Scientists have discovered that modern polar bears are descended from now extinct brown bears that roamed the region we know today as Britain and Ireland. It is thought that polar bears moved into this area just before, or during the last Ice Age, where they mated with female brown bears.
The maternal lineage of the extinct Irish brown bear can still be traced to all modern polar bears today, according to the research published in the latest issue of the journal Current Biology. However, perhaps surprisingly, this DNA sequence does not appear in modern brown bears, but only in polar bears and the ancient brown bears that inhabited Ireland when it was experiencing much cooler temperatures.
The link was established after an international research team analysed mitochondrial sequences of ancient brown bears using DNA extracted from the teeth and skeletons of 17 bears found at eight cave sites across Ireland. The scientists compared the DNA sequence of the ancient Irish brown bears with global data from prehistoric and modern brown bears and polar bears. They discovered that the remains of ten of the Irish brown bears, dated to between 38,000 and 10,000 years ago (the period spanning the last Ice Age), had the maternal line that is now seen in every modern polar bear. Despite the DNA links, a bone isotopic analysis revealed that these ancient Irish brown bears did not have the same marine diet as polar bears.
Mitochondrial DNA is passed from the mother to her offspring. This DNA link between ancient brown bears from Ireland and modern polar bears provides a new insight into the migratory patterns of bears during changing environmental and climatic conditions. The paper suggests that climate changes within the last 100,000 years may have forced brown bears and polar bears to move into each other’s habitat and consequently come together to mate.
The paper’s lead author, Dr. Ceiridwen Edwards, now a researcher in the University of Oxford’s Research Laboratory for Archaeology, is part of the team that carried out the DNA sequencing and analysis. The DNA sequencing was conducted at Trinity College Dublin, using a process known as Polymerase Chain Reaction.
Dr. Edwards said: ‘Hybridisation between ancient Irish brown bears and polar bears has led to the complete replacement of the original polar bear mitochondria. This maternal lineage is now present in all modern polar bears. Environmental conditions appear to have played, and continue to play, a big part in the evolutionary history of polar bears. Today’s warming climate is again bringing modern polar bears into contact with brown bears in places like Canada and Alaska.’
The paper finds that the DNA sequencing of older brown bears that lived in Ireland between 43,000 and 38,000 years ago, before temperatures cooled, have a genetic signature that matches bears found today in eastern parts of Europe. The remains of the last ancient brown bears, dated at 5,000 to 3,000 years ago, have different signatures again, that are like modern western European bears. Only the brown bears from the cooler period, between 38,000-10,000 years ago, share their lineage with modern polar bears, says the paper.
Radiocarbon dating suggests the last brown bear population died out in Ireland around 3,000 years ago, but DNA can survive for tens of thousands of years due to the constant, cool temperatures of the caves where they were found. Study co-author Professor Daniel Bradley of Trinity College Dublin said: ‘Analysing ancient samples provides a means to go back in time and directly measure the movement of species in response to past climate change.’
When analysed together with a collection of modern and ancient bear sequences from around the world, researcher Professor Beth Shapiro, from the Pennsylvania State University in the United States, found a more complex picture of bear history emerging. Professor Shapiro said: ‘We found that brown bears and polar bears, which are hybridising today in the wild, have been hybridising opportunistically throughout the last 100,000 years and probably longer.’