Every polar bear alive today shares a common maternal ancestor, and it isn't even a bear from the same species. Their mitochondrial DNA reveals a 100,000 year story of interbreeding and hybridization...and the story is far from over.
Mitochondrial DNA is passed along exclusively from mothers to their offspring. Because this DNA never mixes with genetic material from the father, it remains pretty much completely unchanged over the generations. By studying the mitochondrial DNA of the living members of a given species, it's possible to work backwards and figure out when their most recent common maternal ancestor - in other words, the individual that supplied that same piece of mitochondrial DNA to all her descendants - must have lived. (For a more complete overview of what mitochondrial DNA is and why it's important, check out this earlier post.)