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ICE AGE | The White Bear's Green Heritage?:

Polar Bears & Ice Age Ireland

DNA research suggests polar bears may be partly descended from ancient Irish brown bears

Our journeys through Ice Age Ireland have already thrown up a few fascinating surprises, such as the fact that spotted hyenas once prowled through the Irish landscape, far north of where their modern relatives now live. However, hyenas are not the only mammals inhabiting far-flung regions of our modern world with links to Ice Age Ireland. In those lands of the deep north in our modern world which are still governed by ice, there prowls a white-furred giant that seems synonymous with the Arctic lands it inhabits – the polar bear. This supreme beast of the Arctic lives its life thousands of kilometres from Ireland, and yet recently is has been discovered that it may carry with it a deep connection to this far more southerly land, an Irish heritage that can be traced back tens of thousands of years, when Ireland too was a land of ice.

A remarkable genetic study published in 2011 found that the closest match to one particular component of the DNA of today's polar bears belonged to Irish brown bears which lived near the end of the last Ice Age.

This possible kinship between modern polar bears and ancient Irish brown bears may come as something of a surprise but, as with explorations of the relationships between other mammals in previous blogs, it becomes less and less surprising the more we delve into the evolutionary relationships of these two ursine species.

Brown bears (Ursus arctos) and polar bears (Ursus maritimus) are actually very closely related, although exactly when their lines diverged from each other has been a matter of debate, with various fossil studies in the past producing divergence dates ranging between 800,000 and 150,000 years ago, while genetic studies have tended to place the split much earlier, somewhere between four to five million years ago and 600,000 years ago. However, a 2014 study settled on a divergence date based on genetic data which fits much more closely with the fossil evidence, concluding that the ancestors of the polar bears split from the brown bears sometime between 479,000 and 343,000 years ago, most likely during a long interglacial which occurred in this time frame.

Ice cores from two sites in Antarctica (blue & green lines) clearly showing that from around 425,000 to 375,000 years ago there was a warm interglacial, while global ice volume at this time (red line) was low

According to the study's authors, the prolonged warmth of this extended interglacial, which lasted from around 425,000 to 375,000 years ago, would have allowed an ancient brown bear population to expand their range far into northern lands that previously would have been uninhabitable. Over the next few hundred thousand years in their new northern domain, the descendants of these original brown bear pioneers would have progressively adapted to the conditions in their new homeland, diverging in form and habit from their ancestors.

A life on the ice, scavenging on the carcasses of sea mammals or hunting seals, promoted a change in their teeth, which diverged from the omnivorous form of their forebears (no pun intended), the cheek teeth becoming sharper, the canines more developed. Their faces also changed, morphing from the dished shape of a brown bear to become longer in design. Similarly, their bodies underwent significant modifications, the coat changing from the perhaps greyish hue of their ancestors to become almost white, this adaptation providing camouflage in their new habitat, aiding them when on the hunt, as did their paws which became large and paddle-like partly to facilitate walking across thin ice (like snowshoes).

These enlarged paws also proved very useful when they took to the water, serving as powerful oars to propel their great bodies, as did changes in the form of the body itself, which became longer and more teardrop-shaped for more efficient swimming. The evolution of a thick layer of body fat not only insulated them from the cold but also aided buoyancy in the water. Other aspects of their behaviour changed too, life in their new environment necessitating a change from denning in earth inland to denning in snow closer to the coast.

Polar bears are well adapted to swimming in the sea

It is thought that certainly by around 110,000 years ago these myriad adaptations had evolved and, thus, a new species, the polar bear, had emerged on Earth. However, although polar bears had evolved to become quite different in many respects to their brown bear relatives, there is plenty of evidence to suggest that interbreeding between the two species occurred a number of times during the late Ice Age.

Of course, such couplings are still known to happen every now and then in our modern world, producing fertile offspring, and when this happens between polar and grizzly bears the young are known as 'grolar bears' or, my own personal favourite, 'pizzly bears'. In any case, it was during one of these late Ice Age encounters with brown bears that the polar bears are thought to have inherited one of the most characteristic aspects of their genetic makeup – their mitochondrial DNA.

A polar bear/brown bear hybrid taking a nap

Mitochondria are tiny power generators that live inside every cell in more complex organisms. They are thought to be the descendants of an ancient bacterium that invaded, or was captured but not ingested by, another single-celled bacteria-like organism called an archaean over two billion years ago.

