For millennia they trudged through the icy landscapes of Eurasia – but at the end of the last ice age the woolly rhinos even disappeared from Siberia. Did humans exterminate them or did the climate change at the time kill them? The results of a genetic study now suggest a rapid change in environmental conditions rather than the cause of extinction. Because the population of woolly rhinos remained stable long after humans had established themselves in their habitat. The decline apparently only occurred with the beginning of a known warm phase.
In addition to the woolly mammoth, the woolly rhinoceros (Coelodonta antiquitatis) another celebrity of the northern ice age megafauna. Its features are well known from fossils from the permafrost of Siberia and also from bone finds in Europe. The wool rhinoceros reached a body length of over 3.60 meters and a weight of almost three tons. The characteristic horn of these animals reached up to 1.23 meters measured across the arch. Due to its stocky build and thick fur, the woolly rhinoceros was well adapted to the rough steppe landscapes of the Ice Age.
Is the human factor or the climate decisive?
It is still unclear why the species completely disappeared at the end of the Ice Age. The climatic and vegetation changes during the transition to today’s warm period come into question, but also the heavy hunting by humans. “Originally it was believed that man appeared in northeast Siberia fourteen or fifteen thousand years ago, around the time the woolly rhinoceros became extinct. But recently traces of much older human habitation have been discovered, the most important of which are around thirty thousand years old, ”says senior author of the study Love Dalén from Stockholm University. “The extinction of the woolly rhinoceros does not coincide so clearly with the first appearance of humans in the region”. This raises doubts about the crucial role of humans in the decline of the species.
In order to gain insight into the development of the size and stability of the woolly rhinoceros population in Siberia, the researchers have now analyzed the genetic material from tissue, bone and hair samples from 14 individuals. Using the genetic diversity of these genomes, the researchers were able to estimate the populations of woolly rhinos for tens of thousands of years before their extinction. “We sequenced an entire nuclear genome to look back in time. We also sequenced fourteen mitochondrial genomes. This genome reflects the effective population sizes of the females, ”explains Edana Lord, first author of the study, from the Center for Paleogenetics in Stockholm.
Stable co-existence with humans
As the scientists report, it became apparent in the genetic characteristics of the animals: After an increase in population size at the beginning of a cold spell about 29,000 years ago, the population size of woolly rhinos remained constant and the extent of inbreeding was low during this time. This stability lasted well after the beginning of human settlement in Siberia. So there is no decline in population density that would be expected if human hunting had reached a critical level, ”say the scientists. “The dates we looked at go back 18,500 years, around 4,500 years before the woolly rhinos went extinct. That means that their population must have decreased at some point during that time, ”says Lord.
As the researchers also report, the DNA data also made it clear how strongly the woolly rhinoceros was adapted to the cold through genetic mutations. One of these genetic characteristics, which affects a type of receptor in the skin for the perception of temperature differences, is also known from the genome of the woolly mammoth. Adaptations such as these indicate that the woolly rhinoceros may have reacted particularly sensitively to periods of warming through its adaptation to the cold north-east Siberian climate.
And precisely one such is also known from the time of decline: the so-called Bølling-Allerød-Interstadial saw significant climatic changes, say the scientists. “Although we cannot rule out human involvement, we suspect that the extinction of the woolly rhinoceros was more related to the climate,” sums up Lord. “We are moving away from the one-sided notion that humans dominated everything as soon as they opened up in a new environment,” says the scientist.
The researchers now hope to be able to examine the DNA of other woolly rhinos, which come from the crucial time gap of 4,500 years before their ultimate end, in order to gain more precise insights into the course of the decline. They also want to investigate the fate of other cold-adapted representatives of the Ice Age megafauna to see what effects the warming, unstable climate had at the end of this era. “We know that the climate has changed a lot, but the question is: How badly were different animal species affected and what do they have in common?” Says Dalén.
Source: Cell Press, technical article: Current Biology: doi: 10.1016 / j.cub.2020.07.046