Polar bears are actually a relatively recent species. Evolving from brown bears no more than two glacial cycles ago. With most evidence only dating back to the last glacial cycle.
I actually believe along with others that they are a species that likely comes and goes during glacial and interglacial cycles. And unlike popular belief, during the ice ages they likely did not live where they are found now. A lot of polar bear fossils have been located in England, Siberia, and coastal Greenland. All of which were relatively ice free when they died.
And their species can only spread as far as it has because of the interglacials. In fact, the species is not native to Alaska and Canada at all. They evolved in Europe, and actually did not enter North America until after humans did, around 10 kya. And have only been allowed to extend their range so far because the ice cap is mostly gone.
They do not actually live for extended periods of time on the ice, but because of their adaptations they are clearly the apex pioneer predator along the edges of the ice fields. And if they were ever to vanish, the advantage of their coloring would also vanish and they would return to being slightly larger brown bears with strange fur.
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