To cut a long story short, this incident turned out to be advantageous for both parties, the bacterium benefitting from living in a protected environment with plentiful nutrients and it eventually evolved to become an organelle – a structure that performs a specialised function – called a mitochondrion, capable of manipulating oxygen in such a way that releases energy from food. The archaean host, in turn, profitted greatly from having such a power generator on board, and it was this new type of single-celled organism that eventually gave rise to all the large multi-celled organisms in our world, such as the plants, fungi, and animals. Indeed, each breath you take as you read this continues to pass oxygen to the mitochondria in your cells, who use it to power your body and without which we could not survive.

In any case, because of their particular evolutionary origins, the mitochondria continue to reproduce in cells as if they were independent bacteria, and this has great advantages when investigating an organism's ancestry.

Although the vast majority of an organism's DNA is stored in the nucleus of the cell, that tiny fraction accounted for by mitochondrial DNA is very informative because mitochondrial DNA mutates much faster than nuclear DNA, meaning it is easier to track genetic changes over time and so determine how individuals in a species are related. Mitochondrial DNA is also different to normal nuclear DNA in that it can only be passed in an unbroken line from mother to daughter, as though males too inherit mitochondria from their mothers they cannot pass theirs on as it is destroyed during the fusion of egg and sperm which creates their own offspring. Mitochondrial DNA, then, can be thought of as a type of genetic surname, albeit unlike Irish surnames it is inherited down the female line.

Mouse cells showing the nucleus in blue, the mitochondria in green, and the cellular skeleton in red (L), and a simple diagram of an animal cell depicting the nucleus and mitochondria (R)

Unlike nuclear DNA, mitochondrial DNA is only passed from mother to daughter

So, one of the most useful aspects of this specific DNA type is that the mitochondrial DNA (mtDNA) in members of a living species can be analysed and a time frame for when the various strains all shared a most recent common ancestor can be produced. Analysis of the mtDNA of modern polar bears reveals that every polar bear on Earth today gained their mtDNA from a female ancestor which lived sometime between around 50,000 and 20,000 years ago, and it is here that we find the connection to Ireland's ancient brown bears.

The aforementioned 2011 genetic study looked at the mitochondrial DNA from modern and ancient brown bears which lived in the last 50,000 years, including those from Alaska as well as those which lived in the vicinity of what is now Britain and Ireland, which up to this point had not been sampled. Older polar bear fossils were also included, with one from Norway dating to around 115,000 years ago and another from Svalbard dating from somewhere between 110,000 and 130,000 years ago.

The results of this study were quite unexpected. The mtDNA of the older polar bear fossils showed that they were closely related but not directly ancestral to modern polar bears. Of the brown bears, the best mtDNA match to modern polar bear mtDNA was not from the modern Alaskan brown bears, which up to this point had been shown to be the most genetically similar to polar bears, but from ancient Irish brown bears which lived during the late Ice Age. This meant that sometime between 50,000 and 20,000 years ago, polar bears had interbred with Irish brown bears, resulting in the mtDNA from a female Irish brown bear passing into the polar bears' genetic makeup.

A likely point in time for such interbreeding to have taken place would have been in and around the Last Glacial Maximum (c. 26,000–19,000 years ago). As the ice sheets extended across Ireland, brown bears would have been driven farther and farther towards the coasts and eventually out of Ireland onto sea ice shelves, perhaps, or land exposed due to the much lower sea levels. Simultaneously, the extension of Arctic conditions southwards would have led to a southward migration of polar bears as there was now an abundance of their preferred habitat in the areas surrounding Ireland, on the western shelf and in the Celtic Sea and Irish Sea basins.

Thus, these were ideal conditions for the meeting and mating of polar bears and brown bears. Following the introduction of the Irish brown bear mtDNA into the polar bears' genetic makeup, subsequent mating between polar bears would have spread this through the species (which would not have been especially difficult if their population was quite low, as has been suggested), this Irish brown bear mtDNA eventually replacing the previous mitochondrial lineage to become the norm.

And as the Ice Age came to an end, the polar bears retreated to their northern lands. But though they are now only found far from Irish shores, the polar bears may yet carry a piece of Ireland with them. It might not look it, but this great white bear, an béar bán, may, in fact, be at least partly green.

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