for their demise.
“That’s the interesting thing,” said Lord. “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 approximately 4,500 years before their extinction, so it implies that they declined sometime in that gap.”
The researchers also discovered various genetic mutations that would have helped the woolly rhinoceros adapt to colder weather. One of these mutations has also been found in woolly mammoths. These types of adaptations suggest the woolly rhinoceros was not suited for the warmer temperatures that abruptly emerged toward 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,” said Lord. “Although we can’t rule out human involvement, we suggest that the woolly rhinoceros’ extinction was more likely related to climate.”
The study is published in the journal Current Biology.