All living polar bears can trace their genetic lineage back to a single, female ancestor — a brown bear from Ireland, who lived around 20,000 to 50,000 years ago.

Thanks to climate change in the North Atlantic ice sheets around the time of the last ice age, the two types of bears would have periodically overlapped. In Ireland, it appears that they interbred, leading to a hybridization event that plopped maternal DNA from brown bears into polar bears.

http://www.wired.com/wiredscience/2011/07/polar-bears-origins